“How Behavior Evolves and Why It Matters,” by Marlene Zuk

Zuk, Marlene. (2022). Dancing Cockatoos and the Dead Man Test: How Behavior Evolves and Why It Matters. NY: W. W. Norton & Company.

Contents

• Introduction, vii–xix

1. Narwhals and the Dead Man: Why ls Behavior So Hard to Define?, 1–23
2. Snakes, Spiders, Bees, and Princesses: How Behavior Evolves, 24–45
3. Clean-Minded Bees and Courtship Genes: The lnheritance of Behavior, 46–68
4. Raised by Wolves — Would It Really Be So Bad?: The First Domestication, 69–91
5. Wild-Mannered: The Other Domestics, 92–115
6. The Anxious Invertebrate: Animal Mental Illness, 116–138
7. Dancing Cockatoos and Thieving Gulls: Bird Brains and the Evolution of Cognition, 139–161
8. A Soft Spot for Hard Creatures: Invertebrate Intelligence, 162–182
9. Talking with the Birds and the Bees. And the Monkeys: Animal Language, 184–206
10. The Faithful Coucal: Animals, Genes, and Sex Roles, 207–230
11. Protect and Defend: Behavior and Disease, 231–256
    Acknowledgments, 257–258
    Notes, 259–280
    Bibliography, 281–318
    Index, 319–330

Introduction, vii–xix

Marlene Zuk starts off her book by answering the nature–nurture debate with a vehement “BOTH!” She goes on to say that it’s nearly impossible to disentangle the intertwining of how genetics affects environment and environment affects genetics, in a constant intermingling over evolutionary time. She gives examples of how this entangling affects all aspects of an organism’s physiology, but this intermingling becomes even more difficult to tease apart when discussing an organism’s behavior. In Zuk’s view, it’s a fool’s errand to try to point to just heredity/nature or solely the environment/nurture in causing, prompting, or determining the behavior of an organism. It’s understandable that people in the media would latch onto one or the other, but she finds it perplexing when scientists do so, too.

One physiological example helps to illustrate this entanglement: Some newborns are born with phenylketonuria (PKU), a disorder in which the newborn inherits two copies of a defective gene that makes it impossible for the infant (and child/adult) to metabolize phenylalanine, an amino acid (part of a protein). If the infant consumes a diet that includes phenylalanine, this un-metabolized amino acid accumulates in its bloodstream, causing severe intellectual disabilities. If, however, PKU is detected soon after birth, the child can be given a special diet offering proteins that do not include phenylalanine, the child will develop completely normally and not suffer any cognitive impairments whatsoever. A “nature” proponent could note that the problem is entirely hereditary; without the defective genes, the child would not have PKU and would develop normally. A “nurture” proponent could argue that the problem is entirely environmental; with the proper diet, the child will not suffer from the symptoms of PKU and will develop normally. To Zuk, both proponents are right, and both are wrong. It is the interaction of heredity and environment that either causes or avoids problems.

Zuk uses the PKU example to illustrate the foolishness of the nature–nurture debate and says such a debate is particularly inadequate when considering behavior and its evolution. “I hope that understanding how behavior evolves helps us see both other animals and ourselves better, and that the battle between nature and nurture is not worth fighting” (p. xix).

1. Narwhals and the Dead Man: Why ls Behavior So Hard to Define?, 1–23

Zuk notes that it’s very hard to define behavior. For instance, when leukocytes (white blood cells) surround and consume pathogens, are they behaving? Nope. Not if you define behavior as an organism’s actions; leukocytes are cells, not independent organisms. When a Venus flytrap is triggered by a fly, snaps shut, and digests the fly, is that behavior? Zuk says Nope. In her view, plant intelligence doesn’t exist, so intentional plant behavior doesn’t either.
Please cf. my blogs on two books that offer a different view: https://bird-brain.org/2025/06/01/planta-sapiens-by-paco-calvo/ and https://bird-brain.org/2025/06/14/the-light-eaters-by-zoe-schlanger/
This is Zuk’s book, however, so we’ll stick to Zuk’s definition of behavior, which excludes behavior by plants and most other non-animal organisms. She does make an exception, however, for slime molds, which she reveals as showing remarkable problem-solving skills, such as navigating mazes. (I think she should rethink plants, but it’s her book, so no plant behavior here.) She also excludes microbes such as bacteria. They may appear to behave, but they don’t fit her definition of behavior.

One set of criteria for defining behavior lists these requirements:

• something a human or other animal does or says
• a movement that has an impact on the organism’s environment
• an action that’s influenced by the environment
• can be observed, described, recorded
• can NOT be done by a dead man (i.e. the “Dead Man Test” — If a dead man, or a teddy bear, can do it, it’s NOT behavior!)

In addition, Zuk holds that both traits (characteristics such as sexuality, size, coloration) and behavior (as she has defined it) evolve. Traits can even change in a single individual, depending on environmental influences. For instance, in a given clownfish family, the female is the largest fish in the family, and the rest of the fish are males, descending in size from largest to smallest. If the female dies, the largest male becomes a female and grows to be as large as the female was; each succeeding size of clownfish grows to be just the right size to assume the correct ranking in size. In meerkats, if one meerkat is fed extra food and grows larger than its mates, the mates — without the extra food — will grow to be equal in size to the overfed meerkat.

Zuk also cited an example of how a bipedal goat, born with just two legs, managed to develop its skeleton and musculature to support walking bipedally. “What genes do isn’t a one-and-done event; it is an ongoing process . . . in which the environment affects the genes, the animal changes its behavior, that behavior then affects which genes are used, and so on” (p. 13). These interactions occur constantly in humans, too: “Your liver is the size that it is because of your genes interacting with your diet” (p. 15).

Narwhals (marine mammals) can dive nearly a mile downward to catch squids and other prey, while holding their breath; during dives, their heartrates slow to 3–6 beats/minute. Like other mammals, narwhals respond to stress in one of three ways: fight, flight, or freeze. Because they live in remote areas of the Arctic, they have nearly no predators (occasional orcas or Inuit hunters), so their stress response is needed infrequently. If, while diving, a narwhal hears the loud noises of a ship (naval or commercial), it may try to flee — causing cardiac arrest when its heartrate has slowed for the dive. Narwhals are dying when these ships intrude on their hunting grounds. Their formerly adaptive behavior is deadly when the environment changes.

Birdsong is another interaction of heredity and environment. Many species of songbirds have characteristic songs, which birders can readily identify. Nonetheless, regional “dialects” appear in their songs, such that a White-crowned Sparrow from Southern California sings slightly differently than one from Northern California. Their genes provide the basic form of the song, but their environments shape those songs.

Chimpanzees also show “cultural” differences in tool use; chimps from one community may use a tool to catch termites slightly differently from how a chimp from another community does so. Some other “culturally transmitted” behaviors include British tits (birds) opening milk-bottle caps, New Caledonian Crows making tools, and dolphins wearing sea sponges on their snouts while fishing.

Ethologist Niko Tinbergen and primatologist Frans de Waal have both noted that drawing a sharp line between human behavior and capabilities and those of other animals may be challenging, if not impossible. Tool use was once deemed the sole province of humans, but evidence points to the contrary. Historian Jacob Bronowski proposed that frontal copulation was distinctly human behavior, but other animals (e.g., dolphins, bonobos) do so, too. Language has been similarly challenged as unique to humans.

2. Snakes, Spiders, Bees, and Princesses: How Behavior Evolves, 24–45

To illustrate how physiological characteristics and behavior can evolve, Zuk describes a viper that has developed a tail that looks remarkably like a spider (physiological characteristic), which the snake moves in spidery ways, enhancing the deception. For spider-eating birds, this lure is irresistible, and many a bird succumbs to the deceit.

How did this successful mimicry evolve? Snakes aren’t particularly brainy, and they have poor eyesight, so it seems like a peculiar evolutionary development. As Zuk points out, over evolutionary time, a series of mutations in physiology and behavior easily explain it. The snake never willfully intended to develop a spider-looking tail or to make spidery manipulations, but over time, as the snake evolved, its tail and its behavior became increasingly successful in luring unsuspecting birds within striking distance, and the well-fed snake reproduced more of its kind.

Zuk also mentioned the bumblebees observed by Consuelo De Moraes, described in my blog about Light Eaters, by Zoë Schlanger: https://bird-brain.org/2025/06/14/the-light-eaters-by-zoe-schlanger/#bumblebees. In the case of the bumblebees, the plant-nibbling behavior (which prompts flowers to produce nutritious pollen) isn’t genetically inherited, but it did evolve through an interaction with the environment.

Camouflage coloration in animals (Zuk highlights birds’ cryptic plumage) helps them to hide from predators. The cryptically plumed birds who hide more successfully are more likely to survive and to reproduce. Over evolutionary time, cryptic plumage becomes increasingly common in these species.

Complex characteristics evolve over time, via myriad intermediate steps. In Zuk’s view, complex behavior evolves similarly, with intermediate steps. Initially, a fidgety viper may simply have flicked its tail a bit and was rewarded by having better success in luring a bird within striking distance. Generation after generation, the fidgeting became more spider-like and led to even better success in attracting birds. Eventually, the viper’s descendants show an astonishing ability to mimic the appearance and behavior of a spider, with resulting great success in attracting its birdy prey.

Paleontologists haven’t found fossilized behavior, but they have found indications of behavior. For instance, fossilized footprints can hint at how an animal moved, how much it weighed, how quickly it moved, and so on. Fossilized teeth can indicate what an animal ate; fossilized skeletons can indicate an animal’s stance, its movements, and even its maximum speed of movement.

Another way to study animal physiology and behavior is to notice homologues, how a similar structure or behavior evolved from a common ancestor. For instance, a dolphin’s flipper, a bird’s wing, and a human arm all evolved from a common ancestor, and their divergences in physiology tell us something about their behavior.

On the other hand, sometimes physiological structures and behavior can arise from separate origins, yet they converge on a similar structure and behavior. For instance, North American flying squirrels and Australian marsupial sugar gliders both have their front and hind limbs connected by furry membranes. In both species, the right front paw and the right rear paw are connected by a skin “sail,” and the same is true for the left side. Both species can easily glide (or “fly”) from one tree to another. Both evolved separately, but both have convergent evolution of this physiological mechanism for gliding.

Many male hummingbirds engage in amazing aerial displays to attract mates, and during their displays, they make distinctive moves, and their wings produce species-specific sounds. For instance, though both Anna’s Hummingbirds and Costa’s Hummingbirds make these aerial displays, Anna’s males make tall arching ovular loops, whereas Costa’s males use shallower paths.

Whereas hummingbirds are endemic to the Americas, manakins are found only in Central and South America. Like hummingbird males, manakin males have distinctive species-specific displays and wing sounds. For the manakins, the physiology and the behavior of their displays co-evolved, so that their bone structure made possible some of their wing sounds. The basic homologue of a male manakin’s display accompanied by distinctive wing sounds took on many variations in each species of manakin.

When trying to understand the role of evolution in physiology and behavior, it can be challenging to differentiate homologous evolution from convergent evolution. Just because two physiological structures or behaviors look alike doesn’t guarantee that they evolved from a common ancestor.

In addition to homologous evolution and convergent evolution, a third way in which evolution occurs is from simple forms to complex ones. For instance, a simple sensor that can detect light from dark, over evolutionary time, can evolve into the myriad kinds of eyes observed in animals alive today. We should not, however, conclude that these increases in complexity signify that evolution involves a linear progression from primitive to sophisticated forms. One-celled organisms continue to thrive, as do many other simpler forms of life. The key is whether they have adapted to their environment and survive to reproduce, not whether they have numerous physiological bells and whistles.

Zuk takes a brief detour to illustrate how un-natural selection can favor particular traits, in sometimes unexpected ways. Australian herders value the Australian kelpie, a dog that happily spends hours herding sheep and cattle. The humans who bred the kelpie doubtless were looking for many particular traits in their dogs. However, a DNA analysis of kelpies’ genes revealed that the most distinctive genetic trait was this: Kelpies have a high tolerance for pain. What? It turns out that the ground covering in the Australian outback is prickly, and dogs that can’t tolerate pain will simply refuse to spend much time running around, herding livestock. Thus, the herders’ selection process ended up selecting for a trait that they didn’t even know they wanted. Zuk suggests that natural selection may work in similar ways, favoring traits in surprising ways.

Many animals adapt flexibly to a variety of environmental niches, and quite a few animals need particular environmental niches in which to live. For instance, as caterpillars, monarch butterflies need to eat milkweed plants (and their parents must lay their eggs on milkweed plants). As adults, monarchs need air temperatures above 55̊ F. in order to fly.

Alternatively, some animals create their own niches. Beavers, for instance, respond to a fast-moving stream or river by building dams, to slow down the flow. Relatively quickly, beavers will build dams, create ponds, and build their lodge homes in the ponds. Their offspring are relatively protected from predators, and they can eat bark and other woody materials. In doing so, the beaver creates its own ecosystem, its own environmental niche, and birds, fish, insects, and mammals thrive in this beaver-built habitat.

“Charles Darwin was extremely interested in how behavior, and emotions, could evolve. He was particularly fascinated by the idea that we could trace similarities in behavior across different kinds of animals much the same way we could see resemblances in their bones or teeth” (p. 42). Even after most scientists grudgingly accepted Darwin’s views on evolution and natural selection, they continued to reject his notions about emotions and behavior among animals other than humans. More recently, primatologist Frans de Waal has affirmed that primates and perhaps other “nonhuman animals have emotions that are virtually identical to those of humans” (p. 43). Many scientists dispute de Waal’s notions, too. Zuk’s view is that we should neither reject the idea that nonhuman animals have emotions similar to our own nor fully embrace the idea that their emotions are identical to ours.

3. Clean-Minded Bees and Courtship Genes: The lnheritance of Behavior, 46–68

According to Zuk, both physiological traits and behavior are influenced by interactions between our genes and our environment.

A nasty pathogen can invade beehives and kill all the larvae, wiping out the bee colony. Within a given species of bees, some bees were able to prevent having the disease spread from infected larvae to the remaining larvae. It turns out that these bees possessed two genes: one gene that led the worker bees to uncap the wax of the hexagonal cell containing an infected larva, and another gene leading the worker bees to remove and discard the infected larva. If the bees possessed only one of the two genes, the disease continued to spread. However, worker bees carrying both genes were able to limit the disease from spreading to all the larvae. These genetic variants interacted with the environment (the pathogens) to ensure that the colony survived.

When Margaret Bastock observed that fruit flies (the often-studied Drosophila melagonaster) showed varying success at reproduction, she wondered why. In particular the yellow-colored fruit flies were less reproductive than the others. After careful study, she noticed that the yellow males engaged in courtship behaviors less frequently and for less time than the other males. It turns out that something on the gene for yellowness also affected the male flies’ courtship behavior. Such differences could eventually lead to evolutionary differences, too.

Genetics isn’t straightforward, however. A “gene” is a particular chunk of DNA on a chromosome. Before that gene is expressed, it must be translated from DNA (deoxyribonucleic acid) to RNA (ribonucleic acid). Then, at the right time for it to be expressed, the RNA will tell the body what proteins to make in order to express the gene. If you’re wondering about how timing comes into play, just recall your adolescence. Situational factors also come into play. Among genetically identical ants, one will become a queen, reproducing all of the eggs and larvae for the colony, and the rest will become workers, tending to the larvae, finding food, and attending to the queen.

Genes also affect behavior among bees. African honeybees live in dangerous environments, so it is advantageous for them to aggressively defend their hives, on a hair-trigger alarm. Sure enough, their brains show greater expression of genes for a hormone that sends up a chemical alarm in case of perceived danger, prompting these bees to follow and sting any perceived intruders to the colony.

“Heritability” can be confusing. It applies to groups, but not to individuals. Within a group of individuals in an identical environment, the differences among them is said to be high in heritability — because environmental factors have been held constant, while the genetic factors would vary. Within a group of individuals who have the same heredity, living in various environments, the differences among them would be low in heritability — because the genetic factors were held constant, but the environment varied. The heritability of a given trait can be assessed in populations, but not in individuals.

Researchers trying to differentiate hereditary influences from environmental influences have often studied the traits of identical twins, reared apart, in different environments (e.g., adopted by separate families), compared with fraternal twins reared together in the same environments. Presumably the traits shared by the identical twins are affected mostly by heredity, and differences between them are presumed to be environmentally caused. The opposite would be true for fraternal twins reared together. If identical twins are more likely to show similar musical abilities, rates of bipolar disorder, and onset of puberty, compared with fraternal twins, these commonalities are presumed to indicate hereditary influences. If fraternal twins are more likely to share puzzle-solving abilities, rates of depression, and age at first marriage, compared with identical twins, these commonalities are presumed to indicate environmental influences. (I made up these examples! These findings indicate nothing other than my own fabrications!)

Heritability interacts with “strength of selection” to determine the evolution of a characteristic. Strength of selection is simply the degree to which the characteristic offers a selective advantage to the individual. A trait that is highly heritable but has little strength of selection is neither more nor less likely to lead to natural selection; same goes for a trait that isn’t highly heritable but offers a strong selective advantage. To lead to natural selection, the characteristic must both be highly heritable and offer a strong selective advantage.

Sophisticated genome studies have often led to a false impression of particular genes causing particular traits or even behaviors. Genetics doesn’t work that straightforwardly. For one thing, most traits arise from multiple genes interacting in complex ways. For another, genes interact with the environment in ways we can’t yet predict. Few single genes have been found to lead to specific characteristics. In addition, it may be unwise to try to attribute behavior to genetics, as such attributions may lead to holding people less accountable for their behaviors.

4. Raised by Wolves —Would It Really Be So Bad?: The First Domestication, 69–91

The process of domestication of animals — that is, UN-natural selection — can give some insights into the process of evolutionary natural selection. In fact, Charles Darwin observed the domestication of animals — especially pigeons — while formulating his ideas about evolution. Contemporary scientists can obtain samples of ancient DNA of animals — such as dogs and wolves — and compare them with the DNA of contemporary animals, including dogs and wolves. Dogs and wolves, ancient and contemporary, share much of their DNA, and they originated in similar locations.

The stories about how dogs evolved from wolf ancestors aren’t as clear-cut as some might say. Some have proposed that one or two hungry wolves would hang out near human encampments, where the humans would toss them some food; this evolved over time such that a human-tolerant wolf became a friendly dog, domesticated by humans. Instead, some evidence suggests that the interaction may have been more mutual, less one-sided. Perhaps occasionally, a solitary wolf may have partnered with a human hunting party, to the mutual benefit of both the wolf and the humans. A wolf could help the humans to bring down larger prey than either the wolf alone or the humans alone could do. Perhaps even this successful interspecies cooperation helped nudge humans toward greater cooperation as a species. This canid–human relationship could be seen as a mutual domestication, with both parties benefitting and both parties contributing. (Vanessa Woods and Brian Hare even suggest that Homo sapiens may have outcompeted Neanderthals because of our alliance with canids.)

Contemporary dogs and wolves share many genetic characteristics, but they’re also distinct, both physiologically and behaviorally. For instance, whereas wolves rely primarily on smell alone, dogs also use vision and hearing to navigate the world.

In thinking about wolves, dogs, and other animals, anthropologist Richard Wrangham distinguishes between reactive aggression and proactive aggression. In reactive aggression, an animal reacts aggressively when another animal (e.g., a human) behaves aggressively, such as by encroaching on its territory, menacing, or threatening. If not threatened, however, this animal doesn’t behave aggressively. In contrast, in proactive aggression, the aggressive behavior is planned, intentional, not just a response to a perceived threat. When taming an animal, such as a dog, we look for reduced reactive aggression, as well as overall aggression.

Charles Darwin observed that in the process of taming an animal — selectively choosing animals with less reactive aggression — other characteristics often accompany the reduced aggression. Even when not selecting for floppy ears, spotted fur, white feet, or curly tails, these traits often appear in conjunction with the characteristics of reduced aggression. Less aggressive animals also tend to be physically smaller, with shortened faces, and relatively smaller brains. (She also mentioned the study of fox domestication in the USSR, which she noted has been questioned more recently.)

Some studies suggest that dogs respond to human speech distinctively, differentiating between “positive” words (e.g., “clever”) and neutral words (e.g., “if”). Dogs appeared to use one part of their brains when responding to the emotional tones of the words and another responding to the actual words. That’s not to say that they understood the words, however. Moreover, Zuk compares dogs with other animals, finding that they show outstanding abilities on some tasks and unremarkable abilities on others. Pigs, chimps, and pigeons likewise have some areas of excellence and some areas of less-than-stellar performance.

Elinor Karlsson has initiated an ambitious research project, “Darwin’s Dogs,” through which she is trying to enlist the help of every dog owner in the world. She hopes to gather samples of dog saliva from every dog, along with answers to questions about each dog, such as its physiological disorders, behavioral disorders, special skills (e.g., exceptional smelling ability), personality (e.g., well suited to being a companion dog), normal behaviors (e.g., fetches or not), distinctive behaviors (e.g., “Does your dog cross its paws before it lies down?”), and more. She then hopes to use these massive data to see how genetics may interplay with each dog’s physiology and behavior.

M. Nagasawa and colleagues have observed that for humans and dogs who own each other, when they gaze into each other’s eyes, their oxytocin levels rise. Sometimes called the “love hormone,” oxytocin is the hormone that rises in mothers and their newborns when together. Whatever else we figure out about dogs, their ancestry, and their genetics, dogs and their humans truly love one another.

5. Wild-Mannered: The Other Domestics, 92–115

Zuk suggests differentiating among animals that are

• tame — animals who are less likely to kill us than their wild counterparts
• domesticated — animals who are bred selectively for traits that we want them to have
• feral — animals that had been domesticated but that have been living in the wild all their lives or even for many generations (e.g., the Rock Pigeon, aka Rock Dove)

Domestication can take various forms — living in our homes, such as dogs; living in out-buildings, such as horses; living in outdoor fenced areas, such as cattle; or moving from place to place, as herds, such as sheep. Cats are considered synanthropes — they live with us, share our homes, and even mutually prompt the same elevations in oxytocin in one another. Even so, we rarely control the behavior of cats or breed them for particular traits. Cats gladly provide a service, though, killing any rodents who get into our homes. Both wild cats and domestic cats groom themselves, mark their scent, and have similar behaviors, but domestic cats are less active and are more reward seeking than wild cats.

If we take a broad view of domestication, it can be said that some species of leaf-cutter ants have domesticated their own fungi, which they farm by feeding bits of leaves to the fungi. These fungi don’t exist anywhere but in their domesticated settings, and they wouldn’t survive without their ant “farmers.”

DNA evidence shows that even domesticated animals aren’t purely bred selectively. DNA shows that most domesticated animals have been interbreeding back and forth with wild animals over the centuries. A wild boar may mate with a domestic pig, a wild wolf may mate with a domestic dog, and so on. Even so, domestication — un-natural selection — typically occurs much more rapidly than natural selection.

The domestication of chickens is thought to have started with some people’s enjoyment of watching cockfighting. In actual cock fights, both cocks almost always survive, and serious injury is rare. When humans instigate the fights, however, the fights are gruesomely brutal, leading to the death of at least one of the cocks, and often grievous injury to the “winner.”

The relationship didn’t end with fights, however. Most birds breed only one season of the year, lay just one egg per day until they reach a given clutch size (usually 1–12 or so, typically 2–4), incubate those eggs until they hatch, and wait for another year to have more eggs. In the Red Junglefowl (ancestor of our domestic chickens) and a few other species, however, if you remove the egg as soon as it’s hatched, the hen will lay another egg the following day. Humans took advantage of this proclivity to induce the hen to lay an egg every day, even after breeding season. And thus began a centuries-long relationship.

Zuk also discussed guinea pigs, which originated as domesticated pet cavies in the Andes. Like other domesticated animals, though raised as pets, they were also eaten as food. After the Spanish conquest of Peru and Colombia, guinea pigs were transported to Europe, where at least one made it to the lap of Queen Elizabeth I.

In the late 1800s, microbiologists found that guinea pigs made docile lab animal subjects for studying diseases such as anthrax and tuberculosis. To study these diseases, vivisection was used. When author and renowned vegetarian George Bernard Shaw heard of these vivisections, he helped turn public opinion against using guinea pigs. Pretty soon, being a “guinea pig” for scientific study was widely criticized. Nowadays, they’re rarely used in biological research, but they are studied for their behavior, and mice, rats, and other rodents are subjects of medical research.

Fun fact: Wild bees have an effective strategy for surviving in cold climates. When the air temperature gets too low, the bees shiver, rapidly vibrating their wing muscles, which raises each bee’s body temperature. If multiple bees shiver, raising their body temperatures, the temperature of the whole hive rises. This strategy works best in a hive located in a tree or other well-insulated place. Commercial beehives are too thin and poorly insulated for this to work for domesticated bees.

Some have suggested that pampered domesticated animals have smaller brains. Zuk questions that. In a study of domesticated hens, “better [egg] layers turned out to be better learners” (p. 114). When chickens are bred to have an increased body weight, their brain mass doesn’t increase by the same amount. These oversized chickens appear to have proportionally smaller brains, but that difference is deceptive. Zuk does, however, agree that domesticated chickens have smaller cerebellums. The cerebellum is the area of the brain that coordinates body movement and balance. Domesticated caged chickens, who rarely roam freely, would have little need for a large cerebellum.

“Animals are domesticated in different ways for different reasons, but they all share . . . entanglement” of genes and environment influencing their behavior (p. 115).

6. The Anxious Invertebrate: Animal Mental Illness, 116–138

So far, scientists haven’t fully studied or understood how mental disorders can arise in other animals — not that we fully understand mental disorders in humans, either. Some scientists dismiss altogether the idea that animals can have mental disorders; others insist that the only reason for animals to experience mental disorders is because of human mistreatment; and others suggest that wild animals can suffer from mental disorders, but evidence is lacking at present.

For most mental disorders in humans, we can’t identify a specific gene or group of genes responsible, and we can’t even point to a region of the brain where the disorder originates. Huntington’s disease is a rare exception. Early indications suggest that Alzheimer’s disease may have a genetic marker, but that doesn’t necessarily mean a genetic cause linking it to particular lesions on the brain.

It seems unlikely that natural selection would favor the emergence of mental disorders, but it may be that any genes contributing to these disorders have accompanied more favorable ones. How could this work? For example, humans are highly susceptible to lower back pain. Doubtless, we don’t possess genes specifically designed to cause this vulnerability. However, lower back pain is linked to our being bipedal. Though we’ve undergone many physiological adaptations to support being bipedal, our easily stressed lower backs have stayed with us. The same might be true of mental disorders. For instance, a modest degree of anxiety might make us more alert to threats in our environment. When anxiety runs amok, however, we view it as a mental disorder.

It may be valuable also to distinguish between mental disorders that arose because of natural selection (e.g., anxiety in a threatening environment) and those that arose despite natural selection (e.g., disorders that don’t appear until after a person has reproduced successfully, such as late-onset Alzheimer’s disease). When viewed in this light, it would seem that other animals might be susceptible to some mental disorders, as well.

Canines have been identified as showing CCD — canine compulsive disorder — repeatedly showing particular behavior, such as tail-chasing, paw-licking, and so on. There also seems to be a genetic contribution to this disorder, in that some breeds (e.g., Dobermans) seem more susceptible to it than others. North American deer mice also sometimes show OCD (obsessive–compulsive disorder).

In humans and in other animals, it can be hard to distinguish between behavior that is normal, mostly normal, and abnormal. Just how often does your dog have to chase its tail for you to call it CCD? Many animals show “stereotyped behavior,” in which they seem to perform a ritualistic behavior for reasons that aren’t apparent. When do these behaviors become a “mental disorder”?

Crayfish have been observed to show great anxiety. Like many other crustaceans, crayfish have a hard exterior exoskeleton, which they must shed periodically as they grow larger. Whenever they molt out of their protective exoskeleton, they’re highly vulnerable to predation, so they seek safe places to hide until they make a newer, bigger exoskeleton out of chitin. Is it really a mental disorder for these crustaceans to be anxious? Interestingly, serotonin, a neurotransmitter in humans, calms an anxious crayfish, just as it does an anxious human. Serotonin also has behavioral effects on some insects, pigeons, lizards, reef fish, worms, and octopuses (pet peeve: please don’t say “octopi”! — unless you’re Ogden Nash).

Animals can also learn to have mental disorders. For instance, when dogs have been prevented from escaping a painful shock over a series of trials, when they were later in a situation to escape the shock, they didn’t do so. They had learned to feel helpless. Learned helplessness is thought to be one contributor to depression in humans.

Mice and rats can’t become alcoholics! Even when induced to drink alcohol to excess, they refuse to do so. On the other hand, many animal species who consume fruit or nectar — which can ferment to become alcohol — have genes for processing alcohol despite its toxicity. Before we jump to conclusions, it should be noted that many animals that eat only insects have similar genes.

Can other animals have mental disorders? Maybe. Are these disorders influenced by heredity or by the environment? Yes.

7. Dancing Cockatoos and Thieving Gulls: Bird Brains and the Evolution of Cognition, 139–161

Among the many instances of bird braininess are the wily gulls of Bristol, England, who appear on school rooftops just before snacktime and lunchtime, so they can swoop down and steal food from hungry schoolchildren. Countless other examples abound about how urban birds cleverly pilfer food from unsuspecting or defenseless humans.

Perhaps the best-known clever bird was Irene Pepperberg’s African Gray Parrot, Alex, who lived to be at least 31 years old, learned and correctly used 150 words, sorted objects by multiple categories, and performed numerous other cognitive tasks. Another parrot, Snowball the Sulphur-crested Cockatoo, spontaneously danced rhythmically to music, engaging in 14 distinct dance moves, most of which her human partner didn’t do. Other cockatoos showed some ability to make and use tools. When wild cockatoos were motivated to solve puzzles, they solved them just as readily as the captive birds.

Corvids, too, show remarkable cognitive skills. Near New Zealand, New Caledonian Crows not only use tools, but also make two different kinds of tools. The crows also teach one another how to make and use these tools. Crows also solve multi-step puzzles, such as a puzzle in which they must obtain a tool, set it aside, perform some other tasks, then use the tool they obtained earlier. In research settings, crows who were allowed to use tools showed more exploratory behavior and were more willing to investigate a box for which they didn’t know its contents. They also showed some understanding about future events, such as by obtaining tokens to be used at a future time.

Zuk cautions against inferring intelligence based on brain size, even brain size in proportion to body size. “The bony-eared assfish [has] a brain that is less than [0.3%] of its body size, thought to be the smallest brain of all vertebrates” (p. 147), but we can’t infer its intelligence based solely on that percentage. Zuk also warns against inferring that speed of evolution indicates some kind of superiority. Microbes and viruses are champs of rapid evolution, and though they can beat us badly from time to time, few would argue that they’re a superior life form.

Bird brains are more similar to mammal brains than was previously thought. Parts of bird brains were previously given distinctive names, but they actually closely resemble comparable parts in mammal brains.

Sociality seems to be associated with problem-solving abilities, and some birds (e.g. Pinyon Jays) that were more sociable, living in flocks, more readily solved some problems than some birds (e.g., scrub jays) that lived mostly alone or in small groups.

Biologist Louis Lefebvre decided to greatly expand his purview of bird behavior by looking at reports by bird-watching citizen scientists. Many bird watchers revel in reporting distinctive behaviors among the bird species they watch, and Lefebvre analyzed these reports, sorting through the species and the behavioral reports. He and his colleagues then analyzed the forebrain sizes (relative to body sizes) of the birds and found that “birds that were more likely to innovate also had larger forebrains” (p. 153). Further study showed that innovations in feeding behavior was linked to larger forebrains, but innovations in nesting wasn’t. Another result of Lefebvre’s research: Birds willing to eat unusual foods were less likely to go extinct than birds with more limited diets.

Why don’t all birds have big brains? Because brains cost a lot to maintain. They consume much more energy for their size than other kinds of body tissue. If they’re going to justify this extraordinary expense, they must offer some clear benefit. It turns out that having a bigger brain (in relation to body size) was more likely in places where the seasons changed dramatically, and the plants and ground cover changed throughout the year. Of course, we can’t know whether these seasonal changes naturally selected bigger brains or birds with bigger brains moved to these locations — or some third factor affected both brain size and location.

An even stronger association appears regarding a bird’s lifespan and its brain size. Birds who live longer tend to have bigger brains, and birds with bigger brains tend to live longer. These long-lived, bigger-brained birds also tended to lay relatively bigger eggs. It’s possible that these parents invest more nutrients in their young, to launch them for a long life. Their young also tend to stay with their parents for longer periods. As an extreme example, Siberian Jay “youngsters” may stay with their parents up to four years after hatching. Given the predation threat of Northern Goshawks, staying with a family for longer is probably a prudent move.

Some tests of cognitive abilities don’t necessarily give the insight researchers might hope for. For instance, many human children and other animals flunk the highly acclaimed “mirror test” — in which the animal is shown a mirror, to see whether the animal recognizes itself — but cleaner fish readily pass it. Numerous other “tests” show similar paradoxical results.

One final thought: It appears that urban mice have bigger brains than rural mice, perhaps “because the greater levels of variability in cities selected for behavioral flexibility” (p. 161). Go figure!

8. A Soft Spot for Hard Creatures: Invertebrate Intelligence, 162–182

Vespa hornets are ginormous wasps, compared with small Asian honeybees, and they don’t bother making their own hives for raising their larvae. Instead, these hornets will attack and invade honeybee hives, kill off all the adult and larval honeybees, and take over the hive. The tiny honeybees can surround and kill a solitary wasp, but they appear to be defenseless against the hornets. But not so fast. These clever bees have come up with a crappy solution to their problem. The bees gather up bits of dung from water buffalo and carefully place these dung bits all around the hive. If they place enough dung around their hive, the dung deters the hornets from attacking. It could be said that the bees innovated using the dung as a tool for deterring hornet attacks.

In another example of invertebrate tool use, ants wanted to gain access to a container of a sugary solution, but doing so risked drowning. So the ants started plastering sand around the edges of the nectar-holding container; the sand wicked the sugary solution, so the ants could safely drink it. As the level of the solution lowered, they adapted the structure to keep having safe access to it. Could the sand be considered a tool? Did the ants show awareness and strategic planning? Did the dung-using bees?

According to Zuk, just as natural selection can lead to complex digestive systems that can process an omnivorous diet, it can lead to behavioral flexibility adapting to a varying and complex environment. As a behavioral ecologist and entomologist, Zuk particularly enjoys studying invertebrates: “We can actually induce evolution in them without having to wait for centuries or millennia to pass . . . . Their small size and short lifespans challenge the conventional wisdom about which animals do complicated things” (p. 165).

Many ants, wasps, and honeybees are known for their sociability. Not all of them have equal social skills, however. Within a given colony of honeybees, a minority of them lacked the social skills of the rest. It turns out that some of the genes that distinguished these less-sociable honeybees are also present in at least some humans who have autism spectrum disorder, but not in humans with schizophrenia or depression.

In another study, it was found that bees readily distinguish among many different flowers when searching for nectar and pollen. Not particularly surprising. It was, however, surprising to find that bees can also distinguish among human faces and can recognize particular individuals. Bees can also associate number symbols with two items or three items; after 50 training trials, they correctly matched the symbol and the numbers 80–90% of the time. (African Gray Parrots, chimpanzees, and some monkeys can also do so.)

Another invertebrate, shore crabs, figured out how to navigate mazes — if offered a bit of crushed mussel as a reward. More impressive are octopuses, which can readily escape confinement, solve puzzles, form relationships with humans, perform simple tasks to obtain rewards, show playful behavior (often considered a hallmark of intelligence), and more. On the other hand, they don’t have complex social interactions or show complex communication. A profound limit to octopus intelligence is their very short life spans, maybe up to 4 or 5 years.

Four factors increase the likelihood of an animal’s having intelligence:

1. Live in a complex environment requiring flexible strategies for obtaining food
2. Live in a social group requiring you to recognize members of your group (and what to expect from individual members) and to communicate with members of your group.
3. Live a long time, so that learning can accumulate and offer an adaptive advantage.
4. Have a prolonged juvenile period, during which you can learn from your family and your community.

According to Haller’s Rule, “among vertebrates, smaller individuals tend to have larger brains relative to their body size” (pp. 173–174). Biologist Bill Eberhard has tried to discover whether the same rule would apply to invertebrates, too. His preliminary evidence suggests that it does. Body size predicted brain size. Even teeny-tiny spiders can produce elaborate orb webs; trying to understand how such a tiny spider could fit a brain into its tiny body, he found out that its brain extends into its legs. It literally thinks on its feet!

On the other hand, insects — or at least bees (93 species) — break the rule about sociability and braininess. Social species of bees have smaller brains than solitary species. Likewise relative brain sizes were bigger among bees that had specialized diets (eating just one kind of plant), and bees whose queens died after 1 year, rather than extending through multiple generations of workers.

Fun fact: Bees learn colors more quickly than vertebrates do (including humans!).

Zuk has a special fondness for crickets, and she highlighted the relatively rapid evolution of a particular cricket species. The males of this species attract females by loudly rubbing their wings together to make a distinctive “song” — the louder, the better, the more females they’ll attract. Unfortunately, the female of a species of parasitic flies loves to lay her larvae on and around the bodies of these crickets; her larvae burrow into the cricket and eat him from the inside. When these females hear the male crickets singing, they can easily find the crickets to parasitize them.

Within a few generations, some of the male crickets acquired a gene mutation that made them unable to sing. That’s great for them to avoid being parasitized, but how can they attract females? These non-singers lurk near the singers, who attract females, and the non-singers mate with the passing females. For the species to continue, the non-singing males will need the singing males to continue to reproduce. Zuk notes that crickets don’t have to have intelligent awareness to lurk near the singers. Rather, cricket males and females probably have a “rule of thumb” to avoid silence and to move closer to cricket sounds.

When considering animal intelligence, especially invertebrate intelligence, Zuk reminds us that humans don’t always show conscious, rational behavior either, citing several examples. You can probably think of a few yourself. Instead, Zuk suggests thinking about behavior as evolving in individual species in just the way that physiological characteristics evolve.

9. Talking with the Birds and the Bees. And the Monkeys: Animal Language, 184–206

Scientists now acknowledge that crows and bees can use tools, and some fish can pass the “mirror test,” but only humans use language, right? Maybe, maybe not. It depends on how we define language. Using language is a complex behavior, and like other complex behavior — or simple behavior, for that matter — it evolved over time, in the same way that simple and complex physiology evolved.

Some of the remarkable developments of evolution include going from unicellular to multicellular organisms, and from asexual to sexual reproduction. Both of these developments paved the way for myriad other developments and evolutionary changes. Similarly, the evolution of language has paved the way for myriad other developments: the ability to use symbols to refer to concepts, to discuss past or future events, to form complex social groups, to engage in current social interactions that consider past interactions, to gossip about people or events not present, and so on.

So far, there are no fossil records of language’s evolution; we can sometimes infer language from artwork or other evidence, but at present, it’s impossible to say how long ago language evolved, let alone how it evolved. There are no stone axes to document the emergence of language. Some prerequisites for human speech can be found. For instance, Zuk points to the importance of becoming bipedal, freeing up the hands — and with free hands for holding things, the mouth is free to chat instead of carrying or holding things. Developments in the physiology of the neck and larynx also facilitated human speech. By observing contemporary primates, we can see how the brain developed to facilitate speech, such as a structure forming a pathway between the frontal cortex and the auditory cortex. Zuk even points to the development of “lip flapping” as a key to human speech.

One way to view possible language emergence is to observe contemporary pidgins, simplified languages that emerge when people from different language groups converge and must find ways to communicate, such as to engage in trade in a port city. Gestures may also have played a role in the development of language. Contemporary primates, such as bonobos and chimpanzees, use gestures to communicate.

Various other forms of animal communication include movements (e.g., bees’ waggle dance), smells (e.g., pheromones), displays, sounds, each of which may have evolved separately over time. Convergent evolution also explains how three families of birds (songbirds, parrots, and hummingbirds) and five families of mammals (humans, dolphins, bats, elephants, seals) developed a form of vocal production that can serve communicative purposes. Among songbirds, vocal learning involves some imitation of others in their own species. Some birds show remarkably flexible mimicry, such as mockingbirds, lyrebirds, and parrots.

So if language isn’t unique to humans, what about syntax — the rules of how meaningful units (“words” in humans) are ordered to communicate meaning? At least two bird species do seem able to combine and recombine calls in meaningful ways — though not in particularly complex ways.

Another way to look at syntax is Zipf’s Law — which generally states that within any given text, the most common word appears about twice as often as the second-most common word, about three times as often as the third-most common word, and so on. For instance, in any large chunk of text (e.g., Tolstoy’s War and Peace), “the” appears twice as often as “of,” three times as often as “and,” and so on. In general, the makes up about 7% of all the words, of makes up about 3.5%, and so on (https://en.wikipedia.org/wiki/Zipf%27s_law). Zipf’s Law applies to all studied human languages — and it applies to the communication units of songbirds, dolphins, and rock hyraxes (tiny mammals closely related to elephants).

Another aspect of syntax is combinatorial signal processing, in which smaller units such as words, are combined to make larger units such as sentences. (Below words are syllables; above sentences are paragraphs, and so on.) Surprisingly, male treehoppers (insect family with 3200 species) use combinatorial signal processing when vibrating on a plant to attract females. When scientists tried to recombine the signals, the females didn’t respond; only the correct combinations elicited female responses. Zuk isn’t suggesting that treehoppers use language, but it’s interesting that they have evolved this form of communication.

Another intriguing example of animal communication occurs among birds who sing duets — beautifully synchronized call-and-response interactions, so closely timed that it can be hard to differentiate one bird from the other without sophisticated equipment. Even songbirds differ in terms of how flexibly they sing: Some acquire their songs almost entirely through vocal learning, and some sing highly structured songs that don’t vary much within a given species. Budgerigars (small parrot species) can make sound discriminations that Zebra Finches can’t detect.

Even your pet dog can distinguish known words (e.g., “Sit”) from unknown or nonsense words, as indicated on electroencephalograph recordings — unless the unknown words sounded similar to known words. (For another perspective on how dogs understand what we say, see “Blah, Blah, Blah, Blah Ginger | Thinking it Through,” https://share.google/xSKy7HQbmCyq0H7tE).

Not to leave out cats entirely, it turns out that when someone (a stranger or its owner) blinks slowly at a cat, the cat will blink back and will be more likely to approach that person.

10. The Faithful Coucal: Animals, Genes, and Sex Roles, 207–230

Coucals are tropical birds related to cuckoos, but they are not nest parasites. They raise their own young in their own nests. Among all coucals, the female is larger than the male — quite a bit larger in some species. White-browed Coucal females are about 13% larger than the males, and Black Coucal females are about 70% bigger than the males. Both species live in pretty much the same habitat, eat the same diet of insects and small reptiles, and must avoid the same predators.

The two species of coucals have different reproductive and parenting styles, though. White-browed Coucals are monogamous mates, and both parents participate in rearing their young. Black Coucals, however, are polyandrous: The female mates with a male, lays his eggs, and leaves him to incubate the eggs and rear the chicks; then she mates with a new male, whom she’ll leave in charge of caring for her next clutch of eggs; then she moves on again; and so on. These coucals aren’t the only birds who are polyandrous, but many of the other polyandrous species (e.g., jacanas and some other shorebirds) have precocial chicks, who are mostly able to fend for themselves soon after hatching. Coucal chicks, however, require intensive parental care, which the male ably provides.

Zuk discusses the differences between sex and gender. Sex refers to physiological differences, which are usually binary male/female, but which can be less clear such as in hermaphroditic species. As an example of how sex differentiation can be more complex, the sex of crocodiles, turtles, and some other reptiles is determined by the temperature at which the eggs hatch.

Males produce abundant numbers of sex cells — sperm — whereas females produce limited numbers of sex cells — ova. These differences may make it more likely that females may be more selective about whom they mate with, and males may be less likely to be as selective. As with most aspects of behavior, there are wide variations in reproductive behavior among males and females.

In contrast to sex, gender refers to cultural differences, not physiological ones. Gender differences are even less clearly binary male/female. What is considered to be gendered behavior may appear to differ between captive settings (e.g., zoos) and wild environments. Though gender more clearly involves environmental input than sex, both sex and gender involve an interaction between genetics and environment.

In the past, medical researchers have studied primarily males (animals or humans) to test the effects of most drugs and other treatments, assuming that females would have comparable responses. Luckily, that has been changing, though studies of males and females are still not equitable. Studies of brain imaging have also revealed sex differences, but how to interpret the findings can be challenging.

Studies of behavioral differences in males and females can also lead to misinterpretation. When males and females live in such differing environments, with males dominant in many contexts, it can’t be known whether the behavior of females is due to their sex or due to their subordination or due to a variety of other influences. Likewise, male behavior can’t be interpreted as due to their sex, their dominance, or other factors.

For instance, it was found that women tend to use more correct grammar than men. Women don’t speak more than men, so they’re not getting any more language practice than men have. Are women just innately better at processing and correctly using grammar? Or are men more casual in their language use because their dominance gives the privilege of being less concerned about using correct grammar? Or does something else explain this difference?

Girls outperform boys in educational achievement. Are girls innately better at educational tasks? Or are boys scorned as being less “masculine” when they excel at educational tasks? As adolescents, girls become less likely to pursue scientific interests than boys. Do emerging sex hormones make girls unable to engage in scientific activities? Are their X chromosomes dictating reduced interest in scientific pursuits? Until cultural variables are considered, no conclusions can be reached about the meanings of apparent sex differences.

“Animals show us that the array of behaviors associated with being male or female goes far beyond what humans might think” (p. 230).

11. Protect and Defend: Behavior and Disease, 231–256

Infectious diseases offer superlative examples of evolution: Our immune systems evolve to fight off these diseases, often aided with vaccines, antimicrobial medications, and other treatments. Likewise, pathogenic microbes (and multicellular parasites) evolve to combat our defenses.

We also develop behaviors to fight infectious diseases: washing hands, eating special foods, getting rest, using toilets and other sanitation devices, keeping ourselves distant from infected individuals, wearing masks or other protective gear, cleaning objects and surfaces, taking medications, seeking professional medical help, and so on. We help members of our community to fight diseases, too — caring for the sick, avoiding direct contact with others when we are sick, practicing good hygiene, and so on.

Other animals also evolve in terms of their immune systems and their behavior. They help sick family or community members, use antimicrobial agents, eat special foods, and so on.

Both humans and other animals also suffer from non-infectious diseases, such as diabetes or hypertension, but only infectious diseases can prompt an evolutionary response in the agent causing the disease. If we treat diabetes with insulin, the cause of the diabetes won’t fight back. If we treat bacterial pneumonia with an antimicrobial medication, however, the bacteria will fight back, sooner or later evolving into a form that can resist the medication. Zuk calls this interaction “an evolutionary arms race” (p. 232).

Some parasites go above and beyond fighting the immune system; in addition, they target the host’s behavior to weaken the host’s defenses against the parasite’s assault. For instance, parasitic fungi will infect an ant, taking direct control over its muscular movements (bypassing the ant’s brain altogether). The parasitic fungus makes the ant climb a blade of glass, then from the peak of the blade, the fungus releases its spores for widespread dispersal. Parasitic worms will prompt fish to swim close to the water’s surface, so birds will easily catch the infected fish and become infected themselves. Parasitic wasps will infect cockroaches, virtually paralyze the roaches, and lay their larvae on the roaches, which then feed the parasitic larvae.

When Toxoplasma parasites infect a mouse, they affect the mouse’s brain, reducing it’s anxiety level, so it’s more willing to explore areas having the scent of cat urine. Cats easily catch and eat these infected mice, becoming infected with toxoplasmosis and providing the parasites their favorite host for reproducing. (Pregnant humans should do all they can to avoid coming into contact with cats or their urine or feces.)

Researchers wondered whether some of the microbes that attack humans might also exert some behavioral influence on their victims. Vaccines typically work by stimulating the immune system to react to a mild or inert form of an infection. They thought that during the initial response to a vaccination, people might show some of the same behavioral responses they would show if they were actually infected. They asked participants to record their activities before and immediately after receiving a flu vaccination, as well as four weeks after that. Though the subjects’ reported they didn’t notice any changes following the vaccinations, their social activities actually increased in the two days after the vaccination, as compared with the other times. From the microbe’s point of view, for the infected person to increase social activity would mean more opportunities to infect more people. This study is only suggestive of a possible effect, but it is interesting.

Regarding treatment of illnesses, many animals have learned how to treat some illnesses. For instance, when chimpanzees appeared to be feeling ill, they were observed to eat a bitter-tasting anti-parasitic plant bark. After that, the number of parasite eggs in their feces dropped. In another observation, chimpanzees rolled up a particular kind of leaf and swallowed it whole; the feces then showed that the leaves appeared to scrape the parasites from the chimpanzees’ digestive tract. Similar medicinal plant treatments have been observed in other great apes (bonobos, gorillas, gibbons, orangutans) and in many monkey species. According to Zuk, these self-medicating behaviors have evolved just as other behaviors have.

Non-primates, too, have been observed self-medicating for parasites and other infectious diseases. To be considered self-medication,

• The animal must eat a plant or other item or apply it topically.
• The medication must be something the animal doesn’t normally eat, or at least doesn’t normally eat in the amounts ingested for self-medication.
• The treatment must occur after the animal is observably ill.
• The medication must help the animal to recover or at least must help the animal avoid spreading illness to other kin.
• The animal must pay a price for taking the medication; it must be unpleasant to consume, such as having a noxious taste, or having toxic side effects, etc.

Goats, for instance, generally avoid plants that are high in bitter-tasting tannins (which can be found in red wine, tea, and other foods). If they’re suffering from roundworms, however, they’ll go out of their way to consume tannin-containing plants, which help to rid the goats of these parasites. Even lambs will eat more tannin-containing plants if they’re infected with intestinal parasitic worms, resulting in a decrease in the infection. Neither the goats nor the lambs ate only tannin-rich plants; they merely increased the number of tannins in their diet. Zuk goes on to say that herders and ranchers might be wise to let their herd animals graze on a wide diversity of plant materials. If allowed to choose from an array of plants, these animals might be less likely to need veterinary medicines.

Mammals aren’t the only animals to use plants to treat infectious agents. Many nesting birds are assaulted by lice, fleas, mites, and other parasites. For adult birds, these invertebrates are a nuisance, but for nestlings, these pests can be deadly if they suck enough blood from young hatchlings. To deter these pests, many birds line or interweave their nests with repellent or aromatic plants. Distressingly, some birds have found that a particularly effective insecticide is nicotine, so they include cigarette butts in their nests. The butts effectively kill the pests, but they also cause other health problems for the chicks, and even for the parents.

A similar trade-off occurs with monarch butterflies, which lay their eggs on toxin-containing milkweed plants. Some milkweed species contain more toxins than others. If the mother butterfly has been infected with a pathogen, she will choose to lay her eggs on a species that contains more toxins. When her eggs hatch and become caterpillars/larvae, they eat the toxin-containing milkweed. The highly toxic milkweed will get rid of the pathogen, but it will also produce toxic effects in the caterpillars. If the mother hasn’t been infected, she’ll lay her eggs on a species containing less toxin, and her caterpillars won’t suffer the ill effects of increased toxins.

Woolly-bear caterpillar moms do something similar. Bumblebees will also preferentially consume nectar that contains infection-fighting alkaloids. Social insects such as ants and bees also provide medicines to their colonies, such as by lining their nests or hives with antimicrobial plant resins. In the case of the resins, the insects don’t consume the resins. Instead, the sticky resins seem to protect the hive from the infection. Beekeepers don’t like dealing with the sticky substance, so they prefer to keep bees that don’t engage in this practice; unfortunately, the kept bees are then more vulnerable to infection.

Fun fact: Ants that engage in battles between colonies are often injured. On the battlefield, ants will perform triage on their injured comrades, leaving on the battlefield any who have no chance of survival but carefully tending to those who can survive if given adequate care. The ant medics will carry the injured comrade back to the colony, where ant nurses will clean the wounds and will use their antimicrobial saliva to lick the wounds. Ants with treated wounds had an 80% better chance of survival. (I’m not sure how they measured that.)

Archeologists studying ancient humans have gleaned a lot of information about them from their dental tartar — the sticky calculus that accumulates on our teeth (much to the dismay of dental hygienists). The tartar traps minute particles of food, which can then be analyzed. By studying this tartar, archeologists have observed that Neanderthals used medicinal herbs such as chamomile and yarrow to treat various illnesses. They even found a penicillin-containing fungus in one Neanderthal’s teeth. We cannot know what was going on then, but it does raise the possibility that Neanderthals had at least some awareness of antibiotics. Researchers observing skeletal remains have also noted that many ancient humans showed skeletal evidence of recovery from infections and from broken bones and other grievous wounds.

Zuk also took a brief side trip to explore whether spicy foods are associated with tropical locations, where the risk of foodborne illness is greater. They’re not. Hot climates are correlated with shorter life expectancies, higher risk of accidents, and higher rates of diseases. However, “climate isn’t related to a cuisine’s spiciness” (p. 252).

Do you still believe that humans are the pinnacle of evolutionary achievement? Do you know any humans who can do this? Sacoglassan sea slugs can decapitate themselves, discard their bodies, and from their heads alone, they can regrow an entire new body, complete with digestive system. Why take such a drastic step? If the body is riddled with parasites, the parasite-free head can discard it, ridding itself of the problem, and can begin life anew. These slugs are also remarkable in how they make a living. They consume algae, which live inside the slugs, photosynthesizing and producing nutrients and energy for the slug. After reading this, I’m guessing you’re a little less impressed by reptiles and amphibians that can regenerate a tail or even a limb.

[Back matter]

• Acknowledgments, 257–258
• Notes, 259–280
  Throughout the book are numerous endnotes, which refer to the notes listed here, chapter by chapter. No chapter has fewer than 14 notes, and most have more than 30.
• Bibliography, 281–318
  Also separated by chapters, making it easier to find particular resources.
• Index, 319–330
  Includes concepts, species, and persons mentioned

Copyright © 2025, Shari Dorantes Hatch. All rights reserved.