Cannibalism: A Perfectly Natural History

Chapter 1 follows Schutt’s firsthand experience studying cannibalistic spadefoot toad larvae in Arizona’s Chiricahua Mountains alongside biologist Dr. David Pfennig. Schutt wades through a pond where tadpoles have transformed into larger, carnivorous forms—a shift driven by environmental factors. Initially, all hatchlings are omnivorous, feeding on organic debris, but within days, some undergo rapid physical changes, developing larger bodies, powerful jaws, and sharp keratin beaks, allowing them to prey on their smaller siblings. The difference between the cannibalistic tadpoles and their brethren makes them seem like a different species. The transformation is an example of phenotypic plasticity, where organisms develop different physical traits in response to environmental conditions. The spadefoot toad’s environment, consisting of temporary ponds that can dry up quickly, favors adaptations that speed up development. By consuming protein-rich prey, cannibalistic tadpoles accelerate their growth and metamorphose faster, increasing their survival chances. A similar phenomenon is observed in tiger salamanders, where larvae transform into cannibal morphs in response to overcrowding, triggered by physical contact with others.

Historically, scientists dismissed cannibalism as abnormal behavior caused by starvation or captivity stress. However, research from the 1970s onwards, beginning with ecologist Laurel Fox, showed that cannibalism is “completely normal response to a variety of environmental factors” (5), occurring in species ranging from insects to mammals. Studies by Fox and later ecologist Gary Polis established “a list of cannibalism-related generalizations” (6): Younger individuals are most often consumed, many species do not recognize kin, females are more likely to be cannibals, and the behavior is influenced by population density and food scarcity.

Schutt defines cannibalism as “the act of one individual of a species consuming all or part of another individual of the same species” (7). He considers what might serve as the developmental stimuli that turns some tadpoles into cannibals, but not others. Cannibalism has both advantages and disadvantages. It provides a high-protein food source, speeds up growth, and helps regulate overcrowded populations. Some species, like the flour beetle, even gain “a reproductive advantage” (15) from cannibalism, as cannibalistic females produce more eggs. However, consuming one’s own species also comes with significant risks, particularly disease transmission. Schutt references the Fore people of New Guinea, whose ritualistic cannibalism led to the spread of kuru, a deadly neurological disease which “nearly [drove them] to extinction” (16).

Kin recognition plays an essential role in the evolution of cannibalistic behavior. Some species, like tiger salamanders and paper wasps, have developed mechanisms for “recognizing cues associated with their kin that [are] absent in non-kin” (17), such as using scent-based cues. However, when food is extremely scarce, even these species will resort to eating kin. This principle extends to human-survival cannibalism, such as the Donner Party. Schutt also connects cannibalism to broader evolutionary theory, particularly developmental plasticity, the idea that environmental pressures can trigger new traits that may eventually lead to the evolution of new species. This aligns with theories from early geneticist Richard Goldschmidt, who controversially argued that major evolutionary changes could arise from developmental shifts rather than gradual micromutations. Schutt opts to investigate “the most dramatic examples of Gary Polis’s cannibalism-related generalization” for thoroughness (20).

Cannibalism is widespread among invertebrates, particularly in species that do not recognize their own kind as more than food. Many mollusks, insects, and arachnids engage in this behavior, often indiscriminately consuming their own species’ eggs and larvae. Filter-feeding aquatic invertebrates, such as clams and corals, unknowingly eat their own planktonic offspring, making them inadvertent cannibals. Some species, like black lace-weaver spiders and rock snails, have evolved trophic eggs—a food source for hatchlings. In extreme cases, such as black lace-weaver spiders, maternal sacrifice occurs when a mother allows her offspring to “eat her alive” (22) after their first molt.

Cannibalism is also common among vertebrates, occurring in fish, amphibians, reptiles, birds, and mammals. Factors like larger body size and longer lifespans have made vertebrate cannibalism easier for scientists to study, leading to the classification of distinct forms, such as filial cannibalism (eating one’s own offspring) and heterocannibalism (eating unrelated members of the same species). Among mammals, filial cannibalism is observed in rodents, rabbits, shrews, and hedgehogs, often due to food scarcity or litter size management.

Fish exhibit some of the highest rates of cannibalism due to their reproductive strategies, which involve producing large numbers of eggs with little parental care. Many fish consume their own eggs and larvae, leading ichthyologists to consider the absence of cannibalism in fish as an exception rather than the rule. Some fish engage in partial or total filial cannibalism, eating a portion of their brood to sustain themselves while guarding eggs. Schutt suggests that male fish have “much less invested in the brood than do females” (27), which may explain their filial cannibalism. In mouthbrooding species such as male cichlids, parents holding eggs or fry in their mouth may eat “a considerable portion of them” (28) since regular eating during this period is difficult. An extreme case of cannibalism occurs in sand tiger sharks, where developing embryos consume their smaller siblings in a phenomenon known as adelphophagy (sibling cannibalism). This process ensures that only the strongest individuals are born, as the cannibal sharks are experienced predators.

In reptiles such as alligators, cannibalism accounts for “significant juvenile mortality” (31). Eggs, neonates, and juveniles are particularly at risk due to their defenselessness. Among birds, cannibalism is less common. Some seabirds, such as herring gulls, engage in heterocannibalism by eating eggs and chicks from neighboring nests as “an integral part of foraging strategy” (32). Raptors and snowy egrets display sibling cannibalism, where stronger chicks kill and consume weaker siblings, especially when food is scarce as it “becomes an efficient way to produce well-nourished offspring (albeit fewer of them) during times of stress” (33). Filial cannibalism, though rare, has been observed in species like barn owls and roadrunners, often as a survival mechanism during periods of extreme resource limitation as “a lifeboat strategy, where the strong cannibalize the weak” (34).

Praying mantises and black widow spiders typically share the title of “Nature’s Most Infamous Cannibal” (35), though myths distort the truth. The praying mantis, a diverse order of over 2,200 species, has a fearsome reputation, but its name originates from the prayer-like position of its forelegs. Revered in various cultures, mantises are ambush predators equipped with spiked forelegs and powerful mandibles capable of consuming small reptiles, birds, and even mammals. The widely held belief that female mantises routinely decapitate their mates during copulation gained traction in the late-19th century. Early entomologists claimed that males “continued to copulate” (36) even after being beheaded, leading to speculation that brain removal enhanced sperm transfer. However, later research found that cannibalism occurs primarily in laboratory settings when females are underfed. Well-fed females rarely attack their mates, suggesting that sexual cannibalism is an opportunistic feeding strategy rather than an evolutionary necessity, though there is likely “some risk [for males] of being attacked by females” (39). Since the death of the male mantis can increase successful reproduction, Schutt frames this as an evolutionary trade-off: While fatal to the individual, the act ultimately supports reproductive success and the passing on of his genes.

Similarly, the black widow spider’s reputation is exaggerated. Though early reports described female black widows consuming their mates post-copulation, later studies revealed that most males escape unharmed and even cohabitate with the female. However, sexual cannibalism is well-documented in other spider species, like the Australian redback spider. Male redbacks actively facilitate their own consumption by somersaulting into the female’s mouth during copulation. This act increases mating duration and fertilization success while making the female less receptive to subsequent mates, so that it “optimizes the likelihood that the cannibalized dad gets to pass his genes on to a new generation” (43).

Size plays a crucial role in sexual cannibalism, as females are often significantly larger than males in many invertebrate species. This sexual dimorphism enhances female reproductive success by allowing them to produce more offspring. In rare cases, such as in wolf spiders and water spiders, males are larger and engage in mate selection, sometimes attacking and consuming unfit females or rivals. In these instances, size determines cannibalistic behavior.

Beyond mantises and spiders, “bizarre sexual encounters” (47) among gastropods can also include cannibalism. Some carnivorous snails bite their mates, leading to rapid and aggressive copulation. Even more unusual, banana slugs practice “apophallation” (48), where individuals chew off their mate’s penis to disengage. In land snails, “love darts” (49) are calcium-rich, hormone-coated projectiles shot into a mate’s body to enhance sperm survival and reduce the likelihood of re-mating with others. Some researchers even suggest that Cupid’s mythological origins may trace back to observations of snail mating.

Cannibalism often increases in response to overcrowding, food shortages, and competition for resources. This pattern is evident in many species, as individuals consume their own kind for survival. Mormon crickets—large, flightless katydids native to North America—demonstrate “crowd-related cannibalism” (52). Biologist Stephen Simpson found that these crickets swarm in massive migrations to avoid predators, forming what he describes as “coherent mass movement” (54). However, when protein and salt become scarce, they resort to cannibalism, chasing and attacking slower, weaker individuals. Simpson’s experiments showed that even minor handicaps, such as attaching small weights to the crickets, caused them to fall behind and be devoured by the swarm.

In birds, overcrowding exacerbates cannibalism, particularly in commercial poultry farms, where stressed chickens engage in aggressive pecking behavior. This example of cannibalism is now thought to be “actually misdirected foraging behavior related to cramped and inadequate housing conditions” (55). To prevent losses, farmers historically resorted to beak-clipping, while in the 1940s, some tried Anti-pix glasses, which tinted their vision to reduce aggressive pecking.

Among mammals, 75 species regularly engage in cannibalism, likely due to mammals’ “relatively low numbers of offspring coupled with a high degree of parental care” (56). However, golden hamsters—solitary desert-dwellers—display extreme filial cannibalism when held in captivity. Female hamsters, stressed by unnatural housing, excessive handling, or food deprivation, frequently eat their own pups. Sibling cannibalism and aggressive adult interactions also occur, often triggered by territorial disputes.

Non-human primates rarely engage in cannibalism, but exceptions exist. Overcrowding and stress have led to infanticide in species such as lemurs, marmosets, and squirrel monkeys, prompting caretakers to isolate pregnant females in breeding colonies. Chimpanzees offer a “chilling and fascinating” (59) case. Males routinely kill and sometimes eat the infants of rival groups, a behavior observed by Dr. Jane Goodall. This practice increases reproductive opportunities by forcing bereaved mothers into estrus. Unusually, female chimpanzees have also been observed committing cannibalistic infanticide. In 1976, Goodall documented a mother-daughter duo, Passion and Pom, who systematically targeted and ate infants in their group.

A similar pattern was later observed in Uganda’s Budongo Forest, where increased female competition for resources may have triggered violent attacks on infant-bearing mothers. With human encroachment reducing wild chimpanzee habitats, some researchers fear that rising population densities and resource scarcity could drive more instances of cannibalism. These cases illustrate how environmental pressures, stress, and survival instincts can override social bonds, meaning that continued human encroachment could cause an increased rate of cannibalism in these groups.