Cannibalism, the act of an animal consuming another individual of the same species, is a widespread biological phenomenon known formally as intraspecific predation. It has been documented across more than 1,500 species, spanning nearly every major phylum of the animal kingdom. This behavior is a complex ecological interaction driven by evolutionary pressures that favor individual survival and reproductive success, often at the expense of a conspecific. The practice occurs in species as diverse as insects, fish, amphibians, and mammals, suggesting it is a survival strategy rooted in nature’s economy.
The Core Motivations Behind Cannibalism
Many instances of cannibalism are directly linked to the immediate need for sustenance, particularly when food is scarce or unpredictable. This nutritional gain provides a dense influx of energy and protein. For instance, tadpoles of the Fletcher’s frog develop in temporary pools that lack other food sources, forcing them to consume their clutch-mates to survive and grow. Cannibalistic wood frog tadpoles experience faster growth rates and higher fitness levels than their non-cannibalistic counterparts, demonstrating the biological advantage of this caloric boost.
Beyond simple hunger, cannibalism frequently functions as a mechanism for population regulation when densities become too high for available resources. It acts as a density-dependent source of mortality that reduces competition for the survivors. In species such as the confused flour beetle, the consumption of eggs and pupae lowers the population density, ensuring that remaining individuals have better access to shelter, territory, and food. This stabilizes the population by reducing the strain on the ecosystem’s carrying capacity.
A distinct category of motivation is sexual selection, where the female consumes the male, often immediately before, during, or after copulation. This provides the female with a large “nuptial gift” of nutrients and energy that she can immediately allocate to egg production, increasing the number or quality of her offspring. From the male’s perspective, this self-sacrifice can be an evolutionary advantage, as the nutritional boost may ensure his paternity and increase the likelihood that his genes are passed on, especially if he has few other mating opportunities.
Filial cannibalism involves parents consuming their own offspring. The most common explanation is the energy-based hypothesis, where the parent consumes part of the brood to replenish energy reserves depleted by parental care. This investment increases the parent’s chances of surviving to future breeding cycles, maximizing their lifetime reproductive output. Parents may also selectively consume sick, weak, or unfertilized offspring to prevent the spread of disease, effectively culling the brood to improve the survival odds of the healthier young.
Diverse Examples Across the Animal Kingdom
Invertebrates
The most widely recognized examples of intraspecific predation occur among invertebrates, particularly arachnids and insects, often in the context of sexual selection. Female praying mantises, which are significantly larger than males, are notorious for consuming their mates, sometimes decapitating the male during copulation. Remarkably, the male’s abdominal nerves can continue the mating process after the head is removed, ensuring sperm transfer while the female gains a nutritional supplement.
In the case of the redback spider, the male often actively facilitates his consumption, positioning his abdomen near the female’s mouthparts after mating in a behavior referred to as “copulatory suicide.” This voluntary act prolongs copulation, increasing the amount of sperm transferred and improving the male’s chances of fertilizing the female’s eggs. Since the male’s chance of finding a second mate is low, this sacrifice represents a high-stakes strategy to ensure his genetic legacy.
Fish and Amphibians
Filial cannibalism is particularly prevalent among fish species that exhibit paternal care, such as male sunfish and tessellated darters. These males guard the eggs in a nest but frequently consume a portion of their own clutch, especially when their energy reserves are low due to reduced foraging while guarding. Genetic analysis confirms that the eggs eaten by the male are indeed his own offspring, providing a direct nutritional benefit.
Amphibians often display size-dependent cannibalism, where larger, faster-developing larvae prey on their smaller counterparts. In the cane toad, for example, the tadpoles consume the eggs of their own species, a behavior that helps regulate population density in small aquatic habitats. This secures food for the cannibalistic tadpoles and eliminates future competitors from the limited water source.
Mammals and Reptiles
Cannibalism in mammals is less frequent but centers on resource stress or parental culling. Female rodents, including house mice, may consume their own young, especially when faced with extreme stress, overcrowding, or when a pup is stillborn or too weak to survive. This parental behavior allows the mother to recover the caloric investment made in a compromised offspring, conserving energy for the remaining, stronger litter members.
Among reptiles, the large female green anaconda may consume a smaller male after mating, using the energy store to sustain her long gestation period. Since the female’s movement is restricted during pregnancy, the male provides an energy reserve that eliminates the need to hunt while she is vulnerable. This links sexual cannibalism to the high energetic demands of reproduction in large, slow-moving species.
The Evolutionary Trade-offs and Risks
While cannibalism offers immediate benefits, the behavior is constrained by evolutionary trade-offs that prevent its universal adoption. One serious risk is disease transmission. Consuming a conspecific means ingesting tissues with a nearly identical biological makeup, which facilitates the rapid spread of pathogens, parasites, and even prions. Diseases that struggle to jump species barriers can easily be transmitted through this shared biological pathway.
The act of preying upon a member of one’s own species also carries a risk of injury or conflict. Unlike hunting a different species, a conspecific prey possesses the same defensive and offensive strategies as the cannibal, making the attack potentially dangerous. Even successful cannibalism can result in wounds that may become infected or hinder future hunting, presenting a high cost for the meal.
A long-term consequence of cannibalism relates to inclusive fitness and the loss of shared genetic material. By consuming a relative—be it an offspring, sibling, or cousin—the cannibal deletes a portion of its own genes from the collective gene pool. This trade-off means that the immediate survival advantage must be large enough to outweigh the cost of reducing the overall genetic representation of the family line in the next generation.