The phenomenon of an animal consuming another individual of its own species is a widespread biological interaction known specifically as cannibalism. This behavior, sometimes termed intraspecific predation, occurs across nearly all major animal groups, from insects and fish to birds and mammals. Scientists recognize cannibalism as a common ecological strategy observed in over 1,500 species globally, rather than a rare act of last resort. This action is driven by complex evolutionary pressures and serves various functions related to survival, nutrition, and reproduction. The study of this behavior reveals a dynamic system of costs and benefits that shape population structure and health in diverse environments.
Defining Cannibalism
Cannibalism is defined as the act of an individual organism consuming all or part of a conspecific—a member of the same species—as a food source. This definition requires consumption and does not include the simple act of killing, known as infanticide, unless the victim is also eaten. The consumption provides a nutritional benefit, which is the core biological driver of the behavior.
It is also distinct from scavenging, which is the consumption of an already dead member of the same species. In a biological context, the individual performing the act is referred to as the cannibal, and the consumed individual is the victim. The behavior is an integrated part of the life cycle for many species.
Biological Drivers of the Behavior
The evolutionary pressures that favor cannibalism typically center on immediate survival and long-term reproductive success. One of the most common drivers is resource limitation, where an individual turns to a conspecific when other food sources are scarce. This is frequently observed in harsh or nutritionally poor environments, such as in the open ocean or temporary desert pools.
Cannibalism can also be a nutritional necessity for an individual during a demanding life stage. For instance, a female spider or insect preparing to lay eggs requires a significant energy boost to produce a large, viable clutch. Consuming a conspecific, such as a mate, provides a readily available source of protein and fat to fuel this reproductive effort.
Another driver relates to competitive advantage, often involving the elimination of rivals’ offspring. In some mammals, such as male lions, a new dominant male may kill and consume the cubs fathered by his predecessor. This behavior causes the females to become receptive to mating sooner, allowing the male to sire his own young and pass on his genes more quickly.
Parental energy management also plays a significant role, particularly in species that invest heavily in offspring care. Parents may consume some of their young to recoup energy reserves, which can then be redirected toward survival or future reproductive attempts. This strategic consumption helps maximize the parent’s overall lifetime reproductive output, especially when the current brood is too small or appears weak.
Classification by Context
Cannibalism manifests in different ways depending on the relationship between the individuals, leading to several distinct classifications.
Sexual Cannibalism
Sexual cannibalism involves the consumption of a mate either before, during, or after copulation, and is most famously associated with arachnids like the praying mantis and the redback spider. In these cases, the female, who is often significantly larger than the male, gains a substantial nutrient meal that directly contributes to the development of her eggs. The male, in turn, may benefit by ensuring his sperm is used to fertilize the eggs, effectively increasing his paternity success through self-sacrifice.
Size-Structured and Chronological Cannibalism
Another common form is size-structured or chronological cannibalism, where larger or older individuals prey upon smaller or younger members of the same population. This is particularly prevalent in aquatic environments, such as among many fish species, where adults readily consume the eggs, larvae, or juveniles of their own kind. Even before birth, intrauterine cannibalism occurs in species like the sand tiger shark, where the strongest embryo consumes its siblings and unfertilized eggs within the mother’s womb.
Parental Cannibalism
Parental cannibalism, also known as filial cannibalism, is the act of a parent consuming its own offspring. This practice is common in species with high reproductive output, such as many fish and rodents. Male fish guarding clutches of eggs may eat a portion of them, particularly those that are diseased or infertile, to maintain their own energy while continuing to guard the remaining viable eggs. Rodent mothers may consume sick or overly numerous young, thereby optimizing the size of the litter to match their available resources and ensure the survival of the healthiest offspring. This behavior functions as a form of parental investment, where current losses are traded for a greater reproductive success in the future.
Ecological Implications and Risks
Cannibalism plays a complex role in the broader ecology of a species. Ecologically, it acts as a self-regulating mechanism for population density by removing individuals, particularly when resources become limited. This thinning of the population can reduce competition for the remaining individuals, ensuring that the fittest survive to reproduce. The behavior can also function to cull weak or infected individuals from the gene pool, indirectly benefiting the overall health of the population.
A biological risk associated with consuming a conspecific is the increased possibility of disease transmission. Pathogens, viruses, and parasites that are adapted to a specific host species can be rapidly and efficiently transferred through this direct trophic connection. For instance, prion diseases, such as the human disease Kuru, are known to spread through the consumption of infected neural tissue.
However, the disease risk is not always straightforward, and in some cases, cannibalism may limit the spread of infection. If a cannibal preferentially consumes an already sick and weakened individual, it is effectively removing a source of horizontal transmission from the population. By removing these low-quality hosts, the cannibalistic act can sometimes reduce the overall prevalence of the disease within the group.