In zoology, cannibalism is the act of one individual of a species consuming all or part of another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded for more than 1500 species. It does not, as once believed, occur only as a result of extreme food shortages or artificial conditions, but commonly occurs under natural conditions in a variety of species. Cannibalism seems to be especially prevalent in aquatic communities, in which up to approximately 90% of the organisms engage in cannibalism at some point of the life cycle. Cannibalism is also not restricted to carnivorous species, but is commonly found in herbivores and detritivores.
Sexual cannibalism is a special case of cannibalism in which a female organism kills and consumes a conspecific male before, during, or after copulation. Rarely, these roles are reversed. Sexual cannibalism has been recorded in the female redback spider, black widow spider, praying mantis, and scorpion, among others.
Size-structured cannibalism, is cannibalism in which older, larger, more mature individuals consume smaller, younger conspecifics. In size-structured populations, (where populations are made of individuals of various sizes, ages, and maturities), cannibalism can be responsible for 8% (Belding's Ground Squirrel) to 95% (dragonfly larvae) of the total mortality, making it a significant and important factor for population and community dynamics.
Size-structured cannibalism has commonly been observed in the wild for a variety of taxa. Vertebrate examples include chimpanzees, where groups of adult males have been observed to attack and consume infants.
Filial cannibalism, is a specific type of size-structured cannibalism in which adults eat their own offspring. Though most often thought of as parents eating live young, filial cannibalism includes parental consumption of stillborn infants and miscarried fetuses as well as infertile and still-incubating eggs. Vertebrate examples include pigs, where savaging accounts for a sizable percentage of total piglet deaths, and cats.
Filial cannibalism is particularly common in teleost fishes, appearing in at least seventeen different families of teleosts. Within this diverse group of fishes, there have been many, variable explanations of the possible adaptive value of filial cannibalism. One of these is the energy-based hypothesis, which suggests that fish eat their offspring when they are low on energy as an investment in future reproductive success. This has been supported by experimental evidence, showing that male three-spined sticklebacks, male tessellated darters, and male sphinx blenny fish all consume or absorb their own eggs to maintain their physical conditions. In other words, when males of a fish species are low on energy, it might sometimes be beneficial for them to feed on own offspring to survive and invest in future reproductive success.
Another hypotheses as to the adaptive value of filial cannibalism in teleosts is that it increases density-dependent egg survivorship. In other words, filial cannibalism simply increases overall reproductive success by helping the other eggs make it to maturity by thinning out the numbers. Possible explanations as to why this is so include increasing oxygen availability to the remaining eggs, the negative effects of accumulating embryo waste, and predation.
Further evidence also suggests that occasionally filial cannibalism might be the unfortunate by-product of cuckoldry in fish. Males consume broods, which may include their own offspring, when they believe a certain percentage of the brood contains genetic material that is not theirs.
Infanticide is often wrongly or inaccurately described as an example of cannibalism, especially size-structured and filial cannibalism. By definition, infanticide occurs when an adult animal kills a non-adult animal of the same species. The adult does not have to be a parent or relative of the juvenile and the young animal does not have to be an infant or newborn. The reason behind the killing - a wandering male killing an unrelated cub so the mother will go into estrous (bear, lion), a hungry adult taking advantage of an easy meal, two fighting males accidentally killing a nearby pup (Northern elephant seal), abandonment or neglect by an inexperienced, injured, or uncaring mother, etc. - is unrelated to the term "infanticide."
Infanticide can occur as a result of an adult's cannibalistic desire, but by itself, infanticide is not cannibalism.
Intrauterine cannibalism is a behaviour in some carnivorous species, in which multiple embryos are created at impregnation, but only one or two are born. The larger or stronger ones consume their less-developed siblings as a source of nutrients.
Intrauterine cannibalism is known to occur in lamnoid sharks such as the sand tiger shark, and in the Fire Salamander, as well as in some teleost fishes. The Carboniferous period chimaera, Delphyodontos dacriformes, is suspected of having practiced intrauterine cannibalism, also, due to the sharp teeth of the recently born (or possibly aborted) juveniles, and the presence of fecal matter in the juveniles' intestines.
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- M. A. Elgar and Bernard J. Crespi (eds.). 1992. Cannibalism: Ecology and Evolution of Cannibalism among Diverse Taxa Oxford University Press, New York. (361pp) ISBN 0-19-854650-5