Evolution of Cannibalism Evaluating the Costs and Benefits

Cannibalism may evolve in situations where the benefits to the inclusive fitness of the cannibal exceed its costs. Cannibalism is beneficial as a source of energy and as a means of reducing the number of competitors. Crowding in high-density populations, with a low availability of alternative prey types, facilitates acts of cannibalism. Filial cannibalism during periods of low food availability converts an individual's reproductive investment back into somatic tissue; the energy gained can then be used for survival by the parent or converted back into eggs. In such cases, energy invested into reproduction is not lost but, rather, is maintained and then redirected into future reproduction.

Acts of cannibalism can also incur significant costs, most notably the risk of retaliation by the intended victim and the risk of acquiring parasites and diseases from infected conspecifics. Perhaps the best-known example of the latter is the occurrence of a prion disease called kuru in both prehistoric and extant human populations. This neurodegenerative disorder is universally fatal and transmitted through cannibalism. Forty years ago it killed almost 10% of a small New Guinea tribe called the Fore. Similarly, cannibalism is thought to be a potential mechanism of direct (nonvector) transmission of West Nile virus in geese, tularemia in voles, and bacterial and viral diseases in larval amphibians. Indeed, for larval tiger salamanders the enhanced risk of disease acquisition may have been a selective force against cannibalism and the propensity to develop specialized cannibal morphologies (see the section titled 'Cannibalistic polyphenism').

Another important cost of cannibalism is the risk of a reduction in inclusive fitness if close relatives are consumed. This potential loss of fitness will depend upon the degree of genetic relatedness between cannibal and victim. All else being equal, the potential for this loss suggests that cannibalism should be directed away from related individuals. The evidence for this, however, is equivocal. We searched the literature for publications since 1985 that addressed kinship and cannibalism. We reviewed 44 studies in which cannibals of different species were presented a choice between kin or nonkin as potential prey. Of these studies, 36.4% reported cannibals avoiding consumption of their close kin (siblings or offspring) and 34.1% reported indiscriminate cannibalism (i.e., kin and nonkin eaten with similar frequencies).

In another 22.7% of the studies, whether kin or nonkin was consumed depended upon the ecological and social contexts in which the study was conducted (i.e., cannibalism was context-dependent). Most frequently, kin cannibalism varied with sex, morphotype, body condition, rearing condition, and stage of development (e.g., instar stage in insects and spiders). For example, in a species of spadefoot toad in which both cannibalistic (carnivorous) and noncannibalistic alternative morphologies coexist (see the section titled 'Cannibalistic polyphenism'), tadpoles avoided eating kin when they expressed the cannibal phenotype, but not when these same individuals reverted to the noncannibalistic morph. Thus, individual tadpoles facultatively adjusted their level of discrimination according to how likely they were to harm kin.

Genetic evidence supports the occurrence of filial cannibalism in nature. In 6.8% of the studies we reviewed, cannibals actually preferentially consumed close kin. Polymorphic microsatellite markers have been used to determine the genetic parentage of freshly cannibalized embryos collected from the stomachs of nest-tending darter and sunfish males. This analysis demonstrated that guardian males do indeed consume their own genetic offspring, even when unrelated embryos also occur within the nest. It is not clear, however, whether the filial cannibalism observed in this study resulted from an overt preference for related embryos, or whether cannibals simply failed to discriminate within the context in which the study was conducted.

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