Intraspecific predation is taxonomically widespread, but few species routinely prey on conspecifics. This is surprising as conspecifics could be a valuable resource for animals limited by food. A potential cost of cannibalism that has been largely unexplored is that it may enhance the risk of acquiring debilitating pathogens or toxins from conspecifics. We examined how pathogens affect variation in the incidence of cannibalism in tiger salamander larvae (Ambystoma tigrinum nebulosum), which occur as two environmentally-induced morphs, typicals and cannibals. Salamanders from one population were more likely than those in another to develop into cannibals, even when reared under identical conditions. Variation in the propensity to become a cannibal may be caused by variation in pathogen density. In the population with cannibals at low frequency, bacterial blooms in late summer correlated with massive die-offs of salamanders. The frequency of cannibals correlated significantly negatively with bacterial density in ten different natural lakes. In the laboratory, cannibals exposed to a diseased conspecific always preyed on the sick animal. As a result, cannibals wre more likely to acquire and die from disease than were typicals that were similarly exposed, or cannibals that were exposed to healthy conspecifics. Since conspecifics often share lethal pathogens, enhanced risk of disease may explain why cannibalism is generally infrequent. Pathogens may constrain not only the tendency to be behaviorally cannibalistic, but also the propensity to develop specialized cannibal morphologies.

Egg dumping, or abandonment of eggs and young to the care of other conspecifics, frees individuals from costs of maternal care while potentially imposing energetic and ecological costs on egg recipients. It is not clear, however, that egg dumping necessarily represents selfish manipulation of egg recipients, and in some ecological contexts, recipients may benefit from enlarged broods. Thus, egg dumping may either be mutually beneficial for dumpers and recipients or entail costs for dumpers that are compensated by other means, such as improving reproduction of genetically related egg recipients. Here I use field experiments to test the relative importance of manipulation (i.e., "parasitism"), mutualism, and kin selection in the evolution of egg dumping in the tingid lace bug Gargaphia solani. In support of mutualism and kin selection, I found that reproduction of egg recipient G. solani benefits from brood enlargement, most likely because eggs and gregarious nymphs find safety in greater numbers. But contrary to both parasitism and mutualism, egg dumper reproduction was not improved by offspring abandonment. Indeed, dumpers laid smaller clutches than recipients, and dumpers did not convert a survival advantage into greater future reproduction. Genetic analyses of a natural G. solani population revealed, however, that dumpers are related to their egg recipients. Moreover, Hamilton's rule showed that egg-dumping G. solani earn sufficient indirect genetic benefits for kin selection to favor the behavior. Thus, egg dumping in some species may be kin-selected cooperation rather than parasitism or mutualism.

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