A test of alternative hypotheses for kin recognition in cannibalistic tiger salamanders

The function of kin recognition is controversial. We investigated the adaptive significance of kin discrimination in cannibalistic tiger salamander larvae, Ambystoma tigrinum. Previous laboratory experiments show that cannibals preferentially consume less related individuals. We hypothesized that this example of kin recognition (1) is a laboratory artifact, (2) is a by-product of sibship-specific variation in escape responses, because cannibals from families with rapid responses may be more likely to cannibalize slowly escaping non-kin, (3) is an epiphenomenon of species recognition, (4) functions in disease avoidance, because kin may be more infectious than non-kin, or (5) is favored by kin selection. We evaluated these five hypotheses by using laboratory and field experiments to test specific predictions made by each hypothesis. We rejected hypotheses 1-4 above because (1) kin recognition was expressed in the wild, (2) escape responses did not reliably predict whether a cannibal would ingest kin or non-kin, (3) kin recognition was not most pronounced in populations where tiger salamanders co-occur with other species of salamanders, and (4) non-kin prey were more likely than kin to transmit pathogens to cannibals. However, we established that the necessary condition for kin selection, Hamilton's rule, was met. Thus, our results implicate kin selection as the overriding reason that cannibalistic tiger salamanders discriminate kin.