INCLUSIVE fitness theory predicts that organisms can often increase their fitness by helping relatives1. Indeed, many animals modify their behaviour towards kin in a fashion consistent with theory2-4. Morphogenesis may also be sensitive to kinship environment, especially in species that facultatively produce distinct morphs that differ in their ability to harm relatives, such as those that produce alternative cannibalistic and non-cannibalistic phenotypes5-9. We tested this hypothesis by examining whether consanguinity affected the probability that structurally distinctive cannibal morphs5,10 would develop in larval Arizona tiger salamanders (Ambystoma tigrinum nebulosum). We report here that when tiger salamander larvae are reared in mixed-brood groups they are significantly more likely to develop the cannibal morphology and at an earlier age than siblings reared in pure-sibship groups. In general, morphogenesis may be responsive to kinship in any species that facultatively develops structures that can be used against conspecifics as weaponry.
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