Context-dependent kin discrimination in larvae of the marbled salamander, Ambystoma opacum

The context-dependent nature of kin discrimination was examined in larvae of the marbled salamander, Ambystoma opacum. In a graphical analysis, relationships between aggression and social and ecological factors were explored. Increasing relatedness between individuals was predicted to reduce aggressive encounters, except when correlates of fitness (e.g. body size) differed between individuals and/or when environmental conditions increased the profitability of aggression. These predictions were tested in a multifactorial experiment in which genetic relatedness, food quantity, relative body size and the potential for physical encounters were manipulated. The effects of each factor and their interactions on larval aggression and correlates of fitness (body size and timing of metamorphosis) were examined. Frequency of aggression depended upon an interaction between kinship, food quantity and relative larval size. Thus, in accordance with our graphical model, kinship-based aggression varied in a context-dependent fashion. Low food level reduced body size and increased the time required to reach metamorphosis. When larvae were not separated by a barrier, growth was inhibited in the smaller, but not the larger, individual of a pair. Moreover, the larval period was significantly shortened in chambers where barriers were absent. Neither kinship nor initial relative size, either alone or in combination with the other factors, significantly affected larval performance. These results show that the frequency of larval aggression in A. opacum depends upon prevailing social and environmental conditions; however, the influence of aggression on larval growth and metamorphosis remains unclear. Previous inferences regarding context-dependent kin discrimination in amphibians have been based on independent, interspecific studies or concurrent, intraspecific comparisons between individuals of alternative morphologies. The present results provide direct evidence that simultaneous variation in multiple socioecological factors may alter aggression in a single amphibian species in which kin discrimination is known to occur. ? 1996 The Association for the Study of Animal Behaviour The ability to discriminate between kin and nonkin (often called kin recognition) is especially germane to kin selection theory and the concept of inclusive fitness (Hamilton 1964a, b). Kin discrimination can enhance aid-giving behaviours or preferential treatment of kin and preclude its misdirection toward unrelated individuals (discussed in Holmes & Sherman 1983). Moreover, kin discrimination can influence key ecological processes that may ultimately affect population and community structure; for example, it may enable individuals to direct competition and intraspecific predation (cannibalism) away from kin (Blaustein & O’Hara 1982). Hamilton (1964a, b) predicted that, all else being equal, an animal’s behaviour towards another individual depends upon the relative costs and benefits of expressing the behaviour, as well as the genetic relatedness of the two individuals. Since Hamilton’s prediction, numerous studies have revealed a diverse array of taxa that appear to distinguish kin from non-kin (e.g. Fletcher & Michener 1987; Blaustein et al. 1988; Hepper 1991). Kin selection is often invoked as the primary force driving the evolution of kin Correspondence: A. R. Blaustein, Department of Zoology, 3029 Cordley Hall, Oregon State University, Corvallis, OR 97331-2914, U.S.A. D. G. Hokit is now at the Department of Biology, Carroll College, Helena, MT 59625-0002, U.S.A. S. C. Walls is now at the Department of Biological Sciences, University of Southern Mississippi, Hattiesburg, MS 39406-5018, U.S.A. 0003–3472/96/010017+15 $18.00/0 ? 1996 The Association for the Study of Animal Behaviour