Group size structure affects patterns of aggression in larval salamanders

The potential importance of intrapopulation phenotypic variability to population-level ecology has been demonstrated in both theoretical and field studies. One way to connect individuals to the dynamics of populations they compose is to study behavioral response (an individual-level characteristic) to variability in conspecific phenotypes (a population-level characteristic). We examined effects of variation in size of individuals on patterns of aggression in larval tiger salamanders (Ambystoma tigrinum nebulosum) by observing aggressive behavior in groups of three larvae in a laboratory experiment. We assessed effects of variability in size of conspecifics independently of mean larval size and larval density. Overall levels of aggression were generally higher in groups in which all individuals were similarly sized than in groups of variably sized individuals. Medium-sized individuals exhibited significantly higher levels of aggression and were attacked significantly more often when in groups consisting only of similarly sized larvae as compared to groups composed of larvae representing a wider range of body sizes. Activity levels of larvae were also generally lower when all individuals were the same size, resulting in a negative correlation between activity and levels of aggression. These results suggest that groups of similarly sized individuals experience a more aggressive social environment than groups of variably sized individuals, and they suggest a promising avenue of research for connecting individual behavioral and physiological responses to size structure (phenotypic variability) with population dynamics.