Size-dependent aggression towards kin in a cannibalistic species

In juveniles extreme intraspecies aggression can seem counter-intuitive, as it might endanger their developmental goal of surviving until reproductive stage. On the other hand, aggression can be vital for survival, although the factors (e.g. genetic or environmental) leading to the expression and intensity of this behavior vary across taxa. Attacking (and sometimes killing) related individuals may reduce inclusive fitness; as a solution to this problem, some species exhibit kin discrimination and preferentially attack unrelated individuals. Here, we used both experimental and modeling approaches to consider how physical traits (e.g. size in relation to opponent) and genetic relatedness mediate aggression in dyads of cannibalistic Dendrobates tinctorius tadpoles. We paired full-sibling, half-sibling, and non-sibling tadpoles of different sizes together in an arena and recorded their aggression and activity. We found that the interaction between relative size and relatedness predicts aggressive behavior: large individuals in non-sibling dyads are significantly more aggressive than large individuals in sibling dyads. Unexpectedly, although siblings tended to attack less overall, in size mismatched pairs they attacked faster than in non-sibling treatments. Using a theoretical model to complement these empirical findings, we propose that larval aggression reflects a balance between relatedness and size where individuals trade-off their own fitness with that of their relatives.

Methods
Pairs of tadpoles of different degrees of relatedness (full sibling, half-sibling, non-sibling) were placed together in an arena (18.5 cm by 12 cm clear plastic container). Initially, each tadpole was placed on either side of an opaque partition dividing the arena; this partition kept tadpoles separated but allowed water to flow throughout the container. After an acclimation period of one hour, tadpole activity (resting, swimming) of the separated individuals was recorded every 15 seconds for 10 minutes.  Following the acclimation and separated observation, the barrier was removed and tadpole interactions were recorded for 60 minutes. Focal behaviors (resting, swimming, biting, and chasing; see supplementary ethogram descriptions) were recorded for both tadpoles every 15 seconds. Tadpoles were visually distinguishable from each other as a result of size differences. Individuals were photographed and weighed before the beginning of each trial to establish initial tadpole condition, and were only used once (nTrial = 15 for each relatedness level, n = 90 tadpoles for the entire experiment).