Size-dependent aggression towards kin in a cannibalistic species

Abstract

In juveniles extreme intraspecies aggression can seem counter-intuitive, as it might endanger their developmental goal of surviving until reproductive stage. Ultimately, 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. Lay SummaryBefore you eat someone, you have to attack them first. Here, we investigated the factors that shape aggression in the cannibalistic tadpoles of the dyeing poison frog. We find that aggression depends on both size and relatedness: when set in pairs, large tadpoles are half as aggressive towards their smaller siblings than to nonsibs. It looks like belonging to the same family provides some protection against aggression, though no one is ever truly safe.

Keywords: cannibalism; kin discrimination; larval aggression; poison frog; tadpole.

Figure 1

Tadpole activity levels before and during experimental trials. Panel (A) shows the post-acclimation activity of tadpoles. We found no difference in swimming between large and small tadpoles or relatedness treatments during this phase. Panel (B) shows experimental activity throughout behavioral trials. Large tadpoles were significantly more active than small tadpoles during assays. N_Trial = 15 for each relatedness level. Large tadpoles are in pink and small tadpoles in blue. Boxplot medians are depicted by thicker lines, whiskers span ± 1.5 * interquartile range.

Figure 2

Differences in aggression across relatedness treatments with respect to relative size between dyads. Point ranges indicate mean for each category with bootstrapped 95% confidence intervals. Pink points represent large tadpoles and blue dots represent small tadpoles. Dashed line indicates mean aggression for large tadpoles from non-sibling dyads. N_Trial = 15 for each relatedness level. There was significantly less aggression by large tadpoles from siblings dyads when compared with large tadpoles from non-siblings dyads.

Figure 3

Latency to first bite between tadpole dyads. Points are colored by the first biter’s relative size within dyads. Lines are fit with a GLM smoother with a y ~ log(x) formula and shaded regions represent 95% confidence intervals. There is an inversion in behavior as weight difference between dyads increases, where sibling pairs with large weight differences attacked significantly faster than non-siblings. Dyads where there were no aggressive behaviors were accounted for by assigning them the maximum time limit (60 min). N_Trial = 15 for each relatedness level.

Figure 4

Optimal aggressiveness of dyads of tadpoles as a function of relative size difference for three different levels of relatedness (represented in panel columns) and three sets of assumptions (represented in panel rows). First row: direct fitness was assumed to be size-independent. Second row: direct fitness was assumed to be proportional to size. Third row: aggressiveness was assumed to be costlier for smaller tadpoles. The smaller tadpole’s size was held fixed at s_i = 0.1; plotted lines show aggression levels in response to the increasing difference in size between dyads. The larger tadpole’s size is shown on the x-axis.