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.

在幼体中，极端种内攻击性（intraspecies aggression）看似反直觉，因其可能危及它们生存至繁殖阶段的发育目标。另一方面，攻击性对生存至关重要，尽管影响该行为表现与强度的因素（如遗传或环境因素）在不同分类群（taxa）间存在差异。

攻击（有时甚至杀死）亲缘个体可能降低广义适合度（inclusive fitness）；作为此问题的解决方案，部分物种表现出亲属识别（kin discrimination），并优先攻击非亲缘个体。

在此，我们结合实验与建模方法，探究物理特征（如相对于对手的体型）及遗传亲缘关系如何调控食同类染色箭毒蛙（Dendrobates tinctorius）蝌蚪二元组（dyads）中的攻击性。我们将不同体型的全同胞、半同胞及非同胞蝌蚪在实验arena中配对，并记录其攻击性与活动情况。

我们发现，相对体型与亲缘关系的交互作用可预测攻击行为：非同胞二元组中的较大个体比同胞二元组中的较大个体攻击性显著更强。意外的是，尽管同胞总体攻击倾向较低，但在体型不匹配的配对中，其攻击速度快于非同胞处理组（treatments）。

通过理论模型补充这些实证结果，我们提出，幼体攻击性反映了亲缘关系与体型间的平衡，个体在此平衡中权衡（trade-off）自身适合度与亲属的适合度。