Abstract
Within-brood or -litter dominance provides fitness-related benefits if dominant siblings selfishly skew access to food provided by parents in their favour. Models of facultative siblicide assume that dominants exert complete control over their subordinate sibling's access to food and that control is maintained, irrespective of the subordinate's hunger level. By contrast, a recent functional hypothesis suggests that subordinates should contest access to food when the cost of not doing so is high. Here, we show that within spotted hyena (Crocuta crocuta) twin litters, dominants most effectively skew access to maternal milk in their favour when their aggression prompts a highly submissive response. When hungry, subordinates were less submissive in response to aggression, thereby decreasing lost suckling time and increasing suckling time lost by dominants. In a species where adult females socially dominate adult males, juvenile females were more often dominant than males in mixed-sex litters, and subordinate sisters used more effective counter-tactics against dominant brothers than subordinate brothers against dominant sisters. Our results provide, to our knowledge, the first evidence in a mammal that dominant offspring in twin litters do not exert complete control over their sibling's access to resources (milk), and that sibling dominance relationships are influenced by sibling sex and training effects.
Highlights
Siblings may provide each other with indirect fitness benefits [1], within-brood or -litter asymmetric competition may result in fitness costs to the subordinate and fitness benefits for the dominant sibling [2]
To our knowledge, the first evidence in a mammal that dominant offspring in twin litters do not exert complete control over their sibling’s access to resources, and that sibling dominance relationships are influenced by sibling sex and training effects
In mixed-sex litters, 61.5 per cent (8/13) of subordinates gaining dominance were females, and this proportion did not deviate from chance, it was in the expected direction (p 1⁄4 0.58, 95% confidence interval (95% CI): 32.7 –83.4%)
Summary
Siblings may provide each other with indirect fitness benefits [1], within-brood or -litter asymmetric competition may result in fitness costs to the subordinate and fitness benefits for the dominant sibling [2]. Avian models of facultative siblicide predict that as parental food provisioning rates decline, dominant chicks should aggressively increase their share of food [2,5,6]. Such a response occurs in several bird species [7,8,12] and at least one mammal, the spotted hyena. A recent hypothesis [18] suggests that in facultatively siblicidal species, subordinates should adjust their submissiveness to their survival prospects by increasing assertiveness towards dominants as the cost of subordination in terms of likelihood of starvation increases.
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