Sibling rivalry: training effects, emergence of dominance and incomplete control

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.

Figure 1.

The relative delay (rel.delay) of dominant and subordinate siblings to return to suckling after aggression by the dominant interrupted suckling, in relation to (a) level of assertiveness of the subordinate (mean.subscore), (b) hunger level of the litter, and (c) age (days), as predicted by model 1. The relative advantage of the dominant is higher when the relative delay tends towards 1, i.e. when the dominant returned much faster to suckle than the subordinate. Tick marks inside the x-axis indicate location of individual data points, dotted lines indicate confidence intervals. Unequal spacing on the y-axis is used to obtain straight lines (see the electronic supplementary material for details on the methods used to produce figures 1–3).

Figure 2.

Mean time(s) lost suckling by the dominant owing to its aggression (domlat) in relation to: (a) time (s) lost suckling by the subordinate (sublat), (b) level of aggression of the dominant (mean.domscore), (c) age (days), and (d) level of assertiveness of the subordinate (mean.subscore) and litter sex composition (scomp2) where (iii) ♀dom♀sub = all-female litter; (iv) ♂dom♂sub = all-male litter; (i) ♀dom♂sub = mixed-sex litter with dominant female; (ii) ♂dom♀sub = mixed-sex litter with dominant male, as predicted by model 2. Tick marks on the x-axis indicate location of individual data points, and dotted lines show CI.

Figure 3.

The mean suckling time(s) lost by the subordinate owing to aggression by the dominant (sublat) in relation to: (a) suckling time(s) lost by the dominant (domlat), (b) level of assertiveness of the subordinate (mean.subscore), (c) age (days), (d) level of aggressiveness of the dominant (mean.domscore) and hunger level of the litter, and (e) level of aggressiveness of the dominant and litter sex composition (scomp2), where ♀dom♀sub = all-female litter; ♂dom♂sub = all-male litter; ♀dom♂sub = mixed-sex litter with dominant female; ♂dom♀sub = mixed-sex litter with dominant male, as predicted by model 3. Tick marks on the x-axis indicate the location of individual data points, and dotted lines show CI.

Figure 4.

Change in the submissive response of the subordinate to repeated aggression by the dominant per focal sample in relation to age (days). Change in behaviour was expressed as the slope of the regression of subscore over consecutive interactions against time. Highly negative values indicate a strong loser effect; values close to zero indicate little change in response. The three most negative values represent three different litters. The vertical dotted line illustrates the change in tendencies after 90 days of age, the mean age before which siblicide occurs [4,27].