How Selfish Should Stronger Sibs Be?

Abstract

We develop a series of models to investigate how selfish stronger progeny should be toward their weaker (full-sib) nest mates. The first represents a sibling hierarchy in which the strongest sib can take any amount of the total resources, the next strongest can take any amount from the remainder, and so on. When there are just two sibs, the stronger sib should increase its share of the total resources until its marginal increase in fitness reduces to half that of the weaker sib. With more than two sibs, the difference in the shares of two consecutive members of the hierarchy should increase down the dominance rank, such that the greatest difference in fitness occurs between the last two members, and the least between the top two. Fitness differences are most exaggerated when resources are relatively scarce. In our second hierarchy model, we show that these conclusions relate to "longterm aims" (i.e., ideal states to be achieved by the end of the nestling period). If we allow that, at a given time, sibs may differ markedly in their profit from food uptake, there quite clearly become instances when a dominant sib should allow a weaker nest mate to take more of a given meal. Roughly, this should occur at the moment during a given feeding bout when the asymptotic increment in fitness for the weaker sib is more than twice that for the stronger sib. In a third model, we investigate a case in which a sib's share of the resource is determined by competitive begging. Each sib can increase its share by a unilateral increase in its begging expenditure. We seek phenotypic strategies for such expenditures that are evolutionarily stable, one for each sib, given that a stronger sib gains more resources for a given effort. For two sibs, the weaker sib always spends more on begging, and the stronger sib's expenditure decreases as the competitive asymmetry between sibs increases. Somewhat counterintuitively, we find that the disparity between begging efforts is greater when resources are plentiful and that, although the stronger sib's share is greater when resources are scarce, the weaker sib may (paradoxically) attain the greater share when resources are plentiful. Despite this latter effect, the net gain (food uptake minus begging cost) of the stronger sib is always higher and its fitness therefore greater. Preliminary field tests of these models, using data from a brood-reducing species (the great egret, Casmerodius albus) are presented. The main prediction from the hierarchy model, that the two elder siblings in three-chick broods should take far larger shares of limited available resources, was supported. However, a basic prediction generated by the begging model was not upheld by the field data, at least for the measure of begging used in this analysis (number of scissors-like grasps of the parent's bill).