Resource exploitation and relatedness: implications for offspring size variation within broods

Much of the theory on offspring size variation within a brood relies on unequal maternal allocation of resources to each embryo. However, maternal allocation strategies are subject to an inherent conflict between mothers and offspring: individual offspring, being more closely related to themselves than to their siblings, should always prefer a larger share of the available resources than that which is optimal from their mother's perspective. Thus, in species where mothers cannot unilaterally impose a resource allocation strategy, offspring can respond to this conflict by competing for more resources than is maternally optimal. Here we show that variation in offspring size within a brood can arise as a by‐product of competition between siblings over a common resource, even when 1) there are no competitive inequalities within families, and 2) maternal investment per brood is fixed. Moreover, we show that size variance among offspring increases with increasing levels of competition, brought about by decreasing relatedness among siblings. Conflict thus offers a simple, testable and, potentially general, explanation for the wide variability in offspring size seen in nature. This extends explanatory hypotheses for offspring size variation beyond those of maternal effects, under which most explanations have been subsumed to date.