Signalling of need by offspring to their parents

Abstract

THE young of birds and mammals often solicit food from their parents in ways that appear to be costly and to reduce their fitness^{1, 2}. Thus birds in the nest beg vigorously for food, incurring energetic costs and possibly attracting predators³; the behaviour of young mammals requiring to suckle (bleating or crying, for example) similarly appears costly^{1, 2}. If solicitation is a means by which the young communicate need to their parents, why has a less expensive form of communication not evolved⁴? An answer to this question is provided by the theory of parent–offspring conflict: natural selection acting on genes expressed in the young will lead to greater demands for parental resources than is optimal for the parent^{1, 5}. The accurate communication of offspring need is evolutionarily unstable as offspring will be selected to demand extra resources. Models of parent–offspring conflict have shown that an evolutionarily stable equilibrium can exist at which an offspring solicits resources in a way that reduces its fitness, and a parent provides extra resources to prevent further expensive solicitation^6–11. I present an alternative explanation for costly solicitation by showing that the level of offspring solicitation can be a true reflection of offspring need as long as solicitation is costly and the benefits of extra resources increase with need. My analysis suggests that the parent normally allocates resources using accurate information about the condition of the young. The requirement that the signalling system is costly is a direct consequence of the potential for parent–offspring conflict.