Abstract
Theoretical models aimed at explaining the evolution of honest, informative begging signals employed by nestling birds to solicit food from their parents, require that dishonest signalers incur a net viability cost in order to prevent runaway escalation of signal intensity over evolutionary time. Previous attempts to determine such a cost empirically have identified two candidate physiological costs associated with exaggerated begging: a growth and an immunological cost. However, they failed to take into account the fact that those costs are potentially offset by the fact that nestlings that invest more in begging are also likely to obtain more food. In this study, we test experimentally whether a 25% increase in ingested food compensates for growth and immunological costs of extra begging in southern shrike (Lanius meridionalis) nestlings. Three nestmates matched by size were given three treatments: low begging, high begging-same food intake, and high begging-extra food intake. We found that, while a higher food intake did effectively compensate for the growth cost, it failed to compensate for the immunological cost, measured as T-cell mediated immune response against an innocuous mitogen. Thus, we show for the first time that escalated begging has an associated physiological net cost likely to affect nestling survival negatively.
Highlights
Nestling birds typically solicit food from their parents by a set of exuberant begging displays which appear to be excessively complex and wasteful to merely accomplish an efficient transfer of food from parents to young [1,2]
Signals that are too cheap to produce should lead to runaway escalation of begging intensity over evolutionary time, which might eventually render the communicative system unstable [9,10]
Several studies have found that chicks experimentally induced to beg at higher rates showed reduced growth rates compared to less-begging controls in some species but not others
Summary
Nestling birds typically solicit food from their parents by a set of exuberant begging displays which appear to be excessively complex and wasteful to merely accomplish an efficient transfer of food from parents to young [1,2]. Ever since Trivers [3], conspicuous begging has been often interpreted as the evolutionary outcome of a genetic conflict of interests between parents and their offspring about the amount of transferred parental resources (Parent-Offspring Conflict), in which offspring are selected to secure more resources from their parents than the latter are selected to give [4,5,6] From this perspective, showy begging signals may have evolved either as selfish attempts to influence parental decisions in scramble sibling competition for parental resources [7,8], and/or as honest signals allowing parents to allocate food in proportion to begging intensity, as begging would be a reliable indicator of nestling nutritional need [9]. In spite of this evidence, the question of whether begging signals are really costly in terms of offspring fitness still remains a troubling area of disparity between theoretical and empirical studies [14,29,30]
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