On the cost of begging vocalization: implications of vigilance

Begging vocalization is thought to have evolved as a consequence of the parent-offspring conflict over parental investment (Mock and Parker, 1997). Under this conflict, parents are reluctant to provide all the resources requested by current offspring because they are saving resources for future broods (Trivers, 1974). In this scenario, begging has evolved as an honest signal of need with the most hungry offspring begging most conspicuously. This information allows parents to optimally adjust reproductive investment, since they can accurately assess offspring food requirement (Godfray, 1991). Given the conflict of interest over resources between parents and offspring, the evolutionary stability of begging would be ensured by its cost for two reasons. First, the benefit of obtaining additional resources from the parents increases with need, and hence the benefit to be fed outweighs the cost of begging only when hungry. Second, if all offspring, independent of need, were to beg at the same level, parents would be rapidly selected to ignore begging solicitation (Godfray, 1991; MacNair and Parker, 1979). The hypothesis that begging is a costly signal of need has prompted numerous experiments to test its predictions. Empiricists have found that parents increase feeding rate when begging level is amplified via play-back experiments (Burford et al., 1998; Davies et al., 1998; Ottosson et al., 1997; Price, 1998; but see Clark and Lee, 1998) and that parents preferentially allocate food to the offspring begging most vigorously (e.g., Hofstetter and Ritchison, 1998; Kilner, 1995; Roulin et al., 2000). Another avenue of experimentation has focused on the cost of begging, and three different costs have been detected so far. These are reviewed below.