Begging for control : when are offspring solicitation behaviours honest ?

Geoff A. Parker Population and Evolutionary Biology Research Group, Nicholson Building, School of Biological Sciences, University of Liverpool, Liverpool, UK L69 3GS. Ever since Trivers [1] showed that parents and their offspring could be in conflict over the amount of parental investment, the evolution and maintenance of begging behaviour has been an area of increasing interest to evolutionary biologists. Trivers considered the relationship from the point of view of the offspring, and saw begging as a form of psychological manipulation by the offspring to obtain a higher amount of parental investment than the parents were selected ideally to give. Subsequent theoretical treatments of begging have tended to model the allocation of parental provisioning to dependent young in terms either of the outcome of scramble competition among siblings (i.e. offspring control parental investment allocation [2–5]), or, more recently, in terms of honest signals [6] of offspring ‘need’ (i.e. parents control parental investment allocation [7,8]). So, are family dynamics regarding provisioning controlled primarily by parents or by their offspring? Since Kilner and Johnstone [9] reviewed the evidence for honest signalling, more recent work has cast some doubt on the stability of the honest-signalling solution [10,11] (but see [12]). Nevertheless, increasing numbers of recent empirical studies claim to support honest signalling [13–16]. Here, we re-evaluate critically the application of honest signalling to the evolution of costly begging displays in the light of recent models of sibling scramble competition [17,18], which show that the primary predictions generated by honest-signalling models and scramble models are not mutually exclusive (Box 1).