Honest begging: signals of need, quality, and/or hunger?

Abstract

Mock et al. (2011) present a very timely review of ideas on the fascinating topic of offspring begging, which is a field of research that has perhaps lost its way over the course of the last 2 decades. They very usefully clarify and separate out theoretical arguments in a number of key areas. In doing so, they perhaps fail to provide a complete answer, but they nevertheless provide a useful framework for discussion, which is used here to further explore certain arguments. Sibling altruism is included in the scheme presented by Mock et al. (2011, Table 1) but only as part of the Signal of Need (SoN) hypothesis. There is also little mention of the central role played by sibling relatedness on variation in begging effort across species, either as a result of extrapair activity (e.g., Briskie et al. 1994) or brood parasitism (e.g., Kilner et al. 1999). In the absence of any other hypothesis, these are often seen as some of the strongest bits of evidence we have in support of the SoN hypothesis. However, the alternative Signal of Quality hypothesis [and perhaps even the Signal of Hunger (SoH) hypothesis] contains exactly the same potential for sibling altruism and so could explain these findings equally well. As with direct sibling competition (Royle et al. 2002), any begging strategy that benefits an offspring over its siblings can (and will) evolve so as to reflect different average levels of kinship, whatever the hypothesis behind such adaptive begging behavior. The SoH hypothesis is unfortunately not very well developed by Mock et al. (2011) in terms of the adaptive basis for such a mechanistic explanation. It seems clear that digestive constraints represent the only reason why parents would care about offspring stomach fullness (i.e., hunger) per se. Digestive efficiency is assumed to decline with stomach fullness, creating the classic diminishing fitness returns curve for successive parental feeding of the same offspring. However, recent research has shown reduced digestive efficiency only when nestlings are approaching satiation (Grodzinski et al. 2009). Even more worrying for the SoH hypothesis is that nestlings in the wild almost never get that close to satiation, and digestive efficiency does not vary within natural ranges of food intake in the way that begging does (Wright et al. 2010). It therefore appears that food is absorbed at a largely similar rate whichever offspring the parents feed. So, begging must be being used to signal stomach fullness because it relates to some other graded physiological or developmental need. At which point, the distinction between a SoH and a SoN becomes much less clear. Mock et al. (2011) severely underestimate the potential utility of what they call ‘‘hunger experiments.’’ When correctly carried out, such experiments are crucial for any real understanding of the evolution of begging via parent–offspring conflict. This is because they allow an assessment of the shape (i.e., elevation and slope) of individual offspring demand functions. Alongside, similar assessments of individual parent supply functions, these can be treated as ‘‘behavioral reaction norms’’ and quantitative genetics methods applied to investigate the coevolution of parent and offspring strategies in natural populations (see Smiseth et al. 2008). This would appear to be one of the most promising and exciting avenues for future research in this field, and so it would be a shame to discourage such basic data collection. Properly carried out hunger experiments are also crucial in explorations of important topics, such as offspring digestion (see above) and learning (e.g., Kedar et al. 2000; Budden and Wright 2005). One such hand-feeding study by Wright et al. (2011) provides key findings in an informative area of begging research that is largely neglected by Mock et al. (2011), which is that senior and junior nestlings within the same brood possess quite different shaped demand functions. Such cryptic strategies in nestling behavior bring into question the suggestion by Mock et al (2011) that it is enough to simply observe whether parents or offspring are ‘‘in control’’ of provisioning and food allocation. The same problem arises from cryptic parental strategies, such as maternal effects and ‘‘laissez faire’’ allocation strategies, which can manipulatively define the arena for begging competition between siblings. Hence, apparent behavioural control at the point of food delivery may not always tell us very much about the specific evolutionary resolution of parent–offspring conflict being observed. As this short commentary hopefully illustrates, the review by Mock et al. (2011) has the potential to breath new life into the study of begging by opening up old arguments to fresh debate and by encouraging new research directions to answer old questions.