The Role of Energetic Costs in the Evolution of Begging Behavior of Nestling Passerines

Abstract

Dramatic advances have been made in our understanding of how elaborate traits and signals, such as the begging behavior of nestling passerines, might evolve. Some of the most fruitful theoretical advances have involved the assumption that such traits must carry a high cost. One of the clearest statements of the importance of this assumption is by Harper (1986), who assumes that begging is costly either in terms of increased predation risk or increased energy expenditures. Most models of begging share the assumption that begging is costly. In an earlier paper (McCarty 1996), I presented results of measurements of the metabolic rates of begging nestlings and found the scope of activity to be 1.05 times resting metabolic rate (RMR) in European Starlings (Sturnus vulgaris) and 1.27 times RMR in Tree Swallows (Tachycineta bicolor). As begging generally has been thought to result in a large increase in energy consumption (e.g. 3.5 x RMR, Beauchamp et al. 1991; or comparable to calling in frogs and insects [6 to 30 x RMR], Redondo and Castro 1992), and because other avian displays have been found to involve increases in metabolic rate as high as 3.9 to 15 x RMR (Vehrencamp et al. 1989, Eberhardt 1994), I concluded that the cost of begging was surprisingly low compared with costs of other activities, and that the assumption that begging was an energetically extravagant trait was not supported. The commentaries by Verhulst and Wiersma (1997) and by Weathers et al. (1997) question my conclusions and make several interesting suggestions for future research. Weathers et al. propose the intriguing hypothesis that energetic costs accrued through anaerobic metabolism may actually make begging an expensive display. Anaerobic metabolism is thought to be most important in terrestrial birds after work rates exceed the ability of aerobic metabolism to sustain activity (estimated to be >5 x BMR in adult birds; Weathers and Sullivan 1989), but the aerobic capacity of nestling passerines is virtually unknown. Although it will be interesting to learn about the use of anaerobic metabolism by nestling passerines in general, I think it is unlikely that its role in fueling begging will be found to change my conclusions. Accumulation of lactic acid in tissues during anaer-