THE EFFECT OF BODY CONDITION ON THE TRADE-OFF BETWEEN VIGILANCE AND FORAGING IN BELDING GROUND-SQUIRRELS

A field experiment was performed to examine the effect of a change in energy reserves on the trade-off between foraging and predator vigilance. Free-living juvenile female Belding's ground squirrels, Spermophilus beldingi, were observed in three-step experiment. First, a squirrel's behavioural response to a tape-recorded conspecific alarm call was observed (playback 1). Then, over a 6-day period, either ad libitum foraging was supplemented with an energy-rich food (peanut butter mixture), or foraging opportunities were restricted for a few hours every day, during which an energy-poor food (lettuce) was available. These two treatments resulted in weight gain ('supplemented' group) or moderate weight loss ('deprived' group), respectively. Finally, a squirrel's response to a second alarm call was observed (playback 2). During playback 2, deprived individuals were less vigilant and foraged more than supplemented individuals. At playback 2, vigilance was positively related and foraging was negatively related both to a measure of body condition and to the mass change during treatment. Body condition and mass change during the previous week were not independent, making it difficult to determine which had a greater impact on foraging activity. However, in the week after playback 2, when all squirrels were foraging freely, the deprived group showed a greater rate of mass gain than the supplemented group. These results suggest that (1) the foraging versus vigilance decision is affected by body mass deviations from a 'set-point', and (2) a moderate reduction in energy stores can influence the trade-off between foraging and vigilance even though there is no short-term threat of energetic shortfall. © 1993 The Association for the Study of Animal Behaviour. All rights reserved.