The effect of body condition on the trade-off between vigilance and foraging in Belding’s ground squirrels [L’effet de la condition corporelle sur la balance entre vigilance et affouragement chez les spermophiles de Belding].

Résumé

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 a 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), of 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 (play-back 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 foragiing freely, the deprived group showed a greater rate of mass gain than the supplemented group. These results suggest that (1) the foraging versus vigilance decisionis 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 energitic shortfall.