How high ambient temperature affects the daily activity and foraging time of a subtropical ungulate, the greater kudu (Tragelaphus strepsiceros)

Abstract

The influence of ambient temperature on daily activity level was assessed for greater kudus observed in two study areas in South Africa. Active time proportion for adult females and subadults of both sexes was significantly negatively correlated with maximum daily temperature, but uninfluenced directly by cloud cover. However, maximum temperature differences between days accounted for only a quarter of the variability in active time. On most days the overall activity level and hence daily foraging time allocation were somewhat lower than the maximum levels observed for the prevailing temperature conditions. The upper limit to the distribution of active time was lower during the mild dry season than over the hot wet season, probably due to a seasonal change in pelage by kudus. Temperature influences were strongest during the early growing season, when food availability was low and conditions frequently hot. The active and foraging times of kudus during daylight hours appeared to be restricted below the target level only when maximum daily temperature exceeded 36 °C in the wet season, and 30 °C in the dry season. Only 15% of days exceeded these levels during the respective mid-seasonal periods, suggesting that foraging activity was effectively constrained by thermal tolerance on only about one day in seven. Optimal foraging models that assume thermal stress to be a consistent daily constraint on foraging time may be misleading.