Consistent behavioural responses to heatwaves provide body condition benefits in rangeland sheep

Abstract

Species are increasingly confronted with extreme climatic conditions such as heatwaves. Behavioural changes are an important response to changed environmental conditions. We investigated how free-ranging sheep (Ovis aries) in large rangeland paddocks respond to heatwave conditions in the arid zone of Australia. We defined heatwaves as a three-day period with each day’s maximum temperature exceeding 40 °C, and its minimum temperature remaining above 25 °C. Hence, the daily maximal temperature exceeded the upper limit of the thermal neutral zone in sheep. We equipped 48 sheep with GPS collars and combined the detailed locational data with georeferenced maps of water locations and tree areas, representing shade. We measured shade use as time spent within areas with trees. We show that individuals were consistent in their shade use behaviour between two separate heatwaves, while there was variation among individuals. Furthermore, shade use was positively related with body condition change between the start and the end of the study. In sheep body condition affects reproductive success, which suggests that shade use behaviour may have flow-on effects on reproductive benefits. We also compared shade use, water use and movement activity between the two heatwaves and two periods of typical ambient conditions. During heatwaves sheep spent on average 14.7 times longer in the shade during the midday interval and 2.0 times longer before dusk than during typical periods. Time spent near the water was high during the midday interval but did not differ between conditions, although, it was greater after dawn and before dusk during heatwaves. Such behavioural changes would assist in avoiding hyperthermia and dehydration. Sheep moved half the distance (which would include grazing) during the midday interval, which they seemed to offset to some extent through increased movement activity before and after dawn and dusk. Our study indicates that shade, and water use behaviours during hot ambient conditions upscale from small yards to large paddocks. Furthermore, it provides deep insight into the behavioural strategies that will allow a free-ranging arid zone ungulate to cope with the challenging ambient conditions that will become more prevalent with a changing climate.