Consistent behavioural responses to heatwaves provide body condition benefits in rangeland sheep
Introduction
Heatwaves, which are one of the more conspicuous consequences of climate change (van de Pol et al., 2017), have increased in both intensity and frequency over the last five decades (Perkins et al., 2012). Phenotypic plasticity, and in particular behavioural changes, are often perceived as a quick response to perturbed environmental conditions, and can play an important role in the acclimatisation to rapid environmental change (Beever et al., 2017; Boutin and Lane, 2014; Merilä and Hendry, 2014; Wong and Candolin, 2014). Consequently, understanding the benefits of these behavioural changes, for instance for maintaining homeothermy, and the extent to which they may drive animal resilience to climate change is of fundamental importance. An example of phenotypic plasticity in response to climate change is the timing of breeding. For instance, bighorn sheep (Ovis canadensis) advanced their median parturition date of the population by 15 days over a period of 26 years, and Renaud et al. (2019) showed that this shift could largely be explained by individual plastic responses to autumn precipitation. In other species, however, such as red squirrels (Tamiasciurus hudsonicus) the timing of breeding has advanced due to both phenotypic changes within generations and genetic changes among generations (Reale et al., 2003).
Regarding the effects of climate change, Levy et al. (2019) suggested that temporal shifts in activity, as extreme as shifting from diurnal to nocturnal activity, may mitigate the costs of heat stress. Spatial shifts of activity have also been reported. For instance, when ambient temperatures increase, male Ibex (Capra ibex) move to areas at higher elevation which are cooler (Aublet et al., 2009). These examples illustrate behavioural responses to extreme temperatures. However, we do not know whether individuals are consistent in their behavioural response, for instance across heatwaves. Furthermore, we are particularly lacking an understanding of whether the expression of these behaviours is beneficial or costly. Benefits include improved thermoregulation and avoidance of hyperthermia and dehydration (Cain et al., 2006) whereas costs include foregone foraging times (Aublet et al., 2009; Fuller et al., 2016). These costs and benefits suggest flow-on effects on body condition. We addressed these questions by tracking free-ranging sheep (Ovis aries) during heatwave conditions in the arid zone of Australia with animal-attached GPS collars.
We investigated whether individuals were consistent in their shade use behaviour across heatwaves. We then quantified how each individual’s shade use relates to changes in body condition over the length of the study. Finally, we compared shade use behaviour, movement activity (which includes grazing) and time spent close to water between heatwave and typical conditions. Extreme conditions, such as heatwaves, are rare by definition, but by contrasting individual behaviour during two heatwaves with two periods of typical ambient conditions, we are able to identify behavioural differences when thermoregulation is of great importance. As ambient conditions vary throughout the day, we investigated behavioural differences for five time intervals per day that differed in their ambient temperatures and hence severity of heat stress experienced by the sheep. We hypothesised that sheep mitigate the energetic costs of physiological thermoregulation during heatwaves by changing their space and resource use. We predicted that during periods of great ambient temperature sheep (1) increase their shade use and spend more time within patches of trees that provide shade. We further predicted that sheep (2) spend more time near the watering point to mitigate dehydration risk from evaporative water loss, and (3) reduce movement activity during hot times of the day, but potentially compensate by increasing activity at times with less challenging ambient conditions.
Using a large GPS dataset, our study quantifies how individual sheep respond to heatwave conditions, whether these responses are consistent across heatwaves and finally how they affect body condition. This provides deep insight into the behavioural strategies of an arid zone ungulate to cope with the challenging ambient conditions that are going to become more common with climate change.
Section snippets
Study site
We conducted our study at Fowlers Gap Arid Zone Research Station (31°05′S, 141°43′E) from December 2017 to March 2018, which is located in the North-West of New South Wales, Australia. We conducted our study in a paddock that is 6 × 1 km in size and includes one water trough at one end of the paddock (31° 5.883′S, 141°84.543′E). The study area is characterised by chenopod shrubland, and consists mostly of blue bush (Maireana spp.) and small trees (Acacia spp., Burrell, 1973).
Sheep movement and space use
In December 2017,
BOM data and heat wave conditions in the paddock
In order to be applicable across different climates, heatwaves are broadly understood as periods with hotter temperatures than normal for consecutive days (Perkins et al., 2012). In line with this understanding, the daily maximal temperatures during both heatwaves were well above the daily maximal temperatures during January and February 2018 (Wilcoxon W = 58, p = 0.003), whereas the maximum temperatures for the two periods of more typical conditions were not (Wilcoxon W = 324, p = 0.999).
We
Consistent individual shade use behaviour and its effect on body condition
Individuals were consistent in their shade use behaviour across two separate heatwaves. At the same time there was considerable variance among individuals. These consistent individual differences in shade use could be interpreted as an individual behavioural type. It is supported by increasing evidence that behavioural types influence animal space use, including resource use (Spiegel et al., 2017, 2015). It is further conceivable that it could be driven by an underlying state and physiological
Declaration of Competing Interest
Author declare no conflict of interest.
Acknowledgments
We thank Garry Dowling (Fowlers Gap station manager), Mark Tilley, Derek Kennedy for their support during sheep handing and mustering. We also thank Oliver Schülke for his insightful comments on an early draft, Molly Gilmour for her assistance in the field, and Héctor Pacheco for measuring the wool samples. This work and STL were supported by an Australian Research Council DECRA Fellowship [DE170101132]. The Exchange Master of Research program at Macquarie University supported KQ. The funding
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