Solar powered Ibex

Winter is a difficult time for most mammals. Food is often in short supply and thermoregulatory costs are high. Small mammals may avoid the worst winter has to offer by entering torpor or seeking more favorable microclimates in underground burrows. Large mammals (except a few that hibernate) lack these options. How do large ungulates (i.e. caribou, elk, mountain goats, etc.) survive harsh winters?

Large ungulates acclimatize as winter approaches by:
1) reducing their body temperature set-point (hypometabolism),
2) reducing food intake,
3) increasing the thickness of fur (i.e. insulation),
4) losing weight (reducing overall energy expenditure), and
5) spending less energy on locomotion.

To date most studies of winter thermoregulation in ungulates have been conducted on captive animals. However, technological advances allowed
Signer and colleagues (2011) to measure energy expenditure in free-ranging alpine ibex (Capra ibex) living at high altitude in the Swiss Alps (Figure 1).

ibex
Figure 1. An Alpine ibex surveys the mountainside in Switzerland (from Flickr/Rick McCharles)

The researchers instrumented 20 ibex with telemetry devices that recorded heart rate, rumen temperature (essentially a core body temperature), and locomotor activity. The data was collected continuously for two years. By correlating telemetry data with weather conditions and direct observations of ibex behavior, the researchers were able to verify that alpine ibex reduce their heart rate and body weight during winter (Figure 2). During the winter, alpine ibex expend less than half the energy they would use during the summer. Ibex also feed less often and move less frequently.

ibex_data
Figure 2. Seasonal variation in ibex stationary heart rate over a two-year period (solid black line), snow height (solid grey line), and body condition (dotted line). The horizontal axis denotes periods when the ibex were in summer (black bar), winter (white bar) fur and molt (gray bar).

All of these thermoregulatory savings together were still only able to account for roughly 40% of the seasonal variation in heart rate. What could account for the remainder? Signer and colleagues realized that ibex spend considerable time basking in the sun during the winter months. Basking reduces heat loss considerably. This study is the first to reveal the importance of solar-assisted re-warming as an overwintering strategy in ungulates.

At night, ibex took shelter from wind and snow among rocky crevices. As the first rays of morning sun appeared, the ibex sought out sunny spots and move from one sunning spot to the next as the day progressed. The authors conclude that, “together with substantially increased insulation, this strategy allowed the animals to significantly down-regulate metabolic rate during winter while avoiding a drastic reduction in body temperature”. They also point out that maintaining an appropriate body temperature is vital to ibex, because lowered body temperature impairs ruminant fermentation. It may be that basking plays an important, and unappreciated, role in the winter survival of many other large mammals.


References

Signer, C., Ruf, T., and W. Arnold. 2011. Hypometabolism and basking: the strategies of Alpine ibex to endure harsh over-wintering conditions.
Functional Ecology, DOI: 10.1111/j.1365-2435.2010.01806.x