Seasonal acclimatization in varying hare (Lepus americanus).

Abstract

Wild hares were trapped in the vicinity of Ottawa, Canada, and tested during summer and winter. After recovery from implantation of subcutaneous and substernal thermocouples, measurements of oxygen uptake and body temperatures were made during successive stepwise lowering of ambient temperature from 20 °C to −45 °C over about 5 hours. Measurements were also made during stepwise elevation of temperature from 12 °C to 38 °C over a 5-hour period and a final single test was made at 40 °C.Hares trapped during the winter had lower O2 uptake than did summer-caught hares at temperatures below thermal neutrality. The change was very similar in magnitude to the seasonal change in insulation of the fur (27% greater in winter). The lower critical temperature was shifted from +10 °C in summer hares to about −5 °C in winter hares.No seasonal differences were noted in substernal temperatures, but subcutaneous temperatures were significantly higher in winter-caught animals. Colonic temperatures were higher than substernal temperatures at all ambient temperatures.Electromyograms recorded from the mid-back showed that shivering was greater in the cold during summer than during winter in accordance with the higher O2 consumption at any given temperature. Shivering was also slightly greater during summer at the same level of O2 consumption.Varying hares showed a considerable tolerance to elevation of temperature and a capacity to maintain approximate equality of body and ambient temperature at 40 °C for some time through effective respiratory evaporative heat loss, which approached 100% of heat production. Stepwise elevation of ambient temperature did not reveal any seasonal differences in upper critical temperature (about 38 °C) or in elevation of body temperature in spite of differences in insulation of the fur. A slightly greater proportion of the total heat loss was by evaporation during the summer.Caloric intake of captive hares outdoors was similar during winter and summer. It is concluded that seasonal acclimatization in the varying hare is largely insulative with respect to cold and that changes in heat tolerance are minor if present at all. Insulative and behavioral modifications appear to compensate for seasonal changes in temperature in the Ottawa area.