|Abstract or Summary
- An investigation was undertaken to determine the role that temperature might play in limiting the distribution of the mountain beaver, Aplodontia rufa. A field study was carried on from August, 1965, through August, 1966, in the coast mountains of Benton County, Oregon, and consisted of measuring the annual temperature cycle of the burrows in conjunction with air temperatures taken at a height of six cm above the soil surface. A laboratory study was conducted to determine the effects of ambient temperature on body temperature,
metabolic rate, thermal conductance, insensible water loss, and heart rate. The burrow system was characterized by a cool and stable microclimate. The temperature above the soil showed a maximum annual fluctuation of 40.5°C, the temperature within the burrows varied 18.3°C, with a weekly variation in burrow temperature of not more than 4°C. Annual range in mean burrow temperature was from 2° to 14°C. The mean body temperature of Aplodontia, over an ambient temperature range of from 4° to 290°C was 37.60°C. Free-living animals had a mean body temperature of 38.0°C. Hyperthermia
began at ambient temperatures above 29°C and a lethal body temperature of approximately 42°C was reached after a two hour exposure
to temperatures between 32° and 35°C. Aplodontia lacked a thermoneutral zone and minimal metabolic rates were recorded at high ambient temperatures when the animals
were hyperthermic. Under conditions of heat stress, mean metabolicrateswere 65 percent of the values predicted by Kleiber's equation. These animals were lethargic, their respiration rates decreased to as low as 18 per minute with minimal heart rates between 60 and 100 beats per minute. The mean minimal recorded heart rates, based on body weight, were 38 percent of that predicted. Aplodontia had a minimal thermal conductance of 0.195 cal/g/hr/°C which could only be increased by a factor of 1.84. Winter
animals had an effective insulation L. 23 times that of summer animals. This raised the lower critical temperature of summer animals by 4°C (19° to 23°C). Aplodontia required a six degree thermal gradient to dissipate its minimal heat production.
Insensible water loss could only be increased by a factor of 2.3 over the ambient temperature range of 12° to 31.5°C. At the latter ambient temperature, evaporative heat loss accounted for only 22
percent of Aplodontia's total heat loss. The results of this investigation indicated that high environmental temperatures may act as an important limiting factor in the
distribution of the mountain beaver which lacks adequate mechanisms to avoid heat stress.