DOES HEAT INCREMENT REPLACE NONSHIVERING THERMOGENESIS OF BROWN ADIPOSE TISSUE IN ARCTIC UNGULATES?

Robert G. White
Large Animal Research Station, Institute of Arctic Biology, University of Alaska Fairbanks, AK 99775-7000, USA.

If appetite is stimulated by cold exposure, particularly within the thermoneutral zone (TNZ), then heat increment of feeding (HIF) can lead to an effective lowering of the lower critical temperature (LCT). This mechanism would be adaptive if the response is sufficient to eliminate the need for shivering thermogenesis at low ambient temperatures. Evidence from studies with reindeer shows that voluntary food intake (appetite) is stimulated by low ambient temperature in winter, and the effect takes place within the TNZ. Theoretical estimates of HIF should cause a left shift in the LCT which expands the TNZ to cover most typical winter temperatures in interior Alaska. Thus, the number of acute thermogenic responses required to meet low ambient temperature would be few. Depending on whole body insulation, implications of not being able to meet cold stimulated appetite, because of chronic food shortage, could be severe. For example HIF would be maintained at a minimal level, and when ambient temperatures decline below the LCT, animals would need to shiver, further exacerbating negative energy balance. Nonshivering thermogenesis could provide an alternate mechanism for acute thermoregulation if such animals could activate synthesis of brown adipose tissue, or induce nonshivering thermogenesis by synthesis of the uncoupling protein. A prediction from this hypothesized model is that animals at risk to chronic hypothermia due to winter food shortage (calves, yearlings, older bulls, nursing cows) could have reserves of brown adipose tissue. Brown adipose tissue has not been reported in reindeer and caribou older than 3 weeks suggesting that developmental changes in the calf suppresses uncoupling protein throughout life, in spite of potential adaptive role for nonshivering thermogenesis.