Abstract:
Mammalian heterotherms, such as hibemators, are known to be more tolerant of low
oxygen tensions than their homeothermic counterparts. It has been suggested that this relative
hypoxia tolerance is related to their ability to deal with dramatic changes in body temperature
during entry to and arousal from torpor. However, hibemators demonstrate dramatic seasonality
in both daily heterothermy and overall torpor expression. It was of interest to test if seasonal
comparisons of normothermic individuals within a single species with the capacity to hibernate
produce changes in the response to hypoxia that would reflect a seasonal change in tolerance to
low oxygen. In particular, the species studied, the Eastern chipmunk {Tamias striatus), is known
to enter into torpor exclusively in the winter. To test for seasonal differences in the metabolic
and thermoregulatory responses to hypoxia, flow-through respirometry was used to compare
metabolic rate, minimum thermal conductance, body temperature, and a thermal gradient used to
assess selected ambient temperature in response to hypoxia in both summer and winter
acclimated animals. Although the animals periodically expressed torpor throughout the winter,
no differences between season in resting metabolic rate, body temperature or minimum thermal
conductance were observed in normoxia. The metabolic trials indicated that chipmunks are less
responsive to hypoxia in the winter than they are in the summer. Although body temperature
dropped in response to hypoxia in both seasons, the decrease was less in the winter, and there
was no corresponding decrease in metabolic rate. Providing the animals with a choice of ambient
temperatures in hypoxia resulted in a blunting of the drop in body temperature in both seasons,
suggesting that the reported fall in body temperature set point in hypoxia is not fully manifested
in the behavioural pathways responsible for thermoregulation in chipmunks. Instead, body
temperature in hypoxia appears to be highly dependent on ambient temperature and oxygen
concentration. The results of this study suggest that the season in which the responses to hypoxia
are measured is important, especially in a heterotherm where seasonality can affect the degree to
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which the animal is tolerant of hypoxia. Winter-acclimated chipmunks appear more capable of
defending metabolic heat production in hypoxia, a response consistent with the increased
thermogenic capacity observed in animals that must periodically enter and arouse from torpor
during hibernation.
