Thermoregulation in the South American grey short-tailed opossum, Monodelphis domestica : the effects of ambient temperature, bacterial endotoxin and hypoxia on behavioural and autonomic body temperature control

This study investigated the concept of thermoregulatory set-point in marsupials, by examining behavioural and autonomic thermoregulation in a laboratory-bred marsupial, the South American grey short-tailed opossum (Monodelphis domestica) with occasional reference to a single captive sugar glider (Petaurus breviceps). Adult animals had clear circadian rhythms of core body temperature (Tb). Such rhythms were not apparent in juvenile M. domestica and rhythms in adult animals were disrupted when ambient temperature (Ta) was reduced, bacterial endotoxin (E. coli LPS) was injected and ambient oxygen levels were lowered to approximately 10% (i.e. during hypoxia). Circadian changes in preferred Ta were observed in individual animals while in a longitudinal thermal gradient but no specific rhythm was observed. M. domestica was able to make small, but insignificant, changes to Tb using thermoregulatory behaviour. This resulted in less variance in Tb in this species while in a thermal gradient. A typical mammalian fever to LPS and a typical hypothermic response to hypoxia was observed in M. domestica and one P. breviceps. Although hypothermic responses to hypoxia have been previously documented in marsupials, this is the first published account of LPS-induced fever in marsupials. Preferred Ta was not significantly affected by hypoxia or fever in M. domestica although a significant reduction in preferred Ta was observed in the sugar glider while hypoxic. This animal also selected a warmer environment while febrile. The hypothermic response to hypo)da in M. domes tica was found to be more significant than the hyperthermic response to bacterial endotoxin. However, the sugar glider utilised thermoregulatory behaviour to maintain a hyperthermic response to bacterial endotoxin even when exposed to hypoxia. These findings show that in marsupials, as in eutherians, thermoregulation involves the interaction of autonomic and behavioural mechanisms. These experiments also highlighted the limitations of using laboratory bred animals to determine the natural thermoregulatory capacities of a species