Thermoregulation by kangaroos from mesic and arid habitats: Influence of temperature on routes of heat loss in eastern grey kangaroos (Macropus giganteus) and red kangaroos (Macropus rufus)

We examined thermoregulation in red kangaroos (Macropus rufus) from deserts and in eastern grey kangaroos (Macropus giganteus) from mesic forests/wood lands. Desert kangaroos have complex evaporative heat loss mechanisms, but the relative importance of these mechanisms is unclear. Little is known of the abilities of grey kangaroos. Our detailed study of these kangaroos' the rmoregulatory responses at air temperatures (T-a) From -5 degrees to 45 deg rees C showed that, while some differences occur, their abilities are funda mentally similar. Both species show the basic marsupial characteristics of relatively low basal metabolism and body temperature (T-b). Within the ther moneutral zone, T-b was 36.3 degrees +/- 0.1 degrees C ((X) over bar +/- SE ) in both species, and except for a small rise at T-a 45 degrees C, T-b was stable over a wide range of T-a. Metabolic hear production was 25% higher in red kangaroos at T-a -5 degrees C. At the highest T-a (45 degrees C), bo th species relied on evaporative heat loss (EHL) to maintain T-b; both pant ing and licking were used. The eastern grey kangaroo utilised panting (76% of EHL) as the principal mode of EHL, and while this was so for red kangaro os, cutaneous evaporative heat loss (CEHL) was significant (40% of EHL). CE HL appeared to be mainly licking, as evidenced from surface temperatures. B oth species utilised peripheral vascular adjustments to control heat flow, as indicated by changes in dry conductance (C-dry). At lower temperatures, C-dry was minimal, but it increased significantly at T-a just below T-b (33 degrees C); in these conditions, the C-dry of red kangaroos was significan tly higher than that of eastern grey kangaroos, indicating a greater relian ce on dry heat loss. Under conditions where heat flows into the body from t he environment (T-a 45 degrees C), there was peripheral vasoconstriction to reduce this inflow; C-dry decreased significantly from the values seen at 33 degrees C in both kangaroos. The results indicated that, while both spec ies have excellent thermoregulatory abilities, the desert red kangaroos may cope better with more extreme temperatures, given that they respond to T-a 45 degrees C with lower respiratory evaporation than do the eastern grey k angaroos.