The role of torpor in the life of Australian arid zone mammals.

Abstract

Approximately half of the Australian continent is arid and is characterised by low primary productivity, limited supply of food and pronounced daily fluctuations of ambient temperature (Ta). Despite these adverse conditions the diversity of small mammals in the Australian arid zone is high, although their abundance is generally low. The most successful groups of small arid zone mammals are the dasyurid marsupials, native rodents, and insectivorous bats. A probable reason for the success of the insectivorous dasyurids and bats, which must cope with strong fluctuations in food and water availability, is their extensive use of torpor. Mammalian torpor is characterised by substantial reductions of body temperature (Tb) metabolic rate (MR) and water loss. Small arid zone dasyurids use exclusively daily torpor, some even during the reproductive season, when most mammals maintain strict homeothermy. Dasyurids reduce Tb from ~ 35ºC during normothermia to ~ 15ºC during torpor, the MR during torpor (TMR) is ~ 30% of basal metabolic rate (BMR). Mass loss, and thus water loss, is related to the duration of torpor bouts. Dasyurids usually enter torpor at night or in the early morning and arouse around midday or in the afternoon. Recent evidence shows that desert dasyurids may bask in the sun during rewarming from torpor. This can minimise energetic cost of arousal to a fraction of that required for endogenous warming. Arid zone bats are also likely to use torpor extensively, but few species, specific to the arid zone, have been studied. Nevertheless, widely distributed bats that occur in the arid zone, such as Nyctophilus geoffroyi, enter brief torpor for part of the day in summer and prolonged torpor (hibernation) for up to two weeks in winter and can reduce Tb to a minimum of 2 - 3ºC and TMR to ~ 3% of BMR; mass loss and water loss are minimal during torpor. Patterns of torpor similar to those in bats also have been observed in the insectivorous echidnas and two species of insectivorous / nectarivorous pygmy-possums, which have distribution ranges that include semi-arid and arid areas. In contrast to these species, no detailed information is available on torpor in native Australian rodents, because little work with respect to torpor has been conducted in Australia. However, many arid zone rodents on other continents employ torpor and it is likely that Australian rodents do as well. In addition to reducing energy expenditure and water loss, use of torpor also appears to prolong life span. This is important for bridging adverse conditions and for subsequent re-colonization of areas after droughts and fires in inland Australia. Thus it appears that the success of small insectivorous/nectarivorous mammals and perhaps rodents in the Australian arid zone is partially due to their use of torpor.