A SMALL Australian marsupial is giving new clues to how the first mammals may have regulated their body temperature.
The fat-tailed antechinus, or Pseudantechinus macdonnellensis, basks in the morning sun to raise its body temperature after waking up from a nightly period of torpor. It’s the first direct evidence of a mammal using basking to increase its body temperature. And because it belongs to a group that can be traced back to some of the earliest mammals, its behaviour could provide insight into how they originally became warm-blooded.
The first mammals were about the size of a shrew and, because of their small size, were likely to lose body heat quickly. Their metabolic rates were probably slower than modern mammals, so how did they maintain their body temperature?
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Fritz Geiser of the University of New England in Armidale, New South Wales, thinks he’s found an explanation (Naturwissenschaften, DOI: 10.1007/s00114-002-0349-4). His team found that antechinus adults were active with body temperatures of between 33 and 37 °C until about midnight, when they cooled to between 16 and 27 °C and dropped into torpor. On clear days, they emerged from their dens at about 9.40 am and basked in the morning sun. The basking helped them to warm up at 0.28 °C per minute, more than twice as fast as when they were out of the sun.
As with the antechinus, Geiser thinks early mammals may have combined periods of torpor, which save energy, with sessions of basking to warm up again quickly. This behaviour may have been one of the first steps towards developing a fully warm-blooded metabolism, Geiser thinks.
Basking probably provided an important advantage for ancestral mammals, Geiser adds. He calculates that a mammal the size of the antechinus would have needed a good five hours to warm to body temperature if it had to rely solely on its internal metabolism, so warming up by basking would have given the animal more time to be active during the day at a lower energy cost.
Many small mammals such as ground squirrels and bats go torpid when they are inactive, but researchers had thought the trait developed independently. Geiser now thinks it may be an ancestral trait that larger mammals ditched because they dissipate heat more slowly. “If you’re very big you don’t have to bother,” Geiser told 91av.
