Small marsupials shown to be cool runners

Published 07 June 2012

stripe-faceddunnartResearchers at the University of New England have shown that small mammals can move from place to place while in a state of torpor – something that was thought to be impossible.

In a paper published online today in the Royal Society journal Biology Letters, they report that mouse-sized marsupials such as kalutas, dunnarts (pictured here) and planigales can run away from a perceived threat – such a predator – at the greatly reduced body temperatures that accompany torpor. (A. Daniella Rojas, Gerhard Körtner and Fritz Geiser: “Cool running: locomotor performance at low body temperature in mammals”. Biol. Lett. doi: 10.1098/rsbl.2012.0269.)

“Lack of movement has been used, in fact, to define the state of torpor,” said Daniella Rojas, the lead author of the paper. “This immobility – particularly when faced by a predator – was thought to be the ‘down side’ of torpor, partly negating its enormous energy-saving advantages.”

“Then a few years ago it was discovered that small marsupials move out of their burrows to bask in the sun while still in a state of torpor,” Ms Rojas said, “thus reducing the energy cost of raising their body temperature at the end of a torpor episode. Now we’ve been able to build on that discovery by showing that a number of small marsupials can run away from a perceived threat while torpid.”

Working with UNE’s Professor Fritz Geiser, an international authority on torpor in mammals and birds, she tested kalutas, dunnarts and planigales – species known to use torpor and basking behaviour in the wild. She filmed the marsupials running along a track both before and after inducing torpor. “All the animals were able to move at body temperatures as low as 14.8 degrees centigrade for planigales, 15.3 degrees for dunnarts, and 17.9 degrees for kalutas,” she said.

However, while they were able to run at speeds of 2 – 3 metres per second at body temperatures between 35 and 40 degrees centigrade, their running speeds dropped to less than 0.5 metres per second at the lowest body temperatures.

Ms Rojas believes that, even at these slower running speeds, the basking mammals would have a chance of escaping from a predator – particularly as they don’t move far from their burrows or rock crevices when basking. “Differences among such mammal species in their ability to run while torpid probably reflect different predation pressures they experience in the wild,” she said.