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Torpid secret of arid-zone bat success

Energy efficient Australia's arid-zone bats owe their success to a fondness for torpor, new research has found.

Artiom Bondarenco, of the University of New England, and colleagues, report their findings in a recent issue of the Journal of Comparative Physiology B.

"At the same temperature, bats in arid areas use longer torpor bouts compared to bats from milder climates," says Bondarenco, who carried out the research for his PhD.

The arid zone comprises half of the Australian continent and is made up of habitats that are very harsh to live in with unpredictable weather patterns and limited food supply, says Bondarenco.

Despite this, microbats are one of the most successful groups of Australian arid-zone mammals.

There are more than 22 species from five families distributed all over arid and semi-arid areas.

So what is the secret to these bats' success?

One hypothesis is that the animals rely on torpor, in which they suppress all their physiological functions, and their body temperature drops down to the ambient temperature.

This can be a useful means of saving energy and water for small bats.

Because of their size they don't have much in the way of energy stores and they can lose a lot of water and energy because of their large surface to volume ratio.

Even just keeping their normal operating temperature, which is above 30°C, the animals must burn a lot of energy.

And the animals need to use a lot of water and energy to keep cool when summer temperatures climb to 48°C or more.

Radio tracking bats

To investigate the thermal biology of arid zone bats, Bondarenco and colleagues used temperature-telemetry to study the tree-roosting inland freetail bat (Mormopterus sp.) at Sturt National Park in New South Wales.

The researchers glued small temperature-sensitive radio transmitters between the shoulder blades of the eight bats in the study.

They then released the animals and tracked them during two summers, capturing temperature measurements at 10-minute intervals.

Bondarenco and colleagues found that overall, animals used torpor on 67.9 per cent of the days they were tracked, with torpor bouts ranging from half an hour to 39.3 hours depending on the weather.

On warm days, the bats spent on average three hours in torpor whereas on colder days they spent longer, says Bondarenco.

On the very hot days they didn't employ torpor at all and relied on energy and water saved from torpor bouts on milder days.

The other smart thing the bats do is when they are coming out of torpor, they tend to warm up with the environment.

Bats in milder climates must burn fat to produce energy to actively warm themselves up from torpor, says Bondarenco.

"Our bats mainly arise from torpor passively. As the ambient temperature increases their skin temperature increases as well. For that process they don't have to burn any energy at all," he says.