Tiny shrews ‘warm up’ before cold dives

American water shrew (Sorex palustris) underwater (c) Roman Gusztak "Silver bullets": Water shrews use speed to reduce the impact of colder waters when diving

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Thumb-sized shrews "warm up" their bodies before taking the plunge into cold water, according to researchers.

Scientists investigated how the shrews, known as the smallest diving mammals, coped with the challenges of diving.

Larger mammals are known to boost their chances of finding prey by staying cool to save energy and dive for as long as possible.

But the shrews raised their body temperature by up to 1.5C and took shorter dives in cold water.

"They were completely contrary to what we predicted," said Dr Kevin Campbell from the University of Manitoba, Winnipeg, Canada who presented the research at the Society for Experimental Biology's annual conference.

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In the past, research into diving ability has focused on large species including seals and penguins.

But Dr Campbell and his colleagues wanted to look at the other end of the scale. They spent over a decade working with tiny American water shrews (Sorex palustris), which weigh a maximum of 17g.

The thumb-sized animals are known for their voracious appetite, consuming their entire body weight in prey every day in order to survive, and Dr Campbell described them as "the most effective, ruthless predators I have ever seen".

Although they live on land they frequently hunt underwater, diving to catch prey such as dragonfly nymphs, crayfish and snails.

Dr Campbell explained that according to existing knowledge, the shrews' diminutive size should make them unsuitable divers.

"Optimal foraging theory would say you want to stay underwater as long as you can because that increases your likelihood of finding food," he said.

"This is why most diving animals are large, because the larger you are, you carry more oxygen and you burn it at a slower rate."

Start Quote

What we're thinking is that they are increasing their body temperature to increase their sensory system”

End Quote Dr Kevin Campbell University of Manitoba

By comparing them with short-tailed shrews, the team found that water shrews do have relatively greater oxygen stores but Dr Campbell described them as "pushing the boundary of size for divers".

To fully understand how the tiny animals survive their lifestyle, the scientists wanted to know if they were sharing any other strategies with larger divers.

"Another thing you can do to dive longer is to let your body temperature drop a little because then your metabolism slows down," explained Dr Campbell.

To test for this the team used miniature implants to record the water shrews' body temperatures before, during and after diving.

Dr Campbell and his colleagues monitored how the shrews' body temperatures changed when diving in a variety of different water temperatures in a tank specially designed to simulate their wild habitat.

The warmest water was 30C, close to the shrews' normal body temperature, but the coldest water was just 3C, mimicking winter water temperatures in their native North America and Canada.

American water shrew (Sorex palustris) diving underwater with bubbles (c) Roman Gusztak Air trapped in their coats allows the water shrews to resurface quickly and makes them appear silver

To their surprise, the scientists found that the animals did not employ the usual strategy of lowering their temperature, choosing instead to minimise their dives in cold water.

The animals sought prey just as frequently but spent less total time in the water and took shorter dives.

Dr Campbell explained that this could be because losing heat poses a much greater risk for the shrews.

"When you're tiny you get to that [critically low] temperature much quicker, so you're much more susceptible to hypothermia," he said.

Instead the shrews maintained their temperature and even employed the opposite tactic; sitting at the water's edge and warming up by 1.5C before diving into the coldest water.

According to scientists the shrews were achieving the heat boost by shivering or using "brown fat": tissue found in many small mammals that can generate heat without them exercising.

"What we're thinking is that they are increasing their body temperature to increase their sensory system," said Dr Campbell.

The water shrews employ sophisticated senses of hearing and touch to make them efficient hunters.

They are even known to "smell" underwater, bubbling air from their nose close to potential prey before sucking it back in.

The researchers suggested any drop in temperature would have a negative impact on these systems.

"We think there's been a trade-off here for the little guys," said Dr Campbell.

"They're going for shorter dives but they're much more efficient."

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