A recent study of American water shrews has surprised researchers by showing that the animals rapidly elevate body temperature immediately before diving into cold water.
This behaviour is unexpected because lower body temperatures enable diving mammals to stay underwater for longer, offering greater opportunity to find food. Heating up doesn’t make sense because a warm body uses oxygen faster than a cooler one.
According to Professor Kevin Campbell of the University of Manitoba, who led the study, “This finding goes against prevailing dogma regarding the physiology of divers. Divers, especially small ones, have always been expected to try to maximize their underwater endurance.”
Compounding this shrew’s curious behaviour is the fact that it is already the world’s briefest diver, so why is it putting further limitations on its hunting opportunity?
Yet the shrews are highly proficient aquatic predators. Indeed, this shrew, Campbell said, is “without a doubt the most impressive (and ruthless) predator I have ever seen.” So an elevated body temperature presumably heightens foraging efficiency somehow.
Cambell is presenting his findings today at the Annual Main Meeting of the Society for Experimental Biology, which began on June 29 at the Salzburg Congress Centre in Salzburg, Austria, and finishes today, July 2, 2012.
Large animal divers, like seals and penguins, have been studied extensively, but these findings show that small diving animals deserve attention as well.
Compared to other diving mammals, the shrews carry the least amount of oxygen under water and use it up the most quickly (one gram of shrew tissue burns oxygen 100 times faster than one gram of tissue from a larger animal like an elephant). Typical dives thus last only 5-7 seconds. Being so small also makes them lose heat the fastest.
Dr. Roman Gusztak, who participated in the study, which was as supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada, said: “The shrews are likely surviving at the limits of what is possible for a diving mammal. They must continually feed to provide for their voracious appetites but have to contend with very short dive durations and the constant threat of hypothermia.”
Exactly how the shrews warm themselves is unknown. Often, the shrews elevated body temperature while they were simply sitting still at the water’s edge before a dive. The researchers believe the shrews are shivering or using their brown fat to generate heat.
As part of this study, the researchers observed the shrews’ behaviour when diving into water of different temperatures. They compared the length of the shrews’ dives in warm and cold water and also monitored the shrews’ body temperatures before, during, and after dives.
About this shrew:
The American water shrew, which is about the size of a human thumb and weighs 10-15g, lives along riverbanks in North America. It feeds on small fish, aquatic invertebrates, beetles, and other animals. Its habitat extends as far north as Canada and Alaska, which means that in winter they must dive many times a day into dark, icy water in search of prey.
The American water shrew is one of just two mammals known to be able to smell underwater (the other being a mole). While submerged, the shrew can also hunt as well in darkness as it does in light and it has a reaction time 10 times faster than a human’s.
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