Muskrats live in an extreme environment where temperatures can soar to 30°C during summer and drop to –40°C in winter. But being partial to plant tubers, winter means that muskrats have to keep diving under the ice covering their marsh to find food. `What really hits you is how much their swimming behaviour changes in the winter', explains Robert MacArthur who has studied muskrats for a number of years. But when he tried to address the question of what made a muskrat a good diver, he found more questions than answers.
Originally MacArthur wanted to understand how such small animals maintained their body temperature in such extremes, and this initial question has led to further studies of their diving. Muskrats dive from a very early age –if a mother is disturbed she will dive into the water dragging her young in with her. `I think of them as little prairie seals,' jokes MacArthur. His work then progressed onto the question of what makes a good diver. It is thought that adult animals are more superior divers than juveniles because experiments predict that they would respire more slowly underwater and store more oxygen. While it is often thought that juveniles acquire these abilities in the normal course of diving, few experiments have looked specifically at whether dive experience improves the physiological capability to dive. So MacArthur and his colleagues in Winnipeg, Canada, trained groups of muskrats to either dive or swim at the surface in a maze in search of food(p. 1153).
Previous experiments on seals and tufted ducks have shown that diving experience alters how oxygen is stored in the body Sure enough, when the Winnipeg group checked the muskrat blood they found levels of the oxygen-carrying protein haemoglobin were raised. But this was the only significant change they found.
`One surprise is that we didn't see the expected change in myoglobin levels', explains MacArthur. Myoglobin, which carries oxygen in the muscles,is thought to be important in the development of diving ability in marine birds and mammals. MacArthur already knew that wild muskrats store significantly more oxygen in their blood and muscles during winter when compared to the summer months. But without seeing altered myoglobin levels, or other changes that they can attribute to diving experience, it is hard for MacArthur and his colleagues to know what causes this seasonal change.
So it's back to the lab for MacArthur. There are many other aspects of diving muskrats that he would like to take a closer look at. While this experiment was carefully designed, water temperature was not considered and he wants to study this further. He would also like to see how longer dive times would affect the results of this study. `Their dive times were typical of nature', explains MacArthur, but they can dive for twice as long as the maze allowed for. This also raises another question: `At what point do they become anaerobic?' MacArthur is already looking at this by examining lactic acid kinetics in these mammals. Another option would to be to train `naïve'muskrats – juveniles that have never dived – in his maze. But there is always the possibility that muskrats are just born to dive.