Do animals exercise to keep fit?

It's a question Dr Lewis Halsey of Roehampton University ponders in a new paper published in the Journal of Animal Ecology. And the surprising answer is that we don't know -- because it is an issue that has gone almost entirely unstudied.

Animals need energy for growth and for locomotion, for attack and defence, and ultimately for reproduction. Yet animals can only obtain energy intermittently by foraging, storing some of it to use later, so the energetic ecology of an animal is fundamental to its success.

As an eco-physiologist, Halsey studies how animals expend energy, and how they adapt their behaviour and physiology to reduce their energy costs. On Sundays he goes running on Wimbledon Common.

"It made me think about my own biology and ecology. If I don't exercise I get less fit, and am less able to do highly active things. So I wondered if some animals need to spend time and energy on voluntary exercise so that they are fit enough to out-run predators, win over mates or hunt down prey," he explains.

But when he searched the literature, he found very few studies on the matter. "Researchers haven't contemplated the idea that some animals may not do enough exercise during their general activities to be suitably fit for infrequent, high-intensity activities such as fleeing from predators. This needs to change," says Halsey.

His new paper, which he hopes will encourage more research, outlines a set of concepts and the experiments that could be used to test them. But despite the lack of direct evidence, he points to some intriguing animal studies -- from polar bears and penguins to giant pandas and barnacle geese -- that suggest the answers might depend on an animal's ecology.

According to Halsey: "We know that animals change their body condition in response to environmental conditions. Songbirds may put on some weight to survive the winter, but not too much if predators are around lest they become slow at escaping. And harbour porpoises, if regularly preyed on by dolphins, become much sleeker and carry less body fat so that they can out-swim their attackers."

There are examples, too, of animals getting fatter when they have no predators to fear. This could explain why laboratory animals pile on the pounds (even though mice and rats will voluntarily run on wheels provided), and why giant pandas are so sedentary. During the day, giant pandas walk on average just 27m in an hour, but their presumed low aerobic fitness may not concern them because they no longer have predators to worry about.

Other species can maintain key aspects of their fitness without doing any voluntary exercise. In the polar regions, polar bears and penguins burn different tissues while fasting. During hibernation polar bears maintain crucial muscles so that they are still physically strong when they wake up. And while king penguins lose lots of muscle during their fasts on land, they seem to be able to get fit very quickly once they return to sea to fish.

Barnacle geese appear to be an extra special case of getting fit quickly. Some populations migrate 2,500km each autumn from Svalbard to Scotland, yet in the run up to migration they fly for only a few minutes each day -- short bursts of flight that perhaps mirror the modern high-intensity training (HIT) regimes human athletes use to boost maximal aerobic capacity.

But according to Halsey: "Barnacle geese appear to get fit for certain predictable, planned events such as migration and yet miraculously seem able to do so with little or no voluntary exercise. So their bodies seem to trigger increased fitness from within -- they get fit automatically when they need to -- enough to make any human with a waning new year's resolution to get fit very jealous."

Finding out more about whether animals exercise to keep fit could have important scientific implications, challenging existing orthodoxy on animal ecology and behaviour, says Halsey.

"If animals are undertaking activities solely or partly to keep fit, this opens up a significant new facet to our understanding and interpretation of animal behaviour. No-one has previously observed animal behaviours and thought 'this behaviour could be associated with keeping fit'," he explains.

"On top of this, if indeed some animals have to 'keep fit' then the activity involved could burn important energy reserves, which feeds into fundamental ideas of optimality, where animals are expected to expend time and energy in ways that maximise their short- or long-term success."