How animals keep fit

No one would dream of running a marathon without first making a serious effort to train for it. But no matter how well they have stuck to their training regime, contestants will find that running non-stop for 42 kilometres is going to hurt.

Now consider the barnacle goose. Every year this bird carries out a 3000-kilometre migration. So how do the birds prepare for this? Do they spend months gradually building up fitness? That’s not really the barnacle goose’s style. Instead, says environmental physiologist Lewis Halsey, ‘They just basically sit on the water and eat a lot.’.

Until recently, nobody had really asked whether exercise is as tightly connected to fitness in the rest of the animal kingdom as it is for us. The question is tied up in a broader assumption: that animals maintain fitness because of the exercise they get finding food and escaping predators.

Halsey points out that this may not necessarily be the case. Take the house cat. Most domestic cats spend much of the day lounging around, apparently doing nothing, rather than hunting for food. But over short distances, even the laziest can move incredibly fast when they want to. Similarly, black and brown bears manage to come out of several months’ hibernation with their muscle mass intact – without having to lift so much as a paw during this time.

Barnacle geese go one better. In the process of sitting around, they don’t just maintain their fitness. They also develop stronger hearts and bigger flight muscles, enabling them to fly for thousands of kilometres in a migration that may last as little as two days.

So, if exercise isn’t necessarily the key to physical strength, then what is? One clue comes from a broader view of the meaning of physical fitness. Biologically speaking, all it means is that the body has undergone changes that make it stronger and more efficient. In animals such as bears these changes appear to be triggered by cues such as falling temperatures or insufficient food. In the months of hibernation, these factors seem to prompt the release of muscle-protecting compounds which are then carried to the bears’ muscles in their blood and prevent muscle loss.

Barnacle geese, Halsey suggests, may be responding to an environmental change such as temperature, which helps their bodies somehow ‘know’ that a big physical challenge is looming.

In other bird species, that cue may be something different. Chris Guglielmo, a physiological ecologist, has studied the effect of subjecting migratory songbirds known as yellow-rumped warblers to changing hours of daylight. ‘We don’t need to take little songbirds and train them up to do a 6- or 10-hour flight,’ he says. If they are subjected to the right daylight cycle, ‘we can take them out of the cage and put them in the wind tunnel, and they fly for 10 hours.’

Unlike migratory birds, however, humans have no biological shortcut to getting fit. Instead, pressures in our evolutionary history made our bodies tie fitness to exercise.

Our ancestors’ lives were unpredictable. They had to do a lot of running to catch food and escape danger, but they also needed to keep muscle mass to a minimum because muscle is biologically expensive. Each kilogram contributes about 10 to 15 kilocalories a day to our metabolism when resting – which doesn’t sound like much until you realise that muscles account for about 40 percent of the average person’s body mass. ‘Most of us are spending 20 percent of our basic energy budget taking care of muscle mass,’ says Daniel Lieberman, an evolutionary biologist and marathon runner.

So our physiology evolved to let our weight and fitness fluctuate depending on how much food was available. ‘This makes us evolutionarily different from most other animals,’ says Lieberman. In general, animals merely need to be capable of short bouts of intense activity, whether it’s the cheetah chasing prey or the gazelle escaping. Cats are fast, but they don’t need to run very far. Perhaps a few mad dashes around the house are all it takes to keep a domestic one fit enough for feline purposes, says Lieberman.

He argues that long ago on the African savannah, natural selection made us into ‘supremely adapted’ endurance athletes, capable of running prey into the ground and ranging over long distances with unusual efficiency. But only, it appears, if we train. Otherwise we quickly degenerate into couch potatoes.

As for speed, even those animals that do cover impressive distances don’t have to be the fastest they can possibly be. Barnacle geese needn’t set world records when crossing the North Atlantic; they just need to be able to get to their destination. ‘And’, says exercise physiologist Ross Tucker, ‘humans may be the only animal that actually cares about reaching peak performance.’. Other than racehorses and greyhounds, both of which we have bred to race, animals aren’t directly competing against one another. ‘I don’t know that all animals are the same, performance-wise … and we don’t know whether training would enhance their ability’, he says.