Big animals move more slowly than small ones to avoid overheating

For animals that weigh more than a tonne, the difficulty of staying cool while travelling over long distances is the key limiting factor for their speed, regardless of whether they run, swim or fly, researchers have found.

The findings suggest that global warming will be an even bigger problem for large animals than previously thought. “If our model is correct, larger animals will have to reduce their activity in general or they will have to shift towards more nocturnal behaviour,” says Alexander Dyer at the German Centre for Integrative Biodiversity Research in Leipzig.

Dyer and his colleagues started by compiling a database of the maximum sustained speeds of more than 500 species of animal, from insects and fish to whales and elephants. They included only studies of freely moving animals in the wild based on, for instance, video recordings, radar or tracking devices, and excluded studies of captive animals.

They found that travelling speeds increased as animals became larger up until their mass reached 1000 kilograms, after which speeds levelled off and began to decrease. The team looked at several possible explanations and concluded that a simple model estimating how much animals would have to slow down to avoid overheating could explain the shape of the curve.

The issue is that muscles are fundamentally very inefficient, says Dyer. “For every 100 joules of chemical energy that gets pumped into your muscles, 70 of those joules are just turned into heat.”

Small animals can rapidly lose this excess heat because of their higher surface area to volume ratio, but for large animals this heat becomes a major issue.

This seems to be just as much of a problem for swimming animals, even though bodies can shed heat faster in water than in air. Dyer thinks this is because large marine animals such as whales have lots of insulation to keep them warm in a resting state. “Heat dissipation is a property of the animal rather than the medium they move in,” he says.

“I think [the team] make a compelling argument that large-bodied animals face an additional metabolic constraint on their maximum speed related to overheating,” says Walter Jetz at Yale University.

“This is an important basic insight, but also has conservation relevance. As human activities increase the distance animals need to travel to get from one foraging patch to another, large-bodied species, which often are already highly threatened, might be in particular peril.”

No living flying animal weighs much more than 15 kilograms. But many flying pterosaurs were larger than this, so shedding excess heat would have been more of an issue for them. That might explain why many had enormous head crests, says Dyer. “I would speculate that they had a role in thermoregulation.”

While rising temperatures are particularly challenging for large animals, staying cool is becoming a problem even for many smaller animals as the planet warms. Some animals are evolving smaller bodies to help them lose heat faster in response.

Some members of the team have previously looked at how size relates to an animal’s all-out speed over short distances. They concluded that the maximum speeds of large animals aren’t limited by the ability of muscles and bones to survive the forces involved, as previously suggested, but by the time it takes to accelerate.

During sprints, muscles rely on stored energy rather than aerobic respiration as during long distances, and large animals run out of energy before they hit their theoretical maximum speeds.