It’s hard for elephants to stay cool when traveling long distances
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For animals that weigh more than a ton, difficulty staying cool while traveling long distances is the biggest factor limiting their speed, whether they’re running, swimming or flying, the researchers found.
The results 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 adopt more nocturnal behavior,” says Alexander Dyer of the German Center for Integrative Biodiversity Research in Leipzig.
Dyer and his colleagues began by compiling a database of the maximum sustained speeds of more than 500 species of animals, from insects and fish to whales and elephants. They only included studies of animals moving freely in the wild based on, for example, video recordings, radar or tracking devices, and excluded studies of animals in captivity.
They found that movement speeds increased as the animals grew larger until their mass reached 1000 kilograms, after which speeds stabilized 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 problem is that muscles are basically very inefficient, says Dyer. “For every 100 joules of chemical energy pumped into your muscles, 70 of those joules are simply turned into heat.”
Small animals can quickly lose this excess heat due to their higher surface area to volume ratio, but for larger animals this heat becomes a major problem.
This seems equally problematic for swimming animals, even though bodies can dissipate heat faster in water than in air. Dyer thinks it’s because large sea animals such as whales have a lot of insulation to keep them warm in a resting state. “Heat dissipation is a property of the animal rather than of the medium in which it moves,” he says.
“I think [the team] make a compelling case that large animals face an additional metabolic stress on their maximum speed from overheating,” says Walter Jetz of Yale University.
“This is an important baseline overview, but also has conservation relevance. As human activities increase the distance animals have to travel to get from one foraging area to another, large species, which are often already highly endangered, could be particularly at risk.
No living flying animal weighs much more than 15 kilograms. But many flying pterosaurs were larger than that, so dissipating excess heat would have been more of a problem for them. That might explain why many had huge head crests, Dyer says. “I guess they played a role in thermoregulation.”
While rising temperatures are particularly difficult for large animals, staying cool is becoming an issue even for many small animals as the planet warms. Some animals develop smaller bodies to help them lose heat faster in response.
Some team members have already looked at the relationship between an animal’s size and top speed over short distances. They concluded that the maximum speeds of large animals are not 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 over long distances, and large animals run out of energy before reaching their theoretical maximum speed.
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