Can wild animals handle the heat?

Dehydrated howler monkeys fall from trees in Mexico. Billions of mussels, oysters and barnacles boil on the shorelines of Canada. Hundreds of Magellanic Penguins drop dead in Argentina in a single day.

These recent cases may have affected different animal species worldwide, but they have one thing in common: they were triggered by extreme heat.

Temperatures across the globe are rising as economies increasingly pump greenhouse gases into the atmosphere 2023 was the hottest year on record. And climate change is only making heat waves more common and more extreme.

The way this affects wildlife depends on several factors: their geography, whether they are experiencing chronic heat or individual heat waves and, of course, what type of animal they are.

Birds and bats falling dead from the sky

In the most extreme cases, the heat can cause mass die-offs.

'This happens especially in already hot and dry areas where animals cannot really take a lot of additional heating,' said Andreas Nord, an ecologist with Lund University in Sweden.

'Australia is a prime example. There, we're talking about the effects of biblical proportions. So birds and flying foxes literally fall dead from the sky.'

Even if the heat doesn't kill the wildlife, it can change their behavior in ways that influence the population size.

There is a 'potentially more insidious way of dying,' says Eric Ridell, an ecologist with the University of North Carolina at Chapell Hill in the US, that leaves no bodies.

'Perhaps because they're more dehydrated or they can't be active enough because it's too hot, they don't breed that year,' he said. 'So the animals still survive, but they're not not creating young.'

A 2023 study found that burying beetles were less likely to reproduce successfully when a heatwave hits during mating.

Adapting to the heat

When animals change the way they act to stay cooler or become warmer, scientists call this thermal regulatory behavior. During heat waves, this can include staying in the shade, entering the water or resting more.

One way koalas in Australia cope with extreme heat is to hug cool tree trunks. Bears in California sometimes take a more unconventional approach, with videos showing them taking dips in people's pools when temperatures soar.

It's not clear, however, to what extent animals can change their behavior to keep up with an ever-warming planet.

'We don't really know that if, say, 100, 1,000 or 10,000 generations from now, animals will be more heat adapted or more tolerable to these extreme temperature fluctuations that we're seeing,' said Nord. 'But it's not looking good. It seems like many animals already live on the maximum of what their physiological systems can tolerate.'

Birds are especially vulnerable

Another open question is which species are the most susceptible to rising temperatures. While many groups suffer, scientists have observed that birds are often especially vulnerable.

'They have, comparatively speaking, relatively poor ways of cooling their bodies. Birds lack sweat glands. That is a very bad thing when it gets very hot,' said Nord.

That is also something Riddell observed in a 2021 study he authored. The research analyzed a 100-year data set of small mammals and birds found in the Mojave Desert in California.

Though the two communities lived in the same ecosystem, eating the same food and drinking the same water, they responded differently to rising temperatures.

The mammal communities stayed stable throughout the century. Meanwhile, the number of bird species in the desert dropped by 43%.

It's not clear if the birds migrated to other environments or died. What is clear is that they are at a disadvantage when temperatures rise.

'Mammals largely live underground. They're mostly nocturnal. And birds are mostly diurnal [and] live above ground. They're experiencing the sun, which really heats them up,' said Riddell. 'These differences played a really big role in responses to climate change over the last 100 years.'

Thinking long-term

There are few emergency interventions that humans can undertake to help animals cope with extreme heat. Some conservationists have tried to help wildlife by spraying them with water or providing shelter, but this is only a band-aid on a much larger problem.

What is even more important looking forward is for governments to conserve natural habitats long-term, say Nord and Riddell. The animals' natural environments will have the resources to buffer the worst effects of scalding temperatures. That could be shady trees, water, nourishment, places to seek refuge.

'This effect that climate change and extreme heat has on the living world really gets worse the less of the living world that we have left,' said Nord.