Climate crisis: Burrowing animals surviving better than birds in overheating deserts, study reveals

As if living in a desert wasn't difficult enough, global warming is turning the heat up to unbearable levels for some animals, writes Harry Cockburn

A raven in Death Valley, part of the Mojave Desert in California (Getty)

The Mojave Desert is a hot, arid and unforgiving environment, but the 50,000 square mile landscape which stretches into four US states is getting hotter quickly due to the climate crisis, presenting new challenges for the animals which call it home.

A new study has found that small burrowing mammals such as the cactus mouse, the kangaroo rat and the white-tailed antelope squirrel are weathering the hotter, drier conditions triggered by global heating much better than birds, researchers at the University of California report.

The desert has been rapidly warming over the last century, with average temperatures up by 2C for the desert as a whole, while there has also been a 10 - 20 per cent decrease in precipitation over the same time period.

This makes a big difference when summer temperatures on valley floors can soar above 49C and have been known to hit 54C at the lowest elevations in places such as Death Valley.

This blazing heat has taken its toll on the desert’s birds, with researchers documenting a collapse in bird populations, “likely resulting from many bird species’ inability to withstand these new hotter temperatures”.

But the same team which documented the birds’ decline has now reported that small mammal populations in the desert have remained relatively stable since the beginning of the 20th century.

Using computer models to simulate how birds and mammals respond to heat, the study team suggest small mammals’ resilience is likely due to their ability to escape the sun in underground burrows and their tendency to be more active at night. 

As a result of these behaviours, these mammals have much lower “cooling costs” than birds, which are less able to escape the desert heat and need extra water to maintain a healthy body temperature.

“Scientists tend to assume that most species in a region experience the same exposure to temperature or precipitation changes, and that they all respond in the same way,” said the study’s senior author Steven Beissinger, a University of California, Berkeley, professor of environmental science.

“But we're finding now that animals have diverse strategies for reducing their exposure to hot and dry conditions that could kill them. 

“You should see these differences most strongly in a harsh environment like the desert, where life is really on the edge.”

The research is part of an effort to revisit and document wildlife at sites in California first studied by UC Berkeley biologist Joseph Grinnell and his colleagues at the Museum of Vertebrate Zoology between 1904 and 1940. 

The detailed field notes of the animal and plant life from the early 20th century have given modern-day biologists a glimpse into the ecology of the past, allowing them to document how climate change has impacted wildlife throughout the state.

“It's becoming clear that animals across the planet are responding to climate change by shifting where they live and shifting when they breed, and we're starting to get really strong evidence of population declines in certain areas that may be associated with warming,” said lead author Eric Riddell, an assistant professor of ecology, evolution and organismal biology at Iowa State University.

“Some estimates now suggest that one in six species will be threatened by climate change over the next century. Figuring out which species those are, what kind of traits they have, will be critical.”

The researchers also warn the disproportionate impact the heating is having could lead to breakdowns in complex food webs which support ecosystems, and the loss of birds was likely to have considerable impacts on other species.

“Desert species have been thought to be relatively invulnerable to climate warming, but many desert species are already at or near their temperature and aridity tolerance limits,” said study co-author Lori Hargrove, an ecologist at the San Diego Natural History Museum.

“Each species also has different degrees of resilience. Climate change may seem minor, only a few degrees, but it has already had, and is having, direct and significant impacts on many species, each of which, in turn, affects other species, with cascading effects yet to be realised.”

The researchers explained how they were able to use the detailed historic work carried out by Grinnell and his colleagues to measure numbers of birds and mammals in the same locations, as the old notes described the landscape features such as hills, mountains and streams.

This allowed the modern scientists to pinpoint the old locations to within less than a kilometre.

"Sometimes they were very nice and left topo maps, but sometimes you have to do a little detective work," said Professor Beissinger, who leads the Grinnell Resurvey Project. 

"For instance, they will say, “we sampled at Horse Mountain’. Well, there are three Horse Mountains in California. So, we track down the Horse Mountain in the study region. And then you find out that they camped at the mouth of a creek. So, then you look at the old topo maps, and you see the creek. Using these details, we're able to recreate more or less where they were.”

Once the researchers identified the location, they then had to take stock of what animals live there. 

To survey bird populations, researchers will pick a specific trail or route to walk for 1.5 miles and stop at defined intervals to record all the birds they see -- or, more likely, hear.

“You have to know your bird songs,” Professor Beissinger said.

But they said surveying the small mammals was “a little trickier”. 

This is because they are often more active at night and usually spend daylight hours in their burrows. For this reason the research team relied on live traps to get an idea of which animals are around.

Co-author James Patton, a professor emeritus of integrative biology and curator in the Museum of Vertebrate Zoology at UC Berkeley, led the trapping efforts with his wife, Carol, in Mojave's Death Valley National Park. 

To sample a given location, Professor Patton said they would set a series of approximately 200 traps each night for four or five nights and record what they caught.

In 2018, the team reported that bird populations have declined precipitously, with the 61 sites losing, on average, 43 per cent of the species that were there a century ago. 

But according to the new trapping data, small mammal populations have not significantly changed since then.

“Mammals have shown this remarkable stability,” Professor Beissinger said. 

“It's really quite interesting that, in the same region, with the same level of climate change, these two very similar taxa have responded very differently to the changes taking place.”

In order to understand the impact the hotter climate was having on both birds and mammals, the research team built physiological models which simulate birds’ body temperature and cooling needs under different desert conditions

Desert animals have a variety of different methods for staying cool in the dry heat. Unlike humans, birds and small mammals do not sweat. Instead some rely on methods of evaporative cooling, such as panting or “gular flutter”, a vibration of a bird’s throat muscles which increases air flow. 

Others will avoid the heat by staying in the shade, or by building dens or burrows to hide from the sun, and some will avoid the heat of the day altogether by being nocturnal.

Dr Riddell built computer models of 49 desert birds. The birds the model predicted would have the greatest increase in cooling costs were also the ones that showed the greatest losses in field surveys - particularly larger birds and those that have an insect or animal diet.

For the new study, Dr Riddell built additional physiological models, this time for small mammals.

“The simulations estimate the head loads that animals experience and then calculates how much heat they need to gain or lose to maintain a stable body temperature,” he said.

Using fur samples warmed on a heated device, and placing thermocouples at the tips of the hairs, he was able to estimate each animal’s thermal conductance.

“Thermal conductance is basically how quickly something transfers heat with its environment,” Dr Riddell said. 

“A big, fluffy mammal might transfer heat very slowly, whereas an animal with very sparse or short fur, like a ground squirrel, might transfer heat quickly.”

According to the models, cooling costs were, on average, about 3.3-fold higher for birds than they were for small mammals. 

Higher temperatures associated with the climate crisis increased cooling costs by 58 per cent for birds, but only 17 per cent for mammals. 

The scientists said this was largely due to the fact that the small mammals can retreat to underground burrows during the hottest parts of the day.

“A lot of the difference in cooling costs is related to what we call microhabitat differences,” Professor Beissinger said. 

“Small mammals are able to go underground where it is much cooler, and they're not being exposed to the direct sunlight, which heats up the bodies of the birds.

“These microgeographic variations in their exposure make a big difference, and these variations need to be considered when we think about how climate change will impact individual species.”

To prevent further losses of wildlife Professor Beissinger said it will be important to identify local refuges in the desert where species will be able to maintain themselves when temperatures soar. 

Reducing the demand on aquifers in the region may also prevent more desert springs from drying up, returning more water to the desert landscape.

“This study has made me realise just how complicated predicting the effects of climate change truly are,” Professor Riddell said. 

“It isn't just about where the landscape is warming and where it isn't warming. It's a really complex process that involves many aspects of an organism’s biology, including their physiology, their behaviour, their evolution - it’s all coupled. You need to take a really integrative approach to understand it.”

The research is published in the journal Science.