Climate Change Is Forcing Animals to Quickly 'Shape-Shift,' Study Suggests

Scientists are noticing widespread body-part growth in animals and believe human-caused climate change is the likely reason.
Scientists are noticing widespread body part growth in animals and believe human-causes climate change is the likely cause.
Mulga parrots near Waikerie, Riverland, South Australia. Image: Auscape/Universal Images Group via Getty Images
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Climate change is causing animals around the world to “shape-shift” in order to better survive rising global temperatures caused by human consumption of fossil fuels, reports a new study.

Scientists have been monitoring the impact of climate change on wildlife for many decades and have established that hotter temperatures will cause some species to shrink, lose fertility, and in some cases, die out completely. Past research has also established that animals in warmer climates tend to evolve larger appendages associated with dissipating body heat, a phenomenon known as Allen’s rule. 


Now, researchers led by Sara Ryding, a Ph.D. student in ornithology at Deakin University in Australia, have demonstrated that certain appendages in birds and mammals, such as beaks and ears, are becoming bigger as a result of human-caused climate change. 

Ryding and her colleagues present “widespread evidence of ‘shape-shifting’” in these warm-blooded animals as a “response to climate change and its associated climatic warming,” according to a study published on Tuesday in Trends in Ecology and Evolution.

“Allen’s rule explains a pattern whereby animals in warmer climates have larger appendages,” Ryding said in an email. “The idea then, that Allen’s rule, could also be applied to the effects of climate change is a logical extrapolation and has been proposed by previous authors,” she added, citing a 2020 study in Current Biology and a 2015 study in the Journal of Biogeography, among others.


“Knowing that Allen’s rule exists for a lot of animals on a spatial scale, this then led to the idea that it could exist on a temporal scale, too: increased appendage size as the climate warms because of climate change,” Ryding noted.

To figure out if Allen’s rule might be amplified by anthropogenic warming, Ryding and her colleagues examined records of animal body proportions in museum collections and reviewed long-term field studies of animals in the wild. The results revealed several instances of rapid growth of appendages in certain species over a short period of time, suggesting that these temperature-related adaptations are occurring faster than would be expected without climate change. 

For instance, the surface area of Australian parrot bills have increased by 4 to 10 percent since 1871, while some shrews and bats display larger relative ear, tail, leg, and wing sizes compared to past counterparts that lived during the 1950s and onward. 

“These types of studies rely on good museum collections, which may not always date back to the 1800s but usually have sufficient coverage to show changes before and after the second half of the 20th century, when climate change started to really be noticeable,” Ryding said.

“I think the most surprising aspect was that the evidence was so widespread, happening in a broad range of animals and across broad geographic scales (the examples we found range from shrews in Alaska to parrots in Australia),” she noted. “Even though this is what we expected, it was surprising to see it happening in such different ways.”


The team emphasized that isolating the role of temperature in these morphological changes can be tricky, because other pressures such as habitat loss or food availability also influence appendage size over time. The exact mechanisms of natural selection driving this shape-shifting are also unclear. For instance, selection for larger beaks or ears may be occurring on a gradual basis, or as the result of sudden die-offs in a population due to extreme weather events such as heatwaves.

“Although evolutionary changes can be a slow process taking thousands (or more) years, we also know that strong selection can drive faster evolutionary change,” Ryding said. “The question is whether the change is fast enough to keep track with the changes in climate that are occurring.” 

“Many have attributed these morphological changes to changes in habitat, diet, and other ecological factors, but, because the effects seems to be widespread across very different animals with different diets and habitats, climate change is the one unifying variable that can explain all of these changes (and can, from what we’ve seen, do so with the most statistical power),” she added. 

While Ryding said that “current research seems to heavily imply that shape-shifting is caused by climate change,” she and her colleagues also emphasize, in the new study, that these correlations need to be bolstered by more evidence collected from the field, museum collections, and other sources. To that end, the team plans to build on their findings by examining birds, a family that is especially prone to shape-shifting.

“Our next research is going to focus largely on birds and the shape-shifting that they show,” Ryding said. “This will include looking at historical records of sea birds, and 3D-scanning museum specimens to collect a large dataset, to see how widespread shape-shifting is across different groups and to answer some of the questions about other ecological factors that I outlined above.” 

“We also hope to investigate more closely how using the beak as a heat exchange organ can predict whether that species will shape-shift in beak size, either now or in the future as the climate crisis continues to worsen,” she concluded.

These efforts, among others, could help to predict which animals will be most likely to rapidly shape-shift in the future, and what the consequences of these sudden adaptations will be for their broader ecosystems.