Some warm-blooded animals are “shapeshifting” in response to a warming climate, growing larger legs, beaks and ears at extremely fast rates to better regulate their body temperatures.
“There are these bio-geographical rules that describe trends in body shape, and one of these is called Allen’s rule,” says Sara Ryding, a bird researcher at Deakin University and lead author of the paper out today in Trends in Ecology and Evolution. “This states that animals that live in colder environments tend to have smaller appendages than those that live in warmer environments, because animals can regulate their temperature through the size of these appendages.”
Ryding says it’s long been suggested that Allen’s rule may hold true for animals whose environments are warming, and that previous research has shown these trends in individual species, but the new study is the first to synthesise the extent of these changes across species.
By reviewing existing literature over a two-year period, Ryding and her team found that this evolutionary shapeshifting has been strongly reported among bird species. Several species of Australian parrot have shown on average a 4% to 10% increase in bill size since 1871, a growth that positively correlates with summertime temperatures year on year. But the pattern is not unique to Australia, nor unique to birds.
In North America, the dark-eyed junco, a small songbird, has experienced an increase in bill size that coincides with short-term temperature extremes in cold environments. Other reports of rapid changes include tail-length increases among wood mice, leg-size increases among masked shrews and growth in the wing size of bats.
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So how did the team rule out other factors?
“While it’s hard to rule out other factors involved in shapeshifting, the unifying factor that we find for all these examples that we uncover in the literature is climate change,” Ryding says. The widespread, geographically disparate and cross-species nature of these changes suggests a globally occurring stimulus: the most obvious one being climate change.
What’s particularly alarming about the research is the extraordinarily rapid pace of change among these species.
“Evolution can be a very slow-going process,” Ryding says. “Instead, we’re finding that it’s happening so quickly over such a short time frame.”
And while it might be tempting to see this evolutionary shapeshifting as a success story of resilience against climate change, Ryding says it’s more a case of desperate times calling for desperate measures.
“We know that climate change is likely to get worse, and we don’t know if animals are going to be able to keep pace,” she says. “Secondly, we don’t actually know how much these increases in appendage size help in survival.”
Moreover, Ryding says there’s always limiting factors in how much an appendage can adapt its size to respond to these changes.
“Animal appendages such as bird beaks are used for functions other than controlling body temperature, and there’s only so much that the beak can change in size before that starts to interfere with their ability to eat their food, so that’s another huge issue to consider.”
Ryding says that humans tend to over-focus on the impacts of climate change on human society, but that this new research highlights how critically impacted animals will be as the world continues to warm.
“A lot of the time when climate change is discussed in mainstream media, people are asking ‘can humans overcome this?’, or ‘what technology can solve this?’,” she says. “It’s high time we recognised that animals also have to adapt to these changes.”
Amalyah Hart has a BA (Hons) in Archaeology and Anthropology from the University of Oxford and an MA in Journalism from the University of Melbourne.
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