Animals 'shapeshifting' to adapt to rising temperatures

Throughout folklore, tales of shapeshifting creatures, from werewolves to vampires, have often provoked horror across a variety of cultures. But these physical transformations are not limited to the pages of fiction.

A review article, published in Trends in Ecology & Evolution, suggests that climate change could be physically altering a variety of species across the planet, as individuals change their shape to suit the fluctuating temperatures across the world.

Features such as tails, legs and ears are changing to provide different levels of heat exchange with the environment.

Sara Ryding, the lead author of the article, says, 'People haven't really thought about how climate change influences the thermal aspect of an animal's life, and if they have, it's to do with extreme heatwaves and animals dying off.  

'Some papers from many years ago postulate that as Allen's rule operates on a spatial scale, it should exist on a temporal scale, but it seems not a lot of people looked into that, so hopefully that will change.'

Big beaks and long legs

 At around the same time that Charles Darwin was publishing On the Origin of Species in 1859, other scientists were also investigating how species varied around the world. 

One of these was Carl Bergmann, whose eponymous rule states that within a group of organisms living across a large range, bigger species will be found in cooler environments. This is because by minimising their surface area to volume ratio they reduce heat loss, while the opposite occurs in warmer environments. For instance the largest species of penguin, the emperor penguin, lives in Antarctica, while the smaller Humboldt penguins are found in South America.

Meanwhile, Joel Asaph Allen formulated his own rule, which describes how organisms adapted to warmer environments have larger limbs and other body parts in order to maximise their heat loss by increasing surface area, such as the larger ears on African elephants when compared to Asian elephants.   

With temperatures set to rise over the coming century, the researchers behind this review looked at a range of studies to see whether adaptations induced by climate change would follow Allen's rule. 

They found that birds in particular are likely to see significant changes as a result of a warming planet. One study found that 58% of all bird species follow Allen's rule when it comes to the size of their bill, which is used to regulate their body temperature. 

Australian parrots, for instance, have seen up to a 10% increase in their bill surface area since 1871, with the amount of increase predicted from the average summer temperatures in the years before the specimens were collected. 

Aside from birds, evidence from a variety of studies demonstrates that bats change their wing size in response to climate change, while mice, pigs and shrews have longer tails in response to a changes in temperature.

Why do these changes happen, and can they go back?

While the changes occur across a range of species, they're not universal across every animal considered. The authors of the review suggest that some species are unable to change certain features as part of a trade-off. Sara says the changes may be masked in species which already exhibit large degrees of variation.

These adaptations may be beneficial in the short to medium term, but as the climate continues to change, animals cannot continually adapt. At some point, the trade-off won't be beneficial anymore, which could lead to population declines and even extinction.

So, while a larger beak may be useful for species wanting to lose more heat, if it makes the beak more impractical for feeding then it may end up being more of a disadvantage for the birds.

There are also a variety of other ways of controlling body temperature that don't require changes in body shape, such as behavioural changes like making use of shade and migrating at warmer times of year.

The researchers also note that a variety of other factors, including food availability and the water cycle, may be affected by climate and can also influence the shape of these body parts. 

One of the significant outstanding questions concerning these impacts are whether or not the changes are permanent. For instance, marine iguanas in the Galápagos can shrink and grow their body length by as much as 20% as temperatures fluctuate and change the amount of food available.

Different studies disagree on the reversibility, and heritability, of these changes, and Sara hopes to continue investigating this as part of her future work. This includes comparing the body changes between species in their native habitat, and where they have been introduced, such as European starlings in Australia. This could lead to a greater understanding of how to separate the environmental causes of the changes from others.

The researchers have called for a variety of studies to take place to look more closely at the changes that are taking place in animals.

'By correlating this work with museum collections, information on the survival of different beak sizes, and observing how beaks are used in thermoregulation by different species, all of this information is giving us a strong indication these changes are caused by climate change,' Sara says.