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Animals changing shape – A warming climate?

When you hear the term “shape-shifting,” your mind immediately goes to a science fiction or horror film, not to the climate. However, scientists believe that is what is happening to some animals as a result of climate change. According to a new study, climate change may be causing animals worldwide to grow more prominent ears, beaks, and tails as our warming planet forces them to rapidly “shape-shift” to survive.

The researchers caution that physiological changes do not necessarily indicate that animals are adapting to climate change. Animals that are unable to regulate their body temperature can become overheated and die. Certain animals that live in warmer climates have historically evolved larger beaks or ears to dissipate heat better. A larger wing, ear, or beak in proportion to body size provides a larger surface area for smaller animals to lose excess heat.

Strong shapeshifting has been observed in birds in particular. Since 1871, several species of Australian parrot have shown an average increase of 4%–10% in bill size, which is positively correlated with the summer temperature each year. Increased bill size was associated with short-term temperature extremes in cold environments in North American dark-eyed juncos, a type of small songbird. Additionally, changes in mammalian species have been reported. Researchers have observed increases in the tail length of wood mice and in the tail and leg lengths of masked shrews.

“Since the increases in appendage size we have observed thus far are quite small—less than 10%—the changes are unlikely to be immediately noticeable,” scientist Sara Ryding explains. Ryding’s next step is to conduct an in-depth investigation of shape-shifting in Australian birds by 3D scanning museum bird specimens from the last century. It will aid her team in determining which birds are experiencing appendage size changes due to climate change and why.


Shapeshifting does not imply that animals are adapting to climate change or that everything is fine,” Ryding explains. “It simply means they are evolving to survive it—but we are unsure of the additional ecological consequences of these changes, or even whether all species are capable of changing and surviving.”

  • Appendages play a critical, but often overlooked, thermoregulatory role in animals as sites of heat exchange
  • This thermoregulatory role results in geographic clines in animal morphology, with animals at lower latitudes and in warmer climates having more prominent appendages (a pattern known as ‘Allen’s rule’).
  • In response to climate change and the associated temperature increases, animals are altering their morphologies to have proportionately larger appendages.
  • This proportional increase in relative appendage size, and thus the subsequent change in body proportions, is referred to as ‘shape-shifting’ and is a frequently overlooked response to climatic warming.


Numerous animal appendages, such as avian beaks and mammalian ears, can conduct heat away from the body. Allen’s rule, which states that animals living in hotter climates have larger appendages to allow for more efficient heat exchange, reflects this. We discovered widespread evidence of endotherms undergoing ‘shape-shifting (changes in appendage size) in response to climate change and the associated climatic warming. We re-examine studies of morphological change over time in a thermoregulatory context and discover evidence that temperature can be a strong predictor of morphological change in the absence of or in combination with other environmental changes. Finally, we discuss how Allen’s rule, the degree of temperature change, and other ecological factors all contribute to morphological change and make predictions about which animals will exhibit shape-shifting behavior.


Franklin, Jonathan. “Climate Change Is Making Some Species of Animals Shape-Shift.” NPR, NPR, 9 Sept. 2021, 

Ravisetti, Monisha. “Scientists Concerned Climate Change Is Causing Animals to ‘Shape-Shift’.” CNET, CNET, 12 Sept. 2021, 

Ryding, Sara, et al. “Shape-Shifting: Changing Animal Morphologies as a Response to Climatic Warming.” Trends in Ecology & Evolution, 2021, doi:10.1016/j.tree.2021.07.006. 

“The Warming Climate Is Causing Animals to ‘Shapeshift’.” ScienceDaily, ScienceDaily, 7 Sept. 2021,