The history of life has been shaped by evolutionary responses to the pressures of natural selection. Temperature in particular has been a pervasive stressor for animals across Earth’s history. It thus comes as no surprise that the current human-caused climate crisis is reshaping animal physiology. What is climate change if not an evolutionary pressure, albeit with a non-natural origin?

Generally, warmer climates tend to favour smaller body sizes for endothermic (warm-blooded) animals and so, many birds have responded to rising temperatures by getting smaller. Even in otherwise-untouched regions of the Amazon, many birds have shrunk in size – with only the heat to blame, as a study of 79 avian species has shown. This phenomenon, known as Bergmann’s rule, arises from the fact that a smaller body size allows for more heat loss.

“What is climate change if not an evolutionary pressure, albeit with a non-natural origin?”

While this isn’t a ubiquitous trend, it is a definite warning sign for ecosystem changes. When temperature isn’t limiting, larger animals tend to have a fitness advantage – a larger body confers advantages in predator defence and resource competition. Smaller birds may be better able to cope with heat stress, but be unable to hunt or forage effectively, especially with the simultaneous loss of habitat and decreasing resource availability that many Amazon species are facing. So, while size adaptations are beneficial for initial climatic change, continuously increasing temperatures will force animals to sacrifice advantageous adaptations to cope with added heat stress through miniaturisation.

The changing climate has also provoked behavioural changes, such as increased cannibalism in Arizona tiger salamanders. Cannibalism is common across salamander species, with the Arizona tiger salamanders able to metamorphose between a normal body plan and a cannibalistic one that has a wider mouth and more sets of teeth. When population density increases, so does cannibalism, in an effort to maintain population balance. However, as the ponds that these salamanders call home dry up, population density increases while numbers stay the same. This promotes cannibalistic behaviours, leading to subsequent population loss caused by an adaptation to initial environmental changes.

“Examples such as the cannibalistic behaviour of Arizona tiger salamanders are an alarm bell”

Many species, including large mammals, are simply not genetically or physiologically plastic enough to evolve fast enough for the changing environment, which means they just won’t be able to function as well at higher temperatures. This has been observed in livestock, with decreases in growth rate and milk production in response to increased heat.

Over long periods of time, cows, goats, and sheep may evolve lighter coat colours, or larger sweat glands, in hotter climates. They may also decrease the production of thyroid hormones, to reduce their metabolic rate and subsequent heat production, and increase production of hormones involved in coping with dehydration. However, the rapid rate of climate change makes it unlikely that many farm species will be able to maintain production rates while responding adequately to higher temperatures. The consequences of animal agriculture are now posing a threat to animal agriculture – but that’s a story for another time.

The selection pressures imposed by climate change allow us to observe evolution at work at a faster rate than often seen in the fossil record. Studying how animals adapt to climate change also sheds light on its pervasive impacts across all facets of ecology, beyond the more obvious examples of extreme weather patterns and altered habitats.

It is important to remember that while some species have made some adaptations, this does not erase the fact that wildlife populations have decreased by an alarming 73% in the past 50 years, and we are in the midst of a major extinction event. Along with being a fascinating case study in evolution, examples such as the cannibalistic behaviour of Arizona tiger salamanders are an alarm bell for the devastating effects of climate change and a reminder of the growing need for climate action.