The zebra in the fairy tale got its striped pattern because it “stands half in the shade and half out in the shade.” Author Rudyard Kipling was not a biologist, but there may be some truth to his story. Research shows that as temperatures rise, the animals’ colors become brighter, resembling the sun-exposed areas of zebras in picture books. On the other hand, in humid shade, darker shades prevail.
As the Earth warms and rain patterns change, the plumage and skin color of many species is changing, often becoming lighter in color. Dutch snails are turning from brown to yellow. In one species of tropical bee in Costa Rica, the proportion of orange and blue individuals is increasing. French lizards are becoming lighter, as are many insects and birds around the world. “We expect that under global warming, dark-colored species and individuals may decline,” says Stefan Pinkert, an ecologist and evolutionary biologist at Yale University.
There are two main ways to color animal skin, fur, and feathers. Some of the hues we perceive result from the interaction of light with the microstructure of feathers and scales. Imagine a hummingbird that changes color depending on the viewing angle. Others are caused by pigments and molecules that absorb light, such as carotenoids, which produce yellow, red, and orange colors, and melanin, which causes black, gray, brown, and rusty hues.
About supporting science journalism
If you enjoyed this article, please consider supporting our award-winning journalism. Currently subscribing. By subscribing, you help ensure future generations of influential stories about the discoveries and ideas that shape the world today.
Melanin, the most common pigment in birds and mammals, can be affected by rising temperatures and changing rain patterns. “The more melanin in the skin, fur, and feathers, the more heat they tend to absorb,” said Matthew Shawkey, an evolutionary biologist at Belgium’s Ghent University. This could be a disadvantage, he says, as rising temperatures could cause animals to overheat. On the other hand, when it rains, pathogens are more likely to breed. In these situations, dark melanin can have a protective effect because it “strengthens the tissue,” Shawkey said.
The rule, proposed by American herpetologist Charles Bogart in a 1949 paper, is that hot climates should have more so-called cold-blooded animals, or cold-blooded animals that are brightly colored and therefore less likely to overheat. I predict that. (Animals such as reptiles and insects cannot regulate their own body temperature and rely on external sources of heat.)
In recent years, science has not only confirmed Bogart’s law, but also extended it to endotherms, or warm-blooded species. Frogs, toads, snakes, and midges aren’t the only species that are lighter in warmer regions. Birds will also be lighter. An analysis of more than 10,000 bird species in 2024 found that white and yellow feathers outperform blue and black feathers in hot climates.
Due to global warming, some animal populations are becoming even lighter. From 1967 to 2010, when temperatures in the Netherlands rose by 1.5 to 2 degrees Celsius, brown land snails were replaced by yellow ones. Dragonflies and damselflies also became progressively lighter in the UK between 1990 and 2020, as Pinkert and his colleagues found in a 2023 paper. And if you have looked closely at some dragonflies, you may have noticed that the black decorations on their wings have diminished.
A recent study conducted in North America found that during the warmest years from 2005 to 2019, males of 10 species of dragonflies had the least melanin-based color spots on their wings. At the same time, the beautiful spots also seemed to fade in the Mediterranean blue tit – a small bird with a yellow chest and a dark blue cap-like pattern on the head. From 2015 to 2019, the blue head spots of the great tit population around Montpellier, France brightened by about 23 percent. This is a change associated with rising local temperatures.
Experiments confirmed observational data that animals’ bodies become lighter when exposed to high temperatures. In some cases, individuals may simply produce more or less pigment, depending on temperature. For example, the bright dance light dragonfly can change color from dark to light and back again as the mercury fluctuates throughout the day. Male chameleon grasshoppers go from black at 50 degrees Fahrenheit (10 degrees Celsius) to blue-green above 77 degrees Fahrenheit (25 degrees Celsius). Evolutionary biologist Kaspar Delhey of the Max Planck Institute for Biointelligence in Seewiesen, Germany, says, “When you keep different types of insects at low temperatures, their colors become darker, and when you keep them at warmer temperatures, their colors become lighter.” . .
Such effects are not limited to insects. A field experiment in Spain found that vultures that hatched in nests exposed to more sunlight had lighter colored plumage than those raised in more sheltered areas. It’s not just that the bird has been bleached by sunlight. The melanin in the feathers was not broken down as it is when it is destroyed by sunlight. Originally, the amount was small.
In addition to an individual’s ability to adjust their color based on temperature, populations of animals living in warming regions may become brighter in color simply as fair-skinned animals migrate to new areas. Pinkert says genetic changes may also be at play, but there are still “significant knowledge gaps” about how such evolution plays out.
Bogart’s law seems simple in areas that remain dry even as temperatures rise, such as the Mediterranean, but if precipitation increases with temperature, seeds can become darker instead of lighter. In 1833, German ornithologist Konstantin Gloger suggested that in humid areas the plumage was more likely to be black than white. One reason may be camouflage. In humid habitats, “there’s more vegetation; the background is darker, so dark animals can be better camouflaged,” Delhaye says. Another explanation for Groeger’s law may be protection against pathogens that are more prevalent in humid climates. A 2020 study of 16 species of birds showed that feathers with more melanin were more resistant to damage from nest bacteria. “The purpose of this molecule is to protect the organism from various sources of stress. For example, black feathers are stronger,” says Alexandre Roulin, an evolutionary biologist at the University of Lausanne in Switzerland who was not involved in the study. . Research suggests that melanin molecules may not only inhibit parasites, but also strengthen cells and form a barrier against pathogens.
DeHay said experiments to see what would happen if both temperature and precipitation increased with climate change found that “the effects of humidity are generally much stronger,” at least for birds. says. Delhey and his colleagues mapped the plumage colors of all of the more than 5,000 species of passerine birds to the climates in which they live. They found that animals were lighter in color in warm, dry locations, and darker in color in warm, humid locations. Roulin and his colleagues found something similar in a 2024 study of thousands of barn owl museum specimens collected around the world between 1901 and 2018. The researchers showed that over time, feather color became lighter where the climate was warmer and drier, and darker where there was both. Temperature and precipitation have increased. “Where the climate change was stronger, the color change was also stronger,” says Roulin.
However, changes in precipitation patterns caused by global warming are not as simple as future increases in temperature. This is why, Delhaye says, if we were to predict a general trend across animals, “based on the effects of temperature, animals should become lighter.” Cold-blooded animals such as insects may also respond more strongly to heat than humidity, but research on this is still lacking, he says.
Overall, color changes in animals are expected to be subtle. “I don’t think there will be any drastic changes that would make the species unrecognizable,” Delhaye said. But from a biological perspective, “that small difference can mean whether a species survives or not,” he says. Meanwhile, animals that change color and adapt can serve as a visual reminder of humanity’s huge environmental footprint, which has unsettled the entire planet. “We can visually track the effects of climate change,” Rulin says.
