In the study of why and how animals look the way they do, color is king—at least, the range of color humans can see. A University of Michigan study has examined a color range that humans can’t see and often ignore: color in the ultraviolet range. Examining snakes, the researchers categorized how the animals used patterns of UV color and tested for factors that promote the evolution of UV color in snakes.
The researchers discovered that UV color is found widely across the snake tree of life, and that it is frequently used for predator avoidance, says study co-author Hayley Crowell, a doctoral student in the U-M Department of Ecology and Evolutionary Biology.
The study, published in Nature Communications, also highlights how researchers might be ignoring the way a whole group of organisms might be using color.
“A lot of UV color work is done in systems that we consider traditionally bright and colorful, like birds, flowers and butterflies, but a lot of this color research is really biased by the human perception of color,” Crowell said.
“This work mainly focuses on either mating or reproductive systems, such as UV ‘nectar guides’ in flowers that help steer insects to the part of the flower necessary for pollination. However, there are a lot of groups, like snakes, that aren’t necessarily on people’s radar as a broadly colorful study system.”
The study examined 110 snake species from regions ranging from Colorado to Peru, many of which have visual systems that can perceive UV color in ways humans can’t. Crowell and colleagues took photos of the snakes using a camera with a specialized lens and light filters to see what kind of ultraviolet color they were reflecting. They did not investigate visible UV fluorescence with black light—rather, they investigated the true UV color invisible to humans.
The researchers then tested many variables to see which correlated to the presence or absence of UV color in different species. These variables included the age and sex of the snakes, what kind of habitat they lived in, the evolutionary history of the species, and how conspicuous a snake’s color makes them to predators like birds, mammals and other snakes.
The biggest tie between UV color and snakes? The snake’s ecology, or the relationship between it and the environment it lives in. For example, arboreal snakes—snakes that live in trees and tend to be nocturnal—had the most UV color. Why? Crowell surmises it has to do with camouflage.
Birds, which can also see UV color, are one of snakes’ biggest predators. Arboreal snakes move around and hunt at night, and sleep during the day. Having a lot of UV color at night isn’t a big deal. But having it during the day is potentially protective: Leaves, lichens and epiphytes—plants and plant-like organisms that grow on other plants, such as ferns and orchids—can also reflect a lot of UV light. Similarly, having UV color would conceal you during the day when birds are looking for something to eat.
Among the study’s unexpected findings was that there were no UV color differences between the sexes in snakes, underscoring the idea that UV color doesn’t relate to reproductive traits like mate choice in snakes, says study co-author Alison Davis Rabosky, U-M associate professor of ecology and evolutionary biology.
“Because reproduction drives UV color evolution in so many other species, the lack of sexual differences in snakes was a surprise,” Davis Rabosky said. “But I don’t think snakes are actually some kind of outlier doing color ‘differently’ than other animals. I think we scientists have simply overlooked a lot of UV coloration in cryptically colored species, especially in insects. They are the next frontier.”
The finding that there was no difference in UV color between the sexes was particularly surprising given snakes’ close relationship to lizards, says study co-author John David Curlis, U-M postdoctoral fellow in ecology and evolutionary biology.
“Sexual dimorphism, where males look different from females, is incredibly common in lizards, with many species characterized by males that display flashy colors and large ornaments and females that are more drab or camouflaged,” he said. “The fact that snake colors did not differ between the sexes may suggest that sexual selection may play less of a role in the evolution of color for snakes than it does for lizards.”
But the findings aren’t black and white. Crowell says another set of snakes in the study that looked nearly identical in the “visible” color spectrum were from the same species, are the same sex, and were collected in the same place. One snake reflects UV color very brightly on its back, and one reflects none at all.
The team found that even though two species of snakes might be closely related, they may not have similar amounts of UV color—in fact, some of the biggest variations of color were within the same genus of snakes. Some of the most and least UV-reflective snakes were vipers, and the researchers found that juvenile snakes often had more UV color than adult snakes.
However, their study helps flesh out what it means for animals to use color—not just the color humans can see, but that which other organisms can see as well.
“I think what’s so exceptional about this study is that we got to look at patterns of UV color across so many species and individuals,” said co-author Hannah Weller, postdoctoral research fellow at the University of Helsinki. “This amazing dataset really helped us start to understand just how variable a trait like this is, even in a group where we wouldn’t expect it.”
The researchers hope their study will inspire more scientists to study UV coloration across organisms.
More information: Hayley L. Crowell et al, Ecological drivers of ultraviolet colour evolution in snakes, Nature Communications (2024). DOI: 10.1038/s41467-024-49506-4
Journal information: Nature Communications
Provided by University of Michigan