Snakes use invisible colors to outsmart predators

When thinking about why animals look the way they do, color is often front and center. However, humans are mostly limited to seeing a certain slice of the spectrum, ignoring the ultraviolet (UV) range that many animals use in their daily lives. 

A recent study by scientists at the University of Michigan (U-M) highlights the importance of UV coloration in snakes, showing that UV patterns are widespread and may play a significant role in evading predators such as birds.

The findings point to a broader issue in the study of color evolution: researchers tend to focus on colors we humans can detect. 

“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,” said Hayley Crowell, a doctoral student in the U-M Department of Ecology and Evolutionary Biology.

Hidden signals in the UV spectrum

Crowell notes that much of the past work on UV color has concentrated on reproductive behaviors – things like UV ‘nectar guides’ in flowers that help steer insects to the part of the flower necessary for pollination.

In snakes, the team suspected UV coloration might be used for different reasons. Because snakes don’t typically rely on bright colors to find mates, the scientists hypothesized that UV might be associated more with camouflage or environmental signaling.

The researchers analyzed 110 snake species, collected in various regions ranging from Colorado to Peru. Many of the species examined in the study can see UV light in ways that humans cannot. 

Using a specialized camera lens and filters, they captured images of the snakes, searching for ultraviolet reflection patterns. Importantly, they did not rely on black lights to observe fluorescence; instead, they looked at the UV color that the snakes naturally reflect – colors invisible to the human eye.

The hidden colors of snakes

Next, the researchers tested how different factors – such as a snake’s age, sex, habitat, evolutionary background, and how visible the snake was to predators – might correlate with having more or less UV color. 

To their surprise, the biggest driver of UV coloration turned out to be where and how the snakes live. Arboreal snakes that spend most of their time in trees and are active at night had the highest levels of UV reflection.

“Birds, which can also see UV color, are one of snakes’ biggest predators,” Crowell explained. 

For a nocturnal, tree-dwelling snake, sporting a heavy UV reflection might be inconsequential at night – but can provide camouflage during the day, when birds are actively hunting. 

Leaves, lichens, and other vegetation are all known to reflect UV light. Thus, a snake displaying UV patterns could blend seamlessly into its sunlit habitat.

Surprising lack of sex differences

One unexpected finding was that UV patterns did not differ substantially between male and female snakes. 

“Because reproduction drives UV color evolution in so many other species, the lack of sexual differences in snakes was a surprise,” said study co-author Alison Davis Rabosky, an associate professor of ecology and evolutionary biology at U-M. 

“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.”

Snakes are closely related to lizards, and lizard species are known for strong sexual dimorphism – males often have flashier colors or bigger ornaments than females. 

In contrast, the lack of color contrast between male and female snakes suggests that sexual selection may not drive color differences as much in snakes as it does in their lizard relatives.

Intriguing individual variations

The researchers also found that some snakes of the same species, same sex, and from the same location could differ dramatically in UV reflectance levels. 

Two individuals might look nearly identical in the visible color spectrum, but one reflects UV strongly and the other does not. This highlights the complexity of color usage – humans simply cannot see it, so the reasons behind these differences remain obscure.

Similarly, evolutionary relationships among snakes didn’t always predict UV usage: some closely related species varied significantly in the presence or intensity of UV reflectance. 

Even within the same genus, the team found both strongly UV-reflective and barely UV-reflective species or populations.

Reassessing our vision of animal color

The new research underscores the concept that just focusing on colors visible to humans could hide a great deal of evolutionary strategy. 

By revealing that snakes employ UV coloration widely – often for camouflage and not for sex-based selection – this study challenges assumptions about how color is used throughout the animal kingdom. 

“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, a 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.”

Ultimately, the researchers hope to inspire further investigations of UV coloration in other less-studied animal groups, especially among those perceived by humans as drab or colorless.

The study is published in the journal Nature Communications. 

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