In the study of why and how animals look the way they do, colour is king—at least, the range of colour humans can see.

A new study by researchers has examined a colour range that humans can't see and often ignore: colour 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 colour is found widely across the snake tree of life, and that it is frequently used for predator avoidance. 

The study, published in Nature Communications, also highlights how researchers might be ignoring the way a whole group of organisms might be using colour.

Two highly-patterned snake species display very different amounts of UV color. The harmless Catesby’s snail eater on the left was much more reflective in the UV spectrum than the venomous South American coral snake on the right. The white circle in each photo is a color standard used to measure UV and visible color reflectance in each photo; in the UV photos, the darker the snake appears, the less UV color it has. Credit: Hayley Crowell, John David Curlis, Hannah Weller, Alison Davis Rabowsky / University of Michigan

A lot of UV colour work is done in systems that we consider traditionally bright and colorful, like birds, flowers and butterflies, but a lot of this colour research is really biased by the human perception of colour.

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 colour in ways humans can't. The researchers  took photos of the snakes using a camera with a specialized lens and light filters to see what kind of ultraviolet colour they were reflecting. They did not investigate visible UV fluorescence with black light—rather, they investigated the true UV colour 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 colour 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 colour. Why? Scientists surmises it has to do with camouflage.

Birds, which can also see UV colour, 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 colour differences between the sexes in snakes, underscoring the idea that UV colour doesn't relate to reproductive traits like mate choice in snakes.

The finding that there was no difference in UV colour between the sexes was particularly surprising given snakes' close relationship to lizards. 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. The fact that snake colours did not differ between the sexes may suggest that sexual selection may play less of a role in the evolution of colour for snakes than it does for lizards.

The findings aren't black and white. Crowell says another set of snakes in the study that looked nearly identical in the "visible" colour spectrum were from the same species, are the same sex, and were collected in the same place. One snake reflects UV colour 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 colour—in fact, some of the biggest variations of colour 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 colour than adult snakes.

However, their study helps flesh out what it means for animals to use colour—not just the colour humans can see, but that which other organisms can see as well.

The researchers hope their study will inspire more scientists to study UV colouration across organisms.