Ecological drivers of ultraviolet colour evolution in snakes

Ultraviolet (UV) colour patterns invisible to humans are widespread in nature. However, research bias favouring species with conspicuous colours under sexual selection can limit our assessment of other ecological drivers of UV colour, like interactions between predators and prey. Here we demonstrate widespread UV colouration across Western Hemisphere snakes and find stronger support for a predator defence function than for reproduction. We find that UV colouration has evolved repeatedly in species with ecologies most sensitive to bird predation, with no sexual dichromatism at any life stage. By modelling visual systems of potential predators, we find that snake conspicuousness correlates with UV colouration and predator cone number, providing a plausible mechanism for selection. Our results suggest that UV reflectance should not be assumed absent in “cryptically coloured” animals, as signalling beyond human visual capacities may be a key outcome of species interactions in many taxa for which UV colour is likely underreported.

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Ecological, evolutionary & environmental sciences study design
All studies must disclose on these points even when the disclosure is negative.We investigated the evolution of ultraviolet (UV) colouration across a broad range of snake species in the Western Hemisphere.First we assesed peer-reviewed literature on UV colouration in nature to quantify biases in reported function of UV colouration.To test the role of ecology on the evolution of UV colouration, we used both phylogenetic linear models and phylogenetic ANOVAs to analyze the effect of primary habitat usage (arboreal, terrestrial, fossorial, and aquatic) and activity patterns (nocturnal and diurnal) on amount of UV reflectance across snake body regions (N=104 species for one tree and 95 species for a second tree, which we used to assess the impact of phylogenetic uncertainty).To test for sex-and age-specific effects, we used mixed effect models with taxonomic Glade as a random effect (N=438 individuals from 5 clades).We ran each analysis as linear and multiple regression models to account for distribution non-normality.To assess interactions between body regions (e.g., dorsal vs. ventral, heads vs. bodies), we used Chi-square analyses for both individual data and species means.To test the relative effects of UV sensitivity and chromacy, we fit a multiple regression model predicting mean colour contrast.
We captured 438 individual snakes (from 110 species) across the Western Hemisphere (Belize, Nicaragua, Peru, USA).We designed our sampling to capture the greatest species diversity and abundance possible, as snakes are highly secretive and difficult to sample.For most species, both male and female snakes were captured, as well as juvenile and adult snakes.
No sample size was predetermined; in order to maximize species diversity and sufficiently sample variation within species, we collected nearly all snakes that were encountered during each field excursion.
We collected snakes through a combination of opportunistic foot surveys, pitfall traps, funnel traps, and driving surveys following the protocol of Davis Rabosky et al. 2021 (Biol.J. Linn.Soc.).We photographed each snake and took spectrometer measurements for a subset of individuals at each site at the time of capture before specimen preservation.All field work was conducted by H.L.C, J.D.C, and A.D.R. with associated field teams (see Acknowledgments).
We collected snakes throughout multiple, multi-week field expeditions between the years 2016-2021.Field sampling locations were Estaciones Biologicos de Los Amigos (Madre de Dios), Villa Carmen (Cusco), Madre Selva (Loreto), and Santa Cruz (Loreto) in Peru We excluded snakes from all colour analyses if their photographs were too poor-quality to accurately score (i.e., under/over exposed, blurry), and we excluded species from phylogenetic analysis if they were not represented by molecular data in a phylogeny.
Our study did not involve an experimental manipulation of organisms.Human observations of snake colors and categorization of UVfunction in the literature review were conducted in a double-blind set up to ensure scoring was consistent and reproducible.All code for analyses was run by at least two authors to ensure repeatability.
Randomization is not relevant to our study because we did not experimentally manipulate organisms.
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