The role of stripes: testing possible drivers of geographical variation in zebra (Equus quagga and Equus zebra) stripe patterns

Abstract

Zebras exhibit one of the most striking patterns in the natural world, yet the adaptive function of their black and white stripes remain largely undetermined. These stripe patterns are unique to each species, with mountain zebras (Equus zebra) displaying thinner and more densely packed stripes and plains zebras (Equus quagga), which display thicker but more spread apart stripes. Furthermore, there is also geographical variation of striping patterns within each species, for example, some individuals of mountain zebras display larger rump stripes and some individuals of plains zebras displaying more contrasted and defined stripes. Multiple hypotheses have been proposed to explain the adaptive functions of zebra stripes, including roles in social interactions, thermoregulation, predator avoidance, and protection against biting flies, but research is yet to reach a consensus on the drivers of these patterns. To test several of these competing hypotheses, I aimed to quantify geographical variation in zebra stripes and to determine whether environmental factors (i.e. temperature, solar radiation, woody cover and rainfall), as well as the probability of lion and biting fly occurrence could explain the geographical variation in stripe patterns for two zebra species, Equus quagga and Equus zebra. Unlike previous studies that assessed single predictors, we used a multimodel approach within an AIC framework to test several competing hypotheses for the function of stripes. I analysed crowd-sourced images (n = 283) of zebras from across their distribution (from Kenya in the north, Namibia in the west, to southern Africa in the south) and used ImageJ to measure features such as the number of stripes, proportion of dark stripes, perimeter/area ratio of stripes, contrast of stripes and presence or absence of shadow stripes. My results showed that 1) certain stripe characteristics such as the number of stripes, perimeter/area ratio, contrast of stripes and presence/absence of shadow stripes varied across geographical space and 2) environmental variables such as solar radiation, woody cover and rainfall may drive this geographic cline. Zebras appeared duller and darker overall in areas with higher solar radiation as the contrast on the torso and rump decreased while shadow stripe presence increased, potentially as a 10 protection mechanism against UV radiation. Increased contrast on belly stripes and presence of shadow stripes in regions with higher woody cover were thought to help zebras better camouflage to their background. Shadow stripe presence was also associated with regions of low rainfall and offered weak support to Gloger’s rule in zebras. My findings indicate that zebra stripe variation is likely driven by multiple selective pressures and highlights the complexity of adaptive traits. My analysis showcases the need for a more integrative approach to understanding one of nature’s most iconic patterns by incorporating multiple ecological, environmental, and evolutionary factors.