Concordance on zebra stripes: a comment on Larison et al. (2015)

Tim Caro,Theodore Stankowich

doi:10.1098/rsos.150323

Abstract

The functional significance of the extraordinary black and white stripes of zebras is still mysterious but now an active field of research. Four major hypotheses have been put forward: stripes are an antipredator defence operating through crypsis [1] or confusion of predators [2], are a means of reinforcing social bonds [3], are defence against ectoparasites [4] or are a means of cooling zebras [5]. Now, in the second multifactorial analysis of striping in zebras to date, Larison et al. [6] investigated the environmental factors that explain geographical variation in striping within a single species, the plains zebra ( Equus quagga or Equus burchellii ). They matched variation in striping patterns at 16 sites across its geographical range to a suite of environmental variables as well as tsetse fly (glossinid) distribution and lion ( Panthera leo ) presence. They found that greater intensities of intraspecific striping were associated with warmer temperatures and high precipitation. Last year we published a similar but phylogenetically controlled analysis using all seven species of equids, namely the three striped zebra species, the African wild ass ( Equus africanus ) which has leg stripes but no body stripes, and the unstriped species of equids ( E. hemionus, E. kiang, E. ferus przewalski ), and 20 of their subspecies, and compared several aspects of the four hypotheses [7]. These were camouflage in woodlands, antipredator defence against both lions and spotted hyaenas ( Crocuta crocuta ), social interactions, tsetse fly distribution, tabanid distribution using both temperature and humidity ranges as a proxy for abundance, and temperature isoclines to assess the cooling hypothesis. First we ran univariate tests on individual factors to identify the hypotheses with the strongest predictive ability, then subsequently used AICc model selection procedures to pit the most promising predictors against each other—a …

Highlights

Our comparative study pertains to the factors that maintain striping across species of equids, whereas 2 Larison and co-workers’ conclusions are specific to the ecocorrelates of variation in striping within plains zebras, and cannot necessarily be extrapolated directly to infer the selective pressures that favoured the original evolution of zebra stripes, which only occurred once in the genus
Our studies found no evidence for striping being associated with woodland or tree cover, refuting the idea that stripes are a form of crypsis against predators
We found many aspects of striping are associated with temperatures lying between 15◦C and 30◦C and humidity between 30 and 85% when these conditions persist for a minimum of six to seven months

Summary

Introduction

Our comparative study pertains to the factors that maintain striping across species of equids, whereas 2 Larison and co-workers’ conclusions are specific to the ecocorrelates of variation in striping within plains zebras, and cannot necessarily be extrapolated directly to infer the selective pressures that favoured the original evolution of zebra stripes, which only occurred once in the genus.

Results

Conclusion