Detection experiments with humans implicate visual predation as a driver of colour polymorphism dynamics in pygmy grasshoppers

Sami Merilaita,Einat Karpestam,Anders Forsman

doi:10.1186/1472-6785-13-17

Abstract

BackgroundAnimal colour patterns offer good model systems for studies of biodiversity and evolution of local adaptations. An increasingly popular approach to study the role of selection for camouflage for evolutionary trajectories of animal colour patterns is to present images of prey on paper or computer screens to human ‘predators’. Yet, few attempts have been made to confirm that rates of detection by humans can predict patterns of selection and evolutionary modifications of prey colour patterns in nature. In this study, we first analyzed encounters between human ‘predators’ and images of natural black, grey and striped colour morphs of the polymorphic Tetrix subulata pygmy grasshoppers presented on background images of unburnt, intermediate or completely burnt natural habitats. Next, we compared detection rates with estimates of capture probabilities and survival of free-ranging grasshoppers, and with estimates of relative morph frequencies in natural populations.ResultsThe proportion of grasshoppers that were detected and time to detection depended on both the colour pattern of the prey and on the type of visual background. Grasshoppers were detected more often and faster on unburnt backgrounds than on 50% and 100% burnt backgrounds. Striped prey were detected less often than grey or black prey on unburnt backgrounds; grey prey were detected more often than black or striped prey on 50% burnt backgrounds; and black prey were detected less often than grey prey on 100% burnt backgrounds. Rates of detection mirrored previously reported rates of capture by humans of free-ranging grasshoppers, as well as morph specific survival in the wild. Rates of detection were also correlated with frequencies of striped, black and grey morphs in samples of T. subulata from natural populations that occupied the three habitat types used for the detection experiment.ConclusionsOur findings demonstrate that crypsis is background-dependent, and implicate visual predation as an important driver of evolutionary modifications of colour polymorphism in pygmy grasshoppers. Our study provides the clearest evidence to date that using humans as ‘predators’ in detection experiments may provide reliable information on the protective values of prey colour patterns and of natural selection and microevolution of camouflage in the wild.

Highlights

Animal colour patterns offer good model systems for studies of biodiversity and evolution of local adaptations
Effects of prey colour pattern and visual background on rates of detection To test how detection was influenced by prey colour pattern and visual backgrounds, we presented human ‘predators’ with images of black, grey or striped grasshoppers on computer screens against photographic samples of either unburnt, 50% burnt or completely burnt grasshopper habitats
As in our previous study [28], we recorded for each prey colour morph and visual background, the number of presented prey images that were detected, time to detection, the number of times that a participant failed to detect the image within 60 seconds, and the number of times the participant clicked somewhere on the screen where there was no grasshopper image

Summary

Introduction

Animal colour patterns offer good model systems for studies of biodiversity and evolution of local adaptations. The efficacy of (most types of ) protective coloration depends on the visual properties of the animal colour pattern relative to properties of the visual background where the animals live, and this offers excellent opportunities for investigations of evolution of local adaptations [4,5,6], evolutionary shifts in response to environmental changes e.g., [7,8,9,10], and for studies of factors that influence the maintenance and dynamics of polymorphisms e.g., [7,11]. [12], the crab spider Xysticus sabulosus [13], the marine crustacean Idotea baltica [14], the walking stick Timema cristinae [15,16], and the pygmy grasshoppers Tetrix japonica [17] and T. subulata [7] In all these species, predation is assumed to be a major factor underlying polymorphism. Conclusive evidence would require demonstrating that colour pattern influences susceptibility to predation, that different colour patterns are favoured in different environments, and that predation contributes to variation in lifetime reproductive success and translates into heritable shifts of colour patterns between generations or populations [21,22,23]

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Conclusion