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

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.

Contrasting patterns of neutral and functional genetic diversity in stable and disturbed environments

“…The higher color morph diversity and the higher genetic diversity of outlier AFLP loci in disturbed compared with stable environments may be attributable in part to different selection regimes. In disturbed environments, oscillating and temporally changing selection associated with succession and habitat modifications resulting in rapid temporal shifts in morph frequencies (Forsman, Karlsson et al., ; Karlsson, Caesar, Ahnesjö, & Forsman, ; Karpestam et al., ) may promote and preserve genetic variation. In stable environments, by contrast, stabilizing selection likely reduces genetic variance (Arnold & Wade, ; Endler, ) and favors the adaptation to optimum phenotypes (de Vladar & Barton, ).…”

Body size influences differently the detectabilities of colour morphs of cryptic prey

“…Furthermore, we have established previously that, in the case of the pygmy grasshopper system, humans may be successfully used as substitutes for natural predators. Thus, the rate at which humans detect images of black, grey, and striped grasshoppers presented on natural visual backgrounds on computer screens mirrors the pattern of selection imposed by natural predators, and reliably predicts associations between morph frequency shifts with different habitat types in the wild (Forsman & Appelqvist, ; Karpestam et al ., , ). Our present study enables usto examine this at a finer scale, by comparing detection and selection on colour morph in males versus females.…”

“…), because their frequencies vary both among populations and over time within populations (Forsman et al ., ), and because estimates of survival probabilities for these morphs have been shown to differ between females and males in a previous study (Forsman & Appelqvist, ). The grasshoppers were photographed from 7 cm above against brownish cardboard in shaded natural daylight conditions (Karpestam et al ., , , ).…”

Background matching and camouflage efficiency predict population density in four-eyed turtle ( Sacalia quadriocellata )

“…However, some authors have suggested that the best method for studying background matching is to test its camouflage efficiency by building a natural predator-prey system (Cuthill et al, 2005). This method, however, is difficult to carry out because of the improbability of witnessing predation events in the wild (Karpestam et al, 2013). One controllable method to study camouflage efficiency is to conduct detection experiments by presenting photographs of prey on paper to human predators (Gendron and Staddon, 1984;Todd, 2009).…”