Biased movement drives local cryptic coloration on distinct urban pavements

Abstract

Explanations of how organisms might adapt to urban environments have mostly focused on divergent natural selection and adaptive plasticity. However, differential habitat choice has been suggested as an alternative. Here, we test for habitat choice in enhancing crypsis in ground-perching grasshoppers colonizing an urbanized environment, composed of a mosaic of four distinctly coloured substrates (asphalt roads and adjacent pavements). Additionally, we determine its relative importance compared to present-day natural selection and phenotypic plasticity. We found that grasshoppers are very mobile, but nevertheless approximately match the colour of their local substrate. By manipulating grasshopper colour, we confirm that grasshoppers increase the usage of those urban substrates that resemble their own colours. This selective movement actively improves crypsis. Colour divergence between grasshoppers on different substrates is not or hardly owing to present-day natural selection, because observed mortality rates are too low to counteract random substrate use. Additional experiments also show negligible contributions from plasticity in colour. Our results confirm that matching habitat choice can be an important driver of adaptation to urban environments. In general, studies should more fully incorporate that individuals are not only selective targets (i.e. selected on by the environment), but also selective agents (i.e. selecting their own environments).

Plasticity and habitat choice match colour to function in an ambush bug

“…The broader pattern of habitat matching appears to have at least one adaptive function—prey capture. Individuals on well‐matched backgrounds were more likely to be found with prey (Figure b), although manipulative experiments of ambush bug colour are needed to definitively link phenotype‐environment matching to increases in prey capture (Edelaar et al, ). Greater prey capture likely confers increased longevity and reproductive output, increasing fitness and providing an adaptive advantage to phenotype–environment matching.…”

“…Note that while ambush bugs may potentially choose their habitat and alter their phenotype, they are unlikely to modify the colour of their habitats (i.e. the third mechanism described in Edelaar et al, ), and accordingly we have not investigated this potential mechanism of phenotype–environment matching.…”

“…Such adaptive plasticity is common, for instance in the many organisms that can change colour to match their background (Anderson & Dodson, ; Duarte et al, ; Johansson & Nilsson‐Örtman, ; Tong et al, ). These two major mechanisms are not mutually exclusive, yet have been studied together only rarely (Edelaar et al, ; Lowe & Addis, ). For instance, when adaptive colour change occurs slowly, this may favour a combined strategy of background matching and plasticity (Stevens & Ruxton, ).…”

Microhabitat contributes to microgeographic divergence in threespine stickleback

“…More recently, Edelaar et al. (2019) showed that grasshoppers inhabiting different anthropogenic substrates have heritable differences in color (matching their various backgrounds), despite being mere meters apart. For natural selection to sustain these differences in such highly mobile organisms would require unrealistically intense selection (∼90% daily mortality rates; Edelaar et al.…”

“…For natural selection to sustain these differences in such highly mobile organisms would require unrealistically intense selection (∼90% daily mortality rates; Edelaar et al. 2019). Such discrepancies between empirical data and migration‐selection theory suggest that we must sometimes invoke additional processes (Edelaar and Bolnick 2019).…”