United colours of the city: A review about urbanisation impact on animal colours

Abstract

Habitat colour gradients characterise urban areas: from green prevailing in the least urbanised areas dominated by lawn, shrubs and trees, to grey characteristic of heavily urbanised areas dominated by impervious surfaces. These changes may promote modifications in community composition and intraspecific colour changes in species occurring in urban areas. This review's objectives were: (1) to analyse reported patterns of animal colour change between urban and non-urban areas; (2) to identify the main mechanisms underlying those patterns; and (3) to determine research bias and suggest future research directions. A bibliometric on animal colour patterns using Scopus resulted in 62 studies being found and their findings assessed. Most studies were focused on birds (N = 38) and were conducted in the Northern Hemisphere (N = 55), with 90% analysing intraspecific colour changes. Intraspecific studies found three main patterns: (1) urban melanism, with polymorphic species of insects, birds and reptiles showing darker colouration in urban areas than in non-urban areas; (2) urban dullness, with carotenoid-based plumage colours being duller in urban than in rural areas; and (3) a reduction in conspicuousness of sexually selected plumages in urban areas. Interspecific studies suggested that urban areas are filters for animal colours, promoting the colonisation of those species that increase camouflage or exhibit warning colours to predators. More studies are needed in the Southern Hemisphere and at the interspecific level to determine the role of animal colours in urban colonisation.