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.

Parallel evolution of urban–rural clines in melanism in a widespread mammal

“…The melanic morph is more visible to vehicle drivers than the gray morph on asphalt roads, which may lead to the melanic morph having greater survival in areas with high car traffic 13 . Additional selective mechanisms may play a role in maintaining urban melanism (e.g., parasite pressure, pollution) 24 , including mechanisms involving correlated traits 25 . For example, melanic squirrels have less phylogenetically variable microbiomes than the gray morph in cities 26 .…”

“…The melanic morph is more visible to vehicle drivers than the gray morph on asphalt roads, which may lead to the melanic morph having greater survival in areas with high car traffic 13 . Additional selective mechanisms may play a role in maintaining urban melanism (e.g., parasite pressure, pollution) 24 , Figure 1. (a) Distribution of coat color morphs of 26,924 eastern gray squirrels (Sciurus carolinensis) among 43 cities in North America.…”

Parallel evolution of urban-rural clines in melanism in a widespread mammal

“…We hypothesize that the dramatic shift from old growth to secondary forests following European settlement in North America 27,28,29 changed the degree to which gray and melanic morphs are concealed from visual predators and hunters, favoring the gray morph in regrown forests 13 . Additional selective factors may play a role in driving urban melanism (e.g., parasite pressure, pollution) 30 , and ultimately genomic tools and field experiments are needed to clarify the role of natural selection in generating urban-rural clines 31 .…”

“…The melanic morph is more visible to vehicle drivers than the gray morph on asphalt roads, which may lead to the melanic morph having greater survival in areas with high car traffic 13 . Additional selective mechanisms may play a role in maintaining urban melanism (e.g., parasite pressure, pollution) 24 , including mechanisms involving correlated traits 25 . For example, melanic squirrels have less phylogenetically variable microbiomes than the gray morph in cities 26 .…”

Multiple carotenoid-based signals are enhanced despite poor body condition in urban male and female Northern Cardinals (Cardinalis cardinalis)

“…Moreover, carotenoids can undergo extensive physiological modification before being deposited in tissue (Goodwin 1984), leaving room for other more indirect effects of urbanization on color (e.g., via stress or parasites; Møller 2009, Hutton and McGraw 2016). Additionally, rural and urban habitats likely differ in lighting regime, which can also affect carotenoid-based color via selection on conspicuousness (Leveau 2021). Importantly, these mechanisms are not mutually exclusive and might either reduce or enhance color in urban populations.…”