Recent articles
Cosentino, B. J., & Gibbs, J. P. (2022). Parallel evolution of urban–rural clines in melanism in a widespread mammal. Scientific Reports, 12, 1752. https://doi.org/10.1038/s41598-022-05746-2
This foundational SquirrelMapper paper, using >60,000 georeferenced observations of gray and melanic squirrels from SquirrelMapper, iNaturalist, and Zooniverse evaluated parallel evolution of urban–rural melanism clines across 43 cities in North America. We showed that in northern cities with cold winters, melanism is consistently more frequent in urban cores and declines toward rural surroundings, and that the strength of these clines is associated with climate and forest cover, consistent with selection related to thermoregulation and habitat. SquirrelMapper was the engine of the entire analysis: volunteers’ photos and reports created the first continent-scale, morph-resolved dataset for a mammal, enabling formal tests of parallel urban evolution that would have been logistically and financially impossible using only researcher-collected data.
Cosentino, B. J., Vanek, J. P., & Gibbs, J. P. (2023). Rural selection drives the evolution of an urban–rural cline in coat color in gray squirrels. Ecology and Evolution, 13, e10544.
In this study, we used remote cameras, visual surveys, and a translocation experiment across 76 sites spanning the Syracuse, NY urbanization gradient to quantify how survival differs between gray and melanic squirrels in city parks versus surrounding rural woodlands. We documented a strong decline in melanism from nearly 50% in the urban core to under 5% in rural forests, and showed that melanic squirrels suffer lower survival than gray squirrels in rural woodlands but not in the city—supporting a “rural selection” scenario in which selection against melanism in forests, combined with relaxed selection in cities, generates the cline.
Parlin, A.F., Cosentino, B.J., Lehtinen, R.M., McDonald, J.E., Sinclair, E.C. and Gibbs, J.P., 2025. Road mortality contributes to the evolution of an urban–rural cline in squirrel coat color. Evolutionary applications, 18(5), p.e70109.
In this paper we integrated field surveys of road-killed squirrels with a large, range-wide dataset of coat-color observations to test whether roads act as selective filters on gray versus melanic morphs. Using >100,000 geo-referenced squirrel photos from SquirrelMapper, iNaturalist, and a Zooniverse project, we showed that melanic squirrels are disproportionately common among roadkill in some parts of the range, and that spatial patterns of road mortality correspond to changes in the strength and direction of urban–rural melanism clines. The paper argues that roads can both generate and erode clines depending on local conditions, making road mortality an underappreciated driver of urban evolution. Citizen scientists via SquirrelMapper were central to producing this massive dataset needed to detect continent-scale associations between traffic, morph frequency, and cline shape that would have been impossible to assemble with traditional field methods alone.
Proctor, J., Bryan, A., Cosentino, B.J. and Gibbs, J.P., 2025. Crypsis in a polymorphic mammal along an urbanization gradient. Urban Ecosystems, 28(2), p.94.
This paper asks whether differences in camouflage (crypsis) between gray and melanic morphs of eastern gray squirrels help explain urban–rural clines in coat color. We photographed both morphs in forested habitats and on road surfaces across the Syracuse, NY urbanization gradient, then quantified how detectable each morph was to human observers via an online browser game and through image-based measures of background matching. We found that melanic squirrels tend to be more conspicuous than gray squirrels on rural forest backgrounds but can be less conspicuous on dark road surfaces, suggesting opposing selection on crypsis along the gradient that may interact with predation and road mortality. This study used SquirrelMapper’s citizen-science platform to engage hundreds of volunteer “players” to score image detectability, thereby turning public participation into a quantitative measure of a key selective mechanism.
Fusco, N. A., Cosentino, B. J., Gibbs, J. P., & coauthors. (2024). Population genomic structure of a widespread, urban-dwelling mammal: The eastern grey squirrel (Sciurus carolinensis). Molecular Ecology, 33, e17230.
This paper used ddRAD sequencing of 44,000 SNPs from 403 gray squirrels sampled across eastern North America to assess how urbanization and land cover shape genetic diversity, population structure, and connectivity. We found moderate genetic diversity, low inbreeding, and generally high connectivity across both urban and non-urban landscapes, with only modest isolation-by-distance and somewhat reduced gene flow in heavily agricultural regions. Landscape-genetic analyses suggest that forested habitats facilitate gene flow, whereas intense agriculture can impede it.
This popular article outlines the fascinating evolution of melanism in gray squirrels in urbanizng areas
This book chapter was our first exploration of the melanism dynamics of gray squirrels in cities
Useful resources
A fascinating overview of the history of squirrels in urban areas: The Urbanization of the Eastern Gray Squirrel in the United States, Journal of American History 100, no. 3 (2013): 691-710 (.pdf available on author Etienne Benson’s website)
This podcast reveals much about our changing relationship with squirrels.
An article by Helen McRobie on gray squirrel morph genetics: The genetic basis of melanism in the gray squirrel (Sciurus carolinensis) Journal of Heredity 100: 709-714.