Fine-scale spatial variation in eastern grey squirrel melanism across a complex urban landscape

Area of Science:

• Urban ecology
• Evolutionary biology
• Animal coloration

Background:

• Urban environments present unique selective pressures driving rapid evolutionary changes in wildlife.
• Previous research often uses broad urban-rural contrasts, overlooking fine-scale spatial variation within cities.

Purpose of the Study:

• To investigate spatial variation in eastern grey squirrel (Sciurus carolinensis) coat color morphs across Montreal.
• To understand the influence of urbanization, local environment, and geographic structure on melanism.

Main Methods:

• Conducted standardized surveys in 197 Montreal parks.
• Utilized citizen science data from iNaturalist and SquirrelMapper.
• Analyzed melanism in relation to urbanization, temperature, canopy cover, and geographic barriers using structural equation modeling.

Main Results:

• Melanism in Montreal grey squirrels was generally negatively correlated with urbanization, contrary to patterns in other cities.
• Abrupt shifts in morph frequency occurred at major waterways, suggesting rivers act as dispersal barriers.
• Melanic morph frequency increased in areas with cooler winters and higher canopy cover; urbanization indirectly suppressed melanism via these factors.

Conclusions:

• Phenotypic patterns within urban populations are shaped by fine-scale environmental heterogeneity and geographic barriers.
• Montreal's complex island landscape and waterways create distinct phenotypic regions for grey squirrel coat color.
• Future studies should consider localized environmental factors and geographic structure beyond simple urban-rural gradients.