Space use and survival of gray and black Eastern Gray Squirrels translocated across an urbanization gradient

Background/Question/Methods

Urban areas are the fastest growing ecosystem on Earth. Ecological conditions vary sharply between cities and rural areas and may cause divergent natural selection between urban and rural populations. Eastern Gray Squirrels (Sciurus carolinensis) are one of the most common and well-known urban mammals in North America and exhibit two Mendelian inherited color morphs, gray and melanic (black). Melanics were the prevailing color morph in northern forests prior to the 1800s and subsequent land use changes, but today the melanic morph is only common in urban areas and portions of the Great Lakes region. Previous work suggests that melanic squirrels are less cryptic than the gray morph and may have fitness advantage in urban areas where predation pressure is relaxed. To test the hypothesis of differential fitness across the urbanization gradient, we used a radio-telemetry experiment to estimate survival rates of each color morph. Because the melanic morph is rare in rural forests, we captured squirrels of both morphs in urban areas and translocated them to urban (n = 28) and rural areas (n = 31) in equal proportions.

Results/Conclusions

Rural squirrels moved greater distances than urban squirrels, and currently surviving rural squirrels all established home ranges in rural housing developments. Using the Kaplan-Meier procedure with a staggered entry design and right-censoring, we found daily survival for rural melanics was lower than that of rural grays (p = 0.02), but found no difference between the morphs at the urban release site (p = 0.12). Low rates of carcass recovery precluded a formal analysis of cause-specific mortality between release sites, but more carcasses were recovered in rural areas. Our preliminary results suggest that melanic squirrels have a fitness disadvantage in rural areas. Future work will include an additional cohort of tracked squirrels, estimation of attack rates on model squirrels, and integration of survival and genetic data.