The edge of a species' range : survival and space-use patterns of raccoons at the northern periphery of their distribution

Abstract

The raccoon (Procyon lotor) is a species that has likely benefited from environmental conditions that followed European settlement of North America. As such, the raccoon has experienced vast range expansion during the past century, moving northward across the continent. However, little is known about the factors that currently govern the northern distribution and the ecology of raccoons at the northern edge of its range. Thus, I studied the population dynamics and social ecology of raccoons in southern Manitoba during spring, summer, and autumn, 2002-2005. To understand how intrinsic and extrinsic factors affected population dynamics, I examined how winter severity, body condition, age, and gender influenced over-winter survival of raccoons. Winter severity (measured by temperature and snow accumulation) was the most important factor influencing survival (β = 1.08, 95% CI = 0.99-1.17). Over-winter survival estimates ranged from 0.51 (95% CI: 0.41, 0.75) during the harshest winter to 0.84 (95% CI: 0.71, 0.97) during the mildest winter on record for Manitoba. There was no apparent relationship between autumn body condition and autumn food indices and no correlation between autumn and spring body condition. Adults experienced higher survival than yearlings while males had a greater chance of dying compared to females. Variation in abundance natural food items thought to be important during autumn hyperphagia are likely overwhelmed by the presence of grain as an alternative food source, as autumn body condition was constant across all years and plateaued at ca. 20% body fat. I conclude that changes in climatic conditions will likely have the greatest impact on raccoon demographics, with milder winters leading to higher survival. I also examined the spatial ecology of raccoons to determine if spacing behaviour could limit population growth and to test hypotheses regarding social tolerance and the formation of male coalition groups. Female home ranges were regularly spaced throughout the study site, with minimal spatio-temporal overlap among adult females. However, there where instances where females did display tolerance among conspecifics as well as the ability to partition areas of overlap to use them dissimilarly. While females were generally non-gregarious, plasticity in social tolerance likely precludes spacing behavior from regulating densities of this population. Male social behavior was more complex than previously described for northern populations; most adults (ca. 80%) formed a coalition pair with another male. There was little overlap among male groups and high overlap within groups. The dynamic interaction tests confirmed association in movements for male dyads. Male coalition groups formed despite females being regularly spaced, which contradicts working hypotheses of mechanisms explaining grouping behavior in male carnivores. I propose that group formation occurred because of the benefits that dominant males received through increased efficiency in territory maintenance and the increased likelihood of territory inheritance by subordinate males.