Figure 1.—North American raccoon, Procyon lotor.
The Hypothesis
Our general hypothesis was that whereas most contemporary procyonids have retained the metabolic characteristics of their warm-adapted ancestors, Procyon lotor possesses a different set of adaptations, which either evolved as characteristics unique to this species or were acquired from its ancestral stock. In either case, its unique adaptations have given Procyon lotor the physiological flexibility to generalize its use of habitats and climates and expand its geographic distribution to a much greater extent than other procyonids.
Hypothesis Testing
We tested our hypothesis by comparing Procyon lotor with several other procyonids (Bassariscus astutus, Nasua nasua, Nasua narica, Procyon cancrivorus, and Potos flavus) on the basis of their (1) basal metabolic rate (Ḣb), (2) minimum wet thermal conductance (Cmw), (3) diversity of diet (Dd), (4) intrinsic rate of natural increase (rmax), and, when data were available, (5) capacity for evaporative cooling (Ec). In a genetic sense each one of these variables is a complex adaptive characteristic, expression of which is determined by the interaction of several genes (Prosser, 1986:110-165). Experience has shown that a given species will express each one of these variables in a specific manner that is relevant to its mass, physiology, behavior, and environmental circumstance. Thus, different expressions of these variables may represent specific climatic adaptations (Prosser, 1986:16) that have been selected-for by evolutionary process. Because these variables are interrelated with respect to regulation of body temperature and energy balance, they have co-evolved in each species to form an adaptive unit. For each species, measured and calculated values for the first four variables were converted into dimensionless numbers and used to derive a composite score that represented its adaptive unit. Climatic distributions of these species were then compared relative to their composite scores.