Basal Metabolism and Intrinsic Rate of Natural Increase
Background.—McNab (1980a) suggested that if food is not restricted during an animal's reproductive period, the factor that will limit growth and reproduction will be the rate at which energy can be used in growth and development. Under these conditions, an increase in Ḣb would actually increase rmax because it would provide a higher rate of biosynthesis, a faster growth rate, and a shorter generation time. Hennemann (1983) tested McNab's (1980a) premise and found a significant correlation between rmax and metabolic rate, independent of body size, for 44 mammal species. A low correlation coefficient for this relationship, however, indicated to him (Hennemann, 1983) that factors such as (1) food supply, (2) thermal characteristics of the environment, and (3) brain size also contribute toward shaping a species' reproductive potential, particularly when these factors strongly influence rates of biosynthesis or growth or for some reason alter generation time. Results of our estimates of rmax for procyonids are presented in [Table 10].
Procyon lotor.—This species had the highest Ḣb and Dd, and also had the highest rmax (1.34; [Table 10]). Such a high rmax may infer that this trait evolved under conditions where food and temperature were not limiting to reproduction. Under these conditions selection could have favored those reproductive characteristics sensitive to a higher Ḣb (biosynthesis, growth, and generation time; McNab, 1980a). Procyon lotor's high reproductive potential is due to its early age of first female reproduction and its large litter size, characteristics that may reflect metabolically driven increases in both biosynthesis and growth.
Bassariscus astutus.—This species has a low Ḣb but an rmax that was 124% of expected ([Table 10]). This suggests that rmax evolved under conditions where food and temperature were not limiting to reproduction. Reduced litter size should restrict this species' reproductive potential and may be a reflection of its low Ḣb. The factor that is responsible for increasing its reproductive potential, however, is its early age of first female reproduction. Bassariscus astutus is the smallest of these procyonids, and even though it has a low Ḣb, its small mass may contribute to its ability to reach adult size and sexual maturity in its first year. The high quality of its diet (a high proportion of small vertebrates; [Table 9]) also may be a factor that is permissive to early female reproduction. Thus, small body size and diet may be factors that have allowed this species to evolve a higher than expected reproductive potential in spite of its low Ḣb.
Nasua narica.—This species is one of the largest procyonids ([Table 7]), and it possesses characteristics that should limit its reproductive potential: lower than predicted Ḣb ([Table 7]), a relatively low-quality diet (Kaufmann, 1962:182-198; [Table 9]), and delayed time of first reproduction ([Table 10]). In spite of this, Nasua narica has a higher than expected rmax (111% of predicted; [Table 10]). The life history feature that enhances Nasua narica's reproductive potential, and increases rmax beyond expected, is its large litter size. In this species females live in bands. Each year just before their young are born these bands break up, and each female seeks out a den for herself and her litter. Once the young are able to leave the den (approximately five weeks), bands reform. In this situation, females not only care for their own young but also for those of other females in the band (Kaufmann, 1962:157-159, 1982, 1987; Russell, 1983). This social structure may contribute to this species' ability to produce large litters and in this way increase its reproductive potential.
Table 10.—Intrinsic rate of natural increase (rmax) of several procyonids. (a = potential age of females producing first young; b = potential annual birth rate of female young (= average litter size/2; average litter size was calculated from the published range of litter sizes for each species); n = potential age of females producing their final young; rmaxe = intrinsic rate of natural increase expected from body mass (Hennemann, 1983); rmaxr = ratio of calculated to expected intrinsic rate of natural increase (rmax/rmaxe).)