Age-Specific Reproduction in Raccoons in Northwestern Oregon

Information on reproductive biology is available for populations of the raccoon (Procyon lotor) throughout most of the range of the species (Dunn and Chapman, 1983). Nevertheless, that available for populations in the northwestern portion of the range of the species (Pacific Northwest) is based on a single study (Scheffer, 1950) or is anecdotal. Litter size is related to maternal age in many species of mammals (Nalbandov, 1976), possibly including raccoons (Dunn and Chapman, 1983; Fritzell et al., 1985; Junge and Sanderson, 1982), but most studies of the reproductive biology of raccoons were based either on two age classes, small samples, or both (reviewed by Dunn and Chapman, 1983; Johnson, 1970). Advances in techniques for estimating age in raccoons (Junge and Hoffmeister, 1980) permit classification of the adult segment into several age classes. The opportunity to obtain large samples of raccoons permitted us to evaluate age-specific reproduction of raccoons in northwestern Oregon. During the 1982-83 and 1983-84 furbearer seasons in Oregon, 1,319 carcasses of raccoons taken north of the Lane-Douglas County line (-43?36'N) and west of the crest of the Cascade Mountains ( 121049'W) were obtained from hunters and trappers. Of these, 628 were females sufficiently intact to be used in this investigation. Female raccoons were assigned to one of five age classes corresponding to approximately 0.5, 1.5, 2.5, 3.5, or 4.5 years of age or over, respectively, on the basis of the sequence of ossification of cranial sutures (Junge and Hoffmeister, 1980). The latter technique was modified slightly because we found that closure of the squamosal-jugal suture was not at the same stage in the sequence among Oregon raccoons (Fiero, 1985) as that reported for raccoons in Illinois (Junge and Hoffmeister, 1980). Uteri were excised, opened longitudinally, and examined for presence of pigmented implantation sites. Counts of implantation sites with similar densities of pigmentation were recorded. We followed Junge and Sanderson (1982) in considering the set of most intensely pigmented sites to have formed during the previous breeding season. Mean litter sizes for the previous breeding season reported herein were calculated from numbers of sites in the darkest set; mean sizes for earlier litters were calculated from sets of sites with discernable differences in pigmentation. Two-way analysis of variance (Kim and Kohout, 1975) and Fisher's protected least significant difference (LSD) test (Ott, 1977) were used to test for and identify significant differences in litter size among and between age classes and years. Chi-square in contingency tables was used to test for significant differences in the proportion of parous females among age classes. Unpaired t tests were used to determine if differences in mean numbers of pigmented sites of implantation between sets of different intensity were significant. In all statistical analyses, P < 0.05 was accepted as the level of significance. Of 313 females in age classes 2 to 5, 210 (67.1%) had pigmented sites of implantation; none of 315 females in age class 1 had borne young (Table 1). The proportion of parous females differed significantly among age classes within years (F = 66.4, d.f. = 3,305), but not among age classes between years (F = 1.4, d.f. = 1,305). Significantly fewer (x2 = 133.6, d.f. = 1) females in age class 2 were parous [41 (31.8%) of 129] than among older females [169 (91.8%) of 184]; however, the proportion of females exhibiting sites of implantation did not differ significantly (x2 = 0.99, d.f. = 2) among age classes 3 to 5 (Table 1). The proportion of female raccoons in age class 2 that bear young was reported to range from none of 17 in Washington (Scheffer, 1950) to 27 (73%) of 37 in Illinois (Junge and Sanderson, 1982). In addition, the proportion of raccoon populations consisting of parous females in age class 2 differed dramatically among years (Fritzell et al., 1985). The proportion of females in age class 3 or older that bear young was reported to range from 17 (74%) of 23 in Washington (Scheffer, 1950) to 42 (95%) of 44 in Illinois (Junge and Sanderson, 1982) and was not found to differ as greatly between years as parity among females in age class 2 (Fritzell et al., 1985). Of the 210 parous females, 85 (40.5%) had uteri with multiple sets of implantation sites discernable on the basis of the intensity of pigmentation (Sanderson and Nalbandov, 1973); 75 had two sets and 10 had three sets (Table 2). Eighty-three (49.1%) of 169 parous females in age class 3 or older had uteri with more February 1986 169