Northern Prairie Wildlife Research Center
Our findings that 74% of female skunks collected in March-July were 1 year old and that only 5% were >3 years old are consistent with findings of others (Casey and Webster, 1975; Schowalter and Gunson, 1982; Verts, 1967) and further demonstrate that high annual turnover is common within populations of striped skunks. High annual turnover also is implicit in our finding that nearly all (95%) females were pregnant or parous when collected and that mortality rates of embryos were low (1.5%). Most other studies also reported high pregnancy rates for striped skunks; 96% in Illinois (Verts, 1967), 88% in Alberta (Bjorge et al., 1981), and 92% in Alberta and Saskatchewan (Schowalter and Gunson, 1982). However, Pengeroth (1991) reported a pregnancy rate of 75% in Montana. In contrast to our finding of low mortality of embryos, Verts (1967) reported resorption of 13% of embryos in 21 striped skunks in Illinois, and Hamilton (1963) reported resorption of 53% of embryos in seven striped skunks in New York. We expect little additional mortality of embryos would have occurred beyond our observed rate because loss of embryos in mammals declines greatly after the first 2 weeks of gestation (Noden, 1986). Our females averaged ca. 6 weeks pregnant when collected; 96% of females in the subsample were >2 weeks pregnant.
Our overall mean estimate of 7.2 and annual mean estimates of 9.5 in 1981 and 8.2 in 1980, all based on live embryos, are higher than that reported among similarly calculated estimates for wild striped skunks: 5.8 (Allen, 1939); 5.8 (Hamilton, 1963); 6.3 (Verts, 1967); 6.7 (Pengeroth, 1991); 6.8 (Schowalter and Gunson, 1982). Our estimates are much higher than the mean of 4.8 reported in captive skunks (Wade-Smith and Richmond, 1978). Based on our estimated date of mean implantation of 4 March (1979-1981 and 1990) and reported range in gestation period for striped skunks of 59-77 days (Wade-Smith and Richmond, 1978), parturition in our females would have occurred during early May. This parturition time is consistent with observations of others summarized by Wade-Smith and Verts (1982).
Our finding that 55% of embryos were female is higher than proportions reported for wild striped skunks: 44% (Hamilton, 1963); 46% (Pengeroth, 1991); 50% (Verts, 1967). Wade-Smith and Richmond (1978), however, found that 53% of newborn captive striped skunks were female, which they believed was an artifact of captivity.
We found that litter size based on counts of corpora lutea underestimated litter size based on counts of embryos by a mean of 0.9 young/female, indicating that some females produced polyovular follicles or identical twins. Leach and Conaway (1963) and Verts (1967), however, believed that striped skunks are not polyovular. We found corpora lutea easy to discern and believe our counts were precise; had we included 13 resorbing embryos, the difference would have been even greater. Verts (1967) reported that corpora lutea were easy to distinguish after 11-12 days of gestation; 96% of our pregnant females in the subsample were more advanced than that. Although Schowalter and Gunson (1982) reported a mean of 0.5 more embryos than corpora per female in striped skunks, it is not clear whether their counts included corpora albicantia, which are not reliable indicators of embryonic litter size (Gilbert, 1987), or their counts indicate presence of polyovular follicles.
High pregnancy rates, large embryonic litters, and low mortality of embryos that we observed demonstrate potential for large annual increases in populations of striped skunks in intensively farmed areas of the upper Midwest. Our findings also suggest the long, harsh winters common to this region have little impact on potential for reproduction in skunks.