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Age-Related Reproduction in Striped Skunks
(Mephitis mephitis) in the Upper Midwest

Results


Of 178 (six to 40 annually) adult female striped skunks examined, 74% (59-89% annually) were age-class 1, and 26% (11-41%) were age-class 2. Only 5% of all females were >3 years old; the two oldest were 5 years old.

At least 95% (168 of 177) of the females were pregnant (123) or parous (45) in the year collected; reproductive status of one female was not determined. Among eight females believed to be barren, seven were age-class 1, and one was age-class 2. Mean date of collection for pregnant females in all years was 18 April ± 0.9 days.

We noted resorbing embryos in 12 females; 11 had one each, and one had two. The resorption rate was 1.5% (13 of 873 embryos from 123 females). Nine females with resorbing embryos were age-class 1, and three were age-class 2.

Mean estimate of litter size for all pregnant females was 7.2 ± 0.4, based on counts of live embryos. This estimate was not affected by interaction between age class and year (F = 1.40; d.f. = 6,109; P = 0.22) or by age class (F = 0.91; d.f. = 1,109; P = 0.34). However, we detected differences among annual estimates for both age classes combined (F = 3.90; d.f. = 6,109; P = 0.001). Annual mean estimates were 7.3 ± 0.3 (1979, n = 22), 8.2 ± 0.5 (1980, n = 20), 9.5 ± 0.9 (1981, n = 4), 7.0 ± 0.5 (1987, n = 15), 5.9 ± 0.6 (1988, n = 12), 6.1 ± 0.4 (1989, n = 25), and 6.7 ± 0.4 (1990, n = 25). Estimates for 1979, 1980, 1981, and 1987 were not different, nor were estimates for 1979, 1987, 1988, 1989, and 1990. The largest estimate of litter size for a female in age-class 1 was 12 and for a female in age-class 2 was 11. We estimated that litter sizes were six and seven for the 5-year-old females.

Mean implantation date for the subsample of 68 pregnant females was 4 March ± 1.6 days. Annual sample sizes are as noted in the previous paragraph, minus two skunks in 1979 and one in 1990 in which embryos were too small to measure. Sixty-five (96%) of the 68 females were >2 weeks pregnant when collected. We observed no effect on implantation date from interaction between age class and year (F = 1.29; d.f. = 3,60; F = 0.29). Differences in implantation dates were not detected between age classes (F = 0.21; d.f. = 1,60; P = 0.64) or among years (F = 2.24; d.f. = 3,60; P = 0.09).

Mean proportion of embryos that were female in litters was 55 4%. We detected an interaction between age-class and year (F = 3.52; d.f. = 3,35; P = 0.03), indicating that sex ratios varied among years or between age-classes. Differences detected between age classes in proportion of embryos that were female were significant (P = 0.05) only in 1980 (Table 1). Differences detected annually in proportion of embryos that were female by age class revealed no consistent relationships.

Table 1.  Least-squares means (± 1 SE) of proportion of embryos that were female in pregnant striped skunks (n) collected in North Dakota and Minnesota by age class (1 = 1 year old, 2 = ≥2 years old).
Year Proportion of females
Age-class 1 Age-class 2
mean of X ± SE n mean of X ± SE n
1979 0.48 ± 0.05 11 0.49 ± 0.06 7
1980a 0.41 ± 0.07 6 0.79 ± 0.10 3
1981 0.72 ± 0.10 3 0.55 ± 0.16 1
1990 0.56 ± 0.05 11 0.43 ± 0.16 1
a Means within year differ significantly between age classes (P < 0.05, Fisher's protected least significance difference—Milliken and Johnson, 1984).

The estimate of the overall mean litter based on corpora lutea (7.1 0.2) was 0.9 smaller than the estimate of the mean litter based on live embryos (8.0 0.2) for the same skunks (F = 14.8; d.f. = 1,62; P = 0.0003). We detected a significant effect on those estimates due to interaction between estimation method and year (F = 2.7; d.f. = 1,62; P = 0.05), but other interactions were not significant (P ≥ 0.3). In 1979 and 1980, the litter estimate based on corpora lutea was significantly lower than the litter estimate based on live embryos (P < 0.05); the same trend was evident in 1981 and 1990 (Table 2). Age class also had a significant effect on litter estimates (F = 4.47; d.f. = 1,64; P = 0.04), implying that differences between age-classes were consistent regardless of estimation method. The estimate of the overall mean litter for age-class 1 was 7.1 ± 0.2 and for age-class 2 was 8.0 ± 0.4 (P < 0.05).

Table 2.  Least-squares means (± 1 SE) of potential litter sizes of striped skunks collected in North Dakota and Minnesota, based on numbers of corpora lutea of macroscopically counted numbers of live embryos in individual pregnant females (n).
Year Litter size
Corpora lutea Live embryos
± SE n ± SE n
1979a 6.6 ± 0.2 20 7.3 ± 0.2 20
1980a 6.3 ± 0.3 20 8.2 ± 0.3 20
1981 8.8 ± 0.6 4 9.5 ± 0.6 4
1990 6.6 ± 0.2 27 7.1 ± 0.2 27
a Means within year differ significantly between estimation methods (P < 0.05, Fisher's protected least significant difference—Milliken and Johnson, 1984).


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