Estrus, Fertility, Early Embryo Development, and Autologous Embryo Transfer in Laboratory Woodchucks (Marmota monax)

Reproductive parameters were studied in female laboratory woodchucks over a 6-year period. The pregnancy rate in adult females after ad libitum exposure to a male (n = 643) was 72%; that after limited (4 to 8 h) exposure with video observations (n = 31) was only 37%. However, limited exposure resulted in a 75% ovulation rate, suggesting a problem with fertilization and not ovulation. The ovulation rate was 0% in the absence of mating (n = 10), suggesting that spontaneous ovulation is not the usual situation in this species. With ad libitum mating, fertility was greater (P < 0.05) for 2- to 4-year-old females (73%) than for 1-year-old (56%) or ≥5-year-old (58%) females. Mean (± SEM) litter size at birth was greater (P < 0.05) for 2- to 5-year-old animals (4.0 ± 0.1) than for 1-year-old animals (2.9 ± 0.2). However, in 1-year-old females the neonatal mortality was low, and the litter size at weaning (2.4 ± 0.3 pups) did not differ from that of older females (2.7 ± 0.1 pups). The measurement of vulval diameter every 3 to 4 days in 31 adult females suggested that enlargement to ≥7 mm was indicative of proestrus or estrus and was a good indicator of willingness to accept mounting by males. Vulval swelling ≥7 mm (n = 31) lasted 5 to 47 days. Fertile mating occurred from 3 to 30 days after vulval diameter reached ≥7 mm, near the time of peak enlargement (10.1 ± 0.2 mm), and was followed in several days by a decrease in vulval size. Fifty-three females were allowed only limited (6 to 10 h) video-observed exposure to males, with (n = 26) or without (n = 27) another 12 to 24 h of unobserved exposure, and were examined surgically for ovulation and pregnancy at known times after mating. There were no consistent differences among nonovulating, ovulating nonfertile, and fertile females in the longest duration of mating (7.2 ± 0.4 min) or in the number of matings lasting ≥1 min (5.7 ± 0.8). Compared with nonfertile females, fertile females experienced more matings ≥3 min in duration (3.3 ± 0.4 versus 1.8 ± 0.2) and ≥5 min in duration (2.9 ± 0.4 versus 1.1 ± 01). At postmating days 1, 2, 3, 4, 5, 6, and 7 the embryos were observed to be 1-, 4-, 4- to 32-cell morulae, 16- to 60-cell morulae, free-floating blastocysts, anti-mesometrial peri-implantation blastocysts, and attached blastocysts respectively. Autologous transfer of day-5 uterine embryos of normal appearance into the noncontiguous contralateral uterine horn was performed in five females that had been subjected to a hemi-ovariectomy. Transfers resulted in pregnancy and litters in four of the five females. The results suggest that this large, circannual sciurid rodent can be successfully and predictably bred as a laboratory animal model, has vulval changes indicative of proestrus, is principally an induced ovulator, has an early embryo development rate similar to that of rats and mice, and may be amenable to reproductive technologies developed for more common laboratory rodents.