ESTRUS, FERTILITY, EARLY EMBRYO DEVELOPMENT, AND AUTOLOGOUS EMBRYO-TRANSFER IN LABORATORY WOODCHUCKS (MARMOTA-MONAX)

Reproductive parameters were studied in female laboratory woodchucks o ver a 6-year period. The pregnancy rate in adult females after ad libi tum 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, lim ited exposure resulted in a 75% ovulation rate, suggesting a problem w ith fertilization and not ovulation. The ovulation rate was 0% in the absence of mating (n = 10), suggesting that spontaneous ovulation is n ot the usual situation in this species. With ad libitum mating, fertil ity was greater (P < 0.05) for 2- to 4-year-old females (73%) than for 1-year-old (56%) or greater than or equal to 5-year old (58%) females . Mean (+/- SEM) litter size at birth was greater (P < 0.05) for 2- to B-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 di ameter every 3 to 4 days in 31 adult females suggested that enlargemen t to greater than or equal to 7 mm was indicative of proestrus or estr us and was a good indicator of willingness to accept mounting by males . Vulval swelling greater than or equal to 7 mm (n = 31) lasted 5 to 4 7 days. Fertile mating occurred from 3 to 30 days after vulval diamete r reached greater than or equal to 7 mm, near the time of peak enlarge ment (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 1 0 h) video observed exposure to males, with (n = 26) or without (n = 2 7) another 12 to 24 h of unobserved exposure, and were examined surgic ally for ovulation and pregnancy at known times after mating. There we re no consistent differences among non-ovulating, ovulating nonfertile , and fertile females in the longest duration of mating (7.2 +/- 0.4 m in) or in the number of matings lasting greater than or equal to 1 min (5.7 +/- 0.8). Compared with nonfertile females, fertile females expe rienced more matings greater than or equal to 3 min in duration (3.3 /- 0.4 versus 1.8 +/- 0.2) and greater than or equal to 5 min in durat ion (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 morula e, 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 int o the noncontiguous contralateral uterine horn was performed in five f emales that had been subjected to a hemi-ovariectomy. Transfers result ed in pregnancy and litters in four of the five females. The results s uggest 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 ea rly embryo development rate similar to that of rats and mice, and may be amenable to reproductive technologies developed for more common lab oratory rodents.