Reproduction of the Brush Mouse and White-Footed Mouse in the Central United States

Abstract

The seasonal reproductive cycles of the brush mouse (Peromyscus boylii) and the white-footed mouse (P. leucopus) were studied for 1Y2 years in southern Missouri. Breeding activity of female brush mice occurred primarily during two periods; February-April and September-November, in 1961 and 1962. Lows in reproduction occurred in the summer and winter. Female white-footed mice followed a similar pattern in 1961, but breeding continued through the midsummer period of 1962. For P. boylii the average number of corpora lutea per ovulation was 3.40?+ 0.42; the mean number of embryos, 3.39?0.81; and placental scars averaged 3.28=?0.35. For P. leucopus the average number of corpora lutea. per ovulation was 4.46?1.24; the mean number of embryos, 3.74?+ 0.51; and placental scars averaged 4.32?0.97. Differences in litter size correlated to parent body length (age) or to the time of pregnancy during the reproductive season could not be demonstrated. Embryonic loss during gestation averaged between 5-10% of ova ovulated in both species. Total litter resorption was not observed. Differences in prenatal loss could not be correlated with body length (age) or with the time of pregnancy during the reproductive season. Transmigration of embryos from one uterine horn to the other was a common phenomenon in these Peromyscus. Synchrony of females in the occurrence of first pregnancy was observed at the onset of the breeding season in August, 1961, for P. boylii. A marked biannual testicular cycle occurred in adult P. boylii, demonstrated by the criteria of testis weight and length, seminal vesicle length, and testis histology. The spring peak occurred in February or March and the fall peak occurred in October during 1961 and 1962. Greatest sexual decline was recorded in June and December of these years. Use of the same criteria on adult P. leucopus revealed peak seasonal activity of the male reproductive tract in March and September, and lows in June and October of 1961. During the summer of 1962 full regression of testes did not occur and spermatogenesis returned to high levels by June. Regressi9n of testicular activity or resumption of spermatogenesis was not synchronous in males of either species; some lagged behind or preceded others by a month or more. The biannual nature of reproductive cycles of the brush mouse and white-footed mouse in Missouri cannot be explained satisfactorily on the basis of photoperiodism, environmental temperatures, or precipitation, although all three are probably of some. importance. For twelve litters of brush mice born in captivity the mean litter size was 2.80?0.41. A single nonlactating gestation period of 23 days was recorded, and the length of lactating gestation ranged from 26-32 days. The earliest age recorded for the appearance of spermatozoa in the testes of known-age P. boylii was 60 days. Studies of the reproductive cycles of several species of Peromyscus in California by Jameson (1953) and McCabe and Blanchard (1950) revealed one reproductive season from spring to fall followed by cessation of breeding during the winter months. In Florida, however, Pournelle (1952) recorded the greatest intensity of breeding for Peromyscus gossypinus in the winter months and little breeding during the summer. In the central United States no thorough study of 1 Present address: Department of Zoology, University of Wyoming, Laramie.