Annual Production by a Cottontail Population

Adult females in a cottontail (Sylvilagus floridanus) population produced seven or eight litters during the 1961 breeding season. A synchrony of the conception dates was maintained throughout the breeding season. Data about ovulation rate, prenatal mortality, and litter size for each litter are presented. The first litter was smallest, averaging about three young, and the remaining litters ranged between five and six young. The last four litters may be slightly smaller than the second and third litters of the season. The total annual production of young by an adult female would be approximately 35. The purpose of this paper is to present information about the number of litters per year and litter size in a cottontail population. The number of litters produced annually by a female cottontail has been estimated variously from two (Allen 1938) to six (Schwartz 1942). The latter author concluded that in Missouri the average number of litters per female was 3.8. Figures for average litter size have also been given by various workers. Schwartz (1942) showed that litters were smallest at the beginning of the breeding season and largest during May and June. Lord (1961) also noted seasonal variation in litter size. Barkalow (1962) and Lord (1960) have reported that smaller litter size is a characteristic of the cottontail in the southern portions of its range. We wish to acknowledge the assistance provided by Faye Grogan and Leroy Heman in making collections of material at the James A. Reed Memorial Wildlife Area. Joan Clifford aided in preparation of laboratory materials. METHODS During 1961, rabbits were collected by shooting from wild populations on the Missouri Conservation Commission's James A. Reed Memorial Wildlife Area. This is a 1,800-acre management area in Jackson County near Kansas City, Missouri. It has a very productive soil and is intensively managed for upland game. During the late winter of 1960 this population suffered heavy mortality, and the onset of the reproductive season was delayed until late March (Wight and Conaway 1961). The population at the beginning of the breeding season in 1961 was still low. An initial collection of 15 female cottontails was made on March 17, 1961. All of these animals were lactating and had conceived again in a postpartum estrus. At the time of collection, 13 were in unimplanted stages and 2 in early implanted stages of their postpartum pregnancy. A second collection of 17 females was made on April 3 to obtain a sample in late gestation of the second pregnancy of the season. Fifteen of the 17 were between 20 days gestation and term. Periodic collections were made throughout the remainder of the year, as follows: (1) May 2, 17 pregnant females of which 1 This study is part of a cooperative project between the Department of Zoology, University of Missouri, and the Missouri Conservation Commission, supported in part by funds under Pittman-Robertson Project, Missouri 13-R-15. 2 Present address: Fish and Wildlife Service, Patuxent Wildlife Research Center, Laurel, Maryland.