A Graphical Method for Estimating Length of Gestation and Estrous Cycle Length from Birth Intervals in Rodents

Abstract

A new method is described to determine length of gestation and estrous cycle in rodents. The method was devised using successive birth intervals for female Rattus lutreolus lutreolus. This technique was tested using data collected later by conventional smearing techniques on the same R. I. lutreolus females. Further tests were made using similar information available in the literature for R. sordidus sordidus and R. tunneyi culmorum (McDougall, 1946), Notomys alexis (Smith et al., 1972), and R. I. velutinus (Green, 1967). The technique can be used only for polyestrous species and is most applicable when a pattern is clearly discernible in the distribution of intervals between births, but is also applicable if approximate estimates of the length of the estrous cycle are available to use with an iterative regression procedure. The method is particularly important when working with native species that may or may not have been bred in captivity before, and may be extremely sensitive to behaviorally stressful experiences such as daily capture for vaginal smearing. Little is known of the breeding biology of the Australian murid rodent Rattus lutreolus lutreolus. There is some limited information on breeding season in the field, from South Australia (Barritt, 1976; Taylor and Horner, 1973a) provided data on ovulation rate and embryo counts from histological studies of field-caught specimens of R. I. lutreolus as part of their review of reproduction in 13 of the 14 subspecies of Australian Rattus that they recognized (Taylor and Horner, 1973b). Breed (1978) examined estrous cycling and ovulation rates for R. I. lutreolus and seven other subspecies of Rattus. Green's (1967) study of the biology of R. I. velutinus provides some information on the number of embryos and litter size for this closely related subspecies. Rattus I. lutreolus proved difficult to breed in the laboratory in the first two years. To minimize disturbance new animals obtained in the third year did not have vaginal smears taken when handled daily. Breeding was then successful and 18 litters, from nine females, were raised for growth studies between October, 1977, and December, 1978 (Fox, 1979). The bulk of the reproductive data were then collected in the form of intervals between births of successive litters, with some observed matings available to supplement these data. Derivation and maintenance of the breeding colony of R. 1. lutreolus were described by Fox (1979). Females were inspected daily between 0800 and 1000 h. Each female was weighed, her reproductive condition noted, and presence of pups recorded. Observations of behavior of paired animals in their home cage were conducted at irregular intervals. A mating was recorded when the male was seen to mount the female and show repeated thrusting and intromission; ejaculation was confirmed by the presence of semen or a copulatory plug in the female. Additionally, mating was recorded when either a copulatory plug was observed at the daily inspection or, in the latter part of the project, when sperm were found in a vaginal smear.