Determination of Reproductive Rates in Rat Populations by Examination of Carcasses

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Abstract
This paper describes a method for calculating the reproductive rate in populations of the brown rat (Rattus norvegicus) as a basis for the study of population mechanics. To determine the stages of pregnancy, a normal table of the progress of pregnancy, based on conspicuous features, was prepd. from a series of albino rats in the laboratory. Pregnancy was divided into 4 phases (illustrated) which have in the wild brown rat durations of 6, 7, 5.5, and 5.5 days. Since the embryos are not visible during the 1st phase, a rat is visibly pregnant at autopsy for only 18 days. To determine the avg. frequency of pregnancy in a series of wild rats, the formula F = Ixt/18 is used, where F is frequency of pregnancy, I is incidence (% visibly pregnant), t is the length of time during which specimens were collected for the series. Birth rate equals F times avg. young per litter. In Baltimore, autopsies of 746 adult (over 250 gms.) wild 9 9 showed 28.4% visibly pregnant and avg. litter size of 9. Hence F is 3.44 which should be reduced by about 3% to account for intrauterine mortality. The pregnancy rate therefore is about 1 litter every 2 months or about 30 young born during the 218-day period.
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