Effect of changing cages, introduction of the male, and other procedures on the oestrus cycle of the rat.

Abstract

Observations were made on the oestrous cycles of 1358 female rats. In females housed at fairly high density in stock colony cages there were relatively few cycles of more than 4 days duration. The animals were changed to experimental cages at lower density and vaginal smears taken daily commencing 0,3, or 10 days later. In most instances, oral dosing with innocuous substances was commenced at the same time. There was a progressive reduction (from 9.7 to 4.6%) in the proportion of animals exhibiting cycles longer than 4 days with increasing intervals between re-caging and beginning to take vaginal smears. Males were introduced 1 week after commencing vaginal smearing. After the introduction of the male, the incidence of oestrus was not random: the number tended to be higher than expectation for the first 3 days (and especially for the first day), lower than expectation for the fourth day.