Sociosexual behaviors during the transition from non-receptivity to receptivity in rats housed in a seminatural environment.

Female behavioral estrus is defined as the period between the first lordosis displayed during the estrus cycle to the lordosis that is not followed by another within 60 min. In a seminatural environment, an estrous female consistently displays lordosis in response to every male mount from the start of behavioral estrus until the end of it. This means that the female suddenly changes from a state of complete non-receptivity to full receptivity and then abruptly changes back to non-receptivity. It is unlikely that these abrupt changes are caused by sudden changes in serum concentration of ovarian steroids. Here, we present the results of a detailed study of sociosexual behaviors during the transition from non-receptivity to receptivity and vice versa. The frequency of male mounting was close to zero before and after estrus. It remained at a constant, high level throughout estrus. Female paracopulatory behavior and male pursuit of the female increased drastically from a very low level before estrus to a high level during estrus. They returned to low levels immediately after estrus. None of the many other behavior patterns registered changed during the transitions. It appears that the sudden increase in male pursuit and female paracopulatory behavior can explain the beginning of behavioral estrus, and their equally sudden disappearance causes it to end. The neurochemical mechanisms behind these almost instantaneous behavioral changes are unknown.