Physiology &Behavior, Vol. 52, pp. 603-608, 1992 0031-9384/92 $5.00 + .00 Printed in the USA. Copyright © 1992PergamonPressLtd. Social Modulation of Behavioral Reproductive Senescence in Female Rats JUDITH A. LEFEVRE ~ AND MARTHA K. McCLINTOCK 2 Department of Psychology, The University of Chicago, Chicago, IL 60637 Received l l February 1991 LEFEVRE, J. A. AND M. K. McCLINTOCK. Social modulation of behavioral reproductivesenescence infemale rats. PHYSIOL BEHAV 52(3) 603-608, 1992.--During aging, the lordosis reflex of female rats changes from a state of cyclic lordosis, when the lordosis reflex can be elicited by manual palpation only on proestrus, to a state of constant lordosis (CL), when it can be elicited daily. Social signals from other females altered this pattern of behavioral senescence. Group living decreased the lordosis reflex intensity in both old and young rats and delayed the onset of CL. Long-term group living delayed CL until late in the life span, while temporary group living delayed it only until rats returned to isolation. Long-term group living also changed the temporal relationship between CL and the acyclic, anovulatory state of constant estrus. Finally, the lordosis reflex intensity, measured by manual palpation, predicted the old rats' receptivity to a male, indicating that constant lordosis is a biomarker for other aspects of behavioral reproductive senescence. Aging Sexual behavior Rats Social environment Social isolation Lordosis reflex REPRODUCTIVE senescence has typically been described in terms of changes in vaginal cytology as well as hypothalamic, pituitary, and ovarian function. These neuroendocrine changes are associated with a sequence of reproductive states in the female rat: regular estrous cycles, irregular cycles, constant estrus (the acyclic state which only half of females enter), persistent diestrus, and finally anestrus (2,9,13,20). Less frequently, reproductive senescence has been described in terms of female sexual behavior. During aging, the temporal coordination between sexual re- ceptivity and the ovarian cycle changes. Receptivity is charac- terized by a lordosis reflex in response to male mounts, a be- havioral posture which facilitates intromission (24). Young rats are receptive only on proestrus, just prior to ovulation, maxi- mizing the likelihood of conception (15,22). Middle-aged rats that are in constant estrus, as defined by vaginal cytology, are often receptive repeatedly on consecutive days (5,12). Finally, old rats that are in persistent diestrus (e.g., repetitive pseudo- pregnancies) are rarely receptive, if at all (5). A similar pattern, of increase and then decrease, emerges when the lordosis reflex is elicited by manual palpation and measured longitudinally throughout reproductive senescence (13). The frequency of the lordosis reflex increases until the reflex can be elicited on every day of the cycle, the state of constant lordosis. Then the frequency of the lordosis reflex decreases, until the reflex can no longer be elicited. In isolated rats, this process of behavioral reproductive se- nescence precedes and predicts changes in the other components of the estrous cycle. As the lordosis reflex increases in frequency, it decouples from the other estrous-cycle components that are still cycling. For example, when the lordosis reflex becomes acyclic, rats still have regular or irregular cycles in vaginal cy- tology (13). In fact, the onset of constant lordosis predicts when rats will stop cycling and enter constant estrus, an acyclic, an- ovulatory state (13,14). In young rats, social signals from other females enhance the temporal coordination of the lordosis reflex with other estrous cycle components. They modulate the frequency and intensity of the lordosis reflex during the estrous cycle, as well as modu- lating cycle length, regularity, and the frequency of ovulation ( 1,6,17,18). However, their effect on behavioral reproductive se- nescence is unknown. There is some evidence that the effect of social signals on behavior declines during aging. For example, social signals from males have less effect on the approach be- havior of old than young females (4). It is unknown, however, whether social signals from other females continue to modulate the lordosis reflex of old rats, just as they modulate that of young rats (14,17). Thus, one purpose of this research was to determine whether living in groups with other females affects the lordosis reflex of old rats to the same extent that it affects that of young rats. If social signals continue to modulate the lordosis reflex during aging, they may delay the onset of constant lordosis, the acyclic behavioral state. However, there may be a limit to how long social signals can delay constant lordosis, just as there is a limit 1Present address: Andrus Gerontology Center, Leonard Davis School of Gerontology, University of Southern California, University Park, MC- 0191, Los Angeles, CA 90089-0191. 2 Requests for reprints should be addressed to Dr. Martha K. McClintock, Department of Psychology, The University of Chicago, 5730 South Woodlawn Ave., Chicago, IL 60637. 603