Exposure to pups influences the strength of maternal motivation in virgin female rats

Following repeated exposure to foster pups, virgin female rats acquire and eventually express a full spectrum of maternal caretaking behaviors directed toward pups. Though these behaviors are vigorous, these females are reportedly less motivated to seek out and interact with pups (i.e. maternally motivated) than parturient females during early postpartum. The present study systematically assesses how the length of pup-exposure and nature of interactions between the female-pup dyad affect maternal motivation in the virgin female rat. Virgin females were exposed to young pups consistently (24 h/day) across a prolonged period (21 days), briefly (1 h/day) across a relatively brief period (7 days), or distally (pups inaccessible in mesh bag). During final pup-exposure days, females were conditioned and tested for their preference for a pup-associated chamber (e.g. maternal motivation) using conditioned place preference. Early postpartum females provided a comparison group. Fully maternal behavior only emerged in females given prolonged pup-exposure; this behavior improved significantly over time and was maximally expressed for a duration equivalent to early postpartum. Females given brief pup-exposure expressed only emergent maternal behaviors initiated by pups; distal pup-exposure evoked pup-avoidance. Virgin females given prolonged or brief pup-exposure expressed substantial pup-associated chamber preference, with more females preferring the pup-associated chamber following longer pup-exposures in a subtle stepwise relationship. Maternal motivation was strikingly similar in prolonged pup-exposure virgin and early postpartum females. Females given distal pup-exposure completely lacked maternal motivation. Maternal behavior did not predict chamber preference. Results suggest that pup-exposure, regardless of length, is sufficient to support strong maternal motivation, whereas parity is not required.