D. Mating and Reproductive Behavior

Three classical reproductive phenomena of mice are thought to either not exist or to be much weaker in rats. Rats do not show the Bruce effect, in which implantation of embryos is delayed with the presence of pheromones from newly introduced males. Likewise, the Whitten effect, in which female mice undergo estrus synchronization when a male is introduced, is lacking in rats. The Lee-Boot effect, in which groups of females housed together become anestrous, is thought to exist in rats, although not as markedly as in mice (Sharp and LaRegina, 1998).

As stated previously, lordosis is a characteristic mating behavior of the female rat. Although both estrogen and progesterone are involved in the reflex, estrogen alone is sufficient to induce the behavior (Maeda et al., 2000). Testosterone is essential to the exhibition of mating behavior in male rats. Castrated males show no sexual behavior (Meisel and Sachs, 1994). Olfactory cues from pheromones are also critical to male sexual behavior (Nelson, 1995). Auditory stimuli play important roles in the reproductive behavior of both sexes. A 50-kHz vocalization is produced by both males and females during copulation. This call is produced by males in response to females in estrus, and by the female to solicit male attention (Maeda et al., 2000). The male rat also produces a 22-kHz vocalization in the post-ejaculatory refractory period (Barfield and Thomas, 1986).

Copulation in rats occurs most often during the latter portion of the dark cycle (Mercier et al., 1987). The male initiates mating behavior with genital sniffing of a female in estrus. The receptive female displays hopping and earquivering, resulting in male mounting, which in turns solicits lordosis from the female. Mounting behavior consists of combinations of intromission and ejaculation, including mounting without intromission; mounting with intromission but without ejaculation, frequently with a backward lunge; and mounting with intromission and ejaculation. An intromission typically consists of two to nine pelvic thrusts and lasts 0.3 to 0.6 seconds (Bennett and Vickery, 1970). Ejaculation is typically preceded by 3 to 44 intromissions, and a non-responsive refractory period occurs after ejaculation. Mating continues intermittently for up to three hours, until 3 to 10 ejaculations have been achieved.

Mating can be confirmed by the presence of sperm in a vaginal smear, by observation of a vaginal plug, or by direct observation of the mating behavior. The vaginal plug does not persist as long in the rat as it does in the mouse, and thus the lack of a plug is not a reliable indicator that mating did not occur. Vaginal smears that detect sperm-positive animals are commonly used to confirm mating, and a 90 to 94% correlation between sperm-positive vaginal smears and pregnancy has been reported (Baker, 1979).