Mating behaviour in the chinchilla

Abstract

Guinea pigs and chinchillas belong to the same suborder of rodents—the Hystrichomorpha; and their mating patterns have several significant similarities. In these common traits the two species differ from representatives of the Myomorpha such as the laboratory rat and mouse, deermouse and hamster. The differences have to do with inter relations between the male and female which result in the male's ejaculation. For the four myomorphic species ejaculation is preceded by a series of brief, closely-spaced penile insertions (rat, hamster, deermouse), or by a very large number of copulatory thrusts during a smaller number of intromissions (mouse). Female myomorphs in full behavioural oestrus are not often resistant to the male. Characteristically they receive multiple insertions and, in the rat, mouse and deermouse, successive ejaculations from the same or different males before losing their receptivity. Male chinchillas and guinea pigs frequently ejaculate during their first insertion after a few seconds of thrusting. It appears that when males take more than one intromission to ejaculate it is usually because the female has forced premature termination of coitus upon preceding insertions. The evidence reveals that many female chinchillas and guinea pigs are characteristically inonsistent in their willingness to receive the male. They often permit him to mount and then dislodge him just before or after the penis enters the vagina. In chinchillas and possibly in guinea pigs one ejaculation by the male frequently terminates the female's receptivity for several hours if not for the remainder of the oestrous period. It is here suggested that these differences in coital patterns may bear a functional relation to differences in female reproductive cycles. The guinea pig and chinchilla spontaneously exhibit a post-ovulatory luteal phase during which corpora lutea secrete progesterone and the uterus is prepared for implantation. Therefore insemination is the principal requirement for successful pregnancy. In contrast, for the rat, and probably for the other Myomorpha mentioned, secretion of effective amounts of progesterone depends upon cervicovaginal stimulation derived from the male's intromissions. Insemination without adequate stimulation leads to fertilization but not to pregnancy since secretion of progesterone is inadequate to permit implantation. It is proposed that consistent receptivity in myomorph females is essential for the male's successive pre-ejaculatory intromissions and these in turn are essential for successful impregnation. Since hystrichomorph females can be impregnated without extensive cervicovaginal stimulation there is in their case no obvious functional value to prolonged or persistent receptivity.