REPRODUCTIVE SUPPRESSION IN EUSOCIAL CRYPTOMYS DAMARENSIS COLONIES - SOCIALLY-INDUCED INFERTILITY IN FEMALES

Inhibition of reproduction occurs in colonies of the Damaraland mole-rat (Cryptomys damnrensis), where one female and usually one male are reproductively active. They remain the sole reproductive animals throughout their stay in the colony, which in the field may exceed five years. An 18-month study on a captive colony of C. damarensis shows that non-reproductive females remain anovulatory and always have concentrations of urinary progesterone lower than that of the reproductive female 10.7 +/- 8.8 nmols/mmol creatinine (n = 85), although their progesterone concentrations are slightly elevated when the reproductive female is early in pregnancy. In contrast the reproductively active female has elevated concentrations of progesterone 63.3,70.1 nmols/mmol creatinine (n = 14). Standard histological together with immunohistochemical examination of the ovarian structure shows that follicular development in the non-reproductive females is halted at varying stages prior to ovulation and that the unruptured follicles luteinize. These unruptured luteinized follicles stain positive for 3 beta hydroxysteroid dehydrogenase-the enzyme catalysing the synthesis of progesterone from pregnenolone. It is possible that the low levels of circulating progesterone produced by the luteinized follicles in the non-reproductive females in the colony are sufficient to feedback on the hypothalamo-pituitary axis and prevent the surge of LH necessary for ovulation. Behavioural studies provide corroborative evidence for sexual suppression. Thus the nonreproductive females in the colony are never involved in courtship or copulatory behaviour, nor do they actively solicit males in the colony. The reproductive female, however, plays an active role in mate selection and is the initiator in courtship behaviour. The suppression of ovulation in non-reproductive females is maintained for as long as there is a reproductive female in the colony.