REPRODUCTIVE SUPPRESSION IN SOCIAL CRYPTOMYS DAMARENSIS COLONIES - A LIFETIME OF SOCIALLY-INDUCED STERILITY IN MALES AND FEMALES (RODENTIA, BATHYERGIDAE)

The Damaraland mole-rat, Cryptomys damafensis, is a highly social subterranean rodent that exhibits reproductive suppression. Inhibition of reproduction occurs in colonies of the Damaraland mole-rat such that only one female and one or two males are reproductively active. A strict breeding system is set up in which the sole reproductive female initiates courtship and selects her mate who is paired with her throughout the year. The present study has shown that in C. damarensis colonies, both adult and juvenile nonreproductive females are anovulatory and have low concentrations of oestrogen, 2.2 +/- 4.6 nmols/mmol creatinine (n = 133) throughout the year. In contrast, the reproductively active female has elevated concentrations of oestrogen during pregnancy, 95.0 +/- 151.0 nmols/ mmol creatinine (n = 17). The non-reproductive females are rendered at least temporarily infertile in that, instead of the secondary follicles maturing and ovulating, they develop into luteinizing follicles. This suppression of ovulation is maintained for as long as there is a reproductive female in the colony. These data support the behavioural findings that nonreproductive females are never involved in courtship or copulatory behaviour nor do they solicit male colony members. Thus a socially induced suppression of reproductive physiology is apparent in females. In the male Damaraland mole-rat, suppression of reproduction is due entirely to an inhibition of reproductive behaviour. The mean urinary concentration of testosterone of the reproductive male was 3.8 +/- 3.7 nmols/mmol creatinine (n = 25) and that of the five non-reproductive males 4.4 +/- 5.3 nmols/mmol creatinine (n = 114). All male mole-rats are capable of undergoing spermatogenesis and thus are physiologically capable of fertilizing the reproductive female. It appears, therefore, that reproductive suppression amongst non-reproductive female mole-rats is more profound than that which occurs in non-reproductive males. The latter are apparently restrained from sexual activity by being subordinate to the reproductive male(s), excluded from the pair bond and by being the progeny of the reproductive animals. Thus a predominantly behavioural control mechanism would be in force on male colony members.