Reproductive suppression and the seasonality of reproduction in the social Natal mole-rat ( Cryptomys hottentotus natalensis)

Abstract

Natal mole-rats are social subterranean rodents which exhibit a reproductive division of labour. Reproduction is confined to a single breeding female and one or more males; the remainder of the colony members is reproductively suppressed by the presence of the breeding animals. Apart from the discovery that female Natal mole-rats are induced ovulators, little is known about the reproductive biology of this species. Natal mole-rats are closely related to common and highveld mole-rats, both of which are induced ovulators and seasonal breeders. We therefore postulated that reproduction in Natal mole-rats is seasonally regulated. However, the results indicate that dominant Natal mole-rats are able to reproduce in the winter as well as in the summer. Furthermore, the increment in plasma LH in response to GnRH does not show marked seasonal differences in reproductive or non-reproductive mole-rats of either sex. Nevertheless, in all reproductive categories the level of plasma LH is significantly higher in the winter than in the summer. Seasonality in plasma LH levels dissociated from seasonality in breeding seems paradoxical. Further investigations will be required to elucidate this finding. We also investigated the processes underlying socially regulated reproduction in this species by determining basal and GnRH-evoked plasma LH in reproductive and non-reproductive animals of each sex in both seasons. The results failed to identify neuroendocrine differences consistent with an inhibited reproductive state in subordinates of either sex. Thus, the present findings suggest that behavioural interactions and/or inbreeding avoidance are the principal factors underlying suppression of reproduction in subordinate Natal mole-rats.