Endocrine control of reproductive suppression in three Cryptomys hottentotus subspecies

Abstract

How social group cohesion is maintained in cooperatively breeding species has been the subject of a significant body of research. In many cooperatively breeding species, subordinate individuals are prevented from breeding through reproductive suppression either by behavioural or physiological means, or a combination of both mechanisms. Although much is known about this phenomenon, the underlying mechanisms that control reproductive suppression are still unclear. The African mole-rats (Bathyergidae) provide an ideal model for studies on reproductive suppression, as they display a wide range of social organisation and cooperative breeding strategies. Many mole-rat species are still understudied with respect to how their reproductive skew is maintained. Several endocrine correlates have been implicated in the maintenance of their reproductive skew. Prolactin was recently proposed to play an essential role in the physiological suppression of subordinate naked mole-rats (Heterocephalus glaber), as prolactin is known to inhibit the reproductive axis and promote cooperative behaviour. Stress hormones (glucocorticoids) are known to inhibit reproductive ability and have been shown to inhibit reproduction and promote cooperative behaviours of subordinates of some social mammals. Furthermore, testosterone (androgens) is associated with aggressive, dominant behaviour and is thought to be important in maintaining behavioural means of suppression in the form of aggressive interactions. In this study, I attempt to address gaps in our knowledge by investigating reproductive suppression in three understudied subspecies of the social mole-rat, Cryptomys hottentotus, namely C. h. natalensis (CHN), C. h. pretoriae (CHP) and C. h. mahali (CHM). To do this, endocrine correlates associated with behavioural and physiological reproductive suppression (prolactin, glucocorticoids and androgens) were investigated in the three subspecies. Previous work on the differences in pituitary sensitivity to GnRH and gonad metrics of these species have hinted at the possible contrasting mechanisms controlling reproduction in the three subspecies, but clear findings are yet to be published. Individuals of the three subspecies were captured at three sites for blood and faecal sampling. Plasma prolactin, faecal glucocorticoid metabolites (fGCMs) and faecal androgen metabolites (fAMs) were determined and compared between breeding and non-breeding males and females of the three subspecies. Comparing CHN and CHP revealed stark differences in reproductive suppression, as CHP showed evidence of physiological suppression through increased levels of circulating prolactin while CHN did not. A possible interactive role of prolactin and fGCMs were also highlighted from this comparison. The difference between these two closely related species led to the investigation of the same mechanisms in more detail in CHM, which revealed that this species also uses physiological suppression, again indicated through increased levels of circulating prolactin and fGCMs. To further investigate the link between prolactin and glucocorticoids, I tested the adrenal sensitivity of both non-breeding male and female CHM to adrenocorticotropic hormone (ACTH). I observed that non-breeding females, that possess higher prolactin levels than their male counterparts, are more sensitive to ACTH, reinforcing the link between prolactin, glucocorticoids, and physiological suppression. This thesis demonstrates how reproductive suppression differs between closely related subspecies and paves the way for similar studies to be done on other cooperatively breeding species. Furthermore, physiological reproductive suppression controlled by elevated prolactin may be important in many other taxa, which could help us understand how social group cohesion and reproductive skew is maintained.