Abstract:
The aim of this study was to elucidate the proximate mechanisms, both social and physiological, responsible for the anovulation observed in non-reproductive, female Damaraland mole-rats (Cryptomys damarensis). To identify the social cues involved in the inhibition of fertility, the histological and endocrine response of the female reproductive axis to different social environments, was investigated. The presence of corpora lutea and increased circulating progesterone concentrations indicates that nonreproductive females ovulate spontaneously when they are housed in the absence of the breeding pair. Since anovulation is due to the inhibition of fertility rather than a lack of copulatory stimulation, it has the potential to play a role in maintaining reproductive skew. The endocrine and neuroendocrine mechanisms responsible for anovulation were also investigated. In comparison to reproductive females (RF), the pituitaries of nonreproductive females (NRF) were found to be desensitised to exogenous gonadotrophin-releasing hormone (GnRH) and contained less luteinizing hormone. However, GnRH priming did not increase pituitary sensitivity. Although there was no apparent difference in the distribution and morphology of GnRH-immunoreactive structures in the brain, a significantly greater concentration of GnRH was found in the brains of NRF compared to RF. This supports the hypothesis that an inhibition of GnRH release, and consequent desensitisation of the pituitary, is responsible for infertility in NRF. Differences in pituitary sensitivity between RF and NRF were apparent following hystero-ovariectomy, in the absence of gonadal steroids. Cortisol concentrations did not differ between RF and NRF. Thus, a gonadal steroid independent pathway, that does not involve the negative effects of stress, appears to be responsible for the inhibition of gonadotrophin secretion. A non-steroidal mechanism does not appear to be mediated through an increased activity of endogenous opioid peptides, since the opioid antagonist, naloxone, had no effect on LH secretion. This study shows that many of the pathways leading to anovulation are common to different forms of natural infertility.
