Female Damaraland Mole-rats Research Articles

A comprehensive profile of reproductive hormones in eusocial Damaraland mole-rats (Fukomys damarensis)

In species where sociality and group cohesion are primarily determined by the maintenance of a reproductive division of labour and cooperative behaviours, the eusocial Damaraland mole-rat (Fukomys damarensis) presents a model which provides behavioural and endocrine distinctions between sex (males and females) and reproductive class (breeders and non-breeders). Although previous studies have demonstrated the endocrine aspects of reproductive suppression and behaviour in Damaraland mole-rats, they have focused on one hormone separately and on different conspecifics and samples across time. Unfortunately, this could introduce extrinsic biases when using these studies to compile complete hormonal profiles for comparisons. This study, therefore, set out to obtain a profile of the reproductive hormones from breeding and non-breeding male and female Damaraland mole-rats at a single point in time, from which circulating plasma prolactin and urinary progesterone, testosterone, and cortisol were measured. As expected, plasma prolactin and urinary cortisol did not differ between the breeders and non-breeders. However, breeders (both male and female) possessed increased urinary testosterone and progesterone concentrations compared to their non-breeding counterparts. These results, in conjunction with the variation in the expression of the respective hormonal receptors within the brains of breeders and non-breeders suggest that elevated testosterone and progesterone in breeders establish a neural dominance phenotype, which ultimately aids in controlling breeding activities. This study has emphasised the need for holistic, comprehensive profiling of reproductive endocrine systems.

Ovulation mitigates fatty liver associated with reproductive suppression and oxidative stress in Damaraland mole-rats (Fukomys damarensis).

Oxidative damage is often linked to reproduction; however, reproducing female Damaraland mole-rats (Fukomys damarensis) exhibit a reduction in oxidative damage relative to their non-reproductive, anovulatory, cohorts. Specifically, liver concentrations of malondialdehyde, a biomarker for lipid peroxidation, are significantly lower in reproducing females. We examined liver histology in reproductive, anovulatory and recently ovulating non-reproductive females, demonstrating an accumulation of lipid droplets only in the livers of anovulatory females and no fibrosis, cell death or inflammatory infiltrates in any group. Our observations suggest that anovulatory females experience a form of non-alcoholic fatty liver disease, which is reversed once they commence ovulation. We propose hormonal interactions that may underlie our observations.

Reproductive status-dependent dynorphin and neurokinin B gene expression in female Damaraland mole-rats

Damaraland mole rats (Fukomys damarensis) are cooperatively breeding, subterranean mammals, which exhibit high reproductive skew. Reproduction is monopolized by the dominant female of the group, while subordinates are physiologically suppressed. The blockade of reproduction results from an inhibition of ovulation, which is caused by inadequate secretion of luteinizing hormone (LH) from the pituitary, which in turn might be brought about by a disruption of the normal GnRH secretion from the hypothalamus. The neuropeptides dynorphin and neurokinin B are expressed together with kisspeptin in a subpopulation of neurons in the arcuate nucleus (ARC). This neuron population is termed KNDy neurons and is considered to constitute the GnRH pulse generator. To assess whether dynorphin (encoded by the Pdyn gene) and neurokinin B (NKB, encoded by the Tac3 gene) are involved in the mechanism of reproductive suppression we investigated the distribution and gene expression of Pdyn and Tac3 by means of in situ hybridisation in wild-caught female Damaraland mole-rats with different reproductive status. In both reproductive phenotypes, substantial Pdyn expression was found in several brain regions of the telencephalon including the cerebral cortex, the striatum, the hippocampus, the amygdala and the olfactory tubercle. Within the hypothalamus Pdyn expression occurred in the paraventricular nucleus, the dorsomedial nucleus, the supraoptic nucleus, the ventromedial nucleus and the ARC. Prominent Tac3 expression was found in the habenula, the bed nucleus of the stria terminalis, the cerebral cortex, the striatum, the hippocampus, the amygdala, the dorsomedial nucleus, the ARC and the lateral mammillary nucleus. Quantification of the gene expression levels in the ARC revealed decreased Pdyn and increased Tac3 expression in breeding compared to nonbreeding females. This suggests that both neuropeptides play a role in the regulation of reproduction in Damaraland mole-rats. Their exact role in mediating the inhibition of GnRH release in nonbreeding females remains to be determined.

Reproductive status affects the expression of prolactin receptor mRNA in the brain of female Damaraland mole-rats

The eusocial Damaraland mole-rat (Fukomys damarensis) represents an extreme example of reproductive skew, in that reproduction is completely blocked in female subordinate group members. It is thought that in these animals normal GnRH secretion from the hypothalamus is disrupted. Prolactin, a peptide hormone secreted from the anterior pituitary gland, has been implicated in a wide variety of functions. Well documented in rodents is its role in mediating lactational infertility. Elevated circulating prolactin levels, such as during lactation, are associated with reduced GnRH release into the portal blood and with a reduction in the frequency and amplitude of LH pulses. The present study aimed at investigating whether such a mechanism could act in reproductively suppressed female Damaraland mole-rats. By means of in situ hybridisation we studied the distribution and gene expression of the prolactin receptor (Prlr) in wild-caught female Damaraland mole-rats with different reproductive status. Substantial Prlr expression was found in several brain regions, with highest levels in the choroid plexus and moderate expression in the preoptic and tuberal hypothalamus. While in reproductive and non-reproductive females plasma prolactin levels were very low and not significantly different, quantification of the Prlr hybridisation signal revealed significant differences in relation to reproductive status. Reproductively suppressed females had increased expression of Prlr in the choroid plexus and in the arcuate nucleus (ARC) when compared to reproductive females. This suggests higher local prolactin levels in the brain of suppressed females. Together with previous findings, it could indicate that prolactin inhibits ARC kisspeptin neurons, which then would lead to reduced activation of GnRH neurons in such females.

Reproductive status-dependent kisspeptin and RFamide-related peptide (Rfrp) gene expression in female Damaraland mole-rats.

Damaraland mole rats (Fukomys damarensis) are cooperatively breeding, subterranean mammals that exhibit a high reproductive skew. Reproduction is monopolised by the dominant female of the group, whereas subordinates are physiologically suppressed to the extent that they are anovulatory. In these latter animals, it is assumed that normal gonadotropin-releasing hormone secretion from the hypothalamus is disrupted. The RFamide peptides kisspeptin (Kiss1) and RFamide-related peptide-3 (RFRP-3) are considered as potent regulators of gonadotropin release. To assess whether these neuropeptides are involved in the mechanism of reproductive suppression, we investigated the distribution and gene expression of Kiss1 and Rfrp by means of in situ hybridisation in wild-caught female Damaraland mole-rats with different reproductive status. In both reproductive phenotypes, substantial Kiss1 expression was found in the arcuate nucleus and only few Kiss1-expressing cells were detected in the anteroventral periventricular nucleus (AVPV), potentially as a result of low circulating oestradiol concentrations in breeding and nonbreeding females. Rfrp gene expression occurred in the dorsomedial nucleus, the paraventricular nucleus and the periventricular nucleus. While in female breeders and nonbreeders, plasma oestradiol levels were low and not significantly different, quantification of the hybridisation signal for both genes revealed significant differences in relation to reproductive status. Reproductively active females had more Kiss1-expressing cells and a higher number of silver grains per cell in the arcuate nucleus compared to nonreproductive females. This difference was most pronounced in the caudal part of the nucleus. No such differences were found in the AVPV. Furthermore, breeding status was associated with a reduced number of Rfrp-expressing cells in the anterior hypothalamus. This reproductive status-dependent expression pattern of Kiss1 and Rfrp suggests that both neuropeptides play a role in the regulation of reproduction in Damaraland mole-rats. Enhanced long-term negative feedback effects of oestradiol could be responsible for the lower Kiss1 expression in the arcuate nucleus of reproductively suppressed females.

Gnrh mRNA expression in the brain of cooperatively breeding female Damaraland mole-rats.

The Damaraland mole-rat (Fukomys damarensis) is a eusocial, subterranean rodent, in which breeding is limited to a single reproductive pair within each colony. Non-reproductive females, while in the confines of the colony, exhibit socially induced infertility. Anovulation is thought to be caused by a disruption in the normal gonadotropin-releasing hormone (GNRH) secretion from the hypothalamus. To assess whether social suppression is associated with altered Gnrh mRNA expression in the brain, we investigated the distribution and gene expression levels by means of in situ hybridization in female breeders and non-breeders from field captured colonies of the Damaraland mole-rat. We found expression of Gnrh mRNA as a loose network in several forebrain areas of female Damaraland mole-rats with the majority of labelling in the preoptic and anterior hypothalamus. The distribution matched previous findings using immunocytochemistry in this and other social mole-rat species. Quantification of the hybridisation signal revealed no difference between breeding and non-breeding females in the average optical density of the hybridization signal and the size of the total area covered by Gnrh mRNA. However, analysis along the rostro-caudal axis revealed significantly elevated Gnrh mRNA expression in the rostral preoptic region of breeders compared to non-breeders, whereas the latter had increased Gnrh mRNA expression at the caudal level of the anterior hypothalamus. This study indicates that social suppression affects the expression of Gnrh mRNA in female Damaraland mole-rats. Furthermore, differential regulation occurs within different neuron subpopulations.

Reproduction is associated with a tissue-dependent reduction of oxidative stress in eusocial female Damaraland mole-rats (Fukomys damarensis).

Oxidative stress has been implicated as both a physiological cost of reproduction and a driving force on an animal's lifespan. Since increased reproductive effort is generally linked with a reduction in survival, it has been proposed that oxidative stress may influence this relationship. Support for this hypothesis is inconsistent, but this may, in part, be due to the type of tissues that have been analyzed. In Damaraland mole-rats the sole reproducing female in the colony is also the longest lived. Therefore, if oxidative stress does impact the trade-off between reproduction and survival in general, this species may possess some form of enhanced defense. We assessed this relationship by comparing markers of oxidative damage (malondialdehyde, MDA; protein carbonyls, PC) and antioxidants (total antioxidant capacity, TAC; superoxide dismutase, SOD) in various tissues including plasma, erythrocytes, heart, liver, kidney and skeletal muscle between wild-caught reproductive and non-reproductive female Damaraland mole-rats. Reproductive females exhibited significantly lower levels of PC across all tissues, and lower levels of MDA in heart, kidney and liver relative to non-reproductive females. Levels of TAC and SOD did not differ significantly according to reproductive state. The reduction in oxidative damage in breeding females may be attributable to the unusual social structure of this species, as similar relationships have been observed between reproductive and non-reproductive eusocial insects.

Breeding status and social environment differentially affect the expression of sex steroid receptor and aromatase mRNA in the brain of female Damaraland mole-rats.

IntroductionThe Damaraland mole-rat (Fukomys damarensis) is a eusocial, subterranean mammal, which exhibits an extreme reproductive skew with a single female (queen) monopolizing reproduction in each colony. Non-reproductive females in the presence of the queen are physiologically suppressed to the extent that they are anovulatory. This blockade is thought to be caused by a disruption in the normal gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus. In order to understand the underlying physiological mechanisms of reproductive suppression in subordinate females we studied the expression of steroid hormone receptors and the androgen-converting enzyme aromatase in forebrain regions involved in the control of reproductive behaviour in female breeders and non-breeders from intact colonies. Additionally, we included in our analysis females that experienced the release from social suppression by being removed from the presence of the queen.ResultsWe found expression of androgen receptor, estrogen receptor α and aromatase in several forebrain regions of female Damaraland mole-rats. Their distribution matches previous findings in other mammals. Quantification of the hybridisation signal revealed that queens had increased expression of androgen receptors compared to non-breeders and removed non-breeders in most brain regions examined, which include the medial preoptic area (MPOA), the principal nucleus of the bed nucleus of the stria terminalis (BSTp), the ventromedial nucleus of the hypothalamus (VMH), the arcuate nucleus (ARC) and the medial amygdala (MeA). Furthermore, breeders had increased estrogen receptor α expression in the anteroventral periventricular nucleus (AVPV) and in the MeA, while aromatase expression in the AVPV was significantly reduced compared to non-breeders. Absence of social suppression was associated with increased androgen receptor expression in the ARC, increased estrogen receptor α expression in the MeA and BSTp and reduced aromatase expression in the AVPV.ConclusionThis study shows that social suppression and breeding differentially affect the neuroendocrine phenotype of female Damaraland mole-rats. The differential expression pattern of estrogen receptor α and aromatase in the AVPV between breeders and non-breeders supports the view that this region plays an important role in mediating the physiological suppression in subordinate females.

Teasing apart socially‐induced infertility in non‐reproductive female Damaraland mole‐rats, Fukomys damarensis (Rodentia: Bathyergidae)

The Damaraland mole-rat is a subterranean mammal exhibiting extreme reproductive skew with a single reproductive female in each colony responsible for procreation. Non-reproductive female colony members are physiologically suppressed while in the colony, exhibiting reduced concentrations of plasma luteinizing hormone (LH) and a decreased response of the pituitary, as measured by the release of bioactive LH, to an exogenous dose of gonadotrophin releasing hormone (GnRH). Removal of the reproductive female from the colony results in an elevation of LH and an enhanced response of the pituitary to a GnRH challenge in non-reproductive females comparable to reproductive females, implying control of reproduction in these individuals by the reproductive female. The Damaraland mole-rat is an ideal model for investigating the physiological and behavioral mechanisms that regulate the hypothalamo-pituitary-gonadal axis. In contrast, we know less about the control of reproduction at the level of the hypothalamus. The immunohistochemistry of the GnRH system of both reproductive and non-reproductive female Damaraland mole-rats has revealed no significant differences with respect to morphology, distribution or numbers of immunoreactive GnRH perikarya. We examined whether the endogenous opioid peptide beta-endorphin was responsible for the inhibition of the release of the GnRH from the neurons indirectly by measuring LH concentrations in these non-reproductive females following single, hourly and 8 hourly injections of the opioid antagonist naloxone. The results imply that the endogenous opioid peptide, beta-endorphin, is not responsible for the inhibition of GnRH release from the perikarya in non-reproductive females. Preliminary data examining the circulating levels of cortisol also do not support a role for circulating glucocorticoids. The possible role of kisspeptin is discussed.

Morphological divergence of breeders and helpers in wild Damaraland mole-rat societies.

The specialization of body shape to an individual's role within society represents a pinnacle of social evolution. Although commonplace among social insects, divergence in the body shapes of breeders and helpers has to date been documented in just one social vertebrate, the naked mole-rat, Heterocephalus glaber; an extraordinary species in which large colony size and frequent inbreeding may have favored the evolution of such specialization. Here, we present new evidence of morphological divergence between breeders and helpers in the Damaraland mole-rat, Fukomys damarensis; a much less socially extreme species that reflects an independent evolutionary origin of sociality. Using longitudinal data from wild populations, we show that dominant female Damaraland mole-rats, like many social insect queens, have a significantly more elongate body shape than subordinates. This difference arises not from a pre-existing difference in the body shapes of subordinates that do, and those that do not, become dominant, but from a modification to the growth trajectory of subordinates on dominance acquisition. Our findings reveal a wider role for morphological divergence within vertebrate societies and, as Damaraland mole-rats neither live in unusually large groups nor inbreed, suggest that circumstances favoring the evolution of such specializations may be more widespread among vertebrates than previously supposed.

Do dispersing non-reproductive female Damaraland mole-rats, Cryptomys damarensis (Rodentia: Bathyergidae) exhibit spontaneous or induced ovulation?

The Damaraland mole-rat is a eusocial, subterranean rodent that exhibits a seasonal breeding. Non-reproductive females show physiological suppression of reproduction whilst in the confines of the natal colony. This study set out to investigate whether dispersing female Damaraland mole-rats exhibit induced or spontaneous ovulation. Fifteen non-reproductive females were removed from their natal colonies and housed individually for a period of 6 weeks. During this period urine was collected from all animals every second day. After this initial period the animals were divided into 3 groups. Females were subjected to 1 of 3 trials: a control group housed separately without a male, allowed non-physical contact, or placed in direct physical contact with vasectomized males. Urine was collected for a further 5 weeks, and urinary progesterone profiles established. All three groups showed a significant difference in urinary progesterone concentrations between the two treatment periods indicating initiation of follicular development in all animals following removal from the colony. Histological results further revealed that at least one animal in the control group and five of the six animals in the vasectomized group had corpora lutea present in the ovaries showing that ovulation has taken place. All groups had similar numbers of Graafian follicles, indicating that all females were likely to ovulate in the near future. This finding suggests that females are capable of spontaneous ovulation, but the act of coitus may advance the onset of ovulation in this arid dwelling mole-rat.

A neuroanatomical and neuroendocrinological study into the relationship between social status and the GnRH system in cooperatively breeding female Damaraland mole-rats, Cryptomys damarensis.

The gonadotrophin-releasing hormone (GnRH) system in female Damaraland mole-rats, Cryptomys damarensis, has been investigated to map the distribution of GnRH-immunoreactive (GnRH-IR) structures in the brain of this species and to assess whether changes in this system may mediate the inhibitory effect of social cues on fertility. The distribution of GnRH-IR cell bodies and fibres was similar to that of other mammals, forming a loose continuum along a septo-preoptico-infundibular pathway. GnRH-IR cell bodies were more abundant in the vicinity of the organum vasculosum of the lamina terminalis than in the medial basal hypothalamus. GnRH-IR cells and fibres were also found in the subfornical organ. The cell bodies were typically unipolar or bipolar. No differences were found in the morphology or size of the cell bodies or in the number of cells between non-reproductive females and reproductive females living together in a colony. However, GnRH concentrations, measured in the brain by radioimmunoassay, were significantly higher in non-reproductive females than in reproductive females; this finding was complemented by the reduced immunoreactivity for GnRH in the median eminence and proximal pituitary stalk of reproductive females. In contrast, the concentrations of GnRH measured by radioimmunoassay in non-reproductive and reproductive males did not differ. These results are consistent with the hypothesis that GnRH release is inhibited in the non-reproductive females but not in the non-reproductive males of this species.

Rainfall, dispersal and reproductive inhibition in eusocial Damaraland mole‐rats (Cryptomys damarensis)

AbstractNon‐reproductive female Damaraland mole‐rats Cryptomys damarensis that were caught before a period of good rainfall (during which 90% of the average annual fell) (Group 1), exhibited a significantly lower pituitary sensitivity to gonadotrophin‐releasing hormone compared to non‐reproductive females (Group 2) caught in the same area, close to the end of the wet period. Group 2 were also significantly heavier than Group 1. Pituitary sensitivity was not significantly correlated to body mass within either group of females, nor within groups of reproductive males and non‐reproductive males from a laboratory held colony. This suggests that rainfall may have resulted in the simultaneous, but unrelated, increase in pituitary sensitivity and body mass. Larger size and reduced sexual inhibition assist dispersal and the probability of successful independent reproduction, during periods when environmental constraints on dispersal are relaxed. These findings support the hypothesis that low rainfall may be an important constraint on dispersal and an important factor promoting the evolution of reproductive inhibition, and consequently eusociality, in this species.

Social Suppression in Nonreproductive Female Damaraland Mole-rats, Cryptomys damarensis: No Apparent Role for Endogenous Opioid Peptides

The role of endogenous opioid peptides (EOPs) on LH secretion was examined to investigate the neuronal mechanisms responsible for the inhibition of GnRH and the resultant infertility in nonreproductive female Damaraland mole-rats, Cryptomys damarensis. The endorphin antagonist naloxone was administered to five groups of females to determine its effect on plasma LH levels: Grouping was determined by social status, social environment, and whether the females were ovariectomized. A single injection of naloxone had no significant effect on LH secretion in either intact or hystero-ovariectomized females. Multiple injections with naloxone failed to affect basal LH concentrations but did result in a decrease in GnRH-stimulated LH secretion in ovariectomized nonreproductive and reproductive females. A significant response to a single naloxone injection following GnRH priming was obtained in both nonreproductive females and in nonreproductive females housed in the absence of the reproductive pair. These results suggest EOPs play a role in sexual function but that socially induced infertility is unlikely to be mediated through the EOP system.

Anovulation in non-reproductive female Damaraland mole-rats (Cryptomys damarensis)

Within colonies of Damaraland mole-rats (Cryptomys damarensis), anovulation in non-reproductive females is thought to play an important role in maintaining reproductive skew. Pituitary sensitivity and ovarian structure were examined in three groups of females that differed with respect to their social environment and breeding status to determine whether anovulation is due to inhibitory social cues or is merely the result of a lack of copulatory stimulation. The contribution of gonadal steroid negative feedback to neuroendocrine differences in the reproductive systems of the respective groups was also investigated. LH secretion after a 0.5 micrograms GnRH challenge in females that had been removed from the presence of the breeding individuals for at least 6 months (removed non-reproductive females) was significantly higher than in non-reproductive females in the colony, but significantly lower than in reproductive females. In both removed non-reproductive females and reproductive females, corpora lutea were observed in ovaries of seven of eight females, indicating that ovulation occurs spontaneously in subordinate females on removal from the breeding pair. Circulating progesterone concentrations in removed non-reproductive females were significantly higher than in non-reproductive females, indicating that circulating progesterone is not responsible for infertility in non-reproductive females. Indeed, after hystero-ovariectomy, reproductive females continued to show significantly greater GnRH-stimulated LH secretion than non-reproductive females. Thus, differential inhibition of gonadotrophin secretion in breeding and non-breeding females occurs independently of gonadal steroids. It is concluded that female Damaraland mole-rats are spontaneous ovulators and that anovulation results from inhibitory social cues within the colony, not a lack of copulatory stimulation. Since non-reproductive females are infertile, inhibition of the hypothalamo-pituitary-gonadal axis has the potential to play a causal role in maintaining reproductive skew in colonies of C. damarensis.

Reproductive suppression in subordinate, non-breeding female Damaraland mole-rats: two components to a lifetime of socially induced infertility.

The Damaraland mole-rat Cryptomys damarensis exhibits an extreme reproductive division of labour. Reproduction in the colony is restricted to a single breeding pair, resulting from a two-fold control: (i) a reduced pituitary synthesis and/or secretion of luteinizing hormone leading to a block to ovulation in non-reproductive females; and (ii) a strong inhibition to breeding with familiar kin. Circulating basal concentrations of luteinizing hormone as well as luteinizing hormone levels measured in response to a single exogenous gonadotrophin releasing hormone challenge, were significantly lower in non-reproductive females in the presence of the reproductive female than those in colonies lacking a reproductive female. Urinary progesterone concentrations before the removal of the reproductive female were significantly higher in non-reproductives than the post removal values. Behavioural studies from sib-sib and non-sib pairings provide evidence for a strong incest avoidance, probably resulting from an inhibition of breeding with familiar colony members. A total of four pairings of non-sibs resulted in copulatory activity and eventual conception. In contrast, four couples of sib-sib combinations failed to produce any sexual activity or offspring. Thus, suppression of reproduction in these non-reproductive, subterranean bathyergids is complicated by the masking effect of familiarity that prevents incest, in addition to the physiological inhibition of fertility in the presence of the reproductive female.