The oestrous cycle of the Damaraland mole‐rat revisited: evidence for induced ovulation

Kin recognition for incest avoidance in Damaraland mole-rats, Fukomys damarensis

Across taxa, breeding among close relatives is usually avoided because it incurs fitness costs to offspring. Incest is often averted through the dispersal of either sex from the natal area to breed. In some philopatric species, association among relatives extends into adulthood, and an ability to discriminate kin may be required for individuals to reduce inbreeding risk. Here, we aim to determine the mechanism of kin recognition for incest avoidance in the Damaraland mole-rat Fukomys damarensis, a cooperative breeder characterized by extreme reproductive skew. Pairs of opposite-sex adults were formed in the laboratory and, within pairs, genetic relatedness and degree of familiarity were manipulated through cross-fostering experiments. We found that unfamiliar pairs were more likely to engage in sexual behaviours and bred more successfully than familiar pairs, regardless of their genetic similarity. Females paired with unfamiliar males were also more likely to exhibit reproductive activation, characterized by increased levels of oestradiol and progesterone. This study shows that in Damaraland mole-rats, inbreeding avoidance can be achieved through a discrimination mechanism that relies on association during rearing, and that ovulation is induced by mating. This study advances our understanding of incest avoidance in species with constrained dispersal.

The role of ambient temperature and light as cues in the control of circadian rhythms of Damaraland mole-rat

Light is considered the primary entrainer for mammalian biological rhythms, including locomotor activity (LA). However, mammals experience different environmental and light conditions, which include those predominantly devoid of light stimuli, such as those experienced in subterranean environments. In this study, we investigated what environmental cue (light or ambient temperature (Ta)) is the strongest modulator of circadian rhythms, by using LA as a proxy, in mammals that experience a lifestyle devoid of light stimuli. To address this question, this study exposed a subterranean African mole-rat species, the Damaraland mole-rat (Fukomys damarensis), to six light and Ta cycles in different combinations. Contrary to previous literature, when provided with a reliable light cue, Damaraland mole rats exhibited nocturnal, diurnal, or arrhythmic LA patterns under constant Ta. While under constant darkness and a 24-hour Ta cycle mimicking the burrow environment, all mole-rats were most active during the coolest 12-hour period. This finding suggests that in a subterranean environment, which receives no reliable photic cue, the limited heat dissipation and energy constraints during digging activity experienced by Damaraland mole-rats make Ta a reliable and consistent "time-keeping" variable. More so, when providing a reliable light cue (12 light: 12 dark) to Damaraland mole-rats under a 24-hour Ta cycle, this study presents the first evidence that cycles of Ta affect the LA rhythm of a subterranean mammal more strongly than cycles of light and darkness. Once again, Damaraland mole-rats were more active during the coolest 12-hour period regardless of whether this fell during the light or dark phase. However, conclusive differentiation of entrainment to Ta from that of masking was not achieved in this study, and as such, we have recommended future research avenues to do so.

Pre- and postcopulatory competition affect testes mass and organization differently in two monophyletic mole-rat species, Georychus capensis and Fukomys damarensis

Sperm competition results from postcopulatory continuation of male-male competition for paternity. The level of sperm competition is predicted to be highest in species with greater polyandry and weakest in monogamous pairs. Sperm competition levels can be indexed using traits that reflect male investment in fertilization, particularly relative testes mass (RTM). However, the relationship between RTM and levels of sperm competition may also be influenced by precopulatory competition selecting for higher levels of testosterone, also produced by the testes. To test the relationship between RTM and both pre- and postcopulatory male-male competition we compared two bathyergid mole-rat species, the promiscuous Georychus capensis and the monogamous eusocial Fukomys damarensis. The promiscuous species had not only larger RTM, but also a greater proportion of spermatogenic tissue, maximizing germ cell production as well. Conversely, the eusocial species had smaller testes, but a higher proportion of interstitial tissue (which contains the androgenic Leydig cells) and higher levels of testosterone. Consequently, testicular traits as well as testes mass may be under selection, but these are not normally measured. More research is required on relative investment in different testicular traits in relation to both pre- and postcopulatory selection pressures.

Differential regulation of Kiss1 gene expression by oestradiol in the hypothalamus of the female Damaraland mole-rat, an induced ovulator

Kisspeptin, a product of the Kiss1 gene is considered a potent stimulator of gonadotropin release, by interacting with its receptor, the G protein-coupled receptor 54. Kiss1 neurons are known to mediate the positive and negative feedback effects of oestradiol on GnRH neurons that control the pulsatile and surge secretion of GnRH. While in spontaneously ovulating mammals the GnRH/LH surge is initiated by a rise in ovarian oestradiol secreted from maturing follicles, in induced ovulators, the primary trigger is the mating stimulus. Damaraland mole rats (Fukomys damarensis) are cooperatively breeding, subterranean rodents that exhibit induced ovulation. We have previously described in this species the distribution and differential expression pattern of Kiss1-expressing neurons in the hypothalamus of males and females. Here we examine whether oestradiol (E2) regulates the hypothalamic Kiss1 expression in a similar way as described for spontaneously ovulating rodent species. By means of in situ hybridisation, we measured Kiss1 mRNA among groups of ovary-intact, ovariectomized (OVX) and OVX females treated with E2 (OVX + E2). In the arcuate nucleus (ARC), Kiss1 expression increased after ovariectomy and decreased with E2 treatment. In the preoptic region, Kiss1 expression after gonadectomy was similar to the level of wild-caught gonad-intact controls, but was dramatically upregulated with E2 treatment. The data suggest that, similar to other species, Kiss1 neurons in the ARC, which are inhibited by E2, play a role in the negative feedback control on GnRH release. The exact role of the Kiss1 neuron population in the preoptic region, which is stimulated by E2, remains to be determined.

The best of both worlds: no apparent trade-off between immunity and reproduction in two group-living African mole-rat species

Co-operatively breeding mammals often exhibit a female reproductive skew and suppression of the subordinate non-breeding group members. According to evolutionary theory and the immunity-fertility axis, an inverse relationship between reproductive investment and survival (through immunocompetence) is expected. As such, this study investigated if a trade-off between immunocompetence and reproduction arises in two co-operatively breeding African mole-rat species, namely the Damaraland mole-rat (Fukomys damarensis) and common mole-rat (Cryptomys hottentotus hottentotus), which possess female reproductive division of labour. This study also attempted to investigate the relationship between the immune and endocrine systems in Damaraland mole-rats. There was no trade-off between reproduction and immunocompetence in co-operatively breeding African mole-rat species, and in the case of the Damaraland mole-rats, breeding females (BFs) possessed increased immunocompetence compared with non-breeding females (NBFs). Furthermore, the increased levels of progesterone possessed by Damaraland mole-rat BFs compared with NBFs appear to be correlated to increased immunocompetence. In comparison, BF and NBF common mole-rats possess similar immunocompetence. The species-specific differences in the immunity-fertility axis may be due to variations in the strengths of reproductive suppression in each species. This article is part of the theme issue 'Evolutionary ecology of inequality'.

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.

Social Isolation Does Not Alter Exploratory Behaviour, Spatial Learning and Memory in Captive Damaraland Mole-Rats (Fukomys damarensis)

Exploratory behaviour, spatial learning and memory affect the survival of animals and appear to be dependent on the specific habitat that a species occupies. Good spatial navigation and memory are particularly important for subterranean animals, as it is energetically expensive to inhabit this niche. Damaraland mole-rats are subterranean mammals that live in colonies with organised social structures. Damaraland mole-rats have been maintained in the laboratory for many years and can be housed in groups or individually. We evaluated the effect of social isolation on the exploratory behaviour and spatial memory of single-housed and colony-housed animals and also considered potential differences in animals with different social statuses. We predicted that solitary housing would increase anxiety-like behaviour and result in higher activity and more errors when solving a maze. Exploration by colony- and single-housed mole-rats was tested in an open-field test, where all individuals explored readily. Single-housed queens and non-breeding females showed increased activity and spent more time in tunnels, which can be explained by increased anxiety. In the Y-maze, improvements in solving the maze were observed in all experimental groups, except in single-housed non-breeding females. In addition, all males showed a decrease in the number of errors in the maze. Spatial learning is thus apparent but could not be conclusively proven. It was possibly underestimated, as magnetic cues that may be used by mole-rats as stimuli for navigation were removed in the experimental setup. Overall, it appears that social isolation has a limited effect on the exploratory behaviour and spatial learning of Damaraland mole-rats.

Socially Induced Infertility in Naked and Damaraland Mole-Rats: A Tale of Two Mechanisms of Social Suppression

Simple Summary

The naked and Damaraland mole-rats are group-living, subterranean mammals in which reproduction is distributed unequally among members of a social group, also referred to as reproductive skew. Only a single female per group, called the queen, produces offspring with the most dominant males of the group. The non-reproductive colony members are physiologically suppressed by the presence of the queen. This is reflected in their low concentration of luteinising hormone released from the pituitary and in their reduced responsiveness of the pituitary to stimulation with gonadotropin releasing hormone. Removal of the queen reverses these effects and leads to endocrine conditions in these females that are similar to those in reproductively active females. Regarding males, the extent of reproductive suppression is different between the two species. Non-reproductive male Damaraland mole-rats show hormonal profiles similar to the breeding males, whereas non-reproductive male naked mole-rats are physiologically suppressed similar to non-reproductive females. Thus, the two species represent ideal models to unravel the physiological, behavioural and neuroendocrine mechanisms regulating the hypothalamic-pituitary-gonadal axis. The recently discovered neuropeptides kisspeptin and RFamide-related peptide-3 are likely candidates to play an important role in the regulation of reproductive functions in the two mole-rat species.

Abstract

The naked mole-rat (Heterocephalus glaber) and the Damaraland mole-rat (Fukomys damarensis) possess extreme reproductive skew with a single reproductive female responsible for reproduction. In this review, we synthesize advances made into African mole-rat reproductive patterns and physiology within the context of the social control of reproduction. Non-reproductive female colony members have low concentrations of luteinising hormone (LH) and a reduced response of the pituitary to a challenge with gonadotropin releasing hormone (GnRH). If the reproductive female is removed from the colony, an increase in the basal plasma LH and increased pituitary response to a GnRH challenge arises in the non-reproductive females, suggesting the reproductive female controls reproduction. Non-reproductive male Damaraland mole-rats have basal LH concentrations and elevated LH concentrations in response to a GnRH challenge comparable to the breeding male, but in non-breeding male naked mole-rats, the basal LH concentrations are low and there is a muted response to a GnRH challenge. This renders these two species ideal models to investigate physiological, behavioural and neuroendocrine mechanisms regulating the hypothalamic-pituitary-gonadal axis. The recently discovered neuropeptides kisspeptin and RFamide-related peptide-3 are likely candidates to play an important role in the regulation of reproductive functions in the two mole-rat species.

The endocrine control of reproductive suppression in an aseasonally breeding social subterranean rodent, the Mahali mole-rat (Cryptomys hottentotus mahali)

Cooperative behaviour, sociality and reproductive suppression in African mole-rats have been extensively studied. Nevertheless, endocrine correlates of some species of social mole-rats have been neglected, and these species may hold the key to understanding the behavioural and physiological complexity that allows the maintenance of social groups in African mole-rats. In this study, we investigated endocrine correlates implicated in the suppression of reproduction and cooperative behaviours, namely glucocorticoids (a stress-related indicator) through faecal glucocorticoid metabolites (fGCMs), plasma testosterone (an indicator of aggression) and plasma prolactin in the Mahali mole-rat (Cryptomys hottentotus mahali) across reproductive classes (breeding females and males, non-breeding females and males) and season (wet and dry). Breeders possessed higher levels of testosterone than non-breeders. In reproductively suppressed non-breeding females, fGCMs were significantly higher than in breeders. Furthermore, an adrenocorticotropic hormone stimulation test (ACTH challenge test) on both male and female non-breeders revealed that female non-breeders show a more significant response to the ACTH challenge than males. At the same time, plasma prolactin levels were equally elevated to similar levels in breeding and non-breeding females. Chronically high levels of prolactin and fGCM are reported to cause reproductive suppression and promote cooperative behaviours in non-breeding animals. Furthermore, there was a negative relationship between plasma prolactin and progesterone in non-breeding females. However, during the wet season, a relaxation of suppression occurs through reduced prolactin which corresponds with elevated levels of plasma progesterone in non-breeding females. Therefore, prolactin is hypothesised to be the primary hormone controlling reproductive suppression and cooperative behaviours in non-breeding females. This study provides new endocrine findings for the maintenance of social suppression in the genus Cryptomys.

Gene expression pattern of Kisspeptin and RFamide-related peptide (Rfrp) in the male Damaraland mole-rat hypothalamus

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 anovulatory. Similarly, male subordinates within the colony show no sexual behaviour although they have functional gonads and do not differ from reproductive males in circulating levels of pituitary hormones and testosterone. However, reproductive status affects the neuroendocrine phenotype of males with breeders possessing increased mRNA expression of androgen and progesterone receptors compared to non-breeders in several forebrain regions implicated in the regulation of reproductive behaviour. The RFamide peptides kisspeptin and RFRP-3, encoded by the Kiss1 and Rfrp gene, are considered potent regulators of gonadotropin release. In females, reproductive inhibition is associated with reduced Kiss1 expression within the arcuate nucleus (ARC) and increased Rfrp expression in the anterior hypothalamus. To assess whether differential gene expression of Kiss1 and Rfrp underlies the difference in reproductive behaviour of males, we studied the expression of both genes by means of in situ hybridisation in wild-caught male Damaraland mole-rats with different reproductive status. The distribution of Kiss1 and Rfrp within the hypothalamus was found to be similar to females. Quantification of the Kiss1 hybridisation signal revealed no significant differences in relation to reproductive status. However, there was a significant positive correlation between testis mass and the number of Kiss1-expressing cells in the ARC and the mRNA content per cell, respectively. Analysis of the Rfrp hybridisation signal along the rostro-caudal extent of the hypothalamus revealed that non-reproductive males possessed an increased number of Rfrp-expressing cells at the level of the dorsomedial hypothalamic nucleus (DMH) than reproductive males. These data suggest the Kiss1 expression within the ARC is not associated with reproductive quiescence in subordinate males but instead, inhibitory effects may be mediated by Rfrp-expressing cells in the DMH.

Plasma oxidative stress in reproduction of two eusocial African mole-rat species, the naked mole-rat and the Damaraland mole-rat

One of the most prominent life-history trade-offs involves the cost of reproduction. Oxidative stress has been proposed to be involved in this trade-off and has been associated with reduced life span. There is currently an unclear relationship between oxidative cost and the reproduction-longevity trade-off. The current study, using a non-lethal and minimally invasive (only a single blood sample and no euthanasia) method, investigated whether an oxidative cost (oxidative stress) to reproduction would be apparent in two long-lived eusocial mole-rats, the naked mole-rat (NMR), Heterocephalus glaber, and the Damaraland mole-rat (DMR), Fukomys damarensis, where breeding colony members live longer than non-breeder conspecifics. We measured the direct redox balance in plasma by measuring the oxidative stress index (OSI) based on the ratio of total oxidant status and total antioxidant activity in breeders and non-breeders of both sexes, in the two species. NMR had significantly higher OSI between breeders and non-breeders of each sex, whereas DMR showed no significant differences except for total antioxidant capacity (TAC). The mode of reproductive suppression and the degree of reproductive investment in NMR may explain to some degree the redox balance difference between breeders and non-breeders. DMR show minimal physiological changes between breeders and non-breeders except for the mode of reproduction, which may explain some variations in TAC and TOS values, but similar OSI between breeders and non-breeders.

Queen pregnancy increases group estradiol levels in cooperatively breeding naked mole-rats

For cooperative species, there can be great value in the synchronization of physiological states to coordinate group behavioral states. This is evident in naked mole-rats (Heterocephalus glaber), which have the most extreme form of cooperative breeding in mammals. Colonies have a single reproductive female, "the queen", and 1-3 breeding males. These breeders are supported by adult "subordinates," which are all socially suppressed into a prepubertal state. Subordinates cooperate in colony maintenance, defense, and alloparental care. Prior work has reported that there may be social sharing of hormones among individuals in the colony because when the queen is pregnant, subordinates of both sexes develop enlarged nipples and female subordinates can develop vaginal perforation. We sought to document the hormonal changes and mechanisms behind these observations. We found that subordinate estradiol levels were elevated during the queen's pregnancy and were correlated with queen levels. To determine if this occurs by direct hormone-sharing, where group members uptake the hormones of conspecifics through excreta or the skin, we then tested whether treating a single subordinate in the colony with estradiol would induce the same effect in other colony members. It did not, which indicates that the influence on group estradiol levels may be specific to cues from the queen. These queen cues may be behavioral in nature, as we found that queens were less aggressive during pregnancy, which prior work has suggested may relax reproductive suppression of subordinates. Yet levels of queen aggression alone were not associated, or were weakly associated, with their colony's estradiol levels, though our sample size examining this particular relationship was low. This is suggestive that additional queen cues of reproductive status, beyond just aggression, may be relevant in influencing the subordinate hormonal change, or that the relationship between aggression and colony estradiol levels is more subtle and would need to be elucidated with a larger sample size. These results have implications for how cooperative breeders coordinate reproduction and alloparental care, and how social cues can influence individual and group physiology.