C. G. Faulkes

Ecological constraints drive social evolution in the African mole–rats

The African mole–rats (family Bathyergidae) are subterranean hystricomorph rodents occurring in a variety of habitats and displaying levels of sociality which range from solitary to eusocial, making them a unique mammalian taxonomic group to test ecological influences on sociality. Here, we use an extensive DNA–based phylogeny and comparative analysis to investigate the relationship between ecology, sociality and evolution within the family. Mitochondrial cytochrome–b and 12s rRNA trees reveal that the solitary species are monophyletic when compared to the social species. The naked mole–rat (Heterocephalus glaber) is ancestral and divergent from the Damaraland mole–rat (Cryptomys damarensis), supporting previous findings that have suggested the multiple evolution of eusociality within the family. The Cryptomys genus is species–rich and contains taxa exhibiting different levels of sociality, which can be divided into two distinct clades. A total of seven independent comparisons were generated within the phylogeny, and three ecological variables were significantly correlated with social group size: geophyte density (p<0.05), mean months per year of rainfall greater than 25 mm (p&0.001), and the coefficient of rainfall variation (p=0.001). These results support the food–aridity hypothesis for the evolution of highly social cooperative behaviour in the Bathyergidae, and are consistent with the current theoretical framework for skew theory.

Dominance and queen succession in captive colonies of the eusocial naked mole–rat, Heterocephalus glaber

Naked mole–rat colonies exhibit a high reproductive skew, breeding being typically restricted to one female (the ‘queen’) and one to three males. Other colony members are reproductively suppressed, although this suppression can be reversed following the removal or death of the queen. We examined dominance and queen succession within captive colonies to investigate the relationship between urinary testosterone and cortisol, dominance rank and reproductive status; and to determine if behavioural and/or physiological parameters can be used as predictors of queen succession. Social structure was characterized by a linear dominance hierarchy before and after queen removal. Prior to queen removal, dominance rank was negatively correlated with body weight and urinary testosterone and cortisol titres in males and females.Queen removal results in social instability and aggression between high ranking individuals. Dominance rank appears to be a good predictor of reproductive status:queens are the highest ranking colony females and are succeeded by the next highest ranking females. The intense dominance–related aggression that accompanies reproductive succession in naked mole–rats provides empirical support for optimal skew theory.

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 familar 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.

Phylogeographical patterns of genetic divergence and speciation in African mole-rats (Family: Bathyergidae)

African mole‐rats are subterranean Hystricomorph rodents, distributed widely throughout sub‐Saharan Africa, and displaying a range of social and reproductive strategies from solitary dwelling to the ‘insect‐like’ sociality of the naked mole‐rat, Heterocephalus glaber. Both molecular systematic studies of Rodentia and the fossil record of bathyergids indicate an ancient origin for the family. This study uses an extensive molecular phylogeny and mitochondrial cytochrome b and 12s rRNA molecular clocks to examine in detail the divergence times, and patterns of speciation of the five extant genera in the context of rift valley formation in Africa. Based on a value of 40–48 million years ago (Myr) for the basal divergence of the family (Heterocephalus), we estimate divergence times of 32–40 Myr for Heliophobius, 20–26 Myr for Georychus/Bathyergus and 12–17 Myr for Cryptomys, the most speciose genus. While early divergences may have been independent of rifting, patterns of distribution of later lineages may have been influenced directly by physical barriers imposed by the formation of the Kenya and Western Rift, and indirectly by accompanying climatic and vegetative changes. Rates of chromosomal evolution and speciation appear to vary markedly within the family. In particular, the genus Cryptomys appears to have undergone an extensive radiation and shows the widest geographical distribution. Of the two distinct clades within this genus, one exhibits considerable karyotypic variation while the other does not, despite comparatively high levels of sequence divergence between some taxa. These different patterns of speciation observed both within the family and within the genus Cryptomys may have been a result of environmental changes associated with rifting.

Hormonal and behavioural correlates of male dominance and reproductive status in captive colonies of the naked mole–rat, Heterocephalus glaber

Naked mole–rat colonies are societies with a high reproductive skew, breeding being restricted to one dominant female (the ‘queen’) and 1 to 3 males. Other colony members of both sexes are reproductively suppressed. Experimental removal of breeding males allowed us to investigate the relationship between urinary testosterone and cortisol, dominance rank, and male reproductive status. Dominance rank was strongly correlated with body weight, age, and urinary testosterone titres in males. No relationship between urinary cortisol levels and male reproductive status or dominance was found. Breeding males were among the highest–ranking, heaviest and oldest males in their respective colonies, and were succeeded by other high–ranking, large, old colony males. In contrast to females, no evidence of competition over breeding status was observed among males. Male–male agonism was low both before and after removal of breeders and mate guarding was not observed. The lower reproductive skew for males compared with female skew or queen control over male reproduction may explain why males compete less strongly than females over breeding status after removal of same–sexed breeders.

Kin discrimination and female mate choice in the naked mole-rat Heterocephalus glaber

Naked mole–rats are fossorial, eusocial rodents that naturally exhibit high levels of inbreeding. Persistent inbreeding in animals often results in a substantial decline in fitness and, thus, dispersal and avoidance of kin as mates are two common inbreeding avoidance mechanisms. In the naked mole–rat evidence for the former has recently been found. Here we address the latter mechanism by investigating kin recognition and female mate choice using a series of choice tests in which the odour, social and mate preferences of females were determined. Discrimination by females appears to be dependent on their reproductive status. Reproductively active females prefer to associate with unfamiliar males, whereas reproductively inactive females do not discriminate. Females do not discriminate between kin and non–kin suggesting that the criterion for recognition is familiarity, not detection of genetic similarity per se. In the wild, naked mole–rats occupy discrete burrow systems and dispersal and mixing with non–kin is thought to be comparatively rare. Thus, recognition by familiarity may function as a highly efficient kin recognition mechanism in the naked mole–rat. A preference by reproductively active females for unfamiliar males is interpreted as inbreeding avoidance. These findings suggest that, despite an evolutionary history of close inbreeding, naked mole–rats may not be exempt from the effects of inbreeding depression and will attempt to outbreed should the opportunity arise.

Eusociality in African mole-rats: new insights from patterns of genetic relatedness in the Damaraland mole-rat (Cryptomys damarensis)

After the discovery of eusociality in the naked mole–rat, it was proposed that inbreeding and high colony relatedness in this species were the major underlying factors driving cooperative breeding in African molerats. By contrast, field and laboratory studies of the eusocial Damaraland mole–rat (Cryptomys damarensis) have raised the possibility that this species is an obligate outbreeder, although the build–up of inbreeding over several generations could still occur. Using microsatellite markers, we show that most breeding pairs in wild colonies of the Damaraland mole–rat are indeed unrelated (R = 0.02 ± 0.04) and that mean colony relatedness (R = 0.46 ± 0.01), determined across 15 colonies from three separate populations, is little more than half that previously identified in naked mole–rats. This finding demonstrates that normal familial levels of relatedness are sufficient for the occurrence of eusociality in mammals. Variation in the mean colony relatedness among populations provides support both for the central role played by ecological constraints in cooperative breeding and for the suggestion that inbreeding in naked mole–rats is a response to extreme constraints on dispersal. Approaches that determine the relative importance of an array of extrinsic factors in driving social evolution in African mole–rats are now required.

Matrilineal genetic structure within and among populations of the cooperatively breeding common marmoset, Callithrix jacchus.

Common marmosets are members of the family Callitrichidae, South American primates characterized by highly social group living and cooperative breeding. In this study we analysed 1112 base pairs (bp) of the mitochondrial control region in 59 Callithrix jacchus individuals, sampled mainly from two geographically distinct field sites in N.E. Brazil. Analysis of molecular variation revealed a highly significant genetic structuring of haplotypes between social groups and between populations. Examination of matrilineal genetic structure within social groups revealed that seven of nine recorded breeding pairs were from different maternal lineages, indicating assortative mating and outbreeding. In addition to the breeders, at least six of 10 groups contained adult individuals from different matrilines, with five haplotypes present in one social group of nine animals. Groups of mixed lineages raise questions about potential reproductive conflicts of interest, and the extent of kin‐selected altruism in the evolution and maintenance of cooperative breeding in this species.

Micro‐ and macrogeographical genetic structure of colonies of naked mole‐rats Heterocephalus glaber

Patterns of genetic structure in eusocial naked mole‐rat populations were quantified within and among geographically distant populations using multilocus DNA fingerprinting and mitochondrial DNA (mtDNA) sequence analysis. Individuals within colonies were genetically almost monomorphic, sharing the same mtDNA control region haplotype and having coefficients of band sharing estimated from DNA fingerprints ranging from 0.93 to 0.99. Family analysis of a hybrid captive colony of naked mole‐rats with increased levels of genetic variability using multilocus DNA fingerprinting gave results consistent with Mendelian inheritance, and has revealed for the first time that multiple paternity can occur. In a survey of wild colonies from Ethiopia, Somalia and locations in northern and southern Kenya, we have examined mtDNA control region sequence variation in 42 individuals from 15 colonies, and together with multilocus DNA fingerprinting and mtDNA cytochrome‐b sequence analysis in selected individuals have shown that these populations show considerable genetic divergence. Most of the variance in sequence divergence was found to be between geographical locations (Φct= 0.68) and there was a significant correlation between sequence divergence and geographical separation of haplotypes. Six colonies from Mtito Andei in southern Kenya shared the same control region haplotype, suggesting a recent common maternal ancestor. In contrast, out of four colonies at Lerata in north Kenya, three haplotypes were identified, and phylogenetic analysis suggests that this area may be a zone where two distinct lineages are in close proximity. Genetic distances were maximal between Ethiopian and southern Kenyan populations at 5.8% for cytochrome‐b, and are approaching interspecific values seen between other Bathyergids.

RNA sequencing reveals differential expression of mitochondrial and oxidation reduction genes in the long-lived naked mole-rat when compared to mice

The naked mole-rat (Heterocephalus glaber) is a long-lived, cancer resistant rodent and there is a great interest in identifying the adaptations responsible for these and other of its unique traits. We employed RNA sequencing to compare liver gene expression profiles between naked mole-rats and wild-derived mice. Our results indicate that genes associated with oxidoreduction and mitochondria were expressed at higher relative levels in naked mole-rats. The largest effect is nearly 300-fold higher expression of epithelial cell adhesion molecule (Epcam), a tumour-associated protein. Also of interest are the protease inhibitor, alpha2-macroglobulin (A2m), and the mitochondrial complex II subunit Sdhc, both ageing-related genes found strongly over-expressed in the naked mole-rat. These results hint at possible candidates for specifying species differences in ageing and cancer, and in particular suggest complex alterations in mitochondrial and oxidation reduction pathways in the naked mole-rat. Our differential gene expression analysis obviated the need for a reference naked mole-rat genome by employing a combination of Illumina/Solexa and 454 platforms for transcriptome sequencing and assembling transcriptome contigs of the non-sequenced species. Overall, our work provides new research foci and methods for studying the naked mole-rats fascinating characteristics.