Morphological and genomic shifts in mole-rat 'queens' increase fecundity but reduce skeletal integrity

In some mammals and many social insects, highly cooperative societies are characterized by reproductive division of labor, in which breeders and nonbreeders become behaviorally and morphologically distinct. While differences in behavior and growth between breeders and nonbreeders have been extensively described, little is known of their molecular underpinnings. Here, we investigate the consequences of breeding for skeletal morphology and gene regulation in highly cooperative Damaraland mole-rats. By experimentally assigning breeding 'queen' status versus nonbreeder status to age-matched littermates, we confirm that queens experience vertebral growth that likely confers advantages to fecundity. However, they also upregulate bone resorption pathways and show reductions in femoral mass, which predicts increased vulnerability to fracture. Together, our results show that, as in eusocial insects, reproductive division of labor in mole-rats leads to gene regulatory rewiring and extensive morphological plasticity. However, in mole-rats, concentrated reproduction is also accompanied by costs to bone strength.

Variation in growth of Damaraland mole-rats is explained by competition rather than by functional specialization for different tasks

In some eusocial insect societies, adaptation to the division of labour results in multimodal size variation among workers. It has been suggested that variation in size and growth among non-breeders in naked and Damaraland mole-rats may similarly reflect functional divergence associated with different cooperative tasks. However, it is unclear whether individual growth rates are multimodally distributed (as would be expected if variation in growth is associated with specialization for different tasks) or whether variation in growth is unimodally distributed, and is related to differences in the social and physical environment (as would be predicted if there are individual differences in growth but no discrete differences in developmental pathways). Here, we show that growth trajectories of non-breeding Damaraland mole-rats vary widely, and that their distribution is unimodal, contrary to the suggestion that variation in growth is the result of differentiation into discrete castes. Though there is no evidence of discrete variation in growth, social factors appear to exert important effects on growth rates and age-specific size, which are both reduced in large social groups.

Tolerance to unfamiliar conspecifics varies with social organization in female African mole-rats

Intolerance to familiar conspecifics characterises solitary mole-rats and distinguishes them from social ones. However, no study has compared the patterns of tolerance to unfamiliar conspecifics. Theoretically, both solitary and social species should react similarly and show intolerance to unfamiliar same-sex conspecifics, unless the evolution of grouping has favoured higher tolerance to conspecifics among the social species. Our study compares tolerance to unfamiliar conspecifics in four African mole-rat species exhibiting varying degrees of sociality. Dyadic encounters between female unfamiliar conspecifics were performed in a neutral arena, and the assessment of social tolerance was based on both behavioural observations of amicable contact behaviour as opposed to aggression and avoidance behaviour and the assessment of a stress response to the encounter measured as an increase in plasma cortisol concentrations. Our results show that the two highly social species and the solitary one presented similar high levels of agonistic behaviours during encounters with unfamiliar conspecifics. Nevertheless, all three social species displayed social tolerance and did not show a stress arousal during encounters with unfamiliar conspecifics, a pattern that contrasted significantly with that evidenced in the solitary species. The results suggest that physiological and behavioural characteristics allow a higher tolerance to unfamiliar conspecifics in the social as opposed to the solitary mole-rat, and the adaptive value of these characteristics are discussed. Finally, we discuss why constraints on social tolerance may be an important limiting factor to take into account in theories concerning the evolution of grouping.

A subterranean mammal uses the magnetic compass for path integration

Path integration allows animals to navigate without landmarks by continuously processing signals generated through locomotion. Insects such as bees and ants have evolved an accurate path integration system, assessing and coding rotations with the help of a general directional reference, the sun azimuth. In mammals, by contrast, this process can take place through purely idiothetic (mainly proprioceptive and vestibular) signals. However, without any stable external reference for measuring direction, path integration is highly affected by cumulative errors and thus has been considered so far as valid only for short-distance navigation. Here we show through two path integration experiments (homing and shortcut finding) that the blind mole rat assesses direction both through internal signals and by estimating its heading in relation to the earth's magnetic field. Further, it is shown that the greater the circumvolution and length of the traveled path, the more the animal relies on the geomagnetic field. This path integration system strongly reduces the accumulation of errors due to inaccuracies in the estimation of rotations and thus allows the mole rat to navigate efficiently in darkness, without the help of any landmark, over both short and long distances.

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.

Reproductive suppression in female Damaraland mole-rats Cryptomys damarensis: dominant control or self-restraint?

Colonies of Damaraland mole-rats Cryptomys damarensis exhibit a high reproductive skew. Typically one female breeds and the others are anovulatory. Two models, the dominant control model (DCM) and the self-restraint model (SRM), have been proposed to account for this reproductive suppression. The DCM proposes that suppression is under the control of the dominant breeder and is imposed by mechanisms such as aggression, pheromones and interference with copulation, whereas the SRM does not involve aggression directed towards non-breeders and may function in order to minimize inbreeding. We investigated potential proximate mechanisms involved in the suppression of females in a series of experiments. Socially induced stress through aggression did not appear to be responsible for anovulation. Nor did breeders actively interfere with subordinate copulation. Females were physiologically suppressed when housed in intact colonies. However, as predicted by the DCM, they did not become reproductively active when removed from the presence of breeders. We found no evidence that pheromonal cues block ovulation. We suggest that the SRM is the basic model found in the Damaraland mole-rat and that self-restraint functions in order to minimize inbreeding by restricting reproduction until an unrelated male is present. This would explain the rapid onset of reproductive activation in females when paired with an unrelated male, as demonstrated in this study.

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 1-3 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.

Social stress in neighboring and encountering blind mole-rats (Spalax ehrenbergi)

Blind mole-rats (Spalax ehrenbergi) are solitary aggressive subterranean rodents. They inhabit individual territories, comprised of branched tunnels. Each such tunnel system is completely separate from that of any neighboring mole-rat. Although intraspecific encounters between neighbors are infrequent, when they do occur, they may result in the injury or death of one or both animals. Avoidance of encounters may be due to the awareness of a neighbor's whereabouts through scent-marking and/or seismic (vibratory) communication. The present study was intended to examine whether encounters between individual mole-rats result in physiological stress. Two experimental conditions were designed to simulate natural situations: a brief encounter between two neighboring mole-rats, taking place either once or several times and long-term residency of neighbors whose only contact was either vibratory or vibratory plus odor communication. Blood samples were taken before, during, and after encounters in the first experiment and at set intervals in the second. The blood variables measured were blood glucose levels (BGL) and neutrophil/lymphocyte ratio (N/L). Blood glucose levels and neutrophil/lymphocyte ratio ratios increased in both members of encountering pairs. Long-term residency with a neighbor resulted in the establishment of a dominant-subordinate relationship through vibratory communication only, with increased neutrophil/lymphocyte ratio ratio in the subordinate males. However, long-term residency of males exposed to both vibrations and odors of neighboring males resulted in the death of both individuals. It seems that brief direct encounters and long-term neighboring conditions without physical contact are sufficient to cause severe stress to mole-rats. It is possible that in the wild, in some situations in which neighboring mole-rats cannot avoid constant exposure to each other's vibratory and odor signals, the consequent extensive stress may result in death.