Naked mole-rats are highly social rodents that live in large groups and exhibit a strict reproductive and social hierarchy. Only a few animals in each colony breed; the remainder are non-reproductive and are socially subordinate to breeders. We have examined androgen receptor immunoreactive (AR+) cells in brain regions comprising the recently described social decision-making network in subordinate and breeder naked mole-rats of both sexes. We find that subordinates have a significantly higher percentage of AR+ cells in all brain regions expressing this protein. By contrast, there were no significant effects of sex and no sex-by-status interactions on the percentage of AR+ cells. Taken together with previous findings, the present data complete a systematic assessment of the distribution of AR protein in the social decision-making network of the eusocial mammalian brain and demonstrate a significant role for social status in the regulation of this protein throughout many nodes of this network.

We previously reported that in a eusocial rodent, the naked mole-rat (Heterocephalus glaber), traditional neural sex differences were absent; instead, neural dimorphisms were associated with breeding status. Here we examined the same neural regions previously studied in naked mole-rats in a second eusocial species, the Damaraland mole-rat (Fukomys damarensis). Damaraland mole-rats live in social groups with breeding restricted to a small number of animals. However, colony sizes are much smaller in Damaraland mole-rats than in naked mole-rats and there is consequently less reproductive skew. In this sense, Damaraland mole-rats may be considered intermediate in social organization between naked mole-rats and more traditional laboratory rodents. We report that, as in naked mole-rats, breeding Damaraland mole-rats have larger volumes of the principal nucleus of the bed nucleus of the stria terminalis and paraventricular nucleus of the hypothalamus than do subordinates, with no effect of sex on these measures. Thus, these structures may play special roles in breeders of eusocial species. However, in contrast to what was seen in naked mole-rats, we also found sex differences in Damaraland mole-rats: volume of the medial amygdala and motoneuron number in Onuf's nucleus were both greater in males than in females, with no significant effect of breeding status. Thus, both sex and breeding status influence neural morphology in Damaraland mole-rats. These findings are in accord with the

Abstract Naked mole‐rats are eusocial rodents that live in large subterranean colonies in which one queen breeds with one to three males. All other animals are nonbreeding subordinates. The external features of male and female subordinates, including their genitalia, are remarkably monomorphic, as is their behavior. Because vasopressin (VP) is associated with social behaviors and sex differences in other species, its distribution in naked mole‐rats was of interest. We used immunohistochemistry to examine VP in the brains of subordinate and breeding naked mole‐rats of both sexes. As in other mammals, VP‐immunoreactive (‐ir) somata were found in the paraventricular (PVN) and supraoptic nuclei (SON) and VP‐ir projections from these nuclei ran through the internal and external zone of the median eminence. However, naked mole‐rats had very few VP‐ir cells in the bed nucleus of the stria terminalis (BST) and none in the suprachiasmatic nucleus (SCN); the extensive network of fine‐caliber VP‐ir fibers usually seen in projection sites of the BST and SCN were also absent. Equally unexpected was the abundance of large‐caliber VP‐ir fibers in the dorsomedial septum. VP immunoreactivity was generally similar in all groups, with the exception of VP‐ir cell number in the dorsomedial hypothalamus (DMH). Breeders had a population of labeled cells in the DMH that was absent, or nearly absent, in subordinates. Future studies on the function of VP in these areas are needed to determine how the

Damaraland and naked mole rat are eusocial mammals that live in crowded burrows in which CO2 is elevated. These species are thought to be highly tolerant of CO2 but their behavioural responses to hypercapnia are poorly understood. We hypothesized that Damaraland and naked mole rats would exhibit blunted behavioural responses to hypercapnia and predicted that their activity levels would be unaffected at low to moderate (2-5%) CO2 but increased at > 7% CO2. To test this, we exposed Damaraland and naked mole rats to stepwise increases in environmental CO2 (0-10%) and measured activity, exploratory behaviour, and body temperature. Surprisingly, we found that both species exhibited no differences in movement velocity, distance travelled, zone transitions (exploration), or body temperature at any level of environmental hypercapnia. Conversely, when carbonic anhydrase was inhibited with acetazolamide (50 mg kg-1 intraperitonially, to increase whole-animal acidosis), exploration was significantly elevated relative to hypercapnic controls in both species at all levels of inhaled CO2, and naked mole rat body temperature decreased in > 7% CO2. We conclude that both species are largely non-responsive to environmental CO2, and that this tolerance may be dependent on bicarbonate buffering at the level of the kidney or within the blood.

Naked mole-rats (Heterocephalus glaber) are small rodents native to east Africa, living in subterranean colonies of up to 300 individuals. Within each colony, reproduction is restricted to a single breeding female and 1-3 breeding males; all other colony members are reproductively suppressed and socially subordinate unless removed from the suppressive cues of the colony. Due to their striking reproductive skew, naked mole-rats are often considered eusocial mammals. Consistent with this idea, there are behavioral specializations and at least some evidence for morphological distinctions within and between the breeding and non-breeding members of the colony. Importantly, naked mole-rats show plasticity in their behavioral phenotype whereby changes in the social environment influence expression of both type and amount of social behavior. Thus, naked mole-rats provide the opportunity to examine the proximate mechanisms controlling individual differences in social behavior, shedding light on how mammals live in complex social groups.