The interplay between social living and the haematological, endocrinological and immunological parameters of the eusocial Damaraland mole-rat, Fukomys damarensis

Abstract

The Damaraland mole-rat is a eusocial, subterranean African bathyergid that exhibits both an extreme reproductive skew and a reproductive division of labour, whereby the breeding female has the highest survivorship. Reproduction is monopolised by a single breeding female (BF), the queen, and a few of the most dominant, non-related males (breeding males; BM’s). The remainder of the colony are reproductively quiescent through physiological suppression and inbreeding avoidance. As an induced ovulator, the absence of copulation as an effect of inbreeding avoidance in non-breeding females (NBF’s) ensures that the reproductive system is physiologically downregulated, in terms of anovulation and a pituitary with reduced sensitivity. Comparatively, non-breeding males (NBMs) are only behaviourally prevented from breeding due to their obligate outbreeding status. However, recent studies have found neuroendocrine divergences between the breeding and non-breeding males that may affect the presence or absence of mating behaviours. As such, the degree of reproductive activity and readiness in each of the reproductive classes (BF, BM, NBF and NBM) has engendered unique endocrine and metabolic profiles. Unfortunately, previous studies that have elucidated these endocrine profiles in Damaraland mole-rats have used different sampling periods whereby season, diet and colony dynamics were not standardised. This study therefore set out to rectify any possible bias by using a synchronous single sampling event, obtained at a single point in time to clarify the testosterone, cortisol, progesterone, and prolactin concentrations in all Damaraland mole-rat reproductive classes. It was found that breeding individuals have higher progesterone and testosterone concentrations than their non-breeding counterparts, in addition to higher testosterone: cortisol ratios, indicating their hierarchical dominance. This is a novel finding specifically in male Damaraland mole-rats as this is the first set of work that has identified a significant difference in the testosterone concentrations between breeding and non-breeding males. Furthermore, this is the first study that has characterised the progesterone concentrations in male Damaraland mole-rats and has thus found that breeding males have similar progesterone concentrations to those recorded in non-breeding females, despite the anatomical differences. The marked reproductive skew in Damaraland mole-rats has resulted in a distinct division of labour between the reproductive classes. Due to their extensive cooperative care duties in the form of pup rearing, foraging, burrow maintenance and colony defence, non-breeding members of the colony perform 95% of the annual labour in Damaraland mole-rat colonies. Previous studies have calculated this activity budget through behavioural, metabolic, and thermoregulatory experiments, whereby the daily energetic expenditure of breeders is 20-30% less than that of the non-breeding individuals. In this study, we thus set out to further clarify the physiological mechanisms underlying the reproductive division of labour experienced in Damaraland mole-rats through determining the erythropoietic indices and thus oxygen-carrying capacity of the blood of each breeding class and sex. Erythropoiesis is the production of mature red blood cells and is characterised by high haemaglobin, haematocrit and red cell count volumes. High endurance levels and performance of activities is both enabled by, and in turn stimulates the synthesis of erythrocytes. Correspondingly, adrenal and sex steroid hormones are capable of alternately enhancing and/or suppressing red blood cell production. Therefore, through the use of complete blood counts and erythropoietic indices, this study demonstrated that non-breeding Damaraland mole-rats that are intrinsically more active, have higher erythrocytic oxygen-carrying capacities. All individuals used in this study were captive-bred and thus had never performed digging activities; as such, the physiological differences observed have likely resulted from inherent biological differences. Additionally, there was a significant relationship between erythropoietic indices and progesterone, whereby the higher progesterone concentrations expressed in breeding Damaraland mole-rats was reflected by lower haemoglobin, haematocrit, and red cell count volumes. The innate immune response is the first line of defence against pathogenic invasions and relies on both physical and chemical barriers, in addition to leukocytic constituents such as neutrophils, monocytes, lymphocytes, basophils, and natural killer cells that phagocytise and destroy foreign bodies. As the endocrine and immune axes are closely linked, leukocyte synthesis, maturation and differentiation can be influenced by adrenal- and reproductively relevant hormones. Previous studies have indicated immune-suppressing roles of testosterone and progesterone on immunity, while prolactin and oestrogen have stimulatory effects on the immune response. Additively, biomedical research on the closely related, only other eusocial mammal, the Naked mole-rat (Hetercephalus glaber), has indicated specific resistance to tumorigenesis and infection as an enabling factor contributing to the elongated lifespan of naked mole-rat queens. This, in addition to the clear physiological and behavioural distinctions between reproductive classes, making Damaraland mole-rats an optimal model to further clarify the inter-related role of reproduction and its resultant hormones on the innate immunity of an immuno-divergent species. This study therefore set out to characterise the innate immunity of Damaraland mole-rats and whether their reproductive state, and thus their respective endocrine profiles influence their individual immunity and resultant survival. Complete blood cell counts were used to quantify the white blood cell components of each Damaraland mole-rat reproductive class, while microbial killing assays were used to determine the bacteriostatic and/or microbiocidal action of their blood. Despite the heavy role of testosterone in common-place immune responses, there were no immunological differences between the breeding and non-breeding Damaraland mole-rat males. There was, however, a negative correlation between the platelet: white cell count ratio (PWR) and both the testosterone concentration in males and their T/C ratio. Comparatively, despite their extremely low concentrations of prolactin, there was a significant correlation between prolactin levels in male Damaraland mole-rats and their immunologically related leukocytic parameters. These findings are in opposition to the central tenets of both the life history theory and the immunocompetence handicap hypothesis (ICHH). Contrastingly, while Damaraland mole-rat queens have been shown to have extended longevity despite the energetic cost of reproduction, very little is known about the immunological mechanisms that enable this extended survival. Through the same immunological measurements as the males above, Damaraland mole-rat breeding females had stronger immune responses than non-breeding females in terms of the haematological antimicrobial capacity and leukocytic ratios. Similarly, Damaraland mole-rat queens had lower body mass indexes (BMI’s) and Lee indexes, indicating they were in a healthier state and had higher survivorship ability than the subordinate non-breeding counterparts. Interestingly, testosterone, progesterone, BMI and the Lee index were each significantly correlated to microbial killing ability of female Damaraland mole-rat blood. This therefore indicates that the individuals with the highest testosterone and progesterone concentrations, the queens, were both the healthiest in terms of BMI and Lee’s index, but also had the highest haematological antimicrobial capacity This study has therefore further elucidated the physiological divergences and underlying mechanisms between reproductive and labour classes in Damaraland mole-rats with regards to their endocrine, haematological, and immunological profiles.