Bathyergus Suillus Research Articles

Nuclear organization and morphology of cholinergic, putative catecholaminergic and serotonergic neurons in the brains of two species of African mole-rat

The distribution, morphology and nuclear subdivisions of the cholinergic, putative catecholaminergic and serotonergic systems within the brains of two species of African mole-rat (Cape dune mole-rat – Bathyergus suillus; highveld mole-rat – Cryptomys hottentotus pretoriae) were identified following immunohistochemistry for acetylcholinesterase, tyrosine hydroxylase and serotonin. The aim of the present study was to investigate possible differences in the complement of nuclear subdivisions of these systems by comparing those of the mole-rats to published studies of other rodents. The mole-rats used exhibit a major reduction of the visual system and live a subterranean lifestyle. These wild caught animals also have differing social systems, the Cape dune mole-rat is strictly solitary whereas the highveld mole-rat occurs in social familial units. While these differences, especially that of phenotype, may lead to the prediction of significant differences in the nuclear complement of these systems, we found that all nuclei identified in all three systems in the laboratory rat and other rodents had direct homologs in the brains of the mole-rats studied. There were no additional nuclei in the brains of the mole-rats that are not found in the laboratory rat or other rodents and vice versa. The mole-rats are phylogenetically distant from the laboratory rat, but are still part of the order Rodentia. We conclude that changes in the nuclear organization of the systems studied appear to demonstrate a form of constraint related to the phylogenetic level of the order.

Craniometric Sexual Dimorphism and Age Variation in the South African Cape Dune Mole-rat (Bathyergus suillus)

Because absolute mammalian age is difficult to measure directly, various methods have been used for its estimation. Among these methods, the degree of molar eruption and wear are considered to be the most reliable indicators of relative age. We used the nature and extent of maxillary molar toothrow eruption and wear to assign individuals of the solitary South African endemic Cape dune mole-rat (Bathyergus suillus) collected from a single population on the grounds of Cape Town International Airport, Cape Town, South Africa, to 9 relative age classes. We then used cranial morphometric analysis, and for comparative purposes, an assessment of the nature and extent of variation in body mass and body length, to investigate the nature and extent of sexual dimorphism and age variation in this little-studied species of mole-rat. Both univariate and multivariate analyses distinguished relative age classes 2 and 3 from 6–9, but age classes 4 and 5 were intermediate between the 2 other age-class groupings, suggesting that individuals of age classes 4 and 5 may be at a point on a hypothetical growth curve where the curve begins to stabilize. Examination of our data showed the absence of sexual dimorphism in younger individuals of age classes 2–5, and its presence in older individuals of age classes 6–9. Together with a proposed study of microsatellites, our analyses may improve our understanding of the population structure of the Cape dune mole-rat.

IS THE CAPE DUNE MOLE-RAT, BATHYERGUS SUILLUS (RODENTIA: BATHYERGIDAE), A SEASONAL OR ASEASONAL BREEDER?

The Cape dune mole-rat (Bathyergus suillus) is a solitary, seasonally breeding, rodent mole. Body mass, reproductive tract morphometrics, ovarian histology, and plasma estrogen and progesterone concentrations were measured in females during 1 calendar year to elucidate if this species had an inclination toward seasonal breeding. Qualitative analyses of ovarian histology revealed that females retain the potential for ovulation and subsequent production of corpora lutea from early austral winter to spring (June–October). Seasonal differences were found in ovarian morphometrics and hormone concentrations that are associated with follicular activation from April and subsequent conceptions from June to November. Body mass, reproductive tract morphometrics, testicular histology, and plasma testosterone concentrations were obtained for males over the period of 1 calendar year. There was a gradual increase in seminiferous tubule diameter from May to October, which was mirrored by fluctuations in testosterone concentration. The current data support a strong seasonality to reproduction in the Cape dune mole-rat from April to October with a peak that is linked to the period of maximal rainfall within the distributional range of this species.

Size matters: genital allometry in an African mole-rat (Family: Bathyergidae)

Typically, sexually selected traits show positive allometry and high coefficients of variation (CV). To date, many studies on the allometry of genitalia have focused on insects. In addition, studies have largely ignored the potential for sexual selection on female genitalia, despite male and female structures presumably co-evolving. Insects tend to show negative allometry in both male and female genitalia, while in contrast, the few studies carried out in mammals (males only) show positive allometry. Reasons for these differences between the taxa still remain unclear. However, in mammals, three main mechanisms have been proposed for genital evolution, namely, sperm competition, female cryptic choice and sexual conflict. In the first such study that we are aware of, we examined intra-specific genital allometry in both males and females of a mammal, the subterranean solitary Cape dune mole-rat, Bathyergus suillus. We found positive allometry occurring in male genitalia, which is consistent with previous vertebrate studies. Similarly, we found that female genitalia also exhibited positive allometry further supporting the notion of co-evolution of male and female genitalia. Although it is difficult to distinguish between the forces or mechanisms determining this directional selection, we suggest that several reproductive advantages are incurred as a result of positive allometric relationship of the genitalia in B. suillus and such advantages are also likely in other subterranean mammals. Our study further highlights the differences in genital allometry across taxa.

The pituitary potential for opportunistic breeding in the Cape dune mole-rat, Bathyergus suillus

In this paper we investigated the effect of sex and season on baseline and post-GnRH challenge luteinising hormone (LH) levels in a solitary, seasonally breeding mole-rat. Circulating basal concentrations of luteinising hormone (LH) were found to differ significantly with season in both sexes. However, no significant difference was found in circulating basal LH concentration between the sexes either within or out of the breeding season. The magnitude of the LH response to an exogenous pharmacological overdose of GnRH both in and out of the breeding season in males and females respectively was not significant. This finding suggests that there is no down regulation of GnRH receptors on the pituitary during the non-breeding season. Cape dune mole-rats thus have the potential for opportunistic breeding outside of the typical breeding period. We argue that this represents an adaptation to limited and brief opportunities for mating in this xenophobic and aggressive species.

Penile morphology of African mole rats (Bathyergidae): structural modification in relation to mode of ovulation and degree of sociality

AbstractThe penile morphology and bacular structure of five species of African mole rat are described in relation to the method of ovulation, degree of sociality and polygynandry. We predicted that, with an increase in sociality, and a concomitant decrease in polygynandry and sperm competition, there would be a decrease in penis ornamentation (spinosity) and baculum size. In solitary species of African mole rat with marked seasonal reproduction and induced ovulation (Bathyergus suillus and Georychus capensis), males have numerous epidermal spines on the penis. Social, seasonally breeding, induced ovulating mole rats Cryptomys hottentotus natalensis possess less elaborate ornamentation in the form of small protrusions that are rounded at the apex. Two aseasonally breeding eusocial species with spontaneous ovulation Cryptomys damarensis and Heterocephalus glaber have ridges on the penis but lack any elaborate ornamentation. Baculae, however, showed a trend to become proportionally smaller in the solitary species. Our prediction that, with an increase in sociality and a move from induced to spontaneous ovulation, the amount and degree of penile ornamentation declines was, therefore, generally supported.

The Topography and Gross Anatomy of the Gastrointestinal Tract of the Cape Dune Mole‐rat (Bathyergus suillus)

The Cape dune mole-rat, Bathyergus suillus, is the largest truly subterranean rodent in the world and is restricted to sandy soil areas of the southern and South-western Cape Province. These animals are herbivorous, feeding mostly on the below ground portions of grass and geophytes. The present study was undertaken to explore whether there are any gross anatomical gastrointestinal adaptations associated with the herbivorous diet of the Cape dune mole-rat and to provide a basis for further investigations into the nutrition and ecology of this animal. The stomach was unilocular (simple) and the entire internal aspect displayed macroscopically visible glandular epithelium. The caecum was voluminous and arranged into a spiral of approximately one and a half turns. The tip of the caecum resembled a vermiform appendix and lay against the left lateral abdominal wall. The ascending colon consisted of proximal and distal portions, arranged in a large double loop and attached to each other by a fat-filled mesenteric fold. The looped, folded ascending colon filled the right side of the abdominal cavity, lying against the right dorsal, lateral and ventral abdominal wall. The internal structure of the ascending colon revealed a shallow groove, lined on either side by a row of papillae. The simple, glandular stomach in conjunction with a prominent caecum and a greatly enlarged ascending colon suggests that this animal is a hindgut fermenting herbivore, rather than a caecal fermenter, as is the case in most rodent species.

Middle and Later Stone Age large mammal and tortoise remains from Die Kelders Cave 1, Western Cape Province, South Africa

Die Kelders Cave 1, South Africa, has provided more than 150,000 taxonomically identifiable mammal and tortoise bones from Middle Stone Age (MSA) and Later Stone Age (LSA) deposits. Cape dune molerats dominate the mammal sample, and they appear to have been accumulated mainly by people during the LSA occupation and mainly by eagle owls in the MSA. In sharp contrast to the LSA fauna, the MSA sample contains extralimital ungulates that imply relatively moist, grassy conditions. The large mean size of the MSA molerats also points to greater humidity, while the large size of the gray mongooses implies cooler temperatures. The sum supports luminescence and ESR dates that place the MSA occupation within the early part of the Last Glaciation (global isotope stage 4). The Die Kelders ungulate bones support those from Klasies River Mouth in suggesting that MSA people obtained dangerous terrestrial prey much less frequently than their LSA successors, probably because MSA people lacked the bow and arrow and other projectile weapons. The Die Kelders tortoise bones constrain the extent of climatic change, since their abundance indicates that warm, dry days remained common, at least seasonally. The tortoises tend to be much larger in the MSA layers than in the LSA ones, suggesting that MSA people collected tortoises less intensively, probably because MSA populations were relatively sparse.

Identifying the Collector: Evidence for Human Processing of the Cape Dune Mole-Rat,Bathyergus suillus, from Blombos Cave, Southern Cape, South Africa

The remains of Cape dune mole-rats are commonly found in archaeological sites in the Cape but identifying the agents responsible for their introduction, namely humans, raptors or other carnivores, has until now been largely speculative. An examination of mole-rats caught and cooked by contemporary farmworkers in the southern Cape showed distinctive burning patterns on the incisors and premaxillae. Similar charring is observable on mole-rat post-cranial bones recovered from the Later Stone Age layers at Blombos Cave suggesting that mole-rats were brought to this site by humans, cooked in the ethnographically observed manner and eaten. Extending this study to other archaeological sites in the Cape may help identify the prehistoric collectors of these animals.

Coefficients of digestibility and nutritional values of geophytes and tubers eaten by southern African mole‐rats (Rodentia: Bathyergidae)

Subterranean rodents have high energy requirements when they are excavating their burrows. This study investigates the energy available to, and the efficiency with which it can be extracted by, four species of bathyergid mole‐rats fed natural diets ranging from the underground storage organs of geophytes to grass roots and leaves.The digestibility coefficients of geophytes ranged from 53% for the fibrous tuber of the gemsbok cucumber to 95–7% for corms and bulbs. One species, Bathyergus suillus whose diet consists of over 80% grass, had a digestibility coefficient of 87% on an all grass diet.All species had similar coefficients of digestibility of > 90% when fed on a uniform diet of sweet potato.Bulbs and corms had a low fibre content (3–3–4%), high calorific value (15–16kJ/g) and high digestibility coefficients (95–7–96%) and on this diet the mole‐rats maintained their body mass. Food of lower digestibility tended to have a high fibre content (8–2–45%) and, with the exception of B. suillus, although the mole‐rats consumed a greater quantity of food, they lost mass. The sweet potato had a low fibre content (4–1 %) but was energetically very similar to bulbs and corms (15–5 kJ/g).Geophytes which have low fibre contents are generally small (1–20 g), whereas geophytes with high fibre contents are much larger (30–2000 g) and often occur in more arid zones.

Allozyme variation and systematics of African mole-rats (Rodentia: Bathyergidae)

Patterns of allozyme variation were examined within and among eight taxa representing the five genera of endemic African mole-rats of the family Bathergidae. Populations of Georychus capensis and Cryptomys hottentotus exhibited greater degrees of intraspecific allozyme divergence than did populations of Cryptomys damarensis, Bathyergus suillus, or Heliophobius argenteocinereus. Past climatic cycles may have affected bathyergid genera differentially, resulting in the current differences among taxa in patterns of allozyme variation. Analyses of allozyme variation among genera consistently placed Heterocephalus glaber at the base of the bathyergid radiation. However, allozyme data did not resolve phylogenetic relationships among the remaining four mole-rat genera, suggesting that bathyergid genera may represent lineages of ancient origin. Phylogenetic relationships among genera may be difficult to determine due to the levels of genetic change that have accumulated within each lineage. Allozyme variation among genera, in conjunction with mitochondrial DNA nucleotide sequence data, lead to the conclusion that a revision in the taxonomy of the family Bathyergidae is warranted. We suggest that the naked mole-rat, Heterocephalus glaber, be placed in the subfamily Heterocephalinae, whereas the remaining four bathyergid genera be placed in the subfamily Bathyerginae.

Size variation in the Cape dune molerat ( Bathyergus suillus) and late Quaternary climatic change in the southwestern Cape Province, South Africa

Within their historic range at the southwestern tip of Africa, Cape dune molerats ( Bathyergus suillus) tend to be significantly larger in areas of higher rainfall. They also tend to be large in late Quaternary fossil samples associated with independent evidence for relatively moist climate and small in samples associated with evidence for relatively dry conditions. Together with sedimentologic/geomorphic and other faunal observations, fluctuations in fossil dune molerat size imply that the regional climate was very moist during an early late Quaternary interval corresponding to marine isotope stage 4 (74,000 to 59,000 yr B.P.) or perhaps to substage 5b or 5d (centered on 110,000 and 90,000 yr B.P., respectively.) It was comparably moist again from roughly 14,000 to 8,000 yr B.P., somewhat drier than at present from about 8000 until 4000 yr B.P., and mainly near the modern average after 4000 yr B.P. Together, the modern and fossil data suggest that dune molerat size is a reliable index of past precipitation, but it may not be useful for revealing extremely arid conditions because these seem to be associated with depositional/occupational gaps in most local paleontological/archeological sites.

Allozyme differentiation and systematics of the endemic subterranean mole rats of South Africa

We have analyzed allozyme differentiation, encoded by 20 putative gene loci, of three genera and four currently accepted species of Southern African mole rats, Bathyergidae ( Cryptomys hottentotus hottentotus, C. hottentotus natalensis, C. hottentotus damarensis, Georychus capensis, Bathyergus suillus and B. janetta). Two major results are indicated: (a) genetically the family reflects distinct homozygosity ( H=0.019, range 0.00–0.05) as is typical for underground fossorial mammals and (b) remarkable large genetic distances exist between the six taxa analyzed ( D= 1.865, range 0.547–2.917). These results suggest that homoselection operates in the underground ecotope in accordance with the niche width variation hypothesis, and that bathyergid systematics needs urgent revision, particularly the genus Cryptomys. The three subspecies of Cryptomys appear to be at least three different species, possibly even falling into two genera. The adaptive radiation of bathyergids started at least in Miocene times, but probably began earlier in the Oligocene as indicated by the fossil record.

The burrow systems and burrowing dynamics of the mole‐rats Bathyergus suillus and Cryptomys hottentotus in the fynbos of the south‐western Cape, South Africa

The configuration and dynamics of six Bathyergus suillus and one Cryptomys hottentotus burrow systems in coastal fynbos vegetation was studied for a year by monitoring mound production and excavating burrows. Bathyergus suillus is a large solitary mole‐rat and C. hottentotus is smaller and social. Our results are compared with those for other subterranean rodents.Most B. suillus burrows (15–22 cm in diameter) were 40–65 cm below ground to prevent their collapse in the seasonally softer and drier surface sand; the burrows were consequently situated deeper than most food plants. Cryptomus hottentotus burrows were narrower (5–7 cm) and shallower (<35 cm), facilitating food location but restricting these mole‐rats to more consolidated soils. Bathyergus suillus and C. hottentotus burrows were of similar length (up to 400 m) but in terms of biomass per metre, C. hottentotus and other small social mole‐rats have relatively longer systems. The mean home range of B. suillus (0.27 ha) was higher than most subterranean rodents, including C. hottentotus (0.16 ha), but their mean density (0.9 ha−1) and biomass (≏ 500 g ha−1) was low. Bathyergus suillus appear to remain resident in an area and to have an optimum burrow length, maintained by blocking old burrows with newly‐excavated soil. This blocking may promote rapid regeneration of food plants within the home range. Daily burrow extension (estimated from mound production alone) averaged 0.7 m for both mole‐rats. The maximum monthly burrow extension was 141 m for B. suillus and 47 m for C. hottentotus. All mole‐rats have a characteristically clumped and patchy distribution, reasons for this are discussed. Bathyergus suillus cats and uses as nesting a wide range of roots, leaves and stems but preferentially selects some plant species. It rarely stores food. Cryptomys hottentotus eats and stores geophytes. Reasons for these dietary differences and the possibility that mole‐rats act as agents of dispersal of geophytes are discussed.