The reproductive biology of the Cape mole-rat, Georychus capensis (Rodentia, Bathyergidae)

Georychus capensis is a seasonal breeder with the reproductive potential of producing two litters during the summer breeding season (August to December). These solitary mole-rats signal to each other through the soil, by drumming with their hind feet. This drumming is probably important in spacing the burrow systems and in triggering the onset of reproductive behaviour. In a captive male the onset of drumming was accompanied by a rise in urinary testosterone concentrations and the enlargement of testes and accessory reproductive glands. The male appeared to drum with a different frequency to the female. Courtship is initiated by the male and copulation involves brief multiple intromissions. The gestation period is about 44 days and the mean litter size is 5.9 with a maximum of 10 pups. Development of the pups is relatively rapid. Inter-sibling aggression begins to develop at 35 days, eventually resulting in the pups dispersing when about 60 days old. Body mass increases exponentially from birth to day 60 and the asymptote is reached around day 260. These features are compared with those of other solitary subterranean rodents and with those of the social Bathyergidae ( Cryptomys hottentotus, Cryptomys damarensis and Heterocephalus glaber ).     

Variation in the digging apparatus of the subterranean silvery mole-rat, Heliophobius argenteocinereus (Rodentia, Bathyergidae): the role of ecology and geography

The skull of most subterranean tooth-digging rodents is markedly affected by their digging mode. In the present study, we investigated the cranial variation in a strictly subterranean, highly specialized Afrotropical tooth-digger, Heliophobius argenteocinereus (Bathyergidae, Rodentia), using a geometric morphometric approach and evaluated the effect of different factors on size and shape differences among four populations. No evidence for sexual dimorphism was found in skull size or shape. The cranial shape variation was large and influenced mainly by the type of habitat (miombo woodland versus farmland and grassland) and the latitudinal gradient. The dorsal side of the skull appears to be more plastic and adaptable to local environments, as well as more independent of size, than the ventral side. Only the shortening of the rostrum is presumably an adaptive process independent of size that leads to an increase of efficacy of the tooth-digging apparatus in Heliophobius , whereas the increase in the in-force and the more procumbent incisors both comprise size-related changes caused by ontogenetic allometric growth.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 822–831.     

Parasites in social subterranean Zambian mole-rats (Cryptomys spp., Bathyergidae, Rodentia)

Three out of 18 examined common mole-rats ( Cryptomys sp., Xkaryotype 2n = 58) from Zambia were infested with cestodes ( Inermicapsifer nmdagascariensis and an undetermined species) and a nematode ( Protospirura muricola ). Four out of 14 examined Zambian giant mole-rats ( Cryptomys mechowi ) hosted cestodes ( Raillietina (R) sp. and an undetermined species) and nematodes ( Protospirura muricola, Capillaria sp.). Helminths and circumstances of findings are briefly described. No ectoparasites were found. The influences of subterranean and social way of life and feeding habits upon occurrence of parasites in mole-rats are discussed. Burrow conditions and social behaviour seem not to favour infestation by parasites.     

Developmental Plasticity in the Ossification of the Proximal Femur of Heterocephalus glaber (Bathyergidae,Rodentia)

The proximal femoral morphology in rodents of different body sizes, locomotor modes, and from the three main rodent lineages (Sciuromorpha, Myomorpha, and Hystricomorpha) exhibit a separated condition of the femoral head and greater trochanter. We assessed the femoral ossification of eight species of all six genera of a subterranean lineage of mammals, the African mole-rats (Bathyergidae), including the naked mole-rat (Heterocephalus glaber). Here we report a surprising level of intraspecific variation in the ossification of the proximal femur of H. glaber, which presents both separated and coalesced conditions, regardless of sex and reproductive status. The other bathyergids, including chisel-tooth and scratch-diggers exhibit a separated condition, similar to the typical rodent condition. Because the coalesced condition is uncommon among rodents, our data suggests that the presence of two femoral morphologies in H. glaber represent developmental plasticity in this species. Such a dual condition may result from a constricted femoral head and greater trochanter morphology and slow skeletal growth rates, which could be also influenced by differential loading histories, such as magnitude and orientation of forces acting on the limb during ontogeny. This is the best documented case of intraspecific variation for this trait amongst non-human vertebrates, and its investigation is important to understanding the mechanisms of skeletal development and phenotypic plasticity in mammals.

     

Factors influencing the distribution of the mole rat, Cryptomys zechi (Rodentia, Bathyergidae) in Ghana

Factors influencing the distribution of the Ghana mole rat Cryptomys zechi were investigated in a small part of the middle Volta basin of Ghana where it is endemic. Soil type appeared to be the most important factor controlling mole rat distribution in the area; the mole rats preferred areas where the soil was silt loam. In areas where mole rats were found, local distribution was influenced by food availability and land preparation methods for farming. The highest concentrations of mole rat colonies were found in farmlands where traditional hoe ploughing is used for land preparation and where plants with underground storage organs were more diverse; the lowest concentration was in farms where mechanized ploughing is used for land preparation.     

Growth and development in six species of African mole-rats (Rodentia: Bathyergidae)

The bathyergid mole-rats provide a unique example of a family of subterranean rodents exhibiting a broad spectrum of sociality. Three genera comprise solitary, strongly territorial individuals whereas two genera are social. This sociality culminates in the eusocial naked mole-rat, Heterocephalus glaber . The pups of solitary mole-rats disperse, establish and thereafter defend their own burrow systems when approximately two months old, whereas those of social genera join an established natal colony. This paper examines whether these different lifestyles are reflected in the early development and rate of growth of pups of mole-rats.
Although the trends are not clear-cut, it is apparent that the pups of solitary genera grow and mature more rapidly than those from social genera. Thus, the growth rate constant ( K ) for the first70–80 days of postnatal growth (using the Gompertz equation) for the solitary genera was between 0.042 and 0.052 day⁻¹, whereas that of the social mole-rats was considerably lower (0.01 5 day⁻¹). Similarly the mean growth rates of solitary genera ranged between 3.3 and 1.227g/day while those of the social mole-rats were 0.229-0.233 g/day.
The pattern of development and the rates of growth in solitary bathyergids are similar to those of other solitary subterranean rodents. One interesting feature common to all the social genera studied to date was that the first pups recruited to a 'new colony', consisting of a reproductive pair of adult mole-rats, grew at a significantly faster rate than pups born to an established colony.     

Aspects of the feeding habits and reproductive biology of the Ghana mole-rat Cryptomys zechi (Rodentia, Bathyergidae)

The feeding habits and reproductive biology of the Ghana mole‐rat, Cryptomys zechi (Matchie), were studied in a Guinea savanna woodland in Ghana. Both tunnel contents and stomach content analysis indicated that bulbs and tubers constituted the commonest and most preferred food items, although some animal food materials were also consumed. Five plant species, Urgenia altissima, Manihot utilisima, Curuligo sp., Oxalis corniculata and Archis hypogea, were the most popular plant food source. Breeding occurred during the rainy season (March–August) and was at its peak in July. There is evidence that the species is capable of producing two litters in a year. In a colony, reproduction is restricted to one female and one male. Males reached sexual maturity at a lower body weights (105 g) than females (155 g). Estimated mean litter size was 1.5 (range 1–2), the smallest among the bathergids. Available data on birth weights of three other species of social Cryptomys indicate that C. zechi has the highest birth weight, which is comparable to that of solitary bathyergids.     

Extended longevity of reproductives appears to be common in Fukomys mole-rats (Rodentia, Bathyergidae)

African mole-rats (Bathyergidae, Rodentia) contain several social, cooperatively breeding species with low extrinsic mortality and unusually high longevity. All social bathyergids live in multigenerational families where reproduction is skewed towards a few breeding individuals. Most of their offspring remain as reproductively inactive "helpers" in their natal families, often for several years. This "reproductive subdivision" of mole-rat societies might be of interest for ageing research, as in at least one social bathyergid (Ansell's mole-rats Fukomys anselli), breeders have been shown to age significantly slower than non-breeders. These animals thus provide excellent conditions for studying the epigenetics of senescence by comparing divergent longevities within the same genotypes without the inescapable short-comings of inter-species comparisons. It has been claimed that many if not all social mole-rat species may have evolved similar ageing patterns, too. However, this remains unclear on account of the scarcity of reliable datasets on the subject. We therefore analyzed a 20-year breeding record of Giant mole-rats Fukomys mechowii, another social bathyergid species. We found that breeders indeed lived significantly longer than helpers (ca. 1.5-2.2fold depending on the sex), irrespective of social rank or other potentially confounding factors. Considering the phylogenetic positions of F. mechowii and F. anselli and unpublished data on a third Fukomys-species (F. damarensis) showing essentially the same pattern, it seems probable that the reversal of the classic trade-off between somatic maintenance and sexual reproduction is characteristic of the whole genus and hence of the vast majority of social mole-rats.     

Taxonomic validity of species groups in the genus Akodon (Rodentia,Cricetidae)

In an effort to evaluate the four Akodon species groups, phylogenetic relationships among individuals of the genus Akodon, selected from throughout South America, were examined using cytochrome b and a concatenated data set consisting of data from cytochrome b, exon 6 of the dentin matrix protein 1 and the nuclear intron thyrotropin. Both the cytochrome b data set and the combined data set were analysed under maximum parsimony, maximum likelihood and Bayesian criteria. Like previous studies, a monophyletic Akodon clade was recovered. Monophyly of the boliviensis and cursor groups was supported, and the two form a strongly supported sister relationship. Akodon azarae is basal to and forms a monophyletic group with the boliviensis + cursor clade, resolving the placement of A. azarae but leaving it unassignable to a current Akodon species group. The aerosus and varius groups are paraphyletic as four members of the varius group (A. glaucinus, A. simulator, A. tartareus and A. varius) fall within the aerosus group. Akodon lindberghi is formally placed in the cursor group. Akodon caenosus is recognized as a species distinct of A. lutescens, A. orientalis is recognized as a species distinct of A. orophilus, and A. aerosus, A. baliolus and A. surdus are recognized as three separate species. Based upon chronophylogenetic analysis, the initial divergence within Akodon likely began during the late Pliocene and ancestors of the four extant species groups (aerosus, boliviensis, cursor and dolores) appeared around the Pleistocene–Pliocene boundary or shortly thereafter.     

Thermal biology of a strictly subterranean mammalian family,the African mole-rats (Bathyergidae,Rodentia) - a review

African mole-rats are subterranean rodents, which rarely if ever leave the safety of their burrow systems. The environment of the burrows is humid, with relatively stable temperatures, and may have a hypoxic and hypercapnic atmosphere. One of crucial problems related to the subterranean way of life in mammals is avoidance of overheating, because traditional mammalian cooling mechanisms are not effective under high humidity. In African mole-rats, a variety of adaptations have evolved in response to this and other challenges of the underground ecotope. Traditionally, attention has been devoted mainly to the naked mole-rat Heterocephalus glaber, which became popular as a result of its eusociality and absence of fur, both being unique phenomena in small mammals. Despite more recent research, information on other species is still relatively limited and patchy. I review the results of studies on African mole-rats that are relevant for the understanding of their energetics and thermal biology. Attention is paid to the parameters of the burrow environment, which represent the main selection pressures shaping their physiology. In addition, an overview is given of the morphological, physiological and behavioural adaptations helping mole-rats to face temperature extremes, mechanisms by which they deal with a surplus of metabolic heat and how changes in ambient temperature influence their daily activity. The naked mole-rat is compared to its furred relatives to determine whether this species is really exceptional from the point of thermal biology. An ordination analysis was conducted using published data on mole-rat body temperature, thermoneutral zone, resting metabolic rate and thermal conductance. Most of the variability in these characteristics was found to be explained by body mass, followed by temperature characteristics of climate, but not precipitation, of the species distributional ranges. This analysis shows that the naked mole-rat is comparable to the other mole-rat species in these physiological characteristics.     

The social structure and dominance hierarchy of the Mashona mole-rat, Cryptomys darlingi (Rodentia: Bathyergidae) from Zimbabwe

Cryptomys darlingi is a social subterranean rodent mole which inhabits the mesic regions of south-eastern and central Africa. Mashona mole-rats live in small colonies (5-9 animals) in which reproduction is normally restricted to the largest male and female in the colony. The non-reproductive members in a mature colony cannot be placed into clearly defined work-related groups based on body mass.
The dominance hierarchy of a young colony was found to be linear, with a value of 1.00 calculated from Landau's linearity index, while that of a mature colony of nine mole-rats was almost linear (0.77). Dominance was found to be related to gender in the mature colony, with males more dominant than females, and to age in the young colony. The reproductive mole-rats are the dominant animals within their respective genders. Dominance appears to correlate positively with body mass (rs = 0.77 in the mature colony and rs = 0.93 in the young colony).
Popularity studies show that smaller animals and females tend to be more popular than the larger massed individuals or males. In the mature colony which contained predominantly adult animals, the reproductive pair was among the least popular. While in the young colony, composed predominantly of sub-adult and juvenile animals, the reproductive pair was the most popular.
Social organization within Mashona mole-rat colonies is compared with other southern African Cryptomys species.     

Taxonomic identity of Chionomys nivalis spitzenbergerae (Mammalia: Rodentia)

The most genetically divergent populations of the European snow vole Chionomys nivalis occupy the extreme eastern range of the Black Sea and the Caspian Sea. It was recently suggested that subspecies C. n. spitzenbergerae from the Central Taurus Mountains (Turkey) represents a highly divergent lineage of C. nivalis from the Alada? Range which induced us to address its status by examining topotype specimens. Two females karyotyped displayed 54 mitotic chromosomes of 25 acrocentric autosomal pairs. One of the smallest autosomal pairs was heteromorphic in both specimens, consisting of a submetacentric and an acrocentric chromosomes (NFa = 53). Cytochrome b sequences however unambiguously clustered both individuals with reference sequences from C. nivalis from Turkey. We conclude that a deviant haplotype from the Alada? Range does not represent C. n. spitzenbergerae. Its taxonomic identity was not resolved in our study. Further attention should be devoted to snow voles from Central Anatolia and Western Iran, which are characterized by cranial peculiarities.