Colony defense in Damaraland mole-rats, Cryptomys damarensis

I examined defense against conspecific intruders in coloniesof the Damaraland mole-rat. I introduced foreign animals tocaptive colonies and examined the effect of the sex and breedingstatus of the intruder on the sex and breeding status of thosethat defended. Intruding mole-rats were almost always attacked.Colony defense against intruders was carried out almost exclusivelyby dominant, reproductive individuals and was directed primarily toward individuals of the same sex. Nonbreeding, subordinatemole-rats participated only rarely. These results contrastsharply with colony defense in the related naked mole-rat andsuggest that investment in defense activities in Damaralandmole-rats primarily reflects repulsion of potential competitorsfor breeding status. Subordinate mole-rats may lack incentiveto participate in potentially costly defense, as they are unlikelyto gain reproductive opportunities within the colony and probablygain fitness primarily through dispersing to breed.     

Social suppression in nonreproductive female Damaraland mole-rats, Cryptomys damarensis: no apparent role for endogenous opioid peptides

The role of endogenous opioid peptides (EOPs) on LH secretion was examined to investigate the neuronal mechanisms responsible for the inhibition of GnRH and the resultant infertility in nonreproductive female Damaraland mole-rats, Cryptomys damarensis. The endorphin antagonist naloxone was administered to five groups of females to determine its effect on plasma LH levels: Grouping was determined by social status, social environment, and whether the females were ovariectomized. A single injection of naloxone had no significant effect on LH secretion in either intact or hystero-ovariectomized females. Multiple injections with naloxone failed to affect basal LH concentrations but did result in a decrease in GnRH-stimulated LH secretion in ovariectomized nonreproductive and reproductive females. A significant response to a single naloxone injection following GnRH priming was obtained in both nonreproductive females and in nonreproductive females housed in the absence of the reproductive pair. These results suggest EOPs play a role in sexual function but that socially induced infertility is unlikely to be mediated through the EOP system.     

Food availability and foraging by wild colonies of Damaraland mole-rats (Cryptomys damarensis): implications for sociality

We investigated some of the ecological determinants of sociality in the Damaraland mole-rat, including the spatial distribution and biomass of resources (geophytes) available to foraging Damaraland mole-rats in partly vegetated sand dunes in the Kalahari and in grasslands near Dordabis, Namibia, and the foraging behaviour and residency characteristics of colonies at Dordabis. In both study areas, the geophytes had a clumped distribution, but the highest coefficients of dispersion and mean biomass occurred in the Kalahari where the principal food was the gemsbok cucumber. However, because the coefficient of digestibility was lower in geophytes from the Kalahari than from Dordabis, and the mole-rats only ate about half of a gemsbok cucumber, there was less energy available to mole-rats in the Kalahari. At Dordabis, large established colonies occur in the areas with the richest resources and remain resident in the same area for many years; within this area they search (blindly) for food during brief periods when the soil, at burrow depth, is moist and easily worked. Initially, long straight burrows are dug and few bulbs are taken; once the soil dries, minor changes are made to the burrow system as the mole-rats exploit the food patches they located immediately after the rain. Our results show that the characteristics of the resources, and the short time interval during which location of new resources is possible, favour group living; however, the constraints imposed by these features affect large and small colonies in different ways. Small colonies are more likely to fail than large ones and some crucial factors in the survival of these newly formed colonies are the richness of the area in which their burrows are located, and the size of the colony work force available to locate the food. Received: 6 May 1997 / Accepted: 21 August 1997     

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.     

Behaviour and social organization in a colony of the Damaraland mole-rat Cryptomys damarensis

This paper reports qualitative and quantitative behavioural studies of the social mole-rat C. damarensis . The behaviours were subdivided into four major categories: nest, maintenance, interactive and auto-behaviours. Social interactions and behavioural types were analysed by sex, age category and caste. The nest area is a focal point in the burrow system with individuals spending more than 70% of the day there. Many of the interactive behaviours occurred there and the mole-rats showed definite preferences with regard to which individuals they rested next to or avoided.
The Damaraland mole-rat colonies show some division of labour, into frequent and infrequent worker groups; these castes differed significantly in the amount of both digging and soil movement undertaken.     

Body temperature patterns and rhythmicity in free-ranging subterranean Damaraland mole-rats, Fukomys damarensis

Body temperature (T(b)) is an important physiological component that affects endotherms from the cellular to whole organism level, but measurements of T(b) in the field have been noticeably skewed towards heterothermic species and seasonal comparisons are largely lacking. Thus, we investigated patterns of T(b) patterns in a homeothermic, free-ranging small mammal, the Damaraland mole-rat (Fukomys damarensis) during both the summer and winter. Variation in T(b) was significantly greater during winter than summer, and greater among males than females. Interestingly, body mass had only a small effect on variation in T(b) and there was no consistent pattern relating ambient temperature to variation in T(b). Generally speaking, it appears that variation in T(b) patterns varies between seasons in much the same way as in heterothermic species, just to a lesser degree. Both cosinor analysis and Fast Fourier Transform analysis revealed substantial individual variation in T(b) rhythms, even within a single colony. Some individuals had no T(b) rhythms, while others appeared to exhibit multiple rhythms. These data corroborate previous laboratory work showing multiplicity of rhythms in mole-rats and suggest the variation seen in the laboratory is a true indicator of the variation seen in the wild.     

The relative age structure and body masses of complete wild-captured colonies of two social mole-rats, the common mole-rat, Cryptomys hottentotus hottentotus and the Damaraland mole-rat, Cryptomys damarensis

The relative ages of individuals in a complete wild-captured colony of 13 Cryptomys hottentotus hottentotus and a complete wild-captured colony of 25 Cryptomys damarensis were determined by means of tooth wear and eruption patterns and through the degree of ossification of their skeletons.
From sequential tooth wear and eruption patterns four relative age classes were discerned in C. h. hottentotus and five in C. damarensis . These relative age class allocations were supported by the studies on the ossification of the skeletons.
In C. damarensis , older animals are not necessarily the larger-sized individuals. In C. h. hottentotus , however, older animals are usually the larger-sized individuals. The reproductive pair in each of the Cryptomys species are the oldest or amongst the oldest individuals in the colony.
Individuals in a second colony of C. damarensis kept under laboratory conditions for three years maintained greatly disparate body masses which were correlated with the role of the individual within the colony and not, where it was known, with the age of the animal.
An analysis of 13 body and skull measurements performed on a complete colony of 13 C. h. hottentotus and a complete colony of 25 C damarensis revealed that only C. damarensis showed sexual dimorphism. This dimorphism was apparent from an early age but became more pronounced in older animals.     

The colony structure and dominance hierarchy of the Damaraland mole-rat, Cryptomys damarensis (Rodentia: Bathyergidae), from Namibia

Cryptomys damarensis is one of the few subterranean rodents which is social. This species is found in the semi-arid regions of southwestern and central Africa. The Damaraland mole-rat occurs in coloniesof up to 25 individuals, in which reproduction is limited to one or two of the largest males and the largest female in the colony.
The mean colony size is 18 mole-rats ( n =6 colonies). The mean colony biomass is 2.32 kg and the sex ratio is female biased (0.71–0.78). The number of mole-rats in each colony, the mean body mass and the sex ratio are described for six field-captured colonies, three of which were captured in their entirety.
The dominance hierarchy of two colonies of C. damarensis was found to be linear with a value of between 0.94 and 1.00 calculated from Landau's linearity index. Dominance was found to be related to gender, with the males more dominant than females. The reproductive individuals are the dominant animals within each respective gender. The non-reproductive females rank lowest in the hierarchy.     

Differential growth patterns between successive litters of the eusocial Damaraland mole-rat, Cryptomys damarensis, from Namibia

Differential mean rates of growth were calculated for successive litters of mole-rats born to genetically unrelated pairs of C. damarensis. The Logistic equation provided the closest fit to the growth data.
Both intra- and inter-colonial variation in mean maximum growth rate between successive litters incorporated into the natal colonies were determined.
The mean asymptote (A), growth rate constant (K) and inflection time (I) were modelled for the first five litters of pups born to pairs of mole-rats. Inter-colonial analysis of recruitment to colonies revealed litters 1, 2, and 5 to grow faster than litters 3 and 4. Litters 1 and 2 had significantly higher asymptotes and inflection times than litters 3, 4, and 5 ( P < 0.001). All litters were assimilated into their natal colonies. There were no significant differences between males and females for the asymptote, growth rate constant, or inflection time ( P < 0.05).
Intra-colonial variation of litters 1 to 4 born to a single colony revealed comparable results to that obtained for inter-colonial comparisons. Multiple comparisons showed litters 1 and 2 to have a significantly higher absolute growth rate than litters 3 and 4. The lowest rate of growth being for litter 4.
The patterns of mean differential growth are discussed in the light of the social organization of the colony. It is postulated that growth rates in litters 1, 2, and 3 are more rigid since they constitute the main functional unit of colony organization. Litters 4 and 5 show a greater plasticity in growth; it is speculated that these colony members become incorporated into the various worker and defender groupings which are characteristic of C. damarensis colonies.     

Fibre utilization by Kalahari dwelling subterranean Damara mole-rats (Cryptomys damarensis) when fed their natural diet of gemsbok cucumber tubers (Acanthosicyos naudinianus)

Kalahari dwelling Damara mole-rats (Cryptomys damarensis) naturally feed on a high fibre diet of underground gemsbok cucumber tubers, Acanthosicyos naudinianus. We investigated the degree of fibre utilization and fermentation on this diet by measuring caecal characteristics (namely temperature, pH and weight) and in vitro rates of gas and short chain fatty acid (SCFA) production in these underground dwelling hind-gut fermentors. Rectal temperatures (33.8±0.6°C) were consistently higher than caecal temperatures (33.3 ± 0.6°C). Furthermore, a 0.8°C gradient of temperatures existed within the caecum, with the lowest temperature occurring in the corpus caeci. Both rates of gas production (4.74 ± 0.6 ml/g dry matter/hr) and SCFA production (266.80 ± 9.251 /i mol/caecum per hr) were high, with proportionately more acetic acid produced than any other SCFA. Nevertheless, the initial concentrations of SCFAs present in the caecum were low (52 ±17 mM) implying a rapid rate of absorption of these SCFAs. The high rates of fermentation provide a considerable amount of energy that would otherwise be trapped in fibre and thus unavailable to the animal. This highly efficient caecal fermentation enables the Damara mole-rat to maximally exploit the underground food resources in the arid-zone ecotope.     

Are non-reproductive male highveld mole-rats, Cryptomys hottentotus pretoriae physiologically suppressed while in the confines of the natal colony?

The highveld mole-rat, Cryptomys hottentous pretoriae is a social, subterranean, co-operative breeder with a high reproductive skew. The relationship between reproductive status and reproductive physiology in male highveld mole-rats was investigated. Testicular morphometrics, histological parameters, plasma testosterone concentrations and sperm motility parameters were studied in 31 males (14 reproductive and 17 non-reproductive males). Reproductive males were significantly larger than non-reproductive males, with testicular mass and volume corrected for body mass considerably larger for the reproductive males. Circulating plasma testosterone concentrations of reproductive males were not significantly higher than non-reproductive males (reproductive males 10.3±1.8 nmol l⁻¹ vs non-reproductive males 7.3±1.74 nmol l⁻¹). Sperm motility parameters were measured, but no significant differences were found between reproductive and non-reproductive males. All 14 reproductive males had motile sperm, whereas 13 of the 17 (76%) of non-reproductive males possessed motile sperm. A typical ejaculate of a reproductive male contains 48.3% motile sperm characterized by a high percentage of flagellar defects, whereas the non-reproductive male has an ejaculate containing 45.6% motile sperm with a high percentage of head defects. It is apparent that non-reproductive males are not physiologically suppressed from reproducing. Moreover, the non-reproductive males are excluded from incestuous matings as a result of being the offspring of the reproductive female.     

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.     

The magnetic compass orientation of the burrows of the Damara mole-rat Cryptomys damarensis (Bathyergidae)

In response to reports claiming that part of the ability of mole-rats (Bathyergidae) to orientate with respect to the geomagnetic field involves orientation of their burrow systems in a southward direction, we measured the orientation of burrows of the Damara mole-rat, Cryptomys damarensis , in the Kalahari Desert. It was found that burrow orientation was not significantly different from that expected for a random distribution of compass orientations.