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.     

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.     

Colony size and structure, activity patterns and foraging behaviour of a colony of the social mole-rat Cryptomys damarensis (Bathyergidae)

A complete colony of 20 Cryptomys damarensis was trapped in the Kalahari Gemsbok National Park, South Africa. The colony consisted of 15 males and five females and formed three distinct size classes, two of which were presumed to represent worker (smallest mole-rats) and soldier (largest males) castes. Body masses ranged from 86–197 g, with the dominant male being the largest mole-rat. The activity patterns of five animals in the colony were monitored by radiotelemetry. Activity patterns were not influenced by diel fluctuations in burrow temperature and photoperiod. It was suggested that the chance of hyperthemia influenced activity, such that in order to maximize the daily distance burrowed (foraging efficiency), the mole-rats had to engage in frequent (5.58 times. day^-1), but short ( c. 60 min) foraging bouts.     

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.     

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.     

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.     

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.     

Soil and burrow temperatures, and the resource characteristics of the social mole-rat Cryptomys damarensis (Bathyergidae) in the Kalahari Desert

The diel temperatures of the soils and burrows of the Damara mole-rat Cryptomys damarensis were measured during winter and summer in the Kalahari Gemsbok National Park, South Africa. Also, the subterranean resource characteristics of C. damarensis , such as food abundance, spatial dispersion patterns and quality, were measured. The mean temperatures of the primary burrows were 33.5 C and 18.8 C during summer and winter, respectively. These temperatures were higher and lower than the upper and lower limits of thermoneutrality of C. damarensis , respectively. Resource characteristics were found to be habitat-specific. Where geophytes occurred in high densities, such as the bulbs of Dipcadi gracillimum (40–118 bulbs.m^-2), they were small (0.41–5.17 g), but represented a good quality diet. Where densities were low, such as for the tubers of Acanlhosicyos naudinianus (0.17-0-407 tubers.m^-2), they were considerably larger (670 g), but represented a poor quality diet. These patterns confirm a general trend with increasing aridity, of increased geophyte sizes and decreased geophyte densities. In the Bathyergidae, these trends are accompanied by increased sociality and decreased body sizes.     

The colony structure and reproductive biology of the afrotropical Mashona mole-rat, Cryptomys darlingi

Cryptomys darlingi occurs in the mesic Miombo woodland of north-eastem Zimbabwe. It occurs in colonies of up to nine individuals, in which reproduction is limited to one of the largest males and the largest female in the colony.
Reproduction and details of colony size and number of breeding animals in a colony are described for five complete field-captured colonies.
In captivity, mating is not confined to a particular season, and up to three litters of pups are orn per annum. The reproductive female initiates the pre-copulatory behaviour. The gestation lengti is 56–61 days ( n = 2 ). The new-born pups are altricial and litter size is small = 1.7 ± 0.5 ( n = 6). In this case, the pups first left the nest 10 days after birth, began to eat solids when 14 days old, and were fully weaned at five weeks. They began to spar with each other when 36–40 days old, but did not disperse and were incorporated into the colony. This suggests that the Mashona mole-rat colonies are composed of a founding pair and at least three successive litters of pups.