The Damaraland mole-rat (Fukomys damarensis), Damara mole rat or Damaraland blesmol
The Damaraland mole-rat (Fukomys damarensis), Damara mole rat or Damaraland blesmol, is a burrowing rodent found in southern Africa. Along with the smaller, less hairy, naked mole rat, it is a species of eusocial mammal.
Description
Like other blesmols, the Damaraland mole-rat has a cylindrical body with short, stout limbs, large feet, and a conical head. It is also similar in size to most other African mole-rats, having a head-body length of 14 to 20 cm (5.5 to 7.9 in), with a short, 2 to 3 cm (0.79 to 1.18 in), tail, and weighing between 100 and 280 grams (3.5 and 9.9 oz). There are no external ears, and the blue-coloured eyes are tiny with thick eyelids. The incisor teeth are large and prominent, with flaps of skin behind them to prevent soil from falling into the throat while the animal is using them to dig.
The fur is short and thick, and varies from fawn to almost black, with shades of brown being most common. There is always a white patch on the top of the head, although its exact shape varies, and there may also be additional blotches of white fur elsewhere on the body. Longer sensory hairs project above the fur over much of the body, with the facial whiskers being particularly long. Females have six teats.
Habitat
Fukomys damarensis is distributed in the arid regions of south-western and central Africa. These rodents are characterised by a remarkable social structure of their colonies, in which there is one breeding female ("queen"), two or three mating males and many workers. Among the latter, two castes are distinguished - the so-called "regular workers" and the larger "irregular workers".
Lifestyle
Traditionally, it was believed that all the work of expanding the network of passages leading to feeding places (and Damar diggers feed on underground parts of plants), lies on the "regular workers", and numerous "irregular" workers - just lazy. However, recently a group of scientists from the Republic of South Africa together with colleagues from Great Britain managed to show that the "irregular workers" of Damara diggers fulfil the most important for any kind of organisms function of dispersal. After rain, when the ground becomes a little softer, they begin to work intensively, laying new underground passages and developing a new territory. In doing so, they may meet a future mate (or mate) and begin to reproduce, establishing a new colony.
Social behaviour
Among the entire class of mammals, there are only two known species whose settlements have a division into castes that resembles the organisation of social insects (bees, ants and termites). Both of these species are rodents in the family Bathyergidae, amazing animals that lead a largely subterranean life and burrow through hard soil with the help of their strong teeth. The Naked mole-rat (Heterocephalus glaber) is the best studied, while very little was known about the second species, the Damaraland mole-rat (Fukomys damarensis), until recently.
In particular, it was believed that the caste of "irregular workers", which account for 25-40% of the colony's population, performed only 5% of the total work. At the same time, these animals themselves are larger than "regular workers" and contain more blubber. A team of researchers from the Institute of Mammalian Studies at the University of Pretoria, together with scientists from the UK, conducted a thorough study of the metabolic rate of representatives of different castes of the Damara shrew during the dry period (when there was no rain for at least a month) and shortly after the rain (when about 40 mm of rain fell). In addition to the level of standard exchange under resting conditions, the researchers assessed active exchange over several days. In the latter case, they used the long-established method of injecting the animal with water containing "double labelled" water, i.e. the hydrogen isotope deuterium 2H and the heavy oxygen isotope 18O. After injections, Fukomys damarensis were released into the wild, captured again after 2-5 days and blood samples were taken from them to determine the concentration of the injected isotopes. The essence of the method is that hydrogen is excreted from the organism only with liquid, and oxygen - both with liquid and as part of carbon dioxide CO2. Accordingly, it is possible to calculate the respiration intensity of an animal in an active state.
It turns out that during normal times, when there is no rain, the energy expenditure to maintain its existence is significantly higher in "regular workers" than in "irregulars" or "queens". However, after rain, during a relatively wet period, the metabolic intensity of "irregular workers" and breeding females increases significantly, while that of "regular" ones remains almost at the same level. Obviously, while the ground is milder, the "lazy workers" are not lazy at all, but work hard. The labouring individuals of both castes are immature males and females. If they get away from the queen, they can reach sexual maturity and start their own reproduction. Thus, "irregular workers" are very important for the population - they fulfil a dispersal function and, in addition, maintain genetic exchange between different colonies. Otherwise, close inbreeding would lead to the accumulation of harmful mutations and eventually to the extinction of the colony.
Reproduction
Reproduction in Fukomys damarensis is year-round. Pregnancy lasts from 11 to 13 weeks, most often 85-87 days. During the last stages of pregnancy, the female is actively engaged in nest building and is assisted by other members of the family group. They leave the nest only on the eve of delivery. No aggressive behaviour by the pregnant female was observed. Females bear up to two broods per year, each with 2 to 4 cubs (average 2.8). Young are fully independent at about 6 weeks of age, eating solid food and digging underground passages. Mortality is very high, and although adults survive to 10 years of age, annual mortality in populations of this species averages about 30%.
Family groups comprise an average of 16 individuals (up to a maximum of 41), including an adult breeding female, one or two adult males and several broods of immature individuals of different ages. Body mass of adult animals reaches 208 g (average 131 g), and there is no sexual dimorphism in body size, in contrast to the Hottentot Sandpiper.
Most young individuals in the family group belong to two "worker castes" (passive and active, or "regular" and "irregular" workers), mainly engaged in digging and clearing underground tunnels and delivering food. Young and semi-adults have little or no concern for cubs; at most, they are capable of dragging a calf into the nest. An adult male (or one of two adult males) dominates the family group. The founders of the family group inhibit the reproduction of young individuals by their presence, but the latter retain the potential to reproduce and realise it when they are removed to a separate cage. A single adult male or, as in some family groups, two adult males mate with a female. Most of the other males in the family group have spermatogenesis, but do not exhibit sexual behaviour and do not take any part in the courtship of the adult female. Daughter females do not ovulate due to reduced secretion of gonadotropic hormones. In fact, incest taboo is manifested in family groups of this species, and reproduction of young animals (in case of eviction from the family group) is possible only by pairing with an unrelated individual.
The disintegration of family groups and dispersal of young animals in most cases occurs only after the death of the founding members of the family. Moreover, if this occurs during the dry season, the young do not disperse until the onset of the rainy season, when the moistened soil becomes more amenable to digging.
A study conducted in 1986-1987 in the Kalahari Desert (South Africa) using radio telemetry showed that a family group of 20 individuals (5 females and 15 males) utilised a complex burrow system over an area of about 13,000 m2. Five feeding areas and three nesting burrows were located within this area. The main food source was tubers of the rare watermelon Acanthosicyos naudinianus. This plant grows sporadically everywhere.
All individuals in this grouping could be divided into three age categories: the oldest - 8 males (weighing 150-200 g), the middle - 2 females and a male (weighing 100-150 g) and the youngest - 3 females and 6 males (weighing 80-100 g). The younger animals were of the "worker caste", while most of the older males (except for the dominant male) belonged to the "soldier caste" (guards). The latter were located in different nesting chambers. Such dispersal facilitates timely detection of "strangers" or predators.
More detailed data on the social behaviour of Fukomys damarensis were obtained from observations of six family groups captured in the wild and placed in artificial structures consisting of several plastic nesting chambers connected by plastic tunnels (Jacobs et al., 1991). Each family contained between 12 and 25 individuals. Analyses of interactions between family group members revealed the presence of a linear dominance hierarchy. Dominant individuals initiated such acts as biting attacks, head thrusts, climbing over the partner and some others. Subordinate individuals are characterised by defensive postures, brushing the partner, lowering the head, climbing under the partner. The absolute dominant in each family group was the largest male mating with the female, and the breeding pair occupied the highest position in the family hierarchy. Immature males occupied the position of subdominants, and the lowest step of the hierarchy was occupied by immature females. The linear nature of the dominance hierarchy is evidence of its high stability.
Thus, the hierarchical status of individuals in family groups of Fukomys damarensis depends, first of all, on their sex, and males, with some exceptions, usually dominate over females. In the social organisation of the family group, cleaning of the mate plays an important role, and the order of grooming serves as an indicator of hierarchical status: the dominant male and breeding female are always recipients of grooming and never clean other individuals; accordingly, the initiator of cleaning always occupies a subordinate position. It is hypothesised that grooming has a pacifying function and suppresses partner aggression.
Genetics
The Damaraland mole-rat's karyotype shows 74 or 78 chromosomes (2n). Its fundamental number is 92.
