A New Laboratory Research Model: The Damaraland Mole-rat and Its Managed Care
The Damaraland mole-rat (Fukomys damarensis) is a subterranean, hypoxia-tolerant, long-lived rodent endemic to southern and central Africa that is increasingly being used in laboratory research. Its husbandry needs and characteristics differ from traditional rodent research models. Here, we provide a brief overview of this species and discuss its captive housing and husbandry requirements for managed care and good health.
Introduction
The speciose Bathyergid African mole-rats are a clade of subterranean dwelling rodents that have garnered considerable attention primarily because of their wide range of social complexity that includes solitary, social, and eusocial species and adaptations to a subterranean lifestyle.27,33,48,51 The early studies of the 1980s primarily examined the ecology and social organization of these rodents, focusing mainly on the 2 eusocial species, the naked mole-rat (NMR; Heterocephalus glaber) and the Damaraland mole-rat (DMR; Fukomys damarensis).1 Recently, however, scientists have dug deeply into the many aspects of their biology that are of biomedical interest. Most of this focus has been on the extraordinarily long-lived, hypoxia-tolerant, and cancer-resistant NMR.10 Indeed, the NMR has been used as a burgeoning nontraditional animal model for research on aging, ischemic injury, pain, neurodegeneration, inflammation, and immunity.7,10 The DMR shares many of the same attributes as the NMR; for example, it lives long for its body size23,38,39 and is hypoxia tolerant.13 The DMR diverged 26 million y ago from the NMR, an evolutionary separation that is similar to that between mice and rats or humans and macaques.15 Research using the DMR has seen an exponential increase in the last 50 y with the number of published papers per year using this species increasing from one paper throughout the 1970s to over 125 papers per year in recent times. Indeed, there have been more than 400 publications since 2020 (Figure 1, accessed from Google Scholar on May 12th, 2024), with a new focus on biomedically pertinent topics. For the most part, however, the bulk of publications regarding this species, even in recent times, address its ecology, ecophysiology, metabolism, behavior, and unusual aspects of its reproductive biology.11,12,14,15,20,22,44,49,51,53 These studies have highlighted that DMRs, like the NMR, require housing and husbandry conditions that are different from those of standard laboratory mice and rats. To conduct reproducible research using captive DMRs, vivarium conditions and standardized or optimal husbandry protocols need to be established for their managed care.
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-052
Appearance
Fukomys damarensis is a medium-sized rodent, with adults weighing between 80 and 280 g (approximately 160 g) in captivity38 while those caught in the wild tend to be larger.3,55 Males tend to be larger than nongravid females (Figure 2; Table 1), although dominance status affects body size in both sexes. Subordinates are generally smaller than the dominant breeders,3 with this difference being more pronounced in larger colonies.55
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-052
| Captive colony size (no. of animals) | 14 ± 5 (2–48) |
| Wild adult male length (mm) | 189 ± 29 (109–222) |
| Wild adult female length (mm) | 181 ± 15 (159–218) |
| Wild adult male weight (g) | 165 ± 45 (100–281) |
| Wild adult female weight (g) | 142 ± 28 (100–230) |
| Predicted max lifespan based on 160-g animal (y) | 10.6 |
| Observed maximum lifespan (y) | 21 |
| Age at sexual maturity (d) | 180 |
| Interbirth interval (d) | 78–92 |
| Litter size | 1–6 |
| Pup weight at birth (g) | 8–10 |
| Pelage present days after birth | 4–6 |
| First eat solid foods (d) | 6 |
| Weaned at (d) | Approximately 28 |
| Time to attain stable adult mass (y) | 1.5–2 |
Their glossy fur is short, and its color may be quite variable, ranging from black, reddish brown to a light gray fawn color (Figure 2A–D). They have distinctive white patches on top of the head, but these irregularly shaped white blotches are also often randomly spread throughout their face and, less so, over their entire body (Figure 2A–D). Widely distributed thicker guard hairs protrude above the pelage. While these vibrissae occur all over the body, they are concentrated around the mouth and tail and are highly innervated sensory tactile organs.3
The DMR body is cylindrically shaped, with a head body of 14 to 20 cm and a short (1 to 3 cm long) stubby tail, disproportionately short limbs, and large feet (Figure 2B–D). They lack external ear pinnae and have very small eyes with thick eyelids, and a blunt pig-like nose with their nares close together and positioned above extruding large procumbent incisors (Figure 2E).
Apart from body size and possibly subtle differences in head shape, DMRs tend to be sexually monomorphic. In subordinate males, the testes are contained within the abdominal cavity, and the penis is hidden within a sheath (Figure 3). In contrast, the penis of breeding males is visible, and while still cryptorchid, their larger testes descend into obvious inguinal pockets.3 Females have loped labial flaps and a clitoris that is only evident during sexual activity (Figure 4). In contrast to that of subordinate females, the vagina of the breeding females is patent, and her 6 pairs of mammae are more prominent.3
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-052
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-052
Taxonomy and Natural Distribution
The DMR, also commonly known as the Damara mole-rat or blesmol, was first described by William Ogilby, a fellow of the Linnean Society and zoologist renowned for his work on species identification and zoological nomenclature. He identified the DMR from the mammals collected for the British museum by Captain/Sir JE Alexander on his coastal travels through the Damaraland region of South West Africa in 1837. Ogilby named this subterranean rodent with the large white patch on the occiput Bathyergus damarensis in 1838, after the area in which it was found.34 Since that time, phylogenetic reassessments placed the DMR in the genus Cryptomys, and more recently, allozyme, nuclear, and mitochondrial DNA markers, rather than morphologic features, resulted in this species being moved to a distinct new lineage Fukomys (Table 2).29
| Class | Mammalia |
| Order | Rodentia |
| Suborder | Hystricomorpha |
| Infraorder | Phiomorpha |
| Family | Bathyergidae |
| Genus | Fukomys |
| Species | F. damarensis |
Phylogeographic analyses suggest that this diversity and speciation have been primarily influenced by the physical, ecological, and climatic changes associated with the formation of the African Rift Valley and the resulting major river systems isolating the various species within the clade.16 DMRs naturally occur across much of southern Africa, including Botswana, Namibia, South Africa, Zambia, and Zimbabwe. Here, they are found predominantly in the arid-thornveld biome. The soil is predominantly coarse and sandy in these arid and semiarid regions, and scrubland, thorny bushes, and grasslands dominate the vegetation of this biome. The coarse hard soil arenosols support a patchily distributed diversity of geophytes, including Curcurbitae, Hyanthaceae, and Portulaceae, of which the starchy tubers of the gemsbok cucumber Acanthosicyos naudinianus form the main component of the DMR diet. Large fluctuations in above-ground temperatures are observed both daily and seasonally, although winters are generally cold and dry. This biome is also often punctuated by prolonged periods of drought. The DMRs avoid the climatic extremes by residing in an underground maze of burrows. Living below ground, the amplitude of daily and seasonal temperature fluctuations is markedly reduced, and the relative humidity is high, especially in the deeper situated poorly ventilated dank nests.
Lifestyle
DMRs, both in the wild and in captivity, live in medium-sized social groups with 2 to 41 animals per colony in the wild and up to 48 in our captivity colonies. The average colony size in both the wild and captivity being about 12 to 14 individuals. The DMR, like the NMR, is widely regarded as being a eusocial, cooperative breeder.2,25 Both these species exhibit a reproductive division of labor such that fewer than 1% of individual NMRs reproduce in captivity, and long-term field studies of DMRs estimate that between 10% and 15% of individuals get the opportunity to reproduce over their lifetime.7,21 Dominant individuals monopolize breeding opportunities such that there is a single breeding pair within a DMR colony.22 Although there is no evidence of task specialization among subordinates, these offspring of the breeding pair assist in tunnel maintenance, and foraging, sharing found food with their conspecifics. They may also assist in caring for pups. Although they do not directly supply the pups with food, they do carry food to the communal nests and provide the pups with fecal pellets to inoculate their microbiome. The subordinates also retrieve the pups from burrows if they go wandering and carry them away from areas of disturbance. Through these actions, they reduce the workload of the dominant breeders so as to increase their successful reproduction.
DMRs in the wild live in a complex maze of underground burrows and small chambers, which they dig with their extrabuccal, large procumbent teeth and powerful jaw muscles (Figure 2E). During digging, the mouth is fully open, and oral bristles around the mouth and 2 flaps of skin block the entrance of soil into the mouth. The excavated soil is pushed to the surface to form a series of mounds, below which the burrow system is sealed off. Most of the burrow system consists of shallow foraging tunnels located approximately 15 to 35 cm below the surface.43 These usually terminate at the roots, tubers, and bulbs of geophytes, which constitute the main food sources.28 Foraging was initially thought to be a blind process in which animals randomly dig new burrows until they encounter food sources. However, it is now thought that Bathyergids including Fukomys species rely on chemical signals (kairomones) from nearby plants to direct their foraging excavation endeavors.21 Soil excavation is a metabolically expensive process using 5-fold more energy than that expended at rest, and this metabolic cost increases even further if the soil is dry and hardened.31 Foraging tunnels connect to a deeper, more permanent burrow architecture, including numerous football-sized chambers that are used for nesting or food storage, and smaller latrine chambers are also present. The main nest area may be as much as 2.5 m below the surface and is often filled with dried roots and husks for bedding material. The hollowed-out burrows are approximately 65 to 75 mm in diameter and may be 1 km in length.43 Since this underground maze is sealed off from the surface, it has its own microclimate, containing warm, moist air with low partial pressures of oxygen levels and high levels of carbon dioxide, especially in the nest areas where members of the colony aggregate to sleep. Although difficult to accurately assess based on the presence of mole-rat mounds, the home range of a burrow system is dependent on both the size of the colony and the location and distribution of food sources, with the largest reported home range of 13,000 m2 based on radiotelemetry for a colony of 20 animals in the Gemsbok Kalahari National Park.30
Dispersal commonly occurs after periods of rainfall when the soil is more malleable. Dispersal appears to be male biased, with males dispersing earlier and more frequently than females. Moreover, males are more likely to join established groups, challenging the resident breeding male.47 Not surprisingly, males exhibit shorter philopatry and shorter tenures within their natal system than females. Breeding females, in contrast, are more philopatric, showing prolonged occupancy in their colony, and have lower mortality than subordinates, contributing to longer lifespans (approximately 20 y) of the dominant breeding females than their subordinates (approximately 15 y).39
Reproduction
DMRs are obligate outbreeders, generally mating with unrelated individuals. Although extremely rare, on 2 occasions, after a dominant breeder has died in our captive colonies, siblings within the remaining colony have started breeding and raised subsequent litters. Moreover, while also rare, siblings separated from their natal colony for prolonged periods have also successfully bred, but for the most part, incest is avoided.5 When females acquire dominant status, they show signs of skeletal remodeling such that the lumbar vertebrae increase in length.45 This increase in abdominal size may occur over the first few pregnancies and confers fecundity benefits in that longer bodied breeding females can produce larger litters with heavier pups.45
The breeding female commences courtship by drumming her hind feet and with distinct chirping-like vocalizations. The male and female then chase each other around the tunnel system with the female’s tail raised. She then adopts a lordosis posture, during which time the male restrains her with his forelimbs and by biting on her neck, thereby initiating copulation. Mating is completed with a loud squeal as the animals disengage. Multiple matings may occur over more than a week, and presumably, these cease after the female has conceived.2
Once a breeding pair is established they are capable of breeding throughout the year and can produce up to 3 litters a year with an interbirth interval ranging between 78 to 92 d.1 Litter size generally ranges from 1 to 6 pups (Table 1; Figure 5); however, in captivity, we have had, albeit rarely, litters as large as 8 pups. Pregnant females may increase body mass by as much as 80% (Figure 5). Litter size tends to increase with parity from a mean of 2 ± 0.4 pups for a pair’s first litter to 4.3 ± 0.4 pups by the fourth litter. Pups are born with little to no hair (Figure 5A), their eyes are closed, and they weigh between 8 and 10 g at birth (Table 1). While they begin to eat solids at 6 d of age, they may continue to nurse for more than 28 d (Figure 5G).
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-052
While growth rates vary widely between individuals, growth trajectories follow an unimodal distribution with a reported average growth rate of approximately 0.233 g/d over the first 80 d.1 Pups from the first 2 litters tend to grow faster and attain a greater body mass than those in subsequent litters. Moreover, pups in large-sized colonies exhibit slower growth rates with the body mass at 1 y of age lower than that of individuals in smaller colonies.55 This slower growth rate is attributed to competition among subordinate individuals within the colony rather than functional specialization phenotypes. Male DMRs tend to grow faster, take longer to attain maximum body size, and have higher maximum body size than females.55 Sex differences are most apparent in established larger colonies. Here males, but not females, take far longer to reach 90% of their maximum body mass than when they are born into smaller colonies.
Interestingly, while animals can breed at 9 mo of age, body mass continues to increase for the first 18 mo to 2 y of life.1
DMRs are induced ovulators, ovulating after mating has occurred. The sterile nature of subordinates is attributed to both a lack of copulation opportunities as well as endocrine-dependent reproductive suppression inducing anovulatory traits in subordinate females and reduced sex drive in subordinates of both sexes.49 Incest avoidance is also a strong behavioral driver of nondominant members in their natal colony. When compared with the dominant breeding female, subordinate females have distinct morphologic and endocrine features. They are smaller in size, have less defined genitalia (Figure 4), and have low sex steroid hormone levels. The latter is attributed to reduced sensitivity of the anterior pituitary to gonadotrophin-releasing hormone and inhibited follicle maturation.49,50 While subordinate males have similar anatomic reproductive features (Figure 3) and comparable spermatozoa production to that of breeding males, their sperm has many more abnormalities and is of poorer quality. They also have different expression levels of androgen and progesterone receptors, altering endocrine profiles and concomitant sexual behavior when compared with breeding males.50
Biology
Living in an environment devoid of light, it is unsurprising that DMRs show asynchronous circadian rhythms to changes in light if ambient temperature is held constant. Recent studies simulating daily changes in either ambient temperature or light suggest that temperature, rather than light, is the zeitgeber regulating circadian rhythms and that animals are predominantly active when the temperatures are cooler.19 Animals may be active both during the day and night, commonly sleeping in short bouts throughout the 24-h period.
DMRs in captivity are homeotherms that maintain a low body temperature (Tb) of 35.1 °C when acutely exposed to an ambient temperature ranging between 12 and 33 °C. Their thermoneutral zone ranges between 28 and 31 °C, and they exhibit a low basal metabolic rate of 0.66 mL O2/g/h, 67% of that predicted based on body size.4,6 These thermogenic traits are shared among many other subterranean dwelling rodents, facilitating adequate heat exchange and gas exchange in sealed, poorly ventilated natural burrow systems.4,6 In the wild, Tb varies by more than 1 °C over the course of a day, with larger diel differences evident during winter. Data loggers on free-living DMR have revealed low minimum Tbs of 30 to 31 °C during winter. The average winter Tbs of these free-living DMRs are 32.3 °C for females and 32.5 °C for males, conferring considerable energetic savings by reducing facultative thermogenesis.41
DMRs are herbivores that meet all their nutrient and water requirements from the underground food storage organs they burrow into. These foods tend to have high water content and are highly fibrous, filled with indigestible cellulose and lignin. DMRs retain partially digested foods in an enlarged cecum and hindgut where they rely on a rich microbiome to ferment the insoluble dietary components into volatile fatty acids,12 thereby contributing to the high assimilation efficiency of these foods.6 To further maximize the energy extracted from their diet, DMRs also engage in coprophagy, recycling the nutrients in fecal pellets and obtaining additional protein by digesting the various microbes that are also voided in the feces. Geophytes also have a high mineral content, and even though these rodents are rarely exposed to sunlight and are in a chronic vitamin D impoverished state,8 DMRs can absorb these minerals very efficiently. In particular, the apparent absorption efficiencies of both calcium and inorganic phosphorus approach a physiologic maximum (approximately 90%), and this is attained using vitamin D-independent pathways.36
The deeper, poorly ventilated burrows and chambers where DMRs tend to rest are thought to be both hypoxic (less than 10% oxygen) and hypercapnic (up to 6% carbon dioxide).45,46 Not surprisingly, DMRs, like the NMR, are very tolerant of such hostile underground gaseous atmospheres. However, their responses to hypoxia diverge with the DMR increasing ventilation rather than modifying their metabolic profile, as is observed in NMRs.24,57 DMRs also exhibit enhanced expression of proteins of the globin family (for example, hemoglobin and neuroglobin), improving oxygen absorption and delivery,15 further contributing to their hypoxia tolerance.
Genomics
The DMR genome was completed and annotated in 2014.15 It has provided important comparative genomic insights into the African mole-rat clade. Among the most unique DMR adaptations were those revolving around hypoxia and hypercapnia, highlighting potential mechanisms that may facilitate their extreme tolerance to hypoxia and hypercapnia.56,57 Especially prominent were alterations in genes involved in oxygen capture and transit in DMRs, which saw a 3.4-fold decrease of hemoglobin α mRNA under a hypoxic exposure of 8% oxygen that was not paralleled in the NMR.15 This could counterintuitively point to the DMR’s hypoxic program being geared toward a higher set-point than the NMR, as no change was seen in the NMR at this level of hypoxia.
Following this increased hypoxia tolerance, a large-scale change was observed in the detoxification of ammonia by the urea cycle in the DMR. Several noteworthy genetic and transcriptomic differences are evident, such as the upregulation of arginase 2 (ARG2) expression in DMR liver.15 ARG2 is generally not expressed in this tissue. Higher expression levels of the mitochondrial ornithine transporter ORNT1 (SLC25A15) known to play a role in the urea cycle were also found in both NMR and DMR.17 There is a point mutation in arginase 1 (ARG1), the terminal step in the urea detoxification pathway in both the DMR and NMR.15 This sequence change was hypothesized to improve catalytic efficiency, optimizing the urea cycle and resulting in greater ammonia detoxification.
Interrogating the DMR genome also revealed several potential longevity-associated adaptations as well as provided insights into their pain tolerance and suspected cancer resistance. The DMR, like their NMR relatives, lack a functional Fas-activated serine/threonine kinase (FASTK) gene, which encodes a regulator of Fas-mediated apoptosis. FASTK is overexpressed in tumors and immune-mediated inflammatory diseases. The absence of FASTK likely gives rise to the attenuated inflammatory and oncogenic phenotype observed in both the NMR and DMR.18,52
Apart from these few published reports, the genomic, epigenomic, and posttranslational landscape of the DMR has received scant attention and is wide open to exploration in the presence of interventions and pathologies. Only with additional work quantifying the changes within DMR under physiologic stressors and correlated across species with their close relatives will their unique adaptations to their ecological and evolutionary history be uncovered.
Housing and Husbandry
While several publications have described housing and husbandry conditions for the NMR in detail,26,35,37,40,54 to date none focus on DMR-managed care. There are 4 striking differences between DMRs and NMRs that may influence their care; most notably, they are considerably larger, have a thick insulatory pelage, and can better maintain body temperature using facultative thermogenesis, and their colonies are markedly smaller than those of the NMR.
The original progenitors of our DMR colonies, as well as those in most academic institutions worldwide, were trapped near the town of Hotazel, South Africa (27°58′S, 17°41′E), and near the town of Dordabis in Namibia (22°58′S, 17°41′E). Buffenstein trapped and collected DMRs at both these locations in the 1990s and has kept colonies of DMRs in the United States since 2002. The animal husbandry and breeding protocol for their managed care was approved by the IACUC committee of the University of Illinois at Chicago.
Environmental conditions.
Naturally living in dark, humid, oxygen-poor environments, it would be ideal to house them under similar conditions. However, this is not feasible due to facility limitations and occupational safety governing regulations. Ideally, they should be maintained in the dark or under red light, thereby not altering light-dependent gene expression or interfering with deregulated photoperiodic or circadian endocrine functions. However, if this is not possible, it is advisable to maintain a set light-dark cycle compared with randomly switching on lights to perform health checks, clean, and feed, thereby standardizing any potential impact that exposure to light may have. We routinely house our DMRs under 12 h light:12 h dark, and using successful breeding as our metric, this light regime does not appear to be deleterious.
In the wild, burrow temperatures vary quite considerably from the more superficial burrows to the deep nests.41 Given that they can maintain Tb in captivity over a wide range of ambient temperatures, we maintain animal holding areas at 25 to 29 °C. We also provide animals with a heating pad (for example, K and H Pet Products Outdoor Heated Small Animal Pad) placed under one of the chambers in their system. Generally, animals tend to use the heated chamber as their nest chamber. Ideally, the relative humidity should be maintained at approximately 40% to 50%, levels higher than this lead to excessive condensation in the room and mold growth. Although this humidity is considerably lower than predicted burrow humidity, it is sufficient to diminish insentient water loss. In circumstances where humidity levels are challenging to maintain, an increase in supplemental chamber moisture can be attempted by feeding foods high in moisture content, providing green corn husks, and, if need be, wet paper towels. As a result, the measured humidities in nest chambers commonly reach these levels of relative humidity.
Housing.
DMRs, like NMRs, are housed within multichambered interconnected systems (Figures 6 and 7). These chambers are static polycarbonate commercially available rat or mouse cages fitted with filtered high-profile lids. The cages can be easily sanitized at high temperatures using standard animal facility cage washers. Cages can also be readily replaced if animals have chewed through them and can be easily modified by drilling holes in their sides through which 3-in. diameter acrylic tubing is installed to create tunnels connecting the various chambers. The number of cages within a system is dependent on how many DMRs are present. At a minimum, a 4-chambered U-shaped system is used for housing 2 to 10 individuals so that each colony has 2 blind ending chambers, one of which is used as a toilet, and the other is used as the feeding chamber (Figure 7). The system is expanded as the size of the colony grows, and the design is quite flexible. The number of chambers in a colony tunnel system is primarily dependent on the state of cleanliness of the system. Regardless of the size of the colony, animals tend to huddle on top of each other in a single nest chamber, but larger colonies may need additional chambers for latrines or for distributing food so that subordinates can readily gain access to food, rather than wait for the remnants of favored foods after the dominant animals have finished feeding.
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-052
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-052
There are several different ways to keep the tubing connecting the various cages in place. These can be permanently fixed by gluing or using plumbing junctions. For ease of cleaning, we have used silicone bands placed around the tubes on the exterior of the cages. These prevent the tubes from being pushed into cages and help maintain the integrity of the ‘pseudo-tunnel’ system.
Bedding material.
Various types of bedding material can be used. We have used bedding material commonly used in traditional laboratory rodent housing, although there are pros and cons associated with each of these. For example, wood shavings from pine and ash may have chemicals or odors that may be irritating, causing scratching of the skin and, if coarse, may even lead to splinters. The advantage of wood shavings is that the shavings are less dusty and tend not to stick to the skin. Some types of paper bedding produce fine particles of dust that may block noses and irritate their eyes. Similar to NMRs, paper bedding is the most commonly used material for DMRs and effectively absorbs moisture from urine and foods. Paper towels can also be used in addition to bedding for behavioral enrichment and insulation. In addition, we provide the animals with various types of nesting material, including corn husks and nestlets.
Feeding.
DMRs do not drink free water and meet their water requirements from preformed water in food and water produced during metabolism. Their staple diet consists of sweet potatoes/yams supplemented with seasonal fruits and vegetables such as cucumbers, melons, grapes, lettuce, apples, corn, carrots, and bell peppers (Figures 2 and 6). They are particularly partial to sweet-tasting fruits and vegetables and tend to ignore more bland foods like potatoes and zucchini. Before feeding, foods are soaked for at least 10 min in approximately 2 gallons of water to which a cup of vinegar or lemon juice is added and then thoroughly washed and scrubbed of dirt. Animals are also given a vitamin and mineral-rich, high-protein cereal (for example, Pronutro, Bokomo, South Africa) weekly. DMRs should be fed approximately 12 to 15 g of food plus a 1.5 in. of yam per animal. Therefore, a pair will receive 1 in. of yam and approximately 24 to 30 g of food each day, and a colony of 20 would receive a 10-in. yam plus approximately 240 to 300 g of food each day. The larger colonies are usually fed whole fruits and vegetables as well as a few small pieces. In large colonies, food is placed in multiple chambers to ensure that every animal gets access, not just those that are dominant. This amount ensures there is always some residual food the next day. If females are pregnant or lactating, the high-protein supplement is given more frequently. Animals tend only to eat fresh foods, and as such, except for the yam, old food should be removed daily. If not, old foods will sit and rot and either dry out or go moldy.
Cleaning.
Before entering each animal room, shoes should be disinfected or covered. Gloves should be put on before handling animals or performing husbandry duties. Gloves should be changed for every colony to prevent the transfer of colony-specific odors and potential infectious pathogens between colonies. Daily cage cleaning involves cleaning out the toilets and removing old food. If other chambers are wet or soiled, these should also be changed. Animals tend to move the bedding around, often sweeping it through the tunnels and piling it up in one chamber. If this is done, dry clean bedding should be redistributed throughout the chambers so that animals can continue to assist in cleaning out the tunnels in this way. The animals may be left in their system while this cleaning takes place and become habituated to human contact. A more intensive full change of the colony cages occurs weekly or every other week in smaller colonies or pairs. Here, all cages, connecting tubes, and lids are replaced.
Pup care.
The first few days of life are the most vulnerable for newborn pups; indeed, neonatal and weanling mortality is common. We have found that if a nest chamber (for example, Kaytee hamster/ferret igloo) or an upside-down mouse cage with drilled holes for easy entrance is placed in the nest chamber, the pups will be kept in this enclosed smaller environment and their survival may be substantially higher than those colonies that do not have a smaller nest area within the nest chamber. For the first 3 d after pups are born, care should be limited to feeding only, such that systems are quickly opened and food placed in a chamber where there are no pups. After 3 d, old food can be removed, and toilets can be changed as long as pups are not present in the cage. When there are pups younger than one month, all forms of husbandry should be done as quietly and as quickly as possible so as not to disturb the nursing colony.
Loud bangs, excessive vibrations, and other disturbances may result in the pups being dumped out of the nest, carried around, swept into the toilet, not nursed, eaten by their parents or siblings, or generally neglected. Should the mother show no interest in the pups, it is possible to foster the abandoned pups in another colony that has recently had pups. We have successfully done this by gently opening the lid of the nest chamber and placing the abandoned pup near others.
Enclosure integrity.
DMRs spend a considerable amount of time gnawing on the corners of their cages and on their tubes, and not surprisingly, what with their powerful chisel-like incisors, holes are made in the cages that if ignored can become large enough for animals to escape through. Temporary adhesives can be used to patch up holes, but damaged cages should be discarded. In addition, tubes may be pushed out or chewed breaching the integrity of the closed system. Also, bedding may be piled into the corner of one cage, enabling animals to reach the lid and push it off and escape; placing 2 lids over a cage or a weight on top is often successful in preventing such escapes. Most escapes can be avoided by caretaker attentiveness, ensuring there are no holes in the cages, that lids are properly placed on the cage, tubes are pushed in, and bedding is not allowed to pile up. Lids of small cages can also be made more difficult to come off by holding them in place with an elastic band or a weight. Nevertheless, despite the best efforts to maintain colony integrity, it is not uncommon for animals to chew through their enclosures, dislodge a tube, or knock a lid off and escape. If escapes do occur, as inevitably they will, DMRs are far easier to reintroduce back into their colony than NMRs.51 We normally take all of the animals that have remained in their tunnel system out, count and check all animals are accounted for, and then introduce animals one at a time in a rat cage, watching to check that while they may intensely sniff each other’s faces and genitals, they do not start fighting. After they have settled down together in a rat cage and the cause of the breach has been rectified, animals can be returned to their system.
Enrichment.
Effective enrichments should stimulate many of the behaviors that animals would show in the wild. These can be provided using caging that simulates a burrow system and by housing animals in family groups so that they can conduct their normal range of social interactions. Additional enrichments involve providing animals with novel objects or different substrates. DMRs are extremely curious and will explore blocks of wood, lucerne blocks, pumice stones, and novel foods. They will also take paper towels, corn husks, and nestlets back to the nest chamber to create for themselves a cozy nest. Blocking burrow tubes with whole corn or yams also stimulates burrowing behavior in the tubes and may possibly reduce gnawing on cages in burrow-expanding activities.
Sex identification.
It is relatively difficult for the untrained eye to determine the sex of nonbreeding DMRs. Males are cryptorchid, and the anogenital distance and genital mound are similar between nonbreeding males and females (Figures 3 and 4). Moreover, the vaginal opening of nonbreeders is usually obscured by a membrane. In DMRs, the easiest and most reliable way to identify gender, even in pups 1 mo or older, is to gently place fingers to the right and left of the penis/labia and gently pull these apart. In males, the penis is solid, and the appearance will not change; in females, you will see a separation of the labia and a small vaginal opening.
Establishing breeding pairs.
Before attempting to establish new breeding pairs, the 2 animals to be paired should be removed from their natal colony and kept on their own for a week or more. This period of time is usually sufficient to reactivate hormonal cycling. Also, since DMRs are social and do not like to be isolated, they generally are quite receptive when introduced to another animal. This should be done in a clean/neutral cage. Successful pairing usually involves intense sniffing and nuzzling of the genitals by both the male and female, coupled with distinct vocalizations. This is followed by animals chasing each other around the cage. Mating usually occurs shortly after introduction.
Handling.
While DMRs tame relatively quickly when regularly held and become less aggressive, enabling you to simply pick them up by gently placing your hand around their abdomen, they can be quite aggressive and bite when subject to microchipping. They are very flexible, and when scruffed, if the skin has enough slack, they can turn around and bite the hand holding them. To carefully restrain a DMR, we first scruff the animal at its rump and place it on a hard surface (for example, a table). Thereafter steady-gentle pressure can be applied to the back of the head/neck of the animal to allow one to gain a position with your hand on the back of the animal and restrain it by placing a thumb underneath its chin and the remainder of the hand around the body of the animal. This will effectively immobilize the animal. An assistant can then examine the animal, give it an injection if need be, or implant the unique identifying microchip as discussed in the subsequent section.
Microchipping.
To provide unique identifications for each animal, pups are microchipped at 3 to 4 mo with a unique radio frequency identification device (RFID) number. At this age, they are fully weaned, weigh about 20 g and their chances of survival are high. These microchip transponders (AVID identification systems) are 11 mm in length and 2.1 mm in diameter. They are sealed in bioglass capsules composed primarily of silicon and coated with a Parylene C polymer to encourage tissue encapsulation and reduce migration of the implant. They are implanted subcutaneously on the rump. When activated by a 125-kHz radiofrequency signal emitted by a handheld scanner, the microchip transmits a unique preprogrammed 9-digit identification number back to the scanner.
Health surveillance and care.
As DMRs are often housed in vivaria facilities containing mice, the health surveillance program is targeted toward monitoring murine pathogens (Table 3). Colonies have been tested for a variety of agents by direct sampling, assessment of soiled bedding mouse sentinels, and sentinel-free soiled bedding PCR testing.
| Agent | Quarterly | Annual |
|---|---|---|
| Helicobacter genus | X | X |
| LCMV | X | |
| LDV | X | |
| Mycoplasma pulmonis | X | X |
| MAV 1 and 2 | X | |
| MHV | X | X |
| Mites | X | X |
| MNV | X | X |
| Mouse parvovirus (MPV/MVM) | X | X |
| Mousepox (Ectromelia) | X | |
| EDIM | X | X |
| Pinworms | X | X |
| POLY | X | |
| PVM | X | X |
| REO | X | X |
| SEND | X | X |
| TMEV | X | X |
EDIM, epizootic diarrhea of infant mice; LCMV, lymphocytic choriomeningitis virus; LDV, lactate dehydrogenase-elevating virus; MHV, mouse hepatitis virus; MNV, murine norovirus; MPV, mouse parvovirus; MVM, minute virus of mice; PVM, pneumonia virus of mice; TMEV, Theiler murine encephalomyelitis virus.
In addition to these agents, other institutions have tested DMRs for the presence of Filobacterium rodentium (CAR bacillus), Chilomastix spp., Citrobacter rodentium, Clostridium piliforme, Encephalitozoon cuniculi, Entamoeba muris, Giardia muris, Klebsiella oxytoca, Klebsiella pneumoniae, murine cytomegalovirus, Pseudomonas aeruginosa, Salmonella enterica, Spironucleus muris, and Tritrichomonas muris. To date, the DMR colonies have tested negative for all these agents.
The most common cause of injury is due to fighting. If animals have superficial bite wounds, we clean the wounds and apply Neosporin to the bitten area until the wound has healed. We monitor the animals to see if an infection is evident, as abscesses commonly may occur in the wounded area. Cultures of the pus exudates have revealed that these are primarily due to Staphylococcus bacterial infections. We have successfully treated abscesses with enrofloxacin (Baytril) daily subcutaneous injections at a dosage of 5 to 10 mg/kg for 5 to 10 d. If the animal appears to be experiencing pain as a result of these wounds, meloxicam may be administered at a dosage of 1 to 5 mg/kg SC once daily.
The teeth of DMR should be monitored for malocclusion. Overgrown incisors are very difficult to correct as the teeth are thick and hard to cut and realign; we recommend euthanasia under these circumstances.
In captivity, we find no evidence of any ectoparasites in our colony. However, in the wild, although DMRs have a comparatively low incidence of parasites relative to other rodents, one study reported 5 species of mites and one species of louse parasitizing DMRs. Parasitic infestation was greater during the wet summer months, possibly as a result of increases in humidity, and/or rainfall-induced changes in host dispersal behavior.32
If a colony gets too dirty, such as an individual’s fur getting sticky from contact with watery/sugary fruits (for example, cantaloupe) or excreta, or if there is evidence of other skin-related problems, we may bathe animals in warm water and wash their fur with baby shampoo. Thereafter, we dry them by vigorously rubbing them with a paper towel.
The oldest animal in our colony is currently 20 y old, a remarkable age for a small rodent, and we encounter very few deaths or diseases in our colony. The latter contribute to their unusual linear rather than exponential mortality risk with advancing age.38 Animals older than 15 tend to show graying hair (Figure 2D), especially around the face, and some may lose weight. These old animals are regularly monitored and assessed for body condition to ensure good health.
Pathology
To date, there are only 2 published reports concerning the pathology of DMR. The first documents neoplasia and granulomas in the tissues surrounding RFID chips when these chips were transplanted in the interscapular region, with carcinogenesis attributed to chronic inflammation in that region.42 We have microchipped all our animals and, to date, have not observed any tumors around the implanted RFID chips. It is possible this is a site sensitivity difference as we place our RFID chips on the rump and not into the more metabolically active, brown fat-enriched, interscapular region.
The second published pathology report is an abstract from the American Association of Zoo Veterinarians Conference in 2013. It focused on the presence of a cestode infection in 2 ‘closed’ DMR colonies housed at the Houston Zoo.46 This infection manifested as soft subcutaneous swellings, and they reported that the Taenia infection was successfully treated with oral praziquantel at approximately 30 mg/kg body weight every other day for 20 d.
Overall, mortality in research and zoo-managed colonies is rare, and like NMRs, the DMRs typically have long health and life spans.38 Of the available morbidity and mortality reports from zoo-held animals, DMRs have had various infectious, inflammatory, degenerative, and neoplastic diseases (personal communication, M. Delaney). In our research colony, external lesions included alopecia or regionally extensive hair loss (Figure 8A), and 3 individuals presented with perioral lesions (Figure 8B–D). One was a 12-y-old female who presented with perioral proliferative dermatitis and crusting along the oral commissures (Figure 8B). Histologically, the haired (epidermis) and nonhaired epithelium of the mucocutaneous junction were markedly hyperplastic with exophytic papillary fronds, reminiscent of a viral papilloma, with superficial Candida sp. and bacterial colonization. Despite efforts, papillomavirus DNA was not detected in fresh frozen tissues. At necropsy, this individual had interesting lung changes, considered to be inspissated pulmonary surfactant, as well as other degenerative, presumably age-related lesions including neuronal lipofuscinosis and ocular lenticular degeneration; all of which were incidental to demise. The second was a discrete, smooth, raised erythematous mass, which was biopsied and confirmed as a foreign body granuloma, presumably from embedded feed material or substrate (Figure 8C), while the third was a fleshy, pedunculated mass microscopically consistent with an acrochordon (skin tag) observed on a 7-y-old individual (Figure 8D). A 10-y-old male DMR presented with head tilt and circling and was confirmed with severe unilateral, chronic otitis interna of bacterial origin. Other than these few cases, to date, we have no other pathology reports from our 20-y-maintained captive colony.
Citation: Journal of the American Association for Laboratory Animal Science 63, 6; 10.30802/AALAS-JAALAS-24-052
Conclusions
Interest in unconventional biomedical animal models has grown considerably in recent times as scientists begin to address the overreliance on the traditional “big 5” model systems: yeast, flies, worms, rats, and mice and the constraints these impose in the translation of biomedical research.9 Mole-rats have evolved in a harsh subterranean niche that imposes physiologically challenging conditions that are similar in multiple disease states (for example, ischemia/reperfusion). They clearly have evolved mechanisms to overcome these challenges and thrive in such a hostile habitat. As such, they are well positioned to drastically increase understanding of the physiologic and molecular mechanisms that can be employed to overcome clinical physicochemical challenges and translate these findings into therapeutics for improving human health. Over the last 5 y, the number of laboratories worldwide using DMRs in their research has more than doubled. While many of these ongoing studies involve cell culture or isolated tissues provided by labs or zoos housing this species, there are numerous requests globally for live animals with the hopes of expanding access to this species for a myriad of biomedical research opportunities. It is hoped that we have addressed here the many questions regarding the care for this unique species so that the DMR may realize its vast biomedically pertinent research potential.
The rapid increase in the number of published papers regarding the biology of F. damarensis starting in the 1950s, when there were 2 publications, in contrast to the 400 publications since 2020, resulting in 125 publications per year.
Photographs of Damaraland mole-rats. Images show (A–E) variable coloration (B) the sex differences in body size between males and females, (C) subtle sex differences in head shape, and (D) that DMRs lack external ear pinnae, have small eyes and a pig-like nose, and show graying around the face as they age and (E) have large procumbent extrabuccal teeth and buccal flaps that keep their mouth closed even when they are using their teeth to excavate burrows. Photographs by JA, MR, RB, and TP.
Genitalia of breeding males and subordinate males. Note animals are cryptorchid, lacking a descended scrotal sac with abdominal testes, although in breeding males, the testes below the skin are quite obvious as they often descend into inguinal pockets. Photographs by JA, MR, MS, and TP.
Breeding females can be identified by a patent vagina and prominent mammae. A vaginal closure membrane is often present in nonreproductive females. A vertical fold is apparent between the anal mound and the genital mound. Photographs by JA, MR, MS, and TP.
Photographs of (A) Damaraland newborn mole-rat pups, (B) 3 d-old-pup, (C) 14-d-old pup and her mother, (D) 3-mo-old pup, (E) pregnant female, (F) nursing pups at 2 wk of age, and (G) nursing pups at 4-wk-old. Photographs by JA, MR, RB, and TP.
Damaraland mole-rat housing consists of multiple rat and mouse cages connected by acrylic tubing.
Schematic examples of a simple and expanded tunnel-housing system for Damaraland mole-rats.
Photographs of observed facial pathologies in our Damaraland colony. (A) Alopecia of unknown cause, (B) perioral proliferative dermatitis and crusting along the oral commissures, (C) a foreign body granuloma, and (D) pedunculated mass microscopically consistent with an acrochordon (skin tag).
Contributor Notes