Natural history and burrow system architecture of the silvery mole-rat from Brachystegia woodland

The silvery mole-rat Heliophobius argenteocinereus (Bathyergidae) is a solitary subterranean rodent, widely distributed throughout eastern and south-eastern Africa in a variety of habitats. Here, we provide the first data on its biology in a typical natural habitat, the Brachystegia woodland. The population density of mole-rats was low (4.6 ha⁻¹) and its distribution across the study site was random. Contrary to subadults and pups, the sex ratio of adult mole-rats was highly female biased (1:5.75), probably due to the higher mortality of males as a consequence of their mating strategy. Reproduction of Heliophobius is seasonal and pups are born at the beginning of the hot dry season. Burrow systems of the silvery mole-rat were long, highly branched, reticulated and comparable in fractal dimension to systems of social bathyergids. Variability in burrow architecture was related to the body mass of the burrow occupants, soil hardness and food supply when tested together. Burrow systems with a higher fractal dimension had inhabitants that had a greater body mass. Longer systems were less branched. Nests were typically deeper than foraging tunnels and experienced negligible temperature fluctuations. The microenvironmental characteristics of the subterranean niche including temperature, humidity and soil characteristics are provided for purpose of comparison with other mole-rat species.     

Evidence from intron 1 of the nuclear transthyretin (Prealbumin) gene for the phylogeny of African mole-rats (Bathyergidae)

A 900- to 1100-bp fragment encompassing intron 1 of the nuclear transthyretin (prealbumin) gene was examined in 12 taxa of Old World hystricognath rodents of the families Bathyergidae, Petromuridae, Thryonomyidae, and Hystricidae. Within the Bathyergidae, Heterocephalus glaber (naked mole-rat) was basal, and the other East African species, Heliophobius argenteocinereus (silvery mole-rat), was sister to a southern African clade containing Bathyergus, Cryptomys, and Georychus (dune, common, and cape mole-rats). These results are congruent with studies using mitochondrial 12S rRNA gene sequences. A combined analysis of transthyretin and 12S rRNA data resulted in a well-supported topology with better resolution than either gene analyzed separately. These data support the findings by M. W. Allard and R. L. Honeycutt (1992, Mol. Biol. Evol. 9: 27-40) and R. L. Honeycutt (1992, Am. Sci. 80: 43-53) that complex social systems evolved independently at least twice, in the common and naked mole-rats.     

Home-Range Dynamics in a Solitary Subterranean Rodent

Despite an important role of subterranean rodents as ecosystem engineers, their belowground mobility is poorly documented. It is supposed that their underground burrow systems, once established, are relatively stable because of high-energy costs of digging. We chose the silvery mole-rat, Heliophobius argenteocinereus (Bathyergidae, Rodentia) from mesic Afrotropics as a representative of solitary subterranean rodents to investigate how, and how fast these rodents process their established burrow systems. We combined radio-tracking of individual animals with subsequent mapping of their burrow systems, and we developed a new method for assessing the rate of burrowing. Mole-rats continuously rebuilt their burrow systems; they excavated approx. 0.7 m of new tunnels per day and backfilled on average 64% of all tunnels. On average, every 32 d they established a new nest. They often completely backfilled newly excavated peripheral burrows, while other parts of their burrow systems were more permanent. Their home-ranges were dynamic and continuously shifted in space. Burrow system processing continued even in the advanced dry season, when soil is difficult to work.     

Metabolism and thermoregulation in the Cabrera vole (Rodentia: Microtus cabrerae)

Metabolism and thermoregulation were studied for the first time in the Cabrera vole (Microtus cabrerae), an endemic and threatened rodent of the Iberian Peninsula. Low values of resting metabolic rate (RMR) were registered (1.13 mlO(2) g(-1) h(-1)) at the lower limit of the thermoneutral zone (TNZ) (around 33.5 degrees C). Body temperature increased near the TNZ up to 37.3 degrees C but remained stable, around 36 degrees C, at ambient temperatures below 25 degrees C. Values of thermal conductance remained quite stable at ambient temperatures of 10-25 degrees C (0.144-0.160 mlO(2) g(-1) h(-1) degrees C) and increased to 0.301 mlO(2) g(-1) h(-1) degrees C at 33.5 degrees C. Data revealed that M. cabrerae developed a highly adaptive ability of conserving energy and lowering the metabolic cost of thermoregulation at high ambient temperatures, allowing the body temperature to approximate that of the environment and exhibiting low resting metabolic rate and high conductance.     

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.

     

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.     

A seasonal difference of daily energy expenditure in a free-living subterranean rodent,the silvery mole-rat (Heliophobius argenteocinereus; Bathyergidae)

In seasonal climatic regimes, animals have to deal with changing environmental conditions. It is reasonable to expect that seasonal changes are reflected in animal overall energetics. The relation between daily energy expenditure (DEE) and seasonally variable ecological determinants has been studied in many free-living small mammals; however with inconsistent results. Subterranean mammals, i.e. fossorial (burrowing) mammals which live and forage underground, live in a seasonally and diurnally thermally stable environment and represent a suitable model to test seasonality in DEE in respect to seasonal changes, particularly those in soil characteristics and access to food supply. Both factors are affected by seasonal rainfall and are supposed to fundamentally determine activity of belowground dwellers. These ecological constraints are pronounced in some tropical regions, where two distinct periods, dry and rainy seasons, regularly alternate. To explore how a tropical mammal responds to an abrupt environmental change, we determined DEE, resting metabolic rate (RMR) and sustained metabolic scope (SusMS) in a solitary subterranean rodent, the silvery mole-rat, at the end of dry season and the onset of rainy season. Whereas RMR did not differ between both periods, mole-rats had 1.4 times higher DEE and SusMS after the first heavy rains. These findings suggest that rainfall is an important environmental factor responsible for higher energy expenditure in mole-rats, probably due to increased burrowing activity. SusMS in the silvery mole-rat is comparable to values in other bathyergids and all bathyergid values rank among the lowest SusMS found in endothermic vertebrates.     

The metabolism of social subterranean rodents: adaptation to aridity

Summary The social Damara mole-rat Cryptomys damarensis (124 g), has a mean (±SD) resting metabolic rate of 0.57±0.09 cm³ O₂ g^-1 h^-1, within a thermoneutral zone of 27–31° C. This rate of metabolism is 43% lower than that predicted by the curve for rodents, and 29% lower than that predicted by the subterranean rodent curve. These data support the hypothesis that the resting metabolic rates of social and solitary subterranean rodents are lower than those of solitary species inhabiting mesic habitats. These low resting metabolic rates may represent an energy-saving adaptation to aridity. The energetic cost of burrowing, in relation to the dispersion patterns of food in arid habitats, may explain these low metabolic rates.     

Nucleotide sequence variation in the mitochondrial 12S rRNA gene and the phylogeny of African mole-rats (Rodentia: Bathyergidae).

Mitochondrial DNA (mtDNA) sequence variation was examined in eight taxa of the African rodent family Bathyergidae, as well as in two taxa representative of the Old-World hystricognathid rodent families Petromyidae and Thryonomyidae. A total of 812 bp, constituting domains I-III of the 12S ribosomal rRNA gene, were compared for each taxon. The high levels of intrafamilial mtDNA sequence divergence observed (average 16.8, range 3.5-23.2) support an ancient origin for the five genera, 20-38 Mya. These data do not support the current subfamilial groupings of the Bathyergidae. The eastern African naked mole-rat, Heterocephalus glaber, is the most basal representative of the family, with the silvery mole-rat, Heliophobius, being the next most basal. South African forms [dune, common, and cape mole-rats (Bathyergus, Cryptomys, and Georychus, respectively)] group together. The independent origin of the common mole-rat, relative to the naked mole-rat, suggests that complex social systems evolved in parallel along different bathyergid lineages. The 12S rRNA gene is not evolving at a higher rate within the rodent lineages, relative to that seen for artiodactyls and primates. Bathyergid rodents appear to fall at an extreme end of the spectrum of mammalian variation, with respect to both transition/transversion ratios and divergence, showing much lower transition/transversion ratios than those previously reported for intrafamilial comparisons.     

Ecological constraints drive social evolution in the African mole-rats.

The African mole-rats (family Bathyergidae) are subterranean hystricomorph rodents occurring in a variety of habitats and displaying levels of sociality which range from solitary to eusocial, making them a unique mammalian taxonomic group to test ecological influences on sociality. Here, we use an extensive DNA-based phylogeny and comparative analysis to investigate the relationship between ecology, sociality and evolution within the family. Mitochondrial cytochrome-b and 12s rRNA trees reveal that the solitary species are monophyletic when compared to the social species. The naked mole-rat (Heterocephalus glaber) is ancestral and divergent from the Damaraland mole-rat (Cryptomys damarensis), supporting previous findings that have suggested the multiple evolution of eusociality within the family. The Cryptomys genus is species-rich and contains taxa exhibiting different levels of sociality, which can be divided into two distinct clades. A total of seven independent comparisons were generated within the phylogeny, and three ecological variables were significantly correlated with social group size: geophyte density (p < 0.05), mean months per year of rainfall greater than 25 mm (p < 0.001), and the coefficient of rainfall variation (p = 0.001). These results support the food-aridity hypothesis for the evolution of highly social cooperative behaviour in the Bathyergidae, and are consistent with the current theoretical framework for skew theory.     

Resource characteristics and foraging adaptations in the silvery mole-rat (<Emphasis Type="Italic">Heliophobius argenteocinereus</Emphasis>), a solitary Afrotropical bathyergid

The African mole rats (Bathyergidae) is a rodent family unique for subterranean life and diverse social systems. Solitary species are thought to be confined to areas with abundant, evenly distributed food resources and easily workable soils, which favors early natal dispersal and independent reproduction. However, there is a paucity of empirical data confirming this assumption. We examined ecological conditions of a typical natural habitat of the solitary silvery mole-rat (Heliophobius argenteocinereus), which is the Miombo woodland, and we identified behavioral and other adaptations which potentially improve its foraging success. We also tested food selectivity of captive mole-rats. In the Miombo, mole-rat food resources were clumped, but relatively ample. This, along with a predictable and relatively short period of year with dry and difficult-to-work soil, creates relatively moderate ecological conditions. Analysis of food stores showed that food storing alone probably does not secure enough food to overcome the advanced dry season in this species. In light of this, several other adaptations, such as food generalism and area-restricted search can assist silvery mole-rats retaining positive energy balance during these times. Food-preference tests showed that silvery mole-rats prefer tubers with high sugar content, followed by those with high water content.     

Genetic Signatures for Enhanced Olfaction in the African Mole-Rats

The Olfactory Receptor (OR) superfamily, the largest in the vertebrate genome, is responsible for vertebrate olfaction and is traditionally subdivided into 17 OR families. Recent studies characterising whole-OR subgenomes revealed a ‘birth and death’ model of evolution for a range of species, however little is known about fine-scale evolutionary dynamics within single-OR families. This study reports the first assessment of fine-scale OR evolution and variation in African mole-rats (Bathyergidae), a family of subterranean rodents endemic to sub-Saharan Africa. Because of the selective pressures of life underground, enhanced olfaction is proposed to be fundamental to the evolutionary success of the Bathyergidae, resulting in a highly diversified OR gene-repertoire. Using a PCR-sequencing approach, we analysed variation in the OR7 family across 14 extant bathyergid species, which revealed enhanced levels of functional polymorphisms concentrated across the receptors’ ligand-binding region. We propose that mole-rats are able to recognise a broad range of odorants and that this diversity is reflected throughout their OR7 gene repertoire. Using both classic tests and tree-based methods to test for signals of selection, we investigate evolutionary forces across the mole-rat OR7 gene tree. Four well-supported clades emerged in the OR phylogeny, with varying signals of selection; from neutrality to positive and purifying selection. Bathyergid life-history traits and environmental niche-specialisation are explored as possible drivers of adaptive OR evolution, emerging as non-exclusive contributors to the positive selection observed at OR7 genes. Our results reveal unexpected complexity of evolutionary mechanisms acting within a single OR family, providing insightful perspectives into OR evolutionary dynamics.     

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.     

Prolonged longevity in naked mole-rats: age-related changes in metabolism,body composition and gastrointestinal function

Aging is characterized by declines in all physiological processes and concomitant changes in body composition. Age-related changes in metabolism, body composition and gastrointestinal function were investigated in naked mole-rats (Heterocephalus glaber), rodents that exhibit extended longevity. Maximum lifespan of these 40 g rodents (>27 year) is approximately 9 times greater than predicted allometrically. We investigated changes in basal metabolic rate (BMR), body composition and intestinal glucose transport in 1, 5, 10 and 20-year-old male individuals. Body composition was measured using dual X-ray absorptiometry and activity of sodium glucose co-transporters (SGLT1) determined using everted gut sleeves. One-year-olds had lower body mass than other age cohorts, as they had not attained full adult form. Among the 5, 10, and 20-year-olds, no age-related changes in body mass, BMR, percentage body fat, fat-free mass or bone mineral density were found. SGLT1 activity declined moderately (<20%) from 5 to 20 years and was similar at 10-20 years, whereas age-related declines are 40-60% in mice. Although mole-rats have low metabolic rates, their prolonged longevity results in a lifetime energy expenditure more than 4 times that of mice. Since lifetime energy expenditure is an important index of potential exposure to oxidative damage, naked mole-rats may be valuable for studying mechanisms of aging.     

PHOTIC INDUCTION OF Fos IN THE SUPRACHIASMATIC NUCLEUS OF AFRICAN MOLE-RATS: RESPONSES TO INCREASING IRRADIANCE

African mole-rats (family Bathyergidae) are strictly subterranean rodent species that are rarely exposed to environmental light. Morphological and physiological adaptations to the underground environment include a severely reduced eye size and regressed visual system. Responses of the circadian system to light, however, appear to be intact, since mole-rats are able to entrain their circadian activity rhythms to the light-dark cycle and light induces Fos expression in the suprachiasmatic nucleus (SCN). Social organization varies from solitary species to highly elaborated eusocial structures, characterized by a distinct division of labor and in which one reproductive female regulates the behavior and reproductive physiology of other individuals in the colony. The authors studied light-induced Fos expression in the SCN to increasing light intensities in four mole-rat species, ranging from strictly solitary to highly social. In the solitary Cape mole-rat, light induces significant Fos expression in the SCN, and the number of Fos-immunopositive cells increases with increasing light intensity. In contrast, Fos induction in the SCN of social species was slightly greater than, but not statistically different from, the dark-control animals as is typical of most rodents. One species showed a trend for an increase in expression with increased light, whereas a second species showed no trend in expression. In the naked mole-rat, Fos expression appeared higher in the dark-controls than in the animals exposed to light, although the differences in Fos expression were not significant. These results suggest a gradient in the sensitivity of the circadian system to light in mole-rats, with a higher percentage of individuals that are unresponsive to light in correlation with the degree of sociality. In highly social species, such as the naked mole-rat that live in a relatively stable subterranean milieu in terms of food availability, temperature, constant darkness, and devoid of 24-h cyclic environmental cues, the temporal coordination of rest-wake activities may be dependent on social interactions and social status rather than on photic regulation of the circadian timing system. (Author correspondence: moosthuizen@zoology.up.ac.za)     

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 ).     

Hypothermia versus torpor in response to cold stress in the native Australian mouse Pseudomys hermannsburgensis and the introduced house mouse Mus musculus

This study compared torpor as a response to food deprivation and low ambient temperature for the introduced house mouse (Mus musculus) and the Australian endemic sandy inland mouse (Pseudomys hermannsburgensis). The house mouse (mass 13.0+/-0.48 g) had a normothermic body temperature of 34.0+/-0.20 degrees C at ambient temperatures from 5 degrees C to 30 degrees C and a basal metabolic rate at 30 degrees C of 2.29+/-0.07 mL O2 g(-1) h(-1). It used torpor with spontaneous arousal at low ambient temperatures; body temperature during torpor was 20.5+/-3.30 degrees C at 15 degrees C. The sandy inland mouse (mass 11.7+/-0.16 g) had a normothermic T(b) of 33.0+/-0.38 degrees C between T(a) of 5 degrees C to 30 degrees C, and a BMR of 1.45+/-0.26 mL O2 g(-1) h(-1) at 30 degrees C. They became hypothermic at low T(a) (T(b) about 17.3 degrees C at T(a)=15 degrees C), but did not spontaneously arouse. They did, however, survive and become normothermic if returned to room temperature (23 degrees C). We conclude that this is hypothermia, not torpor. Consequently, house mice (Subfamily Murinae) appear to use torpor as an energy conservation strategy whereas sandy inland mice (Subfamily Conilurinae) do not, but can survive hypothermia. This may reflect a general phylogenetic pattern of metabolic reduction in rodents. On the other hand, this may be related to differences in the social structure of house mice (solitary) and sandy inland mice (communal).     

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.     

Energy and distribution in subterranean rodents: sympatry between two species of the genus Ctenomys

The low basal metabolic rate (BMR) observed in subterranean rodents, compared to that of surface-dwelling species of comparable size, has been proposed to be an adaptation to underground life. Two main hypotheses have been proposed to explain this finding, the cost of burrowing and the thermal stress. The former states that the low BMR is due to the high cost of extending the tunnel system whereas the other relates it to the possibility of overheating in burrows where evaporative and convective heat exchange are restricted. Additionally, both hypotheses related the energetics of subterranean rodent with spatial distribution. The genus Ctenomys is an excellent model to evaluate the cost of burrowing or thermal stress, since they are widely distributed, with members differing markedly in body mass. The aim of this study was to assess digging and basal energetics in two Ctenomys species that live in sympatry in a coastal grassland, but differ in their microspatial distribution by soil preference. We used the obtained energetic data to test both energy-distribution hypotheses. We measured BMR and digging metabolic rate (DMR) through open flow respirometry in two species exposed to soft and hard soils. In brief, DMR in Ctenomys talarum (100-170 g), as in Ctenomys australis (250-600 g), was unaffected by soil hardness. Within thermoneutral zone of each species, DMR/RMR quotient was lower in the smaller species. Our data did not support the thermal stress hypothesis, but the cost of burrowing hypothesis was not rejected. Other alternative hypotheses are proposed to explain the distribution of C. talarum and C. australis.     

Thermoregulatory patterns of two sympatric rodents: Otomys unisulcatus and Parotomys brantsii

1. The adaptations to an arid environment in two closely related rodent species were investigated. 2. The rate of oxygen consumption (VO2), body temperature (Tb), evaporative water loss and minimal conductance in Otomys unisulcatus and Parotomys brantsii were determined under controlled conditions at ambient temperatures (Ta), ranging from 11-31 C. 3. Physiological features atypical of desert-adapted rodents include a basal metabolic rate higher than predicted by body mass, the low "lower critical temperature" and symptoms of heat stress at 31 degrees C. 4. The low Tb and wide thermoneutral zone recorded for both species are characteristic of desert rodent species. 5. These species' physiological abilities reflect their mesic phylogeny and we suggest that behaviour must play an important role in their survival in semi-arid areas.