J. U. M. Jarvis

Mammalian eusociality: a family affair

Comparative studies of two species of mole-rat are helping to clarify the ecological correlates of mammalian eusociality. Both species live in social groups composed of close kin, within which breeding is restricted to one female and one to three males. They inhabit xeric areas with dispersed, patchy food and unpredictable rainfall. During droughts, they can neither expand their tunnel systems nor disperse. In brief periods after rain the animals must cooperate and dig furiously to locate rich food patches. By living in groups, arid-zone mole-rats can take full advantage of windows of opportunity when conditions are right for burrowing. Thus, ecological factors and kin selection have apparently interacted in the evolution of eusociality in these species.

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.

The Social Structure and Reproductive Biology of Colonies of the Mole-Rat, Cryptomys damarensis (Rodentia, Bathyergidae)

Cryptomys damarensis occurs in semiarid regions of southwestern and central Africa. It lives in colonies in which reproduction is restricted to one or two of the largest-sized males and the largest female in the colony. Some division of labor, into frequent and infrequent workers, occurs within the remaining colony members. Reproduction and details of colony size and the number of breeding animals in a colony are described for two complete and five incomplete wild-captured colonies; one of the complete colonies and mole-rats from the incomplete colonies were maintained in captivity for more than 2 years. The breeding female initiated precopulatory behavior, mating occurred for about 2 weeks, the gestation length was 78–92 days, mean litter size was 2.8 ( n = 10), and a maximum of four pups was born. The pups were precocial, wandered out of the nest within 24 h after birth, began to eat solids when 6 days old, and were fully weaned when 3 weeks old. When 6 weeks old, pups began to spar with each other and with some colony members, but levels of aggression were never high and the pups were incorporated into the colony. In the colony, subordinate and frequent-worker mole-rats weighed less than dominant animals and infrequent workers; mass, therefore, was not necessarily indicative of the age of the animal. During the 2-year study period, three mole-rats that were frequent workers on capture changed their castes to infrequent workers, two of them showed a concomitant increase in body mass. The colony structure and reproduction of C. damarensis are compared with those of the eusocial Heterocephalus glaber .

Daily and seasonal temperatures in the burrows of African rodent moles

Burrow, soil surface and ambient temperatures were compared in mesic and xeric climates in tropical and sub-tropical Africa. In all the areas studied there was a similar daily pattern of change in the temperature of the soil surface and the burrow air but the magnitude of the changes and the time of day at which maximal and minimal temperatures occurred varied. In all areas, changes in burrow temperature were of a lower amplitude than those above ground. Mean monthly subsoil temperatures show that the annual amplitude of temperature fluctuation is greatest at the soil surface and diminishes with increasing depth. At depths greater than 0,6 m mean annual soil temperatures vary minimally. The depth at which this occurs is to some extent dependent upon soil type and ground cover. The mean daily burrow temperatures also show that the daily amplitude of temperature fluctuation is less with increasing depth. The subtropics show marked seasonal changes in mean burrow and above-ground temperatures whereas the ava...

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

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

Phylogeographical patterns of genetic divergence and speciation in African mole-rats (Family: Bathyergidae)

African mole‐rats are subterranean Hystricomorph rodents, distributed widely throughout sub‐Saharan Africa, and displaying a range of social and reproductive strategies from solitary dwelling to the ‘insect‐like’ sociality of the naked mole‐rat, Heterocephalus glaber. Both molecular systematic studies of Rodentia and the fossil record of bathyergids indicate an ancient origin for the family. This study uses an extensive molecular phylogeny and mitochondrial cytochrome b and 12s rRNA molecular clocks to examine in detail the divergence times, and patterns of speciation of the five extant genera in the context of rift valley formation in Africa. Based on a value of 40–48 million years ago (Myr) for the basal divergence of the family (Heterocephalus), we estimate divergence times of 32–40 Myr for Heliophobius, 20–26 Myr for Georychus/Bathyergus and 12–17 Myr for Cryptomys, the most speciose genus. While early divergences may have been independent of rifting, patterns of distribution of later lineages may have been influenced directly by physical barriers imposed by the formation of the Kenya and Western Rift, and indirectly by accompanying climatic and vegetative changes. Rates of chromosomal evolution and speciation appear to vary markedly within the family. In particular, the genus Cryptomys appears to have undergone an extensive radiation and shows the widest geographical distribution. Of the two distinct clades within this genus, one exhibits considerable karyotypic variation while the other does not, despite comparatively high levels of sequence divergence between some taxa. These different patterns of speciation observed both within the family and within the genus Cryptomys may have been a result of environmental changes associated with rifting.

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.

The use of seismic signals by fossorial southern african mammals : A neuroethological gold mine

Behavioral adaptations exhibited by two African fossorial mammals for the reception of vibrational signals are discussed. The Namib Desert golden mole (Eremitalpa granti namibensis) is a functionally blind, nocturnal insectivore in the family Chrysochloridae that surface forages nightly in the Namib desert. Both geophone and microphone recordings in the substrate suggest that the golden mole is able to detect termite colonies and other prey items solely using seismic cues. This animal exhibits a hypertrophied malleus, an adaptation favoring detection of low-frequency signals. In a field study of the Cape mole-rat (Georychus capensis), a subterranean rodent in the family Bathyergidae, both seismic and auditory signals were tested for their propagation characteristics. This solitary animal is entirely fossorial and apparently communicates with its conspecifics by drumming its hind legs on the burrow floor. Auditory signals attenuate rapidly in the substrate, whereas vibratory signals generated in one burrow are easily detectable in neighboring burrows. The sensitivity to substrate vibrations in two orders of burrowing mammals suggests that this sense is likely to be widespread within this taxon and may serve as a neuroethological model for understanding the evolution of vibrational communication. Neuroethological implications of these findings are discussed.

Seismic signal transmission between burrows of the Cape mole-rat, Georychus capensis

SummaryBoth seismic and auditory signals were tested for their propagation characteristics in a field study of the Cape mole-rat (Georychus capensis), a subterranean rodent in the family Bathyergidae. This solitary animal is entirely fossorial and apparently communicates with its conspecifics by alternately drumming its hind legs on the burrow floor. Signal production in this species is sexually dimorphic, and mate attraction is likely mediated primarily by seismic signalling between individuals in neighboring burrows. Measurements within, and at various distances away from, natural burrows suggest that seismic signals propagate at least an order of magnitude better than auditory signals. Moreover, using a mechanical thumper which could be triggered from a tape recording of the mole-rats seismic signals, we established that the vertically-polarized surface wave (Rayleigh wave) propagates with less attenuation than either of the two horizontally-polarized waves. Thus, we tentatively hypothesize that Rayleigh waves subserve intraspecific communication in this species.

Extraordinary life spans of naked mole-rats (Heterocephalus glaber)

Senescence is the internal physiological deterioration that accompanies advancing age. Evolutionary hypotheses predict that rates of senescence should vary directly with extrinsic mortality and inversely with fecundity. If so, naked mole-rats (Heterocephalus glaber) should live a long time (senesce slowly) because in nature they inhabit heavily protected burrows, and large, old breeding females make disproportionate reproductive contributions. In addition H. glaber has an exceptionally low metabolic rate, which may reduce oxidative stress. We have maintained naked mole-rats in captivity since 1974. Here we report that individuals can live a very long time: many are alive after more than 20 years and some are 26 years old (and counting). Although we do not yet know how long naked mole-rats can live, they already are older than the maximum longevity of all but one of 156 rodent species that have been maintained in captivity from birth to death.