Stanton Braude

Burrow sharing by colonial tuco-tucos (Ctenomys sociabilis)

Preliminary studies indicate that the recently described colonial tuco-tuco ( Ctenomys sociabilis ) is social. As part of efforts to characterize the behavioral ecology of this species, we examined patterns of use of burrows by members of a free-living population of C. sociabilis located in southern Neuquen Province, Argentina. As many as five adults (one male, four females) were captured within a single colony (spatially distinct cluster of burrow entrances). Spatial relationships among eight adults from three colonies were monitored using radiotelemetry. Each animal was active in only a single colony. Within each colony, the areas used by different adults overlapped extensively ( X ≥ 68%) and all animals shared a single nest site. Collectively, these data indicate that a colony of C. sociabilis consists of a single burrow system that may be inhabited by multiple adults. We suggest that comparative analyses of C. sociabilis and other colonial taxa (e.g., colonial African mole-rats; Bathyergidae) will significantly improve our understanding of the factors favoring sociality among subterranean rodents.

Survival of Naked Mole-Rats Marked by Implantable Transponders and Toe-Clipping

We compared survival of free-living naked mole-rats, Heterocephalus glaber , marked by toe-clipping and implantable transponder chips. Although survival was marginally higher for toe-clipped animals than for those with transponder chips in five of six colonies, no significant differences were found between the two marking techniques. Comparison of the costs and benefits of the two marking techniques suggests that toe-clipping is preferable for marking small fossorial mammals in remote areas.

Solitary Burrow Use by Adult Patagonian Tuco-tucos (Ctenomys haigi)

Although >100 Patagonian tuco-tucos ( Ctenomys haigi ) were live-trapped in the Limay River Valley, Rio Negro Province, Argentina, during January 1994, December 1994, and January 1996, no more than one adult was captured per burrow system. Radiotelemetry studies of nine adults revealed that areas occupied by different individuals did not overlap. This pattern of burrow use differed markedly from that reported for the colonial tuco-tuco ( C. sociabilis ), which also occurs in the Limay Valley. We suggest that comparative studies of C. haigi and C. sociabilis will yield important new insights into factors associated with interspecific differences in the social systems of subterranean rodents.

Odours underground: subterranean rodents may not forage "blindly"

Abstract. Subterranean rodents were thought to forage underground for edible roots and bulbs without the benefit of any sensory cues. Subterranean rodents representing four genera from three families (East-Mediterranean Spalax, African Cryptomys and Heterocephalus, and South American Spalacopus), tested in T-mazes filled with soil in which edible plants either had or had not been growing, used odours to discriminate between the soils and chose to dig in the soil containing odorous substances (kairomones) released from roots of growing plants. Such discriminations could enable them to orient their digging toward food sources in the field and thus to improve their foraging efficiency. Spalax blind mole-rats also discriminated between the odours from soil in which edible as opposed to poisonous plants had been growing. These species evolved independently on three continents; thus their abilities probably are characteristic of subterranean rodents in general.

Litter Sizes and Mammary Numbers of Naked Mole-Rats: Breaking the One-Half Rule

Among mammals generally and rodents particularly mean litter sizes usually are about one-half the number of mammae, and maximum litter sizes approximate mammary numbers. Naked mole-rats ( Heterocephalus glaber : Bathyergidae) are exceptions to both generalizations. Field-caught litters averaged 11.3 young ± 6.2 SD ( n = 82), and captive-born litters averaged 11.4 ± 5.6 young ( n = 190). Similarly, numbers of mammae on breeding females averaged 11.6 ± 1.1 ( n = 43) in the field and 11.5 ± 2.0 ( n = 29) in captivity. Maximum litter sizes were 28 in the field and 27 in captivity, whereas the maximum number of mammae was 15. More than one-half of field-caught and captive males and females had different numbers of mammae on the two sides of their body. Neither total numbers of mammae nor fluctuating asymmetries in mammary numbers differed significantly between males and females, nor between breeders and nonbreeders. There was no relationship between litter sizes and numbers of mammae or fluctuating asymmetries in mammary numbers. Breeding female naked mole-rats can bear and successfully rear litters that are far more numerous than their mammae because, on a proximate level, young take turns nursing from the same mammary and, on an ultimate level, breeding females are fed and protected by colony mates, enabling them to concentrate their reproductive efforts on gestation and lactation.

The ontogeny and distribution of countershading in colonies of the naked mole-rat ( Heterocephalus glaber )

Most naked mole-rats Heterocephalus glaber are countershaded, with purple-grey dorsal but pale pink ventral skin. The exceptions to this coloration pattern are uniformly pink, and include newborn pups, most queens and breeding males, and very old animals. Countershading begins to appear at 2‐3 weeks of age and begins to disappear at c. 7 years of age. Countershading may provide camouflage when young naked mole-rats are above ground attempting to disperse. Therefore, reproductives and older workers may lose this coloration once they are unlikely to leave the burrow. Alternative hypotheses for pigmentation that we considered include: thermoregulation, and protection from abrasion or from damaging ultraviolet radiation. These hypotheses are not necessarily mutually exclusive, but do lead to different predictions regarding the development of pigmentation and which colony members should be countershaded.

Monitoring health and reproductive status of olms (Proteus anguinus)by ultrasound

The olm (Proteus anguinus) is a troglomorphic, neotenous amphibian with extraordinary life expectancy and unique adaptations that deserve further investigation. A low reproductive rate and habitat decline render it threatened by extinction. Establishing captive populations for maintenance and artificial breeding may one day become crucial to the species. Longitudinal, in-vivo assessment of inner organs is invaluable to our understanding of reproductive physiology, health, and behavior. Using ultrasound, we measured heart rate and assessed health and reproductive status of 13 captive olms at Zagreb Zoo. Heart rate averaged 42.9 ± 4.6 bpm (32–55 bpm), as determined via pulsed-wave Doppler at 4–12 MHz. By using frequencies of up to 70 MHz (ultrasound biomicroscopy), inner organs were visualized in detail. Assessment of the gastrointestinal tract provided insights into feeding status and digestive processes. Several subclinical pathologies were detected, including biliary sludge, subcutaneous edema, ascites, and skin lesions. Detection of skin lesions by ultrasound was more sensitive than visual adspection. Olms with ultrasonographically detected skin lesions tested positive for Saprolegnia and were treated. Three of the four affected individuals survived and subsequently tested negative for Saprolegnia. Sex was reliably determined; only one individual proved male. The reason for this extreme female-biased sex-ratio remains unknown. However, as most of the individuals were flushed from the caves by strong currents in spring, the sample may not be representative of natural populations. In female olms, different stages of ovarian follicular development were observed with diameters ranging between 0.1 and 1.1 mm. Results were confirmed by comparing ultrasound, necropsy, and histological findings of one dead specimen. In summary, ultrasound proved a valuable tool to support conservation and captive breeding programs by allowing non-invasive assessment of physiological parameters, clinical condition, and reproductive status in olms.

A Hypothesis to Explain Cancers in Confined Colonies of Naked Mole Rats

Naked mole rats (NMRs) are subterranean eusocial mammals, known for their virtual absence of aging in their first 20 to 30 years of life, and their apparent resistance to cancer development. As such, this species has become an important biological model for investigating the physiological and molecular mechanisms behind cancer resistance. Two recent studies have discovered middle and late-aged worker (that is, non-breeding) NMRs in captive populations exhibiting neoplasms, consistent with cancer development, challenging the claim that NMRs are cancer resistant. These cases are possibly artefacts of inbreeding or certain rearing conditions in captivity, but they are also consistent with evolutionary theory. We present field data showing that worker NMRs live on average for 1 to 2 years. This, together with considerable knowledge about the biology of this species, provides the basis for an evolutionary explanation for why debilitating cancers in NMRs should be rare in captive populations and absent in the wild. Whereas workers are important for maintaining tunnels, colony defence, brood care, and foraging, they are highly vulnerable to predation. However, surviving workers either replace dead breeders, or assume other less active functions whilst preparing for possible dispersal. These countervailing forces (selection resulting in aging due to early-life investments in worker function, and selection for breeder longevity) along with the fact that all breeders derive from the worker morph, can explain the low levels of cancer observed by these recent studies in captive colonies. Because workers in the field typically never reach ages where cancer becomes a risk to performance or mortality, those rare observations of neoplastic growth should be confined to the artificial environments where workers survive to ages rarely if ever occurring in the wild. Thus, we predict that the worker phenotype fortuitously benefits from anti-aging and cancer protection in captive populations.

Challenging the inbreeding hypothesis in a eusocial mammal: population genetics of the naked mole‐rat, Heterocephalus glaber

The role of genetic relatedness in the evolution of eusociality has been the topic of much debate, especially when contrasting eusocial insects with vertebrates displaying reproductive altruism. The naked mole‐rat, Heterocephalus glaber, was the first described eusocial mammal. Although this discovery was based on an ecological constraints model of eusocial evolution, early genetic studies reported high levels of relatedness in naked mole‐rats, providing a compelling argument that low dispersal rates and consanguineous mating (inbreeding as a mating system) are the driving forces for the evolution of this eusocial species. One caveat to accepting this long‐held view is that the original genetic studies were based on limited sampling from the species’ geographic distribution. A growing body of evidence supports a contrary view, with the original samples not representative of the species—rather reflecting a single founder event, establishing a small population south of the Athi River. Our study is the first to address these competing hypotheses by examining patterns of molecular variation in colonies sampled from north and south of the Athi and Tana rivers, which based on our results, serve to isolate genetically distinct populations of naked mole‐rats. Although colonies south of the Athi River share a single mtDNA haplotype and are fixed at most microsatellite loci, populations north of the Athi River are considerably more variable. Our findings support the position that the low variation observed in naked mole‐rat populations south of the Athi River reflects a founder event, rather than a consequence of this species’ unusual mating system.

The Multiple Meanings of Inbreeding: The Key to Understanding the Social and Genetic Structure of Subterranean Rodent Populations

Elements of social structure such asmating system or differential dispersal can contribute to genetic structuring of a population. Genetic structure can, in turn, influence the evolution of social structure in a population. While kin selection is the most obvious mechanism for this, lack of dispersal and inbreeding have also been suggested as contributing factors. For example, inbreeding is frequently cited as the explanation for the evolution of eusociality in s based on a number of early assumptions about naked mole-rat natural history, dispersal, and demography. The case of the naked mole-rat illustrates that the term “inbreeding” has a number of biologically andmathematically distinct definitions and is used to describe a range of very different phenomena at the individual andpopulation levels. Examining the differences among these terms allows us to understand how populations of naked mole-rats, and a wide variety of other subterranean mammals, can be ‘inbred’ in the drift or pedigree sense, at the same time that individuals avoid ‘inbreeding’ as their system of mating. It is critical to understand these distinct meanings of inbreeding because only system of mating inbreeding would necessarily foster evolution of eusociality.