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

Summary Both 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-rat's 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.Abbreviations PPM pulses per min - SB simulated burrow - SD standard deviation - SPL sound pressure level     

Hearing in blind subterranean Zambian mole-rats (Cryptomys sp.): collective behavioural audiogram in a highly social rodent

Thresholds for pure tone detection were examined in the common mole-rat, Cryptomys sp. (Bathyergidae, Rodentia) using a positive reinforcement procedure. To bypass the problems connected with testing isolated individuals of this extremely social species, a collective behavioural audiogram was determined for a family group of seven mole-rats. Within the tested frequency range of 225 to 18 kHz, the lowest thresholds (as low as 7.5 dB SPL, on average 24 dB SPL) were found at 800 Hz, the upper limit of hearing (at the level of 60 dB SPL) was at 18 kHz. The behavioural audiogram combines the results of previous studies on hearing in this species. It resembles the distortion threshold curve but differs from neurophysiological data as far as the high frequency cutoff is concerned. On the other hand, the region of the best hearing sensitivity is narrow in behavioural audiogram and neurophysiological curves but rather broad in the distortion threshold curve. In general, the behavioural audiogram of Cryptomys is in many aspects comparable with the available audiograms of other subterranean rodents. Accepted: 18 February 1997     

Seismic communication signals in the blind mole-rat (Spalax ehrenbergi ): electrophysiological and behavioral evidence for their processing by the auditory system

Based on morphological and behavioral findings we suggest that the seismic vibratory signals that blind mole-rats (Spalax ehrenbergi) use for intraspecific communication are picked up from the substrate by bone conduction and processed by the auditory system. An alternative hypothesis, raised by others, suggest that these signals are processed by the somatosensory system. We show here that brain stem and middle latency responses evoked by vibrations are similar to those evoked by high-intensity airborne clicks but are larger in their amplitudes, especially when the lower jaw is in close contact with the vibrating substrate. Bilateral deafening of the mole-rat or high-intensity masking noise almost completely eliminated these responses. Deafening also gradually reduced head-drumming behavior until its complete elimination about 4–6 weeks after surgery. Successive vibrations, at a rate of 0.5 vibrations/s, elicited prominent responses. At rates higher than 2 vibrations/s the amplitude of the brain stem response did not change, yet the middle latency response disappeared almost completely. It is concluded that the seismic signals that mole rats use for long distance communication are indeed processed primarily by the auditory system. Accepted: 11 May 1998     

Caecal function provides the energy of fermentation without liberating heat in the poikilothermic mammal,Heterocephalus glaber

Summary This paper presents the first report of a mammalian internal organ with a lower temperature than its abdominal surrounds. Heterocephalus glaber, the naked mole-rat, is a poikilothermic mammal that leads a strictly subterranean existence and consumes a high proportion of fibre in its diet. The fermentation chamber in these animals appears to absorb rather than generate heat and the temperature in it was consistently 1.2±0.5°C (n=17) lower than rectal temperature. A caecum with a lower temperature than its abdominal surrounds provides an internal heat sink which could be advantageous for metabolic heat dissipation in the plugged humid burrows in which the naked mole-rat permanently resides.Abbreviations RH relative humidity - T _b body temperature - T _c caecal temperature - T _i intraperitoneal temperature - T _r rectal temperature     

Existence of serotonin and neuropeptides-immunoreactive endocrine cells in the small and large intestines of the mole-rats (Spalax leucodon)

The present study was conducted to clarify the regional distribution and relative frequency of endocrine cells secreting serotonin, substance P (SP), cholecystokinin-8 (CCK-8), vasoactive intestinal polypeptide (VIP) and neurotensin in the small and large intestine of the mole-rats (Spalax leucodon), by specific immunohistochemical methods. In the small and large intestine of mole-rats (Spalax leucodon), serotonin, SP and VIP were identified with various frequencies, but CCK-8 and neurotensin were not observed. Most of the IR cells in the small and large intestine were located in the intestinal crypt and epithelium however, they were more frequency in the intestinal crypt. Serotonin-IR cells were detected throughout the whole intestinal tract, predominantly in the duodenum and colon. SP-IR cells were demonstrated throughout the whole intestinal tract except for the ileum and rectum with highest frequencies in the cecum. VIP-IR cells were found in all parts of the small intestine except for the large intestine.In conclusion, the general distribution patterns and relative frequency of intestinal endocrine cells of the mole-rats (Spalax leucodon) was similar to those of some rodent species. However, some species-dependent unique distributions and frequencies characteristics of endocrine cells were also observed in the present study.     

Kidney concentrating ability of a subterranean xeric rodent,the naked mole-rat (Heterocephalus glaber)

The naked mole-rat (Heterocephalus glaber) is a strictly subterranean mammal inhabiting the arid zones of north-east Africa. These animals have no access to free water and water balance thus might be facilitated by regulating renal water loss. The urinary concentrating ability of the naked mole-rat was determined using five dietary manipulations in which both water and salt content were altered. Control animals (n=7) received a high quality protein cereal mixed to a thin paste with water (1 g cereal: 85 g water). Water stress was induced by reducing the water content of the diet by either 50% (n=7) or 65% (n=7). Salt loading was facilitated by replacing the water with the same volume of either 0.9% salt (n=7) or 3.0% salt (n=4) solutions. Changes in body mass, food consumption and urine volume were measured daily. The effect of diet on osmolality and electrolyte concentrations of urine and plasma were determined on termination of the diet trials. Although energy intake was not reduced, naked mole-rats lost body weight with both water stress treatments. Urine volume voided per day decreased significantly with both water stress treatments (P<0.05), such that the most extreme water stress led to an 80% reduction in urine volume. Mildly salt-loaded animals gained weight, yet underwent a sodium diuresis, as indicated by a 1.3-fold increase in the daily volume of urine voided (P<0.05). Maximum urine concentration (1521±250 mmol·kg^-1) was achieved with mild water stress and was 4.6±0.9 times that of plasma. Neither further water stress nor salt loading further increased urine osmolality (P>0.05). The naked mole-rat exhibits a moderate kidney concentrating ability and cannot maintain plasma osmolality or body mass with either extreme water stress or salt loading. Although this species succesfully inhabits arid zones, survival in these areas is not facilitated by renal water conservation, but rather by their underground existence in a microhabitat where humidities are high and radiant heat loads low. In this milieu a moderate kidney concentrating ability is adequate.Abbreviations Bm body mass - ESL extreme salt load - EWS extreme water stress - MSL mild salt load - MWS mild water stress     

Thermoregulatory differences in African mole-rat species from disparate habitats: Responses and limitations

Individuals and populations possess physiological adaptations to survive local environmental conditions. To occur in different regions where ambient temperature varies, animals must adopt appropriate thermoregulatory mechanisms. Failure to adjust to environmental challenges may result in species distributional range shifts or decreased viability. African mole-rats (Bathyergidae) occupy various habitats in sub-Saharan Africa from deserts to montane regions to mesic coastal areas. We examined thermoregulatory characteristics of three African mole-rat species originating from disparate (montane, savannah, and arid/semi-arid) habitats. Animals were exposed to various ambient temperatures, whilst core body temperature and the surface temperature of different body parts were measured. Oxygen consumption was determined as a measure of heat production. Core body temperatures of Natal (montane) mole-rats (Cryptomys hottentotus natalensis) increased significantly at ambient temperatures >24.5 °C, while those of the highveld (Cryptomys hottentotus pretoriae) (savannah) and Damaraland (Fukomys damarensis) (arid/semi-arid) mole-rats remained within narrower ranges. In terms of surface temperature variation, while pedal surfaces were important in regulating heat loss in Natal and Damaraland mole-rats at high ambient temperatures, the ventral surface was important for heat dissipation in Damaraland and highveld mole-rats. This study provides evidence of the variation and limitations of thermo-physiological mechanisms for three mole-rat species relative to their habitats. Information on physiological adaptations to particular habitats may inform predictive modelling of species movements, declines, and extinctions in response to a changing environment, such as climate change.