Acoustic distortion products from the cochlea of the blind African mole rat, Cryptomys spec.

The measurement of distortion-product otoacoustic emissions is a noninvasive method that can be used for assessing the sensitivity and the frequency tuning of nonlinear cochlear mechanics. During stimulation with two pure tones f1 and f2, the acoustic 2f1-f2 distortion was recorded in the ear canal of Cryptomys spec. to study specializations in cochlear mechanics that could be associated with the presence of a frequency expanded cochlear region between 0.8–1 kHz. In addition, a distortion threshold curve was obtained which describes relative threshold of nonlinear cochlear mechanics. Sensitive distortion thresholds could be measured for stimulus frequencies between 0.4 to 18 kHz with a broad minimum between 0.75 to 2.5 kHz. The distortion threshold curve extends to higher frequencies than previous neuronal data indicated.As a measure of mechanical tuning sharpness in the cochlea, suppression tuning curves of 2f1-f2 were recorded. The tuning curves reflected the typical mammalian pattern with shallow low frequency and steep high frequency slopes. Their tuning sharpness was poor with Q10dB values between 0.3 and 1.88. In the range of the frequency expanded region, the Q10dB values were below 0.5. This finding emphasizes that the presence of frequency expansion does not necessarily lead to enhanced mechanical tuning in the cochlea and one has to consider if in certain bat species with cochlear frequency expansion and particularly sharp cochlear tuning, the two phenomena may not be interlinked.Abbreviations CF constant frequency component of echolocation call - STC suppression tuning curve     

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     

Intercolonial encounters and xenophobia in the common mole rat, Cryptomys hottentotus hottentotus (Bathyergidae): the effects of aridity, sex, and reproductive status

The ecological constraints prevalent in arid environments havepromoted the evolution of social groups with a high reproductiveskew in mole rat species occurring there. Outbred social bathyergidsface conflict between maintaining colony integrity to enhancepersonal foraging success and hence survival, and dispersalto maximum individual lifetime reproductive success (LRS). Thisconflict will be a crucial determinant of the response of colonymembers to the presence of foreign conspecifics. We investigatedhow ecological constraints, sex, and reproductive status influencethe outcome of meetings between foreign common mole rats (Cryptomyshottentotus hottentotus) in a series of dyadic encounters. Individualsfrom two localities, at the extremes of an aridity gradient,were used to assess the effects of aridity. The effects of sexand reproductive status were investigated by trials betweenindividuals of differing sex and status. The arid populationrevealed substantially higher levels of rejection than the mesicpopulation. Sex and status played a significant role in moderatingindividual response. For both populations, encounters betweendifferent-sexed individuals produced lower levels of rejectionthan those between same-sexed individuals. For the mesic site,rejection was greatest for encounters between reproductive animals.Conversely, for the arid site, the levels of rejection werecomparatively high and comparable for all combinations of reproductivestatus. Ecological constraints, sex, and reproductive statusare significant factors in interactions between foreign commonmole rats, ultimately influencing individual survival, colonyintegrity and the differential LRS of all colony members. Ourresults provide insight into the evolution and maintenance offamily groups within the bathyergids.     

Constraints of pregnancy and evolution of sociality in mole-rats With special reference to reproductive and social patterns in Cryptomys hottentotus (Bathyergidae,Rodentia)1

Described: basic characteristics of reproduction and pre- and postnatal growth rate and development in Zambian mole-rats, Crypromys hottentotus (Bathyergidae), and compared with the available data on other small subterranean hystricognathous rodents. It is demonstrated that C. hottentotus from relatively rnesic habitats are monoamous and eusocial. The animals in each colony may be divided into several cateories according to their body size and pelage colouration (polymorhim) and prevailing beiavioural pattern (polyethism). Particularly breeding animals are sexually dimorphic. Freuent sexual interactions can be noted in the breeding pair even outside the female's estrus. Growth and sexual behaviour are pheromonally suppressed in the offspring (subordinate animals by their mutual contact and, particularly, by the parents (dominant animals). Most eflicient supression of growth is accomplished by a lactating female. It is arued that eusociality of the naked mole-rat, Heterocephalus glaber, is not qualitatively unique and has not evolved as an immediate response to aridity and dispersed food resources as generally considered. Evidence is presented that eusociality may have evolved as a direct consequence of decreasing the body size combined with retention of a long developmental time (phylogenetic constraint), limitations to store fat reserves, reduction of female's activit and consequent constraints of prenancy. Gestation had to be shortened at the cost of bearing altricial young and prolongation of tie postnatal development. Monogamy and establishment of a caste of helpers may be understood as energy-saving mechanisms of breeding females. Eusociality may be useful for survival in arid habitats, yet reasons for origin should te distinguished from the current (manifold) utility.     

Hearing in coruros (<Emphasis Type=Italic>Spalacopus cyanus</Emphasis>): special audiogram features of a subterranean rodent

Learning curves and behavioural audiograms of subterranean, socially living coruros (Spalacopus cyanus) were obtained using a positive reinforcement conditioning procedure. The individually varying audiograms revealed best hearing at frequencies between 1.25 and 1.6 kHz, which corresponds with the common pattern established in subterranean rodents studied so far. However, the broad hearing range covering frequencies at least between 0.25 and 20 kHz coupled with the high sensitivity (average minimum 7 dB) that is found in coruros are atypical features for audiograms of subterranean rodents, which usually show restricted high-frequency hearing ranges and very poor sensitivity. Hearing at low frequencies (peaks at frequencies <1 kHz), which may be related to sound transmission in underground burrows, and high sensitivity at 1.25/1.6 kHz are discussed in relation to vocalization. In addition to these peaks, a third peak at 8 kHz—probably a plesiomorphic feature of mammals—may be of significance in aboveground communication.     

Factors influencing the distribution of the mole rat, Cryptomys zechi (Rodentia, Bathyergidae) in Ghana

Factors influencing the distribution of the Ghana mole rat Cryptomys zechi were investigated in a small part of the middle Volta basin of Ghana where it is endemic. Soil type appeared to be the most important factor controlling mole rat distribution in the area; the mole rats preferred areas where the soil was silt loam. In areas where mole rats were found, local distribution was influenced by food availability and land preparation methods for farming. The highest concentrations of mole rat colonies were found in farmlands where traditional hoe ploughing is used for land preparation and where plants with underground storage organs were more diverse; the lowest concentration was in farms where mechanized ploughing is used for land preparation.     

Adaptive variation of pelage color within and between species of the subterranean mole rat (Spalax ehrenbergi) in Israel

Summary Color notations of dorsal pelage were analyzed in 451 adult subterranean mole rats, comprising 19 populations and 4 chromosomal species (2n=52, 54, 58 and 60) of the Spalax ehrenbergi superspecies in Israel. In addition, the color notations of soil samples from the collection sites were measured. In an attempt to evaluate the degree of correlation between pelage color of mole rats and the local soil color, each color was compared both macro- and microgeographically. The macrogeographic comparisons were among populations of the 4 chromosomal species, and the microgeographic comparisons were among pairs of geographically neighboring subpopulations which occur under similar climatic conditions but differ in soil type and color. The results indicate that 1. no differences in pelage color notation exist between the sexes; 2. pelage of mole rats is generally characterized by an over-all grayish color, but its shades vary regionally and locally in accordance with varying soil color; 3. a positive correlation was found between pelage and local soil colors in the macrogeographic analysis and this correlation was verified in a microgeographic comparison; 4. specimens from populations of the northern chromosomal species (2n=52, 54) mainly inhabit terra rossa and basalt soils, respectively, with reddish brown tones, and they tend to have a more reddish shade, whereas animals from the central species (2n=58), which inhabits mainly dark soils including alluvial clays, terra rossa and brown rendzina and the souther species 2n=60, which occurs mainly in light soil types such as pale rendzina, sandy loams and loess, tend to have a more yellowish shade. The general appearance of specimens from 2n=58 was dark gray while that of 2n=60 specimens was light gray; 5. mole rats living in xeric environments (particularly 2n=60, but also 2n=54) are lighter than those which live in mesic environments (2n=52, 58). The results support the conclusion that pelage color of strictly subterranean mole rats is subject to selection pressures of overground predation of disharmonious types with their background soil color. The possibility that the pelage color variation and patterns also contributes to better thermoregulation while mole rats are above ground is discussed.     

Foraging in the subterranean social highveld mole-rat (Cryptomys hottentotus pretoriae): an investigation into mass-dependent geophyte use and foraging patterns

The foraging behaviour of captive colonies of the highveld mole-rat Cryptomys hottentotus pretoriae was investigated in an artificial soil-filled burrow system provided with four trays (patches) that varied in geophyte density and mass. An initial trial involving empty trays (only soil) revealed that there was no preference for any specific tray. There were no statistically significant preferences for excavating in any of the patches of different geophyte density. No preferences were evident for excavation in patches containing geophytes of different mass classes. Empty patches seemed to be preferred over patches containing geophytes when combinations of geophyte density/mass were investigated. The duration of handling and the rate of consumption of geophytes were recorded for 23 individuals of two mass classes. Handling time of geophytes was not related to mole-rat sex, but was strongly linked to mole-rat mass class. Handling time of geophytes was related to geophyte mass class. Small geophytes were less profitable to consume. These findings are considered in light of optimal foraging theory and the situation in the field. It was concluded that the mole-rats generally followed the qualitative predictions of optimal foraging theory, although falling short of being energy maximizers.     

The ultrastructure of pinealocytes in the golden mole (Amblysomus hottentotus) with special reference to the granular vesicles

Summary The ultrastructure of the pinealocytes of the golden mole (Amblysomus hottentotus), a blind subterranean mammal living in complete darkness, was examined and compared with that of pinealocytes of other mammals. On the basis of the presence of granular vesicles and glycogen granules, only one population of pinealocytes was found. Large mitochondria, ribosomes, lipid inclusions and scarce lysosomes were observed in the perikaryon. Numerous glycogen granules, often forming typical accumulations, were frequently found to be associated with typical vacuoles. An extraordinary large number of granular vesicles is characteristic of the golden mole pinealocyte. Phenomena of excretion of compound(s) stored in the granular vesicles were identified. Morphologically this is characterized by dissociation of its granular core in situ while its limiting membrane is preserved. The physiological significance of the secretory process characterized by the formation of these granular vesicles is discussed.A portion of these observations were presented at the International Symposium on the Pineal Gland, Jerusalem, Israel, November 14–17, 1977     

Eusociality in African mole-rats: new insights from patterns of genetic relatedness in the Damaraland mole-rat (Cryptomys damarensis)

After the discovery of eusociality in the naked mole-rat, it was proposed that inbreeding and high colony relatedness in this species were the major underlying factors driving cooperative breeding in African molerats. By contrast, field and laboratory studies of the eusocial Damaraland mole-rat (Cryptomys damarensis) have raised the possibility that this species is an obligate outbreeder, although the build-up of inbreeding over several generations could still occur. Using microsatellite markers, we show that most breeding pairs in wild colonies of the Damaraland mole-rat are indeed unrelated (R = 0.02 +/- 0.04) and that mean colony relatedness (R = 0.46 +/- 0.01), determined across 15 colonies from three separate populations, is little more than half that previously identified in naked mole-rats. This finding demonstrates that normal familial levels of relatedness are sufficient for the occurrence of eusociality in mammals. Variation in the mean colony relatedness among populations provides support both for the central role played by ecological constraints in cooperative breeding and for the suggestion that inbreeding in naked mole-rats is a response to extreme constraints on dispersal. Approaches that determine the relative importance of an array of extrinsic factors in driving social evolution in African mole-rats are now required.     

Evolution of crystallins: expression of lens-specific proteins in the blind mammals mole (Talpa europaea) and mole rat (Spalax ehrenbergi)

The mole (Talpa europaea; Insectivora) and the mole rat (Spalax ehrenbergi; Rodentia) both have degenerated eyes as a convergent adaptation to subterranean life. The rudimentary eye lenses of these blind mammals no longer function in a visual process. The crystallin genes, which display a lens-specific expression pattern, were studied in these blind mammals and in related species with normal eyes by hybridizing their genomic DNAs with probes obtained from cDNA clones for alpha A-, alpha B-, and beta Bp-crystallins from calf and gamma 3- crystallin from the rat. For all crystallin genes examined, the hybridization signals of mole and mole rat genomic DNA were comparable, respectively, with those of shrew and of rat and mouse, normal-vision representatives of the orders Insectivora and Rodentia. The expression of the crystallins at the protein level was tested by using antiserum specific for alpha-crystallin in immunofluorescence reactions on lens sections of mole and mole rat eyes and by using antisera against the beta- and gamma-crystallins on sections of the mole eye. All antisera gave positive fluorescence reactions exclusively with lens tissue of these blind mammals, indicating that the crystallins are still normally expressed despite the fact that these lenses have had no function in a visual process in these mammals for at least many million years. These findings apparently imply that some unknown selective advantage has conserved the crystallin genes and their expression after the loss of normal function of the lenses.      

Distribution of NTPDase5 and NTPDase6 and the regulation of P2Y receptor signalling in the rat cochlea

Membrane-bound ectonucleoside triphosphate diphosphohydrolases (E-NTPDases) in the inner ear regulate complex extracellular purinergic type-2 (P2) receptor signalling pathways through hydrolysis of extracellular nucleoside 5′-triphosphates and diphosphates. This study investigated the distribution of NTPDase5 and NTPDase6, two intracellular members of the E-NTPDase family, and linked this to regulation of P2 receptor signalling in the adult rat cochlea. These extracellular ectonucleotidases preferentially hydrolyse nucleoside 5′-diphosphates such as UDP and GDP. Expression of both enzymes at mRNA and protein level was detected in cochlear tissues and there was in vivo release of soluble NTPDase5 and 6 into cochlear fluids. Strong NTPDase5 immunostaining was found in the spiral ganglion neurones and supporting Deiters’ cells of the organ of Corti, while NTPDase6 was confined to the inner hair cells. Upregulation of NTPDase5 after exposure to loud sound indicates a dynamic role for NTPDase5 in cochlear response to stress, whereas NTPDase6 may have more limited extracellular roles. Noise-induced upregulation of co-localised UDP-preferring P2Y₆ receptors in the spiral ganglion neurons further supports the involvement of NTPDase5 in regulation of P2Y receptor signalling. Noise stress also induced P2Y₁₄ (UDP- and UDP-glucose preferring) receptor expression in the root processes of the outer sulcus cells, but this was not associated with localization of the E-NTPDases.     

Electrophysiological responses in guinea pig cochlea to low frequency sound stimuli: distortion of cochlear microphonic (CM) wave form

The present experiment investigated whether or not auditory responses of the middle and/or inner ear in guinea pigs to low frequency sound stimuli [ 60 Hz-2 kHz at 90-120 dB(SPL) ] exhibited the harmonic distortion phenomenon resulting from cochlear microphonics (CM). Measurement of CM leading in turn I by the differential electrode recording method involved measurement of 50 microV isopotential responses, output voltages and CM wave form distortion at each constant sound pressure. The results obtained were as follows: (1) On the 50 microV isopotential response curve and the output voltage curves, the changes at 60-90 Hz were different from those at higher frequencies. (2) At stimuli of 90 or 100 dB(SPL), CM wave form distortion appeared frequently at frequencies below 120 Hz, but were less pronounced above approximately 200 Hz. (3) When raised to 110 and 120 dB(SPL), almost all CM wave forms were distorted at all test frequencies between 60 and 500 Hz. (4) The patterns of CM wave form distortion at frequencies below approximately 120 Hz showed peak clipping and triangular wave distortions, while those at frequencies above approximately 200 Hz showed little of these distortions.