Topography of vibration frequency responses on the bony tympano-periotic complex of the pilot whale Globicephala macrorhynchus.

In modern Cetacea, the ear bone complex comprises the tympanic and periotic bones forming the tympano-periotic complex (TPC), differing from temporal bone complexes of other mammals in form, construction, position, and possibly function. To elucidate its functioning in sound transmission, we studied the vibration response of 32 pairs of formaldehyde-glutaraldehyde-fixed TPCs of Globicephala macrorhynchus, the short-finned pilot whale (legally obtained in Taiji, Japan). A piezoelectric-crystal-based vibrator was surgically attached to a location on the cochlea near the exit of the acoustic nerve. The crystal delivered vibrational pulses through continuous sweeps from 5 to 50 kHz. The vibration response was measured as a function of frequency by Laser Doppler Vibrometry at five points on the TPC. The aim of the experiment was to clarify how the vibration amplitudes produced by different frequencies are distributed on the TPC. At the lowest frequencies (<12 kHz), no clear differential pattern emerged. At higher frequencies the anterolateral lip of the TP responded most sensitively with the highest displacement amplitudes, and response amplitudes decreased in orderly fashion towards the posterior part of the TPC. We propose that this works as a lever: high-frequency sounds are most sensitively received and cause the largest vibration amplitudes at the anterior part of the TP, driving movements with lower amplitude but greater force near the posteriorly located contact to the ossicular chain, which transmits the movements into the inner ear. Although force (pressure) amplification is not needed for impedance matching in water, it may be useful for driving the stiffly connected ossicles at the high frequencies used in echolocation.

Effects of age and size in the ears of gekkotan lizards: auditory sensitivity, its determinants, and new insights into tetrapod middle-ear function.

Do related, differently sized species differ in size-related structural or functional traits merely because they mature at different points of a uniform allometric ontogenetic growth curve, or do they evolutionarily diverge? We tested ears of gekkotan lizards through experiments distinguishing the two. Auditory sensitivity was assessed by compound action potential (CAP) thresholds in eight species. The best thresholds characterizing species ranged 22-72 dB sound pressure level at 0.5-1.0 kHz. Direct acoustic stimulation of the columella footplate elevated thresholds by 25-50 dB. Intraspecific CAP sensitivity was primarily affected by body length in Eublepharis macularius, but by tympanic-membrane velocity in Oedura marmorata. The chief factor determining middle-ear function (difference in CAP sensitivity before and after middle-ear ablation) was body length in both species. A secondary factor was the middle-ear hydraulic lever ratio in E. macularius, but the mechanical lever in O. marmorata. When intra- and interspecific data were compared, the relation of CAP thresholds to body size in E. macularius resembled the interspecific regression. The intraspecific regression of auditory sensitivity over tympanic membrane velocity in O. marmorata differed from that calculated interspecifically. Hence, the evolutionary contribution to size effects on CAP sensitivity exceeds the ontogenetic contribution. Putatively, body length affects CAP sensitivity through absolute sizes of tympanic membrane and columella footplate. These newly discovered effects join those of the hydraulic lever and (interspecifically) hair-cell number to improve the hearing of larger species that vocally communicate across wider spaces, apparently throughout the Tetrapoda.

First evidence of unicellular glands in the general epidermis of terrestrial reptiles.

The general integument of reptiles is traditionally defined as being dry, but we report here the discovery of unicellular mucoid glands (UCMG) in the dorsal skin of lizards of the genus Phelsuma (Gekkonidae). To this end, the skin of these lizards and of some others for comparison was studied by scanning electron microscopy and transmission electron microscopy. These photographs showed that the development and function of the UCMGs are related to the skin's sloughing cycle. The UCMGs differentiate at scattered locations from Oberhäutchen cells of the inner (new) epidermal generation, above the differentiating beta-keratin layer. While the inner generation matures, the UCMG increases in size; unlike the surrounding Oberhäutchen cells, it does not develop the spinules that characterize gecko skin. When, upon sloughing, the inner generation becomes the new outer generation, and the Oberhäutchen forms the skin surface, the UCMGs, several per scale, dot the surface as mucus-inflated "blebs" projecting from the surrounding spinulate Oberhäutchen, each nesting in a shallow pit of the underlying beta-keratin. On the surface, the UCMGs rupture and the mucus appears to dissipate in cords, flowing over the tips of the spinules, and incorporating minute foreign bodies. It is concluded that, due to the low wettability of the spinulate surface (derived from the spacing of the spinules), the cords brush off easily, with the mucus functioning as a cleaning agent.

Analysis of the locomotor activity of a nocturnal desert lizard (Reptilia: Gekkonidae: Teratoscincus scincus) under varying moonlight.

1. This project seeks to identify determinants of the variation observed in the foraging behavior of predatory animals, especially in moonlight, using a lizard as a model. 2. Moonlight generally enhances the foraging efficiency of nocturnal visual predators and often depresses the locomotor activity of prey animals. Previous evidence has indicated for three different nocturnal species of smallish gecko lizards that they respond to moonlight by increasing their activity. 3. In this study some aspects of the foraging activity of the somewhat larger nocturnal psammophilous Teratoscincus scincus, observed near Repetek and Ashgabat, Turkmenistan, were significantly depressed by moonlight, while several confounding factors (sex, maturity, size, sand temperature, hour, prior handling and observer effect) were taken into account. 4. This behavioral difference may relate to the eye size of the various species. 5. Additionally, a novel method of analyzing foraging behavior shows that in this species the duration of moves increases the duration of subsequent stationary pauses. Measurement of locomotor speed, yielding an average speed of 220% of the maximum aerobic speed, indicates a need for these pauses. Secondarily, pause duration decreases the duration of subsequent moves, precluding escalation of move duration. 6. The results of this and related projects advocate the taking into account of physiological and environmental factors that may affect an animal's foraging behavior.

Eye size in geckos: Asymmetry, allometry, sexual dimorphism, and behavioral correlates.

The function of the vertebrate eye depends on its absolute size, and the size is presumably adapted to specific needs. We studied the variation of eye size at all levels, from intra-individual to inter-specific, in lid- less, spectacled, gecko lizards (Gekkonomorpha). We mea sured 1,408 museum specimens of 62 species, representing subfamilies Diplodactylinae, Gekkoninae, and Sphaerodactylinae. Intra-individually, eye size showed significant directional asymmetry in Stenodactylus sthenodactylus. A latitudinal study of six species confirmed that during postnatal ontogeny eye size undergoes conventional negative allometry; the slope is steeper among adults than among juveniles, expressing the need of juveniles for relatively larger eyes. Within species with sexual size dimorphism, commonly the larger sex possessed larger eyes in absolute terms but not relative to head-and-body length. Interspecifically, eye size showed negative allometry, with slope significantly steeper than those of intraspecific ontogenetic allometry, again expressing the need of juveniles for relatively larger eyes and showing that eye-size differences among species do not merely result from body-size differences. Finally, adult eye size varied interspecifically in correlation with parameters of behavioral ecology: eyes were significantly larger in nocturnal than in diurnal species, and significantly larger in cursorial than in scansorial species.

Effects of age and size in the ears of gekkonomorph lizards: middle-ear morphology with evolutionary implications.

The function of the ear depends in part on its absolute size and internal proportions. Thus, in both young individuals and small species, the middle ear is expected to be allometrically enlarged despite its smaller absolute size. Here we aim to compare the ontogenetic allometry of relevant middle-ear structures as observed within gecko (gekkonomorph lizards) species, with the evolutionary allometry observed interspecifically. These observations also provide middle-ear data for future evaluation of variation in auditory sensitivity. The material comprised 84 museum specimens of geckos, representing nine species of three gekkonomorph subfamilies. The results of dissections and measurements show that different reports notwithstanding, the middle-ear ossicular chain is indeed structured as described for geckos by Werner and Wever. Some sexual dimorphism is indicated, but this requires further study. During postnatal ontogeny, the allometric growth in the ratio of the columellar footplate area to body length differed between the intraspecific and interspecific levels, hence species differences in the middle ear do not merely result from animal size. The ratio of the tympanic membrane area to the columellar footplate area increased during ontogeny. In this, geckos resemble birds and probably also mammals. Similarly, when the comparison was among adults representing different species, the ratio of the tympanic membrane area to the columellar footplate area increased with body size. In this, however, the geckos differed from birds and mammals, in which this ratio varied taxonomically, irrespective of body size. It would thus seem that middle-ear proportions have evolved among geckos to produce small interspecific differences, but among amniote tetrapods they have evolved according to different principles in the classes reptiles, birds, and mammals.

Age effects and size effects in the ears of gekkonomorph lizards: inner ear.

Audiograms have indicated greater auditory sensitivity in larger than in smaller geckos; part of this difference, interspecifically and intraspecifically, is explained by middle-ear proportions. To investigate the contribution of the inner ear to the variation in sensitivity, we examined it in museum specimens representing 11 species and three subfamilies. We measured papilla basilaris length, and, when intact, the saccular otoconial mass. Papilla length approximated 1% of rostrum-anus length in large geckos but 2% in small geckos; in some species some inter-aural difference was indicated. Over the lumped material, relative papilla length varied as a function of body length, with highly significant correlation. Similar relations prevailed within each subfamily. However, intraspecifically the correlation of papilla basilaris length with animal size was usually nonsignificant. Hair cell populations assessed from SEM photographs were larger in the larger species but intraspecifically did not relate to an individual's size. Hence interspecifically, the dependence of auditory sensitivity on animal size seems supported by inner-ear differences but intraspecifically this relation derives only from the middle ear. Otoconial mass, as measured by its volume, was correlated with animal length both interspecifically and intraspecifically.

Mechanical leverage in the middle ear of the American bullfrog, Rana catesbeiana.

Textbooks lump the middle ears of 'submammalian Tetrapoda' as being 'one-ossicle ears'. Conventionally the anuran middle ear is depicted with a shaft-like skeletal unit connecting the tympanic membrane to the inner ear. This shaft comprises mediad a long bony columella and laterad a short cartilaginous extracolumella. But dissection of Rana catesbeiana ears showed: the extracolumella, as long as the columella, is proximally expanded in the vertical plane, forming dorsal and ventral heads. The medio-dorsal head is movably jointed to the columella, between these two there is an obtuse angle ventrad; the extracolumellar medio-ventral head is anchored by a ligament to the middle-ear cavity ceiling. When the tympanic membrane moves outwards, pulling the extracolumella, the medio-dorsal head of the extracolumella must be forced inwards, rotating on the ventral anchorage, pushing the columella towards the inner ear. The ossicular chain thus includes a mechanical lever, possessing the magnitude of the ratio length:width of the extracolumella; this is additional to the lever known from the columellar footplate, which rotates on its firm ventral attachment. These levers are confirmed physiologically, by the difference between the inner-ear sensitivity (shown by isopotential audiograms of microphonic potentials) when stimulated by a vibrator first at the tympanic membrane, then at the proximal stump of the amputated columella. Perusal of the primary literature showed that this morphology is widespread among anuran ears.

Effects of age and size in the ears of gekkonomorph lizards: middle-ear sensitivity.

Previous studies of electrophysiological audiograms in gekkonomorph lizards revealed greater sensitivity in adults than in juveniles. We investigated whether this difference, as far as it is affected by the middle ear, is due to animal age or size. The velocity transfer function of the tympanic membrane (TM) was examined using laser interferometry in nine samples: adults of three large gekkonomorph species, adults of three small species (each related to one of the former), and juveniles of the large species, their sizes matching those of the small-species adults. Each transfer function exhibited an inverted 'V' or 'U' shape, with the velocity of TM motion peaking in the mid-frequency range and becoming poorer at lower and higher frequencies. Among samples, maximum TM velocity correlated with animal length, perhaps because of a damping change in the larger TM. The frequency at maximum velocity negatively correlated with measurements of TM area. Presumably, with a larger TM area, the best frequency shifted downward because of increased middle-ear mass or decreased stiffness. The bandwidth of the transfer function negatively correlated with animal length, being broader in smaller animals and sharper in larger animals. This effect can be attributed to increased mass, decreased damping, or both. Among the middle-ear morphological measurements, the one most closely correlated with body length was the length of the extracolumellar anchorage at the TM. Among the physiological variables investigated, maximum velocity was negatively correlated with the frequency at which it occurred. The dependence of these transfer function variables on animal and ear size was similar, regardless of whether the comparison was among adults of species of different sizes, or among age classes within a species, so that age differences appear to be largely the result of size differences.

The middle ear of gekkonoid lizards: interspecific variation of structure in relation to body size and to auditory sensitivity.

Wishing to assess the effects of the dimensions of the middle ear on the auditory sensitivity of gekkonoid lizards, we measured middle ear components in preserved geckos, which in life had yielded 'cochlear microphonics' audiograms. We examined two to seven specimens of 14 species. The measures of middle ear elements varied relative to head or body length similarly within species and among species. The areas of the external ear opening, tympanic membrane, and columellar footplate, and the ratio between the last two ('hydraulic lever'), were correlated with animal length. The hydraulic and mechanical (extracolumellar) lever ratios appeared to complement each other, the former being emphasized in large animals, the latter in small animals. The apparent auditory sensitivity correlated with the sizes of the animal, head and external ear opening, and negatively (insignificantly) correlated with the mechanical lever ratio. The correlation of sensitivity with the hydraulic lever was insignificant, perhaps due to a 'tympanic membrane lever' (catenary effect). The most sensitive frequency negatively correlated with the area of the external ear opening, the area of the tympanic membrane, and with the level of greatest sensitivity. It was positively correlated with the relative length of the cartilaginous portion of the ossicular chain. However, the number of hair cells in the basilar papilla, too, is known to correlate with animal size. Moreover, the least sensitive species were not only the smallest species, they were also the species known to lack a zone of unidirectional hair cells in the basilar papilla. Hence the apparent sensitivity hypothetically depends on both middle ear dimensions and summation of inner ear output. This hypothesis requires verification by other methods.