Proc. Nat. Acad. Sci. USA Vol. 72, No. 4, pp. 1487-1490, April 1975

The Amphisbaenian Ear: Blanus cinereus and Diplometopon zarudnyi (reptiles/hearing/cochlear potentials/temperature effects) CARL GANS AND ERNEST GLEN WEVER Department of Zoology, The University of Michigan, Ann Arbor, Mich. 48104; and Auditory Research Laboratories, Princeton University, Princeton, New Jersey 08540 Contributed by E. G. Wever, January 24, 1975

ABSTRACT Observations on the structure and func- RESULTS tion ofthe ear in amphisbaenians have been extended to two Anatomical observations new species: to Blanus cinereus of the family Amphisbaen- idae and Diplometopon zarudnyi of the family Trogono- 1. Blanus cinereus. Blanus cinereus is regarded as one of the phidae. most primitive members of the family Amphisbaenidae. Its Blanus, considered one of the most primitive of this is distinctive, differing from that group of reptiles, shows a distinctive form of sound-recep- sound-receptive mechanism tive mechanism. The usual extracolumella is lacking, and of other amphisbaenids thus far studied and more nearly the columella attaches to a cartilaginous plate beneath the resembling that of the bipedid, Bipes biporus (4, 5). Except for skin posterior to the facial area. Diplometopon zarudnyi, Blanus cinereus (and Blanus strauchi), all members of the a highly modified trogonophid, shows a columella and ex- tracolumella of massive dimensions, with considerable cal- family Amphisbaenidae have a columellar system consisting cification of the latter process. of a largely osseous columella (or stapes) and a cartilaginous Cochlear potential measurements revealed the levels of extracolumella. The extracolumella is a long, slender rod auditory sensitivity in these species. A peculiar feature is connecting the head of the columella to a thickened skin layer the degree of stability of the ear's responses in the presence far anteriorly and adjacent to the lower jaw. Its interruption of large variations in body temperature. reduces auditory sensitivity, from which it is evident that This report continues our series of observations on the struc- sound is transmitted from the facial plates of skin through the

ture and function of the amphisbaenian ear with results on extracolumellar rod to the columella and finally via the coch- two additional species. Blanus cinereus (Vandelli) 1797 may lear fluids to the hair cells. well be the most primitive species of the family Amphis- Bipes biporus lacks an extracolumella. The columella alone baenidae and Diplometopon zarudnyi Nikolski, 1907 is a makes a connection between the receptive skin surface and the derived form of the family Trogonophidae. Three specimens inner ear (4). The region of optimum sound reception in this of each species were examined, first for auditory sensitivity species is far back on the head; the columella runs outward in terms of cochlear potentials, and then, after histological and a little posteriorly and its end rests against the inner preparation, for details of anatomical structure. Some of the surface of the skin near the neck constriction. Vibrations of the cochlear potential results have already been published in a integumentary segments of the neck region are apparently discussion of the effects of body temperature on the electrical conveyed directly to the inner ear. potentials of the cochlea (1). Cochlear potential measure- Blanus cinereus (and B. strauchi) resembles Bipes in lacking ments on these animals, in contrast to observations by similar the forward-running extracolumellar rod and in retaining an methods in other reptiles, have shown a high degree of uni- epihyal horn lying adjacent, but free in the otic region. The formity in the presence of wide changes in body temperature. oval window faces almost ventrally, and the shaft of the columella extends ventrolaterally from its footplate (Fig. 1). METHOD The end portion of this shaft changes from bone to cartilage The Blanus cinereus were obtained from Spain and the Diplo- without any other discontinuity, and ends well below the skin metopon from Kuwait and Dharan, Saudi Arabia (1). Various surface, medial to a muscle mass representing an anterior specimens including Blanus strauchi were dissected f6r com- expansion of the constrictor colli. A thin plate of cartilage parison. The animals, under anesthesia, were stimulated with attaches to the outer end of this head portion of the columella. a series of pure tones while cochlear potentials were recorded As represented in the cross-sectional view of Fig. 2, this plate with an electrode in contact with the perilymph of the saccule. extends a little posteriorly but slants mainly anteriorly and The stimulating sounds were produced by a loudspeaker and laterally, passing anterior of the muscle just referred to, to conducted through a tube whose end was located over the become embedded in a dense mass of connective tissue be- lateral surface of the head, and sealed there with a narrow neath the dermal layer of the skin. The form of the columella is ring of cotton fibers impregnated with petroleum jelly. shown in the sketch (Fig. 3). At the end of the electrical measurements, the animals were Evidently sound passes from the surface of the skin through perfused through the circulatory system by means of a can- the underlying mass of connective tissue to the terminal plate, nula inserted through the ventricle into ascending aortic then through this plate to the head and body of the columella, vessels, and the tissues of the head were then processed by the and finally to the inner ear. In Blanus the receptive surface celloidin method and sectioned serially. lies relatively anterior to that in Bipes, but still well posterior For further procedural details, see our earlier reports (2, 3). to the facial area utilized by other amphisbaenians. 1487 Downloaded by guest on October 2, 2021 1488 Zoology: Gans and Wever Proc. Nat. Acad. Sci. USA 72 (1975)

t Anterior - Lateral Terminal plate

Mandible -

Headpiece FIG. 3. The columellar system of Blanus cinereus, including the Columella terminal plate. Scale X 17.5.

3. The Inner Ear. The inner ears of Blanus cinereus and Quadrate Diplometopon zarudnyi exhibit no outstanding departures from these structures in other amphisbaenians. Counts of hair cells in the auditory papilla are shown in Table 1. Cochlear potential studies FIG. 1. The ear region of Blanus cinereus. Right side, ventral view. Scale X4. 1. Blanus cinereus. At the time the cochlear potentials of Blanus cinereus were studied, we were still uncertain about 2. Diplometopon zarudnyi. In the trogonophid Diplometopon the form and location of the sound-receptive mechanism. Ac- zarudnyi the otic capsules form lateral extensions in the ex- cordingly the sound tube was given a variety of locations along treme posterior region of the skull (compare ref. 6). The oval the side of the head in different trials. Fig. 5 shows results ob- window lies in a niche and faces ventrolaterally. The sturdy tained in one of the animals with this tube applied at three columella has a conical shaft extending anteroventrally and locations: far anteriorly over the lower lip (curve a), over both also laterally. To this attaches a large extracolumella, shaped lips, close to the angle of the mouth (curve b), and over the like a roughly triangular blade, that extends anteriorly over posterior edge of the lower lip (curve c). the lateral surface of the skull (see Fig. 4). These curves indicate the sound pressure in decibels relative The posterior third of the extracolumella is cartilaginous to 1 dyne/cm2 required for a standard response of 0.1 ,NV. with a heavily calcified outer surface. More anteriorly there The three functions show variations, but have much the same is a core of hyaline cartilage and an outer layer of bone. The general form, with best sensitivity indicated over the low- anterior edge is solid bone, and is embedded in the deep part frequency range up to 2000 Hz, and a very rapid decline of of a greatly thickened dermal layer in the region of the second sensitivity for the higher tones. and third infralabial segments. This extracolumella completely In another specimen (Fig. 6), two positions of the sound traverses the third infralabial segment and enters the second tube were used, one about the middle of the lower lip (curve infralabial for a short distance, reaching a point well anterior a), and another far posteriorly below the corner of the mouth to the quadrato-mandibular articulation. Because of the (curve b). The second position proved superior to the other, great thickening of the skin, the mechanical coupling of this and indeed gave the best results of any of the trials, with points element appears to contact the first infralabial and extend around -20 decibels for tones between 800 and 1500 Hz. through the entire region of the second, as well as connecting The curve continues to indicate a fair degree of sensitivity to the supralabial region (6). for the lower tones, but rises rapidly, showing a progressive

Footplate Headpiece Terminal plate \ ~~~I

- Skin surface Otic capsule

Brain

FIG. 2. Cross-sectional view of the columellar system of Blanus cinereus. Anterior is above, lateral to the right. Scale X25. Downloaded by guest on October 2, 2021 Proc. Nat. Acad. Sci. USA 72 (1975) The Amphisbaenian Ear 1489 TABLE 1. Number of hair cells in auditory papilla Number of hair cells Species Animal no. Left ear Right ear Blanus cinereus 2019 51 48 2020 53 49 Mean of 4 ears 50.2 Columella / Diplometopon zarudnyi 2021 85 76 2022 73 84 2023 71 76 Mean of 6 ears Extracolumella 77.5 FIG. 4. The columellar system of Diplometopon zarudnyi, in a lateral view. Scale X 12.5. columellar system as a conductor of vibrations in Diplometo- pon zarudnyi was tested by cutting the connection between columella and extracolumella, while leaving all other struc- loss of sensitivity, as the frequency is increased beyond 2000 tures intact. The resulting loss of sensitivity is represented Hz. in Fig. 9. As will be seen, this loss varies with frequency, with These functional observations are in harmony with the abrupt changes around 500-600 Hz where it goes from 31 to location of the terminal plate of the columellar system poste- 10 decibels; it averages around 20 decibels. It is of interest rior to the facial area. that the disrupted columellar system is still useful for tones as 2. Diplometopon zarudnyi. Sensitivity curves for two speci- high as 7000 Hz. mens of Diplometopon zarudnyi are given in Fig. 7. In both animals the sound tube was located over the facial area just DISCUSSION anterior to the angulUs oris, covering the two lips about The two species investigated add further to our understanding equally in one (solid line), but shifted a little more ventrally of the curious transformations of the auditory apparatus that in the other (broken line). For both curves the sensitivity is have taken place among amphisbaenians. These transforma- greatest in the region of 700-1500 Hz, is rather well main- tions appear to reflect two conflicting influences adaptations tained for the tones below this range, but falls off rapidly as of skull structure that have occurred in connection with the the frequency is increased beyond 1500 Hz. adoption of a burrowing habit coupled with the conversion Results for a third specimen are shown in Fig. 8. The sound of the skUll into an excavating tool, and the overbearing need tube covered the lips about equally and runs were made at for an animal living in dark subterranean tunnels to retain its cloacal temperatures of 28° and 37°. As reported previously, capacity for the use of auditory cues to locate prey. there appears to be no systematic variation with temperature In a sense, the genus Blanus stands intermediate between within the range investigated. The curves show a region of Bipes biporus and the other amphisbaenids in the form of the good sensitivity around 500-1500 Hz, as well as another favor- sound-receptive mechanism, as its sound-receptive surface able region at 200 Hz. As in the preceding specimens, the lies anterior to that employed in Bipes, but well posterior to sensitivity declines rapidly in the high frequencies. the facial area used in other amphisbaenids. Both Bipes and The efficacy of the skin as a receptive surface and of the Blanus lack the advantage of having the sound-receptive sur- face near the anterior end, where acoustic signals from preda- 1 2 3 4 5 6 7 891 2 3 4 5 6 7 891 tor or prey located ahead in the tunnel would best be received. +80 _ ~~~~~~~~~~~~~~~ACTTT'1 C While auditory signals reach the sensitive skin areas after passing alongside the head, they will be somewhat attenuated, +60A and removed from the jaws relative to which the information has to be processed. 'a b

2 3 4 5 6 7 891 2 3 4 5 6 7 891 +60 - 1000101

~+40 _

100 1000 10,000 0 Frequency c~ FIG. 5. Auditory sensitivity functions in a specimen of Blanus -20~~~~~~~~~U cinereus, obtained with three locations of the sound tube: a, an- 100 1000 10,000 teriorly over the lower lip; b, over both lips close to the angulis oris; Frequency and c, over the posterior edge of the lower lip. These curves indi- FIG. 6. Sensitivity functions in a second specimen of Blanus cate the sound pressure, in decibels relative to 1 dyne/cm2 re- cinereus, with the sound tube in two locations: a, middle of lower quired to produce a standard response to i AsV. lip, and b, below the corner of the mouth. Downloaded by guest on October 2, 2021 1490 Zoology: Gans and Wever Proc. Nat. Acad. Sci. USA 72 (1976)

12 3 4 5 6 7 891 2 3 4 5 6 +40 0 3 5 6 78891 3 5 7 891 r T T T -1- 56+8910 I I I I 'Ai zo + 20 09 20 I4 ( 0 401 in 100 1000 10,000 a) _ Frequency ai -20 FIG. 9. The loss of sensitivity in a specimen of Diplometopon ~~~~=rs s 4 A A~5 b~ L71ll~~~~~~ zarudnyi resulting from severing the connection between colu- mella and extracolumella. -40 100 1000 10,000 Frequency baenians, and is superior to a number of them. The hair-cell FIG. 7. Sensitivity functions in two specimens of Diplometopon population of 77.5 (the mean of six ears) compares favorably zarudnyi. with others for which determinations have been made, which extend from 38.2 in Chirindia langi to 153.6 in Amphisbaena The size of the hair-cell population in Blanus cinereus, alba and average 76.8 for the nine species reported earlier (3). determined as 50.2 (mean of four ears) is comparable with Diplometopon zarudnyi, as noted above, represents one of that of Bipes biporus, reported earlier as 54.1 (4). All of these the most derived members of the family Trogonophidae, observations support suggestions in the literature that Blanus characterized by a shortened and highly angled skull. It is here represents the most primitive surviving amphisbaenids. compared with Trogonophis wiegmanni, the least modified The ear of Diplometopon zarudnyi is remarkable in the large member of this group and the only other one for which sensi- size of the columella and extracolumella, and the considerable tivity measurements have been carried out. In earlier experi- ossification of the surface portions of the latter process. This ments (2) one specimen of Trogonophis wiegmanni, which has conductive structure therefore is massive in relation to that a short, simple rod-shaped columella, was found to show a of other species (6). This massiveness is of course relative, maximum of -36 decibels for a tone of 500 Hz, and another and in such small animals its absolute magnitude is not very specimen gave best points at -33 decibels for the region 500- great, and in some degree this factor is balanced by an increase 700 Hz. The performance of Diplometopon zarudnyi as re- in the effective surface of the overlying skin. As the results ported here is on about the same level with the best ear shown show, this ear performs as well as that of most other amphis- in Fig. 7 reaching -37 decibels at 900 Hz. The frequency regions for best response are also similar, with the region for Diplometopon perhaps extending a little higher, though ir- regularities in the curves make the comparison difficult. Thus, the high modification of the extracolumella in Diplometopon appears to incorporate an effective compensation for the structural changes in the skull associated with a new method of tunnelling (5). 1. Gans, C. & Wever, E. G. (1974) "Temperature effects on hear- ing in two species of Amphisbaenia," Nature 250, 79. 2. Gans, C. & Wever, E. G. (1972) "The ear and hearing in Amphisbaenia (Reptilia)," J. Exp. Zool. 179, 17-34. 3. Wever, E. G. & Gans, C. (1973) "The ear in Amphisbaenia (Reptilia); further anatomical observations," J. Zool. 171, 189-206. 4. Wever, E. G. & Gans, C. (1972) "The ear and hearing in Bipes biporus (Amphisbaenia: Reptilia)," Proc. Nat. Acad. Sci. USA 69, 2714-2716. 5. Gans, C. (1974) Biomechanics, an Approach to Vertebrate 100 1000 10,000 Biology (J. P. Lippincott Co., Philadelphia, Pa.). Frequency 6. Gans, C. (1960) "Studies on amphisbaenids (Amphisbaenia, FIG. 8. Sensitivity functions for a specimen of Diplometopon Reptilia). 1. A taxonomic revision of the Trogonophinae," zarudnyi obtained at two body temperatures as indicated. Bull. Amer. Mus. Nat. Hist. 119, 131-204. Downloaded by guest on October 2, 2021