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Home , Ear canal, Outer ear, Round window, Spiral ligament, Spiral limbus, Tympanic cavity

7

PREFACE

The first International Symposium on the Biology papers were read by title and are included either in of the Seal was held at the University of Guelph, On­ full or abstract form in this volume. The 139 particip­ tario, Canada from 13 to 17 August 1972. The sym­ ants represented 16 countries, permitting scientific posium developed from discussions originating in Dub­ interchange of a truly international nature. lin in 1969 at the meeting of the Marine In his opening address, V. B. Scheffer suggested that Committee of the International Council for the Ex­ a dream was becoming a reality with a meeting of ploration of the Sea (ICES). The culmination of such a large group of pinniped biologists. This he felt three years’ organization resulted in the first interna­ was very relevant at a time when the relationship of tional meeting, and this volume. The president of ICES marine mammals and man was being closely examined Professor W. Cieglewicz, offered admirable support as on biological, political and ethical grounds. well as honouring the participants by attending the The scientific session commenced with a seven paper symposium. section on evolution chaired by E. D. Mitchell which The programme committee was composed of experts showed the origins and subsequent development of representing the major international sponsors. W. N. this amphibious group of higher vertebrates. Many of Bonner, Head, Seals Research Division, Institute for the arguments for particular evolutionary trends are Marine Environmental Research (IMER), represented speculative in nature and different interpretations can ICES; A. W. Mansfield, Director, Arctic Biological be attached to the same fossil material. Readers of this Station, Fisheries Research Board of Canada (FRB) volume should be aware of such differences when read­ represented the International Commission for North­ ing the papers in this section. The twelve papers of west Atlantic Fisheries (ICNAF); and K. S. Norris, S. H. Ridgway’s section on functional illus­ Director, Marine Council Executive Com­ trated the fundamental structure of the seal, as well mittee, represented the International Biological Pro­ as its associated control mechanisms. R. J. Schusterman gram (IBP). The Food and Agriculture Organization followed this theme by introducing ten papers on be­ of the United Nations (FAO) also offered its support haviour. He established a major focus on social or­ to the programme and ICNAF has contributed to the ganization and communication and their association financing of this volume. with the functional anatomy of the pinnipeds. D. E. Sponsors of national origin were the Fisheries Re­ Sergeant chaired the population dynamics section of search Board of Canada (FRB), the National Re­ seven papers, covering the modelling of populations search Council of Canada (NRCC), the Canadian and method of analysis of seal populations around the National Sportsmen’s Show (CNSS), the World Wild­ world. In the fifth section, J. R. Geraci, by means of life Fund (Canada) (WWF), and the University of papers and a panel discussion dealt with the care and Guelph. management of captive pinnipeds. W. N. Bonner co­ In his preliminary remarks Professor Ronald intro­ ordinated a presentation in the broad area of ecology, duced the representatives of these groups; namely J. R. and was able to bring together studies on environmen­ Weir, Chairman, Fisheries Research Board of Canada; tal factors and their associated behavioural and gene­ S. Bata, International Director and J. S. McCormack, tic control systems. The physiology section was chaired Director, World Wildlife Fund (Canada); and R. T. by H. T. Andersen, his introductory remarks forming D. Birchall, President, Canadian National Sportsmen’s the initial paper of the section. The other six papers Show and a Director of WWF (Canada). of his section emphasized the underwater responses of W. C. Winegard, President of the University of seals. The final and general section, chaired by J. E. Guelph, welcomed participants to the symposium and King, offered a broad coverage of several of the more commented particularly on how pleased he was to interesting areas in various disciplines. welcome representatives from so many countries. Later, A. W. Mansfield acted as rapporteur for the entire at a banquet sponsored by the Department of the En­ programme, and his report stressed the need for con­ vironment, Canada, he offered an invitation to the tinued cooperation by all biologists so that they might group to return in 1975 for a Second International understand seals and their importance to environmen­ Seal Symposium. tal studies. Altogether 62 papers were presented. A further 14 This volume includes with one exception, those pa- 8 K. Ronald pers either presented, read by title, or abstracted, but mammals of the world’ by D. W. Rice and V. B. the continuing discussion on the biology of the seals Scheffer (U.S. Fish and Wildlife Service, Washing­ led to one further paper that is included here. Some ton, 1968) has been used as the standard reference on of the discussion was formal and, where recordable, is nomenclature. included here, but by far the greater part of discussion The work of the chairmen of each of the seven sec­ was informal and hence must remain as extremely tions of this volume is especially recognized. As well, valuable, but merely mental recollections of the par­ the convenor wishes to thank the programme com­ ticipants in the symposium. mittee for their ability to support a somewhat unortho­ The symposium achieved its purpose of bringing dox procedural system, and particularly the sponsors together scientists interested in the Pinnipedia and it ICES, ICNAF, IBP, CNSS, FRB, NRCC, WWF (Ca­ offered leads into the international examination of nada), FAO, and the University of Guelph for their marine mammals. valuable financial assistance. The editors with little apology recognized that they The convenor is most grateful to Mr. H. Tambs- have not reached a completely uniform format in this Lyche, General Secretary of ICES, for his advice and volume since they have allowed use of both English encouragement from the embryonic stages of the sym­ and metric systems of measurement and both English posium to the publication of the proceedings; he also and North American word usage for the sake of har­ recognizes the considerable amount of expert help pro­ mony. The main editorial structure has been the con­ vided by A. W. Mansfield in co-editing this volume. sistency of usage throughout a particular paper. Finally, the effort put into both the symposium and Attempts have also been made to attain a fairly this volume by Mrs. Ginny Bandesen has been beyond uniform taxonomy for the species, but where there has measure, but I hope that she will accept the results of been any doubt caution has not overridden clarity. As the symposium recorded here as tangible proof of her in other mammalian groups, the systematics of the most valuable contribution. To the members of the Pinnipedia are still open to much interpretation. The Dean of the College of Biological Science’s office, the references are cited according to an Annotated Biblio- university support staff and our host Dr. W. C. Wine- praphy on the Pinnipedia*. The ‘List of the marine gard, I express on behalf of the participants and my­ self, our sincerest thanks. * Ronald, K., L. M. Hanly and P. J. Healey, College of Bio­ K . Ronald, logical Science, University of Guelph, Ontario, Canada. Convenor

The following have kindly acted as Discussion Care and Management Section Leaders of the different Sections and also assisted in J. R. Geraci the editing of the contributions: Department of Zoology, University of Guelph, Guelph, Ontario, Canada. Evolution Section Ecology Section E. D. Mitchell Arctic Biological Station, Fisheries Research Board W. N. Bonner of Canada, Ste. Anne de Bellevue, Quebec, Canada. Seals Research Division IMER, c/o Fisheries Labora­ tories, Lowestoft, Suffolk, England.

Functional Anatomy Section Physiology Section S. H. Ridgway H. T. Andersen School of Anatomy, University of Cambridge, Nutrition Institute, University of Oslo, Blindern, Cambridge, England. Oslo, Norway.

Behaviour Section General Session R. J. Schusterman J. E. King Department of Psychology, California State University Department of Zoology, University of New South Hayward, California 94542, U.S.A. Wales, Kensington, N.S.W., Australia.

Population Dynamics Section Summary D. E. Sergeant A. W. Mansfield (Rapporteur) Arctic Biological Station, Fisheries Research Board of Arctic Biological Station, Fisheries Research Board Canada, Ste. Anne de Bellevue, Quebec, Canada. of Canada, Ste. Anne de Bellevue, Quebec, Canada. 102

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 169: 102-111. 1975.

AQUATIC ADAPTATIONS IN THE OF THE HARP SEAL PAGOPHILUS GROENLANDICUS (ERXLEBEN, 1777)

F. R a m p r a s h a d Department of Zoology, College of Biological Science, University of Guelph, Guelph, Ontario NIG 2WI, Canada

INTRODUCTION strands of the extrinsic auricular muscles are also It is difficult to carry out an anatomical study of inserted on to an aponeurosis in the membranous part any organ without making some reference to the of the . The action of the extrinsic muscles function of the organ. This is clearly illustrated in all is to open and close the external orifice, i.e. the the anatomical studies of the human ear, and also in membranous part of the outer ear. the studies of the cetacean ear by Reysenbach de The intrinsic auricular muscles, mainly the well- Haan (1957), Fraser and Purves (1960) and more developed helicis and antitragicus (muscles number 7 recent studies by Wever et al (1971, 1971a, 1971b) and 8, Fig. 61), constrict the membranous part of the and Ridgway et al (1972). It is even more interesting outer ear at the internal pinna. The contraction of to compare the structure of the ear of aquatic mam­ the tragicus medialis (muscle number 6) opens this mals with that of terrestrial mammals. Differences in part of the meatus (Fig. 61 b). The opening and clos­ the structure between the two types of reflect ing of the outer is a two-fold process. First­ adaptations to differences in the animals’ habitats. ly, an internal constriction of the membranous part of Since phocids are considered to be both aquatic and the outer ear at the internal pinna occurs, and this is terrestrial, it would be expected that their ears would followed by a later closing of the external orifice. This reflect a transition between aquatic and terrestrially internal constriction may manifest itself in the cres­ adapted ears. The aim of this paper is to present the centic appearance of the eternal orifice. This mechan­ differences between the structure of the ear of the ism may explain the delay in the closing of the external harp seal Pagophilus groenlandicus (Erxleben, 1777) orifice after the seal has submerged. An open external and that of terrestrial mammals, and the possible sig­ orifice therefore, does not indicate the outer ear canal nificance of their function in the ear of the seal. A is open to water, since it may be closed internally. more detailed anatomical description of the ear of the Another modification is the presence of large blood harp seal can be obtained from other publications sinuses within the wall of the outer ear and the thick (Ramprasad et al, 1971, 1972, 1972a, 1973). collagenous and elastic fibers within the wall of the outer ear canal. The cartilaginous wall of the outer ear canal consists of a single strip of auricular carti­ DISCUSSION lage, which is further subdivided into four confluent Outer ear plates wrapped incompletely around the ear canal One of the major modifications in the ear of the (Fig. 62 a). These plates correspond to the course of harp seal is the loss of the external pinna, and the the external ear canal. Thick fibrous connective tissue, changes in the auricular muscles which open and close comprised of thick collagenous and elastic fibers, the outer ear. The external orifice of the outer ear completes the skeletal wall of the cartilaginous part of is usually 15-20 mm behind the eyes in pups and the outer ear. Two sets of elastic fibers originate from 42-44 mm in adults. Its shape varies from a true oval the perichondrium, one extends radially, and the other to an ellipse, closing to a vertical slit. When closed set circumferentially around the ear canal, both inter­ the area around the external orifice is depressed. The connect the plates of auricular cartilage (Figs. 63 and opening and closing of the external orifice is under 64). This elastiscity of the cartilaginous outer ear the control of the auricular muscles (Fig. 60). The would make it easier to accomodate the increased auricular muscles are inserted on the helical plate pressures during diving. which can be considered a reduced, internal homo­ Large blood sinuses are also present within the logue of the pinna of terrestrial mammals. Tendinous cartilaginous wall of the outer ear canal (Fig. 62 d Aquatic adaptations in the ear of the harp seal Pagophilus groenlandicus (Erxleben, 1777) 103

Figure 60. Lateral view of dissection of the head of a harp seal (10-year-old female) showing the muscles associated with the outer ear. The extrinsic and auricular muscles are prominent and well developed. (1) anterior, (2) superior, (3) posterior, (5) cervico- mandibular, (4) zygomaticus is a thin band of muscle. (M.Or.) Meatal orifice has been pulled open (M.Me.) Membranous part of the outer ear (C.M.) Cartilaginous part of the outer ear (B.M.) Bony part of the outer ear The deep are not shown (redrawn from Ramprashad et al, 1972a).

and e). These sinuses are absent in the membranous regulating device. The engorged blood sinuses might wall of the outer ear (Figs 62 b and c). Longitudinal also form a fluid wall around the outer ear canal, blood sinuses with numerous bifurcating side arms thereby preventing complete collapse of the ear canal form the hypodermis of the cartilaginous outer ear during diving. and are especially prominent in the osseous wall of the outer ear canal (Fig. 62 f and g). These sinuses M iddle ear extend into the tympanic sulcus of the pars tensa of A major difference between the of seals the tympanic membrane (Fig. 65). These sinuses when and the middle ear of terrestrial mammals is the pres­ engorged with blood, could reduce the internal volume ence of cavernous tissue within the middle ear mucosa of the outer ear canal and therefore act as a pressure and the thick wall of the auditory tube. The presence of cavernous tissue in the middle ear mucosa in seals is well established. However, in the harp seal, the 1 cavernous tissue is confined to the lateral, medial 3 wall of the middle ear cavity and around the tym­ panic orifice of the auditory tube. No large blood 4&2 sinuses are found in the sub-mucosa on the floor of the tympanic bulla. The function of the cavernous tis­ sue has been attributed to a pressure-regulating mechanism by Tandler (1899), and the auditory tube has always been assumed to be blocked (Møhi, 1968; 5 Kooyman et al, 1970). A detailed study of the struc­ ture of the auditory tube might add to the under­ standing of the pressure equilibration in the middle ear of the harp seal.

Figure 61. Lateral and cranial view of the cartilaginous part of Auditory tube the outer ear showing insertion of the auricular muscles. Note The auditory tube of the harp seal consists of a the well developed helicis (7) and the antitragicus (8) which are bony section and a cartilaginous section as it does in the intrinsic muscles which close the outer ear canal internally. The tragicus medialis (6) opens the ear canal at the internal other mammals. The medial wall of the auditory tube pinna (redrawn from Ramprashad et al, 1972a). consists of a thick fibrous block of connective tissue 104 F. Ramprashad

e f g Figure 62. a) Relation of the outer ear to the . b)-g) Outline drawings of cross-sections at various levels along the course of the outer ear canal. These drawings show the distribution of the blood sinuses (indicated in black) along the outer ear canal. The levels of the sections are indicated to the left of Fig. 62a by the numbers 5-10. Note the absence of blood sinuses in the membranous part of the outer b) and d) ; also in the fibrous connective tissue (F.ct) which completes the skeletal wall of the outer ear canal (redrawn from Ramprashad et al, 1972a). (M.Or.) Meatal orifice (M.Me.) Membranous part of the outer ear (H. P.) Helical plate (H.Pr.) Helical process (B.M.) Bony wall of the outer ear (c) Cartilate

(Fig. 66). This connective tissue block extends from the harp seal may be an important modification for the bony part of the tube to the cartilaginous part of pressure equilibration of the middle ear. the tube, ending at the nasopharynx. The lumen in Studies by Kooyman (1970) have shown that the the bony section is wide, while it is very narrow in the lungs and trachea of seals are severely compressed by cartilaginous part of the tube (Fig. 67a-c). In the the increased pressure during diving. Proper function­ cartilaginous part thick collagenous and elastic fibers ing of the auditory tube of the harp seal would depend comprise this thick fibrous block; and the cartilage on its walls being able to withstand the pressure forms the core of this block (Fig. 67d-f). Thick colla­ changes which occur in the nasopharynx during deep genous and elastic fibers encircle the lumen of the au­ dives. The robust nature of the wall of the auditory ditory tube to form the lateral wall of the tube. Two tube may be a modification that would enable the muscles are associated with the auditory tube, the tube to be opened by muscular contraction during div­ muscle and the tensor veli palatini ing. The distension of the cavernous sinuses of the muscle which are inserted in the ventral and lateral middle ear mucosa would then play an accessory role wall of the auditory tube respectively (Fig. 67d-f). in pressure equilibration by preventing a large pres­ The robust nature of the wall of the auditory tube in sure difference between the middle-ear cavity and the Aquatic adaptations in the ear of the harp seal Pagophilus groenlandicus (Erxleben, 1777) 105

O-S.'tn-ffi;

Figure 63. Cross section of the posterior cartilaginous arm show­ Figure 64. Cross section of the anterior open arm of the cartila­ ing elastic fibers (E) extending radially into the dermis and an ginous wall of the outer ear canal. Note the convergence of the outer set completing the medial skeletal wall of the outer ear canal. hypodermal and perichordial elastic fibers which will complete (Vacuum embedded Paraplast, Gomori aldehyde fuchsin 10 y.) the medial skeletal wall of the outer ear canal. Note the elastic fibers extending from the perichondrium. (Vacuum embedded Paraplast, Gomori aldehyde fuchsin 10 |j.) nasopharynx. This reduction in the pressure differ­ ence between the middle-ear cavity and nasopharynx 1-2-2 1 mm, which is larger than that of the human would facilitate an easier opening of the auditory which may reflect a necessary adaptation to the aqua­ tube. In humans the inability to equilibrate this pres­ tic habitat. Since the seal moves about in an aerobatic sure difference results in the ‘‘locking phenomenon”, fashion through the water, inertial information about i.e. collapse of the cartilaginous part of the tube. The its body axis must be vital for correct orientation. Dur­ structure of the auditory tube and the cavernous tis­ ing deep dives, when visual cues may be absent or sue in the middle ear of the harp seal makes this “lock­ vague, stimulation of the otolithic receptors in res­ ing phenomenon” unlikely during diving. ponse to gravity could be vital for correct orientation. In the diving harp seal pressure equilibration may Stimulation of the otolithic receptors, as in terrestrial therefore result from the muscular opening of the mammals, would be due to the movement of the oto­ auditory tube, aided by the distension of the cavern­ conia and in response to gravity ous tissue in the middle ear. If this is so, the opening or linear acceleration, thereby causing a deflection of of the auditory tube would permit air at ambient pres­ the sensory hairs. This deflection of the sensory hairs sure to enter the from the respiratory would cause an alteration in the passage of nervous system (Møhi, 1968). impulses sent to the brain. Hence it is not surprising to find that the structure of these receptors is similar to other mammals (Fig. 68). The vestibular organ of the harp seal is similar to The crista neglecta is located 0 75 mm from the that of terrestrial mammals. However, a major differ­ posterior crista near the entry of the posterior limb of ence occurs in the size of the macula utriculi and the the lateral semi-circular canal into the (Fig. presence of an extra sensory area, the crista neglecta. 69). The crista neglecta is similar to an ampullary The harp seal’s macula utriculi measures 3 0-31 by crista (Figs 70 and 71) and consists of two patches of 106 F. Ramprashad

Figure 66. Cross section of the bony section of auditory tube showing the thick fibrous block (FB) of connective tissue which Figure 65. Horizontal section of the middle ear showing the forms the medial wall of the tube. (Celloidin H. & E. 20|ji). extension of the blood sinus (bv) into the tympanic sulcus of the tympanic membrane. (Celloidin embedded H. & E., 25(ji). always on the same level as the cells of Hensen. The cells of Claudius are not as well developed as they are neuroepithelium covered by a gelatinous cap. Sensory in cetaceans. The average length of the Organ of Cor­ hairs from the neuroepithelium project into the gela­ ti, obtained from graphic reconstructions (Guild, 1921, tinous layer. The structure and the location of the and Schuknecht, 1953), is 33-7 mm (4 ) and crista neglecta suggest that it may act in a similar is within the range of the length of in manner to the ampullary cristae i.e. responding to humans. The microscopic structure of the Organ of angular acceleration although Lowenstein and Ro­ Corti is typically mammalian (Fig. 73). berts (1950, 1951) have shown that the crista neg­ It is likely that the presence of a short basilar mem­ lecta is sensitive to sound vibrations in the Thornback brane and secondary osseous lamina are essential for ray (Raja clavata). It may be significant that the reception of high frequencies. These factors may pro­ harp seal has two neglectas, as is the case in some vide the extra support which enables shearing action fishes (Retzius, 1880). to occur between the sensory hairs and the tectorial The microscopic structure of the of the harp membrane in the high frequency range of the cochlea. seal is similar to the cochlea of mammals capable of The anatomical findings correlate well with the high at high frequencies. In the large lower basal frequency hearing ability as is shown in the audio­ coil of the cochlea there is a well developed spiral liga­ grams of the harp seal (Terhune and Ronald, 1971, ment and an external spiral osseous lamina which ex­ 1972). tends to the (Fig. 72). The external A major modification in the cochlea of the harp osseous lamina and the reduce in size seal is the position and size of the . The from the round window to the apical coil of the large round window membrane, instead of facing the cochlea. A major difference between the cochlea of tympanic cavity, is directed posteriorly and faces a pit the harp seal and that of cetaceans is that the sensory which opens laterally as the hair cells of the Organ of Corti in the harp seal are (Fig. 74). This pit is filled with loose connective tis- Aquatic adaptations in the ear of the harp seal Pagophilus groenlandicus (Erxleben, 1777) 107

i in i Figure 67. A series of outline drawings of cross sections of the auditory tube of the harp seal. Note the change in diameter of the lumen (L) of the auditory tube from the bony section to the cartilaginous section. Note also the thick medial wall and the shape of the cartilage (c) of the auditory tube. Two muscles are associated with the auditory tube : the tensor veli palatini muscle (T.P.) is inserted on the lateral wall of the auditory tube, whereas the levator veli palatini muscle (L.P.) is inserted on the ventral wall of the tube (redrawn from Ramprashad et al, 1972a). (T.B.) Tympanic bulla (L.) Lumen of the auditory tube (F.C.T.) Fibrous connective tissue 108 F. Ramprashad

Figure 68. Photomicrograph showing the structure of the macula Figure 69. Horizontal section at the posterior ampulla showing sacculi. Note the darkly stained otoconia (Oc) on the otolithic the location of the crista neglecta (C.N.), P.c. - posterior crista membrane (O.M.). Sensory hairs of the neuroepithelium project (Celloidin H. & E., 25|j). into the otolithic membrane. (Celloidin H. & E., 25p).

0-25 m-m- Figure 71. Horizontal section at the lateral ampulla showing the typical mammalian structure of the crista of the harp seal. Figure 70. Section of the crista neglecta showing two patches of (Cu - cupula; Ne - neuroepithelium). (N) endings of the neuroepithelium. Note the nervous elements (Ne) associated with lateral ampullar branch of the . (Celloidin H. & the neuroepithelium, Cu - cupula. (Celloidin H. & E., 25^). E., 25n). Aquatic adaptations in the ear of the harp seal Pagophilus groenlandicus (Erxleben, 1777) 109

0'25 m-m-

Figure 73. Horizontal section at the middle coil of the cochlea showing typical microscopic structure of the organ of Corti. Note the well developed cells of Hensen (C.H.), large cells of Claudius (C.C.) and the thick basilar membrane below the sensory hairs and their large supporting cells. (T.M.) (R.M.) Reisner’s membrane. (Celloidin H. & E.,25|a).

Figure 72. Section at the basal coil of cochlea approximately 10 mm from the round window. This section shows the large spiral osseous lamina (S.O.L.) and the thick spiral ligament (S.L.). Arrow shows that the hair cells are on the same level as the cells of Hensen. Note also the high , a feature in the basal coil and the short length of the basilar membrane and wide inner tunnel. (Celloidin H. & E., 25y).

sue within which are large blood sinuses extending into the connective tissue of the round window mem­ RW.M brane (Fig. 75). Closely associated with the vestibular surface of the round window membrane is the amor­ phous connective tissue of the large (Fig. 76) a feature which may play a part in the hear­ ing mechanism of the seal.

CONCLUSIONS The significance of these anatomical structures in the hearing mechanism of the harp seal becomes ap­ parent when the entire head is considered. The clos­ ing of the external orifice of the outer ear canal dur­ ing submergence makes it unlikely that the seal can

Figure 74. Horizontal section of the temporal bone of the harp seal showing the round window membrane (R.W.M.) opening into a pit which opens into the stylomastoid foramen. (T.B.) is the posterior part of the tympanic bulla. Note the blood sinuses (b.v.) in the connective tissue. S.T. - Scala tympani. (Celloidin H. & E. 25n). 110 F. Ramprashad

R-W-M

1 m-m

Figure 75. Section of the round window membrane (R.W.M.) Figure 76. Horizontal section at the vestibular orifice of the showing the large blood sinuses (b.v.) within the connective tissue. cochlear aqueduct (C.Aq.) : Note the blood vessels (b.v.) extend- (Celloidin H. & E. 25u). ing into the round window membrane (R.W.M.) and the close association of the tissue of the cochlear aqueduct with the round window membrane. (Celloidin H. & E. 25^).

Transmission T ransmission From The From The Skull Contents Through The Cochlear Aqueduct

Sound Transmission Via From The W ater____ Via Ear Canal Ossicular Chain

T ransmission Nervous Via Wall Of The MIDDLE INNER OUTER Across BRAIN EAR Ear Canal And EAR EAR Tympanic Cavity Impulses

Blood Sinuses Sound Transmission Cavernous Tissue From The Entire Facial Region T ransmission From The Mastoid Bone Figure 77. Diagramatic representation of some of the possible pathways of sound to the inner ear by bone conduction. Aquatic adaptations in the ear of the harp seal Pagophilus groenlandicus (Erxleben, 1777) 111

hear by the conventional air conduction mechanism. branch labyrinth. A contribution to the problem of the evolu­ Experiments on terrestrial mammals by Tonndorf tion of hearing in vertebrates. J. Physiol., 114:471-89. (1966, 1968) have indicated that the wall of the outer Møhi, B. 1968. Hearing in seals, pp. 172-195. In The behavior and physiology of pinnipeds. Ed. by R.J. Harrison, R. C. ear canal can conduct sound energy to the tympanic Hubbard, R. S. Peterson, C. E. Rice and R. S. Schusterman. membrane when the ear canal is occluded. Therefore, Appleton-Century-Crofts, New York. blood sinuses within the wall of the outer ear of the Ramprashad, F., Corey, S. & Ronald, K. 1971. The harp seal harp seal may enhance the transmission of sound Pagophilus groenlandicus (Erxleben, 1777). X III. Gross and microscopic structure of the auditory meatus. Can. T. Zool energy to the tympanic membrane. Sound energy can 49:241-48. then be transmitted via the heavy ossicular chain to Ramprashad, F., Money, K. E. & Ronald, K. 1972a. The harp the cochlea. Similarly, the cavernous tissue of the seal Pagophilus groenlandicus (Erxleben, 1777). X X I. The struc­ middle mucosa may also serve to transmit sound ener­ ture of the vestibular apparatus. Can. J. Zool., 50:1357-61. Ramprashad, F., Corey, S. & Ronald, K. 1972. The anatomy of gy across the tympanic cavity to the cochlear shell. the seal’s ear (Pagophilus groenlandicus) (Erxleben, 1777). Vol. Tonndorf (1966) has also suggested that sound energy 1:264-305. In Functional anatomy of marine mammals. Ed. can reach the cochlea by travelling through the inter­ by R .J. Harrison. Academic Press, London. ior of the skull in the form of pressure waves via the Ramprashad, F., Corey, S. & Ronald, K. 1973. The harp seal, Pagophilus groenlandicus (Erxleben, 1777). X IV . The gross and cochlear aqueduct. The large cochlear aqueduct may microscopic structure of the middle ear. Can. J. Zool., 51: also indicate another pathway for sound energy to 589-600. reach the cochlea. One of the prime requirements for Retzius, G. 1880. Zur Kenntnis des innerer Gehörorgans der bone conduction is a compliant round window mem­ Wirbeltiere. Arch. Anat. Physiol. Anat., Abst., [1880] :235-44. Reysenbach de Haan, F. W. 1957. Hearing in whales. Acta brane (Tonndorf 1968). Perhaps the isolation of the Otolaryngol., Suppl., 134. round window membrane from the tympanic cavity Ridgway, S. H., Scance, B. L. & Kanwisher,J. 1969. Respiration in the harp seal may be another modification for a bone and deep diving in bottlenose porpoise. Science, (New York), conduction mechanism. These pathways are summar­ 166:1651-54. Schuknecht, H. F. 1953. Techniques for study of cochlear ized in Figure 77. function and pathology in experimental animals. Arch. Otolaryngol., 58:377-97. Tandler, J. 1899. Ueber ein corpus cavernosum tympanicum beim ACKNOWLEDGEMENTS Seehund. Monatsschr. Ohrenheilkd. Kehlkopf-, Nasen-, Rachenkr., Organ Oestesr. Otol., Ges., 33:437-40. This work was supported in part by the National Terhune, J. M. & Ronald, K. 1971. The harp seal, Pagophilus Research Council of Canada through operating and groenlandicus (Erxleben, 1777). X. The air . Can. development grants to Professor Ronald. J. Zool., 49:385-90. Terhune, J. M. & Ronald, K. 1972. The harp seal, Pagophilus groenlandicus (Erxleben, 1777). III. Underwater audiogram. Can. J. Zool., 50:565-69. REFERENCES Tonndorf, J. 1966. Bone conduction studies in experimental animals. Acta Otolaryngol., SuppL, 213. Fraser, F. C. & Purves, P. E. 1960. Hearing in cetaceans. Br. Mus. Tonndorf, J. 1968. A new concept in bone conduction. Arch. (Nat. Hist.), Bull., Zool., 7:1-140. Otolaryngol., 87:595-600. Guild, S. R. 1921. A graphic reconstruction method for the study Wever, E. G., McCormick, J. C., Plain, J. & Ridgway, S. 1971. of the organ of Corti. Anat. Rec., 22:141-57. The cochlea of the dolphin Tursiops truncatus: general mor­ Kooyman, G. L. 1970. Bronchograms and tracheograms of seals phology. Natl. Acad. Sei. U.S.A., Proc., 68(10) :2381-85. under pressure. Science, (New York), 169:82-84. Wever, E. G., McCormick, J. C., Plain, J. & Ridgway, S. 1971a. Lowenstein, O. & Roberts, T. D. M. 1950. The equilibrium The cochlea of the dolphin Tursiops truncatus : the basilar mem­ function of the auditory organs of the thornback ray. J. Physiol., brane. Natl. Acad. Sei. U.S.A., Proc., 68(11) :2708-l 1. 110:392-415. Wever, E.G., McCormick, J. C., Plain, J. & Ridgway, S. 1971b. Lowenstein, O. & Roberts, T. D.M. 1951. The localization and The cochlea of the dolphin Tursiops truncatus: and analysis of the responses to vibration from the isolated elasmo- ganglion cells. Natl. Acad. Sei. U.S.A., Proc., 68(12) :2908—12.