Pacific Science (1975), Vol. 29, No.2, p. 153-157Printed in Great Britain

Skin Structure of the Hawaiian Monk Seal (Monachus schauinslandi)I

G. C. WHITTOW,2 J. SZEKERCZES,2 E. KRIDLER,3 AND D. L. OLSEN3

ABSTRACT: Skin samples from the dorsal region of the torsos of Hawaiian MonkSeals were examined histologically to determine if there were any features of thestructure of the skin that might explain the reputed heat tolerance of these seals.The skin structure was compatible with an animal exposed to strong solar radia­tion, in which nonevaporative heat loss was promoted, but little could be discernedto suggest the existence of functional evaporative cooling mechanisms.

DATA COLLECTED from Hawaiian Monk Seals,hauled out on the sand, in their natural habitatin the Hawaiian Islands National WildlifeRefuge, indicated that they were not hyper­thermic during the day (Kridler, Olsen, andWhittow 1971). Substantially similar resultswere obtained in a more extensive study of asingle specimen of Monk Seal at the WaikikiAquarium in Honolulu (Ohata et al. 1972).These findings are consistent with reports thatHawaiian Monk Seals are able to tolerate veryhot conditions for prolonged periods of time~Kenyon and Rice 1959), but they are difficultto explain in the light of the demonstrable heatintolerance of other species of pinnipeds(Whittow, Ohata, and Matsuura 1971 ; Whittow,Matsuura, and Lin 1972). One possible explana­tion for th~ apparent heat tolerance of HawaiianMonk Seals is that they are able to lose heat byevaporation of moisture from the skin. Asignificant amount of heat can be dissipated inthis way only by the active participation of thesweat glands. The present study was under­taken to determine whether any evidence forthe functional activity of the sweat glands, orany explanation for the reputed heat toleranceof Monk Seals, could be obtained by a histo­logical examination of their skins.

I This work was supported by National ScienceFoundation grant no. GB-29287X. Manuscript received25 June 1974.

2 University of Hawaii, School of Medicine, Depart­ment of Physiology, Honolulu, Hawaii, 96822.

3 United States Department of the Interior, Fish andWildlife Service, Bureauof Sports Fisheries and Wildlife,Kailua, Hawaii 96734.

METHODS

Specimens of skin were obtained in April1970 from two live seals on Southeast Island,Pearl and Hermes Reef, in the Hawaiian IslandsNational Wildlife Refuge. The skin sampleswere obtained from the dorsal surface of thetorso of the animals, approximately 45 cmrostral to the base of the tail. The seals werehauled out on the sand, asleep, before thesamples were taken. The skin was fixed in10-percent formalin solution and washed inrunning tap water for 2 hours. The sampleswere then dehydrated in three changes ofacetone for 8-16 hr, and subsequently clearedin three changes of benzene for 8-16 hr.Following this treatment, the specimens wereinfiltrated with, and embedded in, paraplast(melting point, 56°_58° C). Sections, 10 I,/,thick, were cut and stained with hematoxylin(Delafield) and eosin. The sections wereexamined with a light microscope and photo­graphed with a Zeiss camera attachment.

RESULTS

The epidermis of the skin was pigmented inits basal layers ; it did not appear to be exten­sively invaginated by the papillary layer of thedermis. The dermis was higWy vascularized.Only separate hair follicles were seen (Figure1); there appeared to be no secondary follicles,confirming previous warle-by Scheffer (1964).A sweat gland and a simple, rather elongated,sebaceous gland were associated with each hairfollicle. The duct of the sweat gland was astraight, slender, tubular structure, leading into

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154 PACIFIC SCIENCE, Volume 29, April 1975

FIGURE 1. General view of the skin showing separate hair follicles (h), (hI)' and (h2). Photomicrograph. Tissueswere stained with hematoxylin and eosin.

the rather long body of the gland. The body ofthe gland was convoluted (Figure 2), the lumenwas small, and the wall of the gland was madeup of a single layer of cuboidal cells sur­rounded by a single layer of myoepithelial cells.The body of the sweat gland was at approxi­mately the level of the hair papilla. The entry ofthe sebaceous gland into the hair follicle wasmore superficial than that of the sweat gland(Figure 3). No arrectores pilorum muscles wereevident.

DISCUSSION

Several features of the histology of the skinof the Monk Seal are consistent with an animalexposed to a hot environment. The pigmenta­tion of the eI'jgermis is to be expected in ananimal exposed to intense sOlar ra-diation.Presumably it serves to reduce the penetrationof short-wave radiation into the skin (Mount1968). The short, bristly, hair coat, withoutsecondary hairs, would permit maximal heat

loss to the air from the skin surface, andpossibly would reflect environmental thermalradiation at the same time. Unfortunately,nothing is .known about the reflectance of theMonk Seal's hair coat. In the Harp Seal, anArctic species, the hair coat is believed topromote· the absorption of thermal radiationby the skin (Oritsland 1971). However, the hairstructure of the Harp Seal differs from that ofthe Monk Seal (Whittow, Szekerczes, andRonald, unpublished data), and there may wellbe functional differences in the coats of the twospecies, related to their very different thermalenvironments.

The marked vascularity of the skin of theHawaiian Monk Seal would be expected tofacilitate the transfer of heat from the deeptissues to the periphery. Although informationon thEfunci:ion of glands cahnot be unequivo­cally deduced from their structure, it seemspermissible to suggest that the sweat glands ofthe Hawaiian Monk Seal are not active. Theonly sweat glands known to be functional in

Skin Structure of the Hawaiian Monk Seal-WHITTow ET AL. 155

FIGURE 2. The body of the sweat gland (sw) in relation to a hair follicle (h). Photomicrograph. Tissues werestained with hematoxylin and eosin.

pinnipeds are in the flippers of the NorthernFur Seal and the California Sea Lion (Bartholo­mew and Wilke 1956, Matsuura and Whittow1974). These glands are considerably largerthan those of the Hawaiian Monk Seal. It is

possible, however, that the sweat glands of theHawaiian Monk Seal vary in size and functionalactivity at different times of the year. The entryof the sweat gland duct into the pilary canalbelow the opening of the sebaceous gland con-

156 PACIFIC SCIENCE, Volume 29, April 1975

FIGURE 3. Entry of the ducts of the sebaceous gland (s) and the sweat gland (sw) into the hair follicle (h). Photo­micrograph. Tissues were stained with hematoxylin and eosin.

forms with the situation in other Phocidae(Montagna and Harrison 1957, Ling 1965,Harrison and Kooyman 1968). According toMontagna and Harrison (1957), the secretionsof the sweat glands and sebaceous glands are

relatively immiscible in the Harbor Seal. Ifthis is true for the Monk Seal, then anysecretion from the sweat glands is likely toemerge onto the surface of the skin beneath alayer of oily sebum, an arrangement not con-

Skin Structure of the Hawaiian Monk Seal-WHITTow ET AL. 157

ducive to evaporation. However, the sebaceousgland was a relatively small, simple structure,suggesting the possibility that it also is not veryactive.

LITERATURE CITED

BARTHOLOMEW, G. A., and F. WILKE. 1956.Body temperature in the Northern Fur Seal(Callorhinusursinus).]. Mammal. 37: 327-337.

HARRISON, R. ]., and G. L. KOOYMAN. 1968.General physiology of the Pinnipedia. Pages211-296 in R. ]. Harrison, R. C. Hubbard,R. S. Peterson, C. E. Rice, and R. ].Schusterman, eds. The behavior and physiol­ogy of pinnipeds. Appleton-Century-Crofts,New York.

KENYON, K. W., and D. W. RICE. 1959. Lifehistory of the Hawaiian Monk Seal. PaciE.Sci. 13: 215-252.

KRIDLER, E., D. L. OLSEN, andG. C. WHITTOW.1971. Body temperature of the HawaiianMonk Seal (Monachus schauinslandi). ]. Mam­mal. 52: 476.

LING, ]. K. 1965. Functional significance ofsweat glands and sebaceous glands in seals.Nature 208: 560-562.

MATSUURA, D. T., and G. C. WHITTOW. 1974.Evaporative heat loss in the California SeaLIon and Harbor Seal. Compo Biochem.Physiol. 48A: 9-20.

MONTAGNA, W., and R. ]. HARRISON. 1957.Specializations in the skin of the seal (Phocavitulina). Amer. ]. Anat. 100: 81-113.

MOUNT, L. E. 1968. The climatic physiology ofthe pig. Edward Arnold, London.

OHATA, C. A., D. T. MATSUURA, G. C.WHITTOW, and S. W. TINKER. 1972. Diurnalrhythm of body temperature in the HawaiianMonk Seal (Monachus schauinslandi). Pacif.Sci. 26: 117-120.

ORITSLAND, N. A. 1971. Wavelength-dependentsolar heating of Harp Seals (Pagophilusgroenlandicus). Compo Biochem. Physiol: 40A:359-361.

SCHEFFER, V. B. 1964. Hair patterns in seals(Pinnipedia). ]. Morph. 115: 291-304.

WHITTOW, G. c., D. T. MATSUURA, and Y. C.LIN. 1972. Temperature regulation in theCalifornia Sea Lion (Zalophus californianus).Physiol. Zool. 45: 68-77.

WHITTOW, G. c., C. A. OHATA, and D. T.MATSUURA. 1971. Behavioral control of bodytemperature in the unrestrained CaliforniaSea Lion. Commun. behav. BioI. 6: 87-91.