Thermoregulatory Behavior of the Hawaiian Monk Seal (

The behavior of Hawaiian monk seals at French Frigate Shoals was studied in order to obtain information on their adaptation to a tropical climate. The seals were unable to remain on the dry beach platform during the day except during very high winds, extensive cloud cover, or rain. The seals characteristically moved down to wet sand on the beach slope during the day and returned to the beach platform at night. The frequency with which the seals changed their posture appeared to be related to the prevailing microclimatic conditions. For the most part, the seals lay in postures that exposed their ventral pale-colored hair coat to the atmosphere. The temperature of this surface was significantly lower than that of the darker dorsal coat. The seals were extremely inactive while ashore; their respiratory pattern included long periods of breath-holding, and the heart rate during breath-holding was low. These features were considered to be compatible with a low level of metabolic heat production and to be adaptive to heat exposure. THE HAWAIIAN MONK SEAL (Monachus schauinslandi, Matschie) is the most tropical of all the pinnipeds. It is, in fact, one of the few species to have penetrated the tropics, which are believed to be a major barrier to the geographical distribution of seals (Scheffer 1958). For the most part, the anatomical, physiological, and behavioral features of the adaptation of monk seals to the tropics have not been identified. Although tropical oceans are relatively warm, it is unlikely that monk seals have any difficulty in dissipating metabolic heat when they are in the water. Young harbor seals (Phoca vitulina) have been shown to lose heat efficiently in water at 36.7 to 37.5° C, which is close to body temperature (Miller et al. 1976), and there is little evidence that tropical cetaceans are ever heat-stressed (Whittow 1976). When they are ashore, however, monk seals experience high levels of solar radiation and relatively high air _.1 This .research. was supported bya grant -from the National Geographic Society. Manuscript received 20 September 1977. 2 University of Hawaii, Kewalo Marine Laboratory, Pacific Biomedical Research Center, 41 Ahui Street, Honolulu, Hawaii 96813. 47 temperatures; there is usually little shade and no fresh water to replace water lost by any physiological evaporative heat-loss mechanisms. Kenyon and Rice (1959:218), in their pioneer study of the Hawaiian monk seal, reported that the layer of blubber in Hawaiian monk seals is "about as thick as on arctic and antarctic seals," suggesting that a reduction of tissue insulation has not occurred in seals in response to exposure to heat. No evidence has been obtained to show that Hawaiian monk seals become hyperthermic on land, either in their natural habitat in the northwestern Hawaiian Islands (Kridler et al. 1971), or in captivity at the Waikiki Aquarium in Honolulu (Ohata et al. 1972). The hair coat of Hawaiian monk seals would appear to permit maximal convective and conductive heat loss from the skin surface to the air, but the histological structure of the sweat glands did not suggest that they were active (Whittow et al. 1975). Monk seals have been observed to lie in depressions· in the sand (Kenyon and Rice 1959), and this may facilitate conductive heat loss to the sand (Ohata and Whittow 1974). The studies reported here were designed