Cape fur seal (Arctocephalus pusillus) pups spend the first weeks of life exclusively or mainly ashore. They are exposed to intense solar radiation and high temperatures for long time periods, which results in temperatures up to at least 80°C on their black natal coat. To test the hypothesis that the natal coat has a crucial function in coping with these extreme conditions, we investigated the insulating properties of the natal coat in six captive newborn Cape fur seals during the first 50 days after birth. The natal fur differs from the adult fur not only in colour, but also in density, structure, and water repellence. We measured temperature on the fur surface and within the fur, as well as skin and rectal temperature under varying environmental conditions, comparable to the species' habitat. Experiments were designed to not influence the spontaneous behaviour of the pups. Rectal temperature was constant as long as the pups stayed dry, even during long-lasting intense solar radiation for up to 3 h. Skin temperature remained close to rectal temperature as long as the fur was dry, while with wet fur, skin temperature was significantly reduced as well. Our results show that the natal coat provides an effective insulation against overheating. The severely reduced insulation of wet natal fur against cold supports the assumption that the natal fur is an adaptation to the pups' terrestrial phase of life.

Citation: Erdsack N, Dehnhardt G, Hanke W (2013) Coping with Heat: Function of The Natal Coat of Cape Fur Seal (Arctocephalus Pusillus Pusillus) Pups in Maintaining Core Body Temperature. PLoS ONE 8(8): e72081. doi:10.1371/journal.pone.0072081

Editor: Andreas Fahlman, Texas A&M University-Corpus Christi, United States of America

Received: February 15, 2013; Accepted: July 8, 2013; Published: August 8, 2013

Introduction
Due to their global distribution, pinnipeds have to face diverse thermoregulatory requirements imposed by their environment. Phocid seals, highly adapted to the aquatic environment, rely on their blubber as thermal insulation, while their fur is generally of minor thermoregulatory relevance [1], [2], [3], [4] due to low hair density and the lack of underwool [5], [6]. The more terrestrial fur seals contrarily are insulated by a dense water repellent fur. For the pups of seal species distributed in the polar and sub polar regions, particularly ice breeding species, the greatest demand is the prevention of heat loss during the first days of life. The pups of the spotted seal (Phoca largha Pallas, 1811) and the harp seal (Pagophilus groenlandicus Erxleben, 1777), for example, are born with a light coloured natal fur, the lanugo, that protects them against the cold until they have developed an adequate blubber layer, as long as they stay ashore and dry [7], [8]. The pups of the harp seal additionally have thermogenic brown adipose tissue (BAT) [9], where fat is oxidized with intense heat production instead of ATP synthesis [10]. By contrast, harbour seal (Phoca vitulina Linnaeus, 1758) pups shed the lanugo in utero [7] and are born with a blubber layer and an adult-type fur, since they have to be able to swim shortly after birth.

For the pups of species distributed in moderate and tropical climate zones, such as many otariids, protection against heat absorption is presumably as essential as protection against heat loss. Sea lion and fur seal pups are born with a black or dark brown natal coat which they start moulting not before the sixth week [11]. Like the lanugo of phocids, the otariid natal fur lacks the water-repellent properties of the adult fur, so that the pups drench to the skin while staying in the water or in the rain [12], [13], [14].. Newborn fur seal pups indeed are able to keep afloat right after birth but they are not really capable of swimming [13] before the fifth week ([15] and own observations).

The breeding areas of the South African or Cape fur seal (Arctocephalus pusillus pusillus Schreber, 1776) range from Algoa Bay, South East Africa, southwards to the Cape of Good Hope and north westwards to the north coast of Namibia. This distribution comprises a huge climate range from humid with moderate temperatures, e. g. at Bird Island, Algoa Bay at the south-east coast of South Africa [16] up to arid desert climate at the north-west coast of South Africa and the south and north coasts of Namibia. Most breeding sites are on rocky islands or cliff coasts, but some sandy beaches at the Namibian coast are occupied by fur seals as well. Due to the poor swimming abilities of fur seal pups, most pupping sites are not suitable for them to enter the water, so that the pups have to spend the first weeks on shore. Furthermore shaded space is rare at most breeding sites, so that the pups are exposed to intense solar radiation for many hours. As it is rarely possible for the pups to cool down behaviourally by entering the water [17] or drenching their fur [18] like adults, other heat protection mechanisms are required. Adult California sea lions (Zalophus californianus (Lesson, 1828)), although less insulated than other otariids [19], become hyperthermic after about 100 min at air temperatures ≥30°C, even without physical activity [20]. Though Cape fur seals, in contrast to Z. californianus, possess functioning sweat glands in the naked skin areas of the flippers [21], it is not known how functional these glands are in newborns. Even if they were functional, intensive sweating would easily lead to dehydration since the mothers leave their pups as soon as a few days after parturition for foraging trips of up to ten days [22], [23]. So how do fur seal pups cope with long periods of exposure to intense solar radiation and high ambient temperatures without the opportunity to cool down? One option would be for the pups to live with an increased core body temperature during their first postnatal weeks, as observed e. g. in Southern elephant seal (Mirounga leonina (Linnaeus, 1758)) pups [24]. This seems rather unlikely considering that Limberger et al. [25] found constant normal core body temperatures in the pups of the Galapagos fur seal (Arctocephalus galapagoensis (Heller, 1904)), which are exposed to a hot and dry environment close to the equator, but with the opportunity to take shelter from solar radiation in natural caves. Here we set up and supported an alternative hypothesis, that is, as long as the pups stay on land, their natal fur acts as such an effective thermal barrier against heat transfer that neither skin nor core body temperature are affected. To test this hypothesis, we investigated six Cape fur seal pups, born and kept in Zoo Rostock, Germany, within their first fifty days after birth under various environmental conditions that they also face in nature. We measured rectal temperature with a veterinary thermometer and the temperature of the skin and the air temperature inside the fur using a mantle thermocouple. The temperature on the outer surface of the fur was measured by infrared thermography (IRT). We tested for correlations between these measured temperatures as well as possible influences of other parameters like air temperature, time of day, age and sex of the pups.