Coping with heat: function of the natal coat of cape fur seal (Arctocephalus Pusillus Pusillus) pups in maintaining core body temperature.

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

T_body and T_skin of the pups with wet and dry fur.

<p>T_body with dry fur (green diamonds, green regression line, N = 23) and wet fur (turquoise filled circles, N = 3) as well as T_skin with dry fur (red circles, red regression line, N = 279) and wet fur (blue circles, blue regression line, N = 28) against T_air. Mean T_skin with dry fur is 38.4±1.7°C, mean T_skin with wet fur is 35.8±1.3°C, which is significantly lower than with dry fur (<i>p</i>≤0.0001). T_body is constant with no significant difference between wet and dry fur. The slope of regression of T_body is with 0.002 again practically constant, while the regressions of T_skin ascend by 0.32 (dry fur) and 0.13 (wet fur).</p

T_body, T_skin, T_fur and T_surface of the pups with wet and dry fur.

<p>Minimum (T_min), maximum (T_max) and mean (T_mean) temperatures of the pups' fur surface, air inside the fur, skin and body core along with temperature range, standard deviation (s.d.) and number of measurements (N). Total includes the data of all investigated pups with dry and wet fur. Beneath the data of all pups are sorted by wet or dry fur conditions.</p

Cape fur seal pups hauling out under intense solar radiation.

<p>The pups, aged 18 and 28 days, hauled out in the sun for 1.5 h, T_air  = 31.3°C. T_surface increased up to 78.7°C on their backs.</p

Reduction of T_surface on hauled out pups by wind forced convection.

<p>Pups hauling out under solar radiation with sudden appearance of a light breeze of wind force 1–2 (Beaufort), corresponding to 0.2–3.3 m/s. (A) Pup with T_max = 61.2°C on the surface, T_air = 17.3°C. (B) Decreased T_surface after light breeze for 5 s to T_max = 58.8°C (3.9%). (C) Pup with T_max = 72.3°C on the surface, T_air = 23.8°C. (D) After 42 s of light breeze T_surface decreased to T_max = 47.3°C (34.6%).</p

Fur seal pups hauling out with dry and wet fur.

<p>(A) Pup with dry fur. The hairs are curled, but constitute a continuous surface. (B) Pup with wet fur, 5 min after leaving the water. The guard hair is divided into strands, opening gaps through which the light grey under fur is visible.</p