DEVELOPMENT OF THERMOREGULATORY ABILITIES IN XANTUS MURRELET CHICKS SYNTHLIBORAMPHUS-HYPOLEUCUS

Thermoregulatory abilities of Xantus'' murrelet chicks (S. hypoleucus) from hatching to 2 wk of age were investigated in the laboratory. Newly hatched Xantus'' murrelet chicks adopt a pelagic existence, and chicks < 1 wk old are constantly exposed to ambient temperatures below their lower critical temperature. The basal metabolic rate (BMR) of hatchlings (mean body mass, 24.8 g) was 8.65 cal(g .cntdot. h)-1, similar to the value predicted for adult nonpasserine birds of comparable size. Hatchlings increased their metabolism to 3.5 times BMR at cooler temperatures. BMR increased by a factor of 2.7 during the next 2 wk. The body temperature (Tb) of hatchlings exposed to thermoneutral temperatures averaged 38.7.degree. C. Thermal conductance of hatchings was 2.6 times the value predicted for adult nonpasserine birds of comparable size but approached predicted values within 4 days. The large drop in conductance occurred around the age when chicks go to sea (2-4 day old) without concomitant changes in mass or plumage, and may be associated with increasing vasomotor control. At 3 days of age, chicks experienced similar rates of heat loss in chilled seawater compared with dry air showing no difference in either metabolic rate or body temperature under the 2 conditions. Murrelet chicks used partial hypothermia at low temperatures to reduce energetic costs. They showed tolerance of severe hypothermia, sustaining Tb as low as 26.degree. C without motor impairment. The quantitative features of Xantus'' murrelet thermoregulatory system are compared with other birds along taxonomic and ecologial lines.