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Behavioral thermoregulation of New Zealand sea lions (Phocarctos hookeri) was studied at a male haul-out ground at Papanui Beach, Otago Peninsula, New Zealand. The proportion of time spent by sea lions in each of five postures (prone, curled, oblique, ventral-up, dorsal-up) and also with the number of flippers exposed or tucked (hind and fore) at different black-bulb temperature (Tbb°C) ranges was recorded. Use of prone and curled postures (0–1 flippers exposed) declined as Tbb increased, suggesting that these are adopted to conserve heat; oblique and dorsal-up postures (3–4 flippers exposed) use increased with Tbb indicating a role in heat dissipation. The transition between heat conserving and heat dissipating postures occurred at about 14°–20°C (Tbb). Both foreflipper and hind flipper exposure increased with Tbb and the trends were similar, but overall hind flipper exposure was 89% of foreflipper exposure. The results show that surface area of flippers exposed to air is largely controlled by postural adjustment. The increase in flipper exposure with Tbb provides evidence of behavioral thermoregulation and that flippers are major sites for heat loss in the New Zealand sea lion, as observed for other otariid species. Nonpostural thermoregulatory behaviors such as flipper waving and sand flipping increased with Tbb, and may provide additional means of dissipating heat. Total body surface areas of six sea lions ranged from 1.72 to 3.39 m2 (curvilinear length range from 1.60 to 2.35 m), and total flipper surface area averaged 22.7% of total body surface area. As otariids do not employ their hind limbs for aquatic propulsion, their role in behavioral thermoregulation may provide an explanation for the relatively large size of the hind flippers of the New Zealand sea lion.