Dear colleagues: We are happy to announce the publication of two studies on thermoregulatory abilities of northern fur seals in Marine Mammal Science. Dalton AJM, Rosen DAS, Trites AW (2014) Broad thermal capacity facilitates the primarily pelagic existence of northern fur seals (Callorhinus ursinus). Marine Mammal Science 30: 994-1013 Rosen DAS, Trites AW (2014) Thermal limits in young northern fur seals, Callorhinus ursinus. Marine Mammal Science 30: 1014-1028 Electronic copies can be obtained by writing me at: rosen@zoology.ubc.ca Regards, Dave Rosen Abstract: Dalton et al. Thermoregulatory capacity may constrain the distribution of marine mammals despite having anatomical and physiological adaptations to compensate for the thermal challenges of an aquatic lifestyle. We tested whether subadult female northern fur seals (Callorhinus ursinus) experience increased thermoregulatory costs in water temperatures potentially encountered during their annual migration in the Bering Sea and North Pacific Ocean. Metabolic rates were measured seasonally in 6 captive female northern fur seals (2.75–3.5 yr old) in ambient air and controlled water temperatures of 2°C, 10°C, and 18°C. Rates of oxygen consumption in ambient air (1°C–18°C) were not related to environmental temperature except below 2.5°C (winter only). However, metabolism was significantly higher during the fall seasonal trials (September–October) compared to other times of year, perhaps due to the costs of molting. The fur seals appeared thermally neutral in all seasons for all water temperatures tested (2°C–18°C) except during the summer when metabolic rates were higher in the 2°C water. Comparing this broad thermal neutral zone to the average sea surface temperatures potentially encountered during annual migrations indicates wild fur seals can likely exploit a large geographic area without added thermal metabolic costs. Abstract: Rosen and Trites The thermoregulatory abilities of northern fur seals (Callorhinus ursinus) during their first two years in the frigid waters of the North Pacific Ocean may limit their geographic distribution and alter the costs for exploiting different species of prey. We determined the thermoneutral zone of six young northern fur seals by measuring their metabolism in ambient air and controlled water temperatures (0°C–12°C) from ages 8 to 24 mo. We found that the ambient air temperatures within our study (overall 1.5°C–23.9°C) did not affect resting metabolic rates. Calculated lower critical temperatures in water varied between 3.9°C and 8.0°C, while an upper critical temperature in water was only discernible during a single set of trials. These thermal responses provide insight into the possible physiological constraints on foraging ecology in young northern fur seals, as well as the potential energetic consequences of ocean climate change and altered prey distributions.