Dear Marmamers: We have a new, exciting manuscript online at Marine Mammal Science. In addition to describing the physiological limitations of a unique cryptic Arctic species (the narwhal), this paper presents a "Biological Achilles Heel" approach for assessing the vulnerability of marine mammals to climate change. Often we examine the behavioral response of wild animals and subsequently try to infer the impact of environmental perturbation. Here we first examine the physiological capacity of the animal and use it to predict the species ability to respond. Such a method enables investigators to predict rather than just react to the effects of environmental disturbance on an animal population. The paper is online at Marine Mammal Science 26 Aug, 2010. DOI: 10.1111/j.1748-7692.2010.00408.x and will be in print in October. Best Regards, Terrie M. Williams McMurdo, Antarctica Extreme Physiological Adaptations as Predictors of Climate-Change Sensitivity in the Narwhal, Monodon monoceros Terrie M. Williams*1, Shawn R. Noren1 and Mike Glenn2 ABSTRACT Rapid changes in sea ice cover associated with global warming are poised to have marked impacts on polar marine mammals. Here we examine skeletal muscle characteristics supporting swimming and diving in one polar species, the narwhal, and use these attributes to further document this cetacean’s vulnerability to unpredictable sea ice conditions and changing ecosystems. We found that extreme morphological and physiological adaptations enabling year-round Arctic residency by narwhals limits behavioral flexibility for responding to alternations in sea ice. In contrast to the greyhound-like muscle profile of acrobatic odontocetes, the longissimus dorsi of narwhals is comprised of 86.8 7.7% slow twitch oxidative fibers, resembling the endurance morph of human marathoners. Myoglobin content, 7.87 1.72 g (100 g wet muscle)-1, is one of the highest levels measured for marine mammals. Calculated maximum aerobic swimming distance between breathing holes in ice is <1450 m, which permits routine use of only 2.6 - 10.4% of ice-packed foraging grounds in Baffin Bay. These first measurements of narwhal exercise physiology reveal extreme specialization of skeletal muscles for moving in a challenging ecological niche. This study also demonstrates the power of using basic physiological attributes to predict species vulnerabilities to environmental perturbation before critical population disturbance occurs.