East Greenland narwhals exhibit both "freeze" and "flee" responses when escaping from threats, researchers report. This paradoxical reaction is highly costly, and it challenges scientists' current understanding of mammalian escape activity, which states that mammals cannot simultaneously freeze and flee when frightened. Historically, narwhals have not been bothered much by humans because their habits are under Arctic sea ice. In recent decades, however, as Arctic sea ice has declined, this is changing, leaving this species unusually vulnerable to disturbance. To begin to predict anthropogenic impacts to these animals, Terrie Williams and colleagues attached custom electrocardiograph, depth and acceleration recorder devices to nine East Greenland narwhals that had been captured in nets or stranded. The researchers monitored their physiological and behavioral responses immediately after being freed, as well as during routine behaviors in ensuing days (for a subset of these animals). The narwhals exhibited both a downregulated heartbeat (characteristic of a "freeze" response) as well as an upregulated swimming stroke speed (characteristic of "flight"), the authors say. These joint responses place extreme stress on the cardiovascular system, providing insights into why deep-diving marine mammals can be so affected by, and vulnerable to, manmade disturbances.