Flight Risk

A swimming narwhal (Monodon monoceros)

Dr. Kristin Laidre, Polar Science Center, UW NOAA/OAR/OER

When mammals are exposed to a threat, fear responses are typically classified as active—fight or flight—or passive—freezing in place. Both reactions provide opportunities to avoid predation but elicit markedly different bodily responses: fighting or fleeing increases heart and respiration rate, while freezing does the opposite. In general, mechanisms exist to prevent the two kinds of responses from overlapping. A new study has shown that frightened narwhals, Monodon monoceros, exhibit a complex escape response that incorporates elements of both, raising concerns for their health.

A team of ecophysiologists, led by Terrie Williams from the University of California, Santa Cruz, affixed submersible recorders to East Greenland narwhals after they were entangled in nets and stranded. Electrocardiographs on the recorders monitored the narwhals’ heart rates, while accelerometers and depth sensors tracked their movements.

The recorders showed that, as the narwhals fled in a series of escape dives, their heart rates plummeted to as low as three to four beats per minute—much lower than observed as part of their normal dive response and more typical of fearful freeze reactions. The narwhals’ heart rates stayed low for long descents and did not increase until the final moments of ascent, sometimes more than ten minutes later. This went on for repeated dives for up to forty-five to ninety minutes, until their fear response began to subside.

Superimposed on this freeze response was significant flight behavior—the narwhals swam hard to get away by approximately doubling their stroke rate. In a normal dive, heart rate would increase to compensate for this activity and deliver needed oxygen to the organs, tissues and the brain. During the narwhals’ escape dives, however, their freeze reaction caused the opposite to occur. Williams suspects that this causes the animals to use up their oxygen stores rapidly and stresses their cardiovascular systems as they try to both freeze and flee.

Until recently, narwhals have had little sustained exposure to predators or to the kind of human disturbances that would trigger this extreme response. However, melting sea ice may affect predator-prey dynamics and increase exposure to other human activities, such as oil and gas drilling. “This escape response sets them up to be vulnerable when there’s any kind of disturbance in their environment,” says Williams. (Science)