Extreme bradycardia and tachycardia in the world’s largest animal

The biology of the blue whale has long fascinated physiologists
because of the animal’s extreme size. Despite high energetic demands
from a large body, low mass-specific metabolic rates are
likely powered by low heart rates. Diving bradycardia should slow
blood oxygen depletion and enhance dive time available for foraging
at depth. However, blue whales exhibit a high-cost feeding
mechanism, lunge feeding, whereby large volumes of prey-laden
water are intermittently engulfed and filtered during dives. This
paradox of such a large, slowly beating heart and the high cost of
lunge feeding represents a unique test of our understanding of
cardiac function, hemodynamics, and physiological limits to body size. Here, we used an electrocardiogram (ECG)-depth recorder tag
to measure blue whale heart rates during foraging dives as deep
as 184 m and as long as 16.5 min. Heart rates during dives were
typically 4 to 8 beats min−1 (bpm) and as low as 2 bpm, while
post-dive surface heart rates were 25 to 37 bpm, near the estimated
maximum heart rate possible. Despite extreme bradycardia, we
recorded a 2.5-fold increase above diving heart rate minima during
the powered ascent phase of feeding lunges followed by a gradual
decrease of heart rate during the prolonged glide as engulfed water
is filtered. These heart rate dynamics explain the unique hemodynamic
design in rorqual whales consisting of a large-diameter,
highly compliant, elastic aortic arch that allows the aorta to accommodate
blood ejected by the heart and maintain blood flow during
the long and variable pauses between heartbeats.