A BLUE whale鈥檚 heart drums at roughly 20 beats per minute, a human heart at 60 to 80 bpm, but eavesdrop on a hummingbird鈥檚 tiny ticker and you鈥檒l find more of an incessant whir than a pulse. In birds and mammals, the bigger the animal, the slower its heart. But fish, it turns out, do not follow this law.
There is simply no relationship between their body mass and heart rate, say and of the University of British Columbia, Vancouver, Canada. They implanted biologgers in nine wild male Chinook salmon to measure their heart rate every 5 minutes for nine days. Regardless of body mass, all maintained between 30 and 43 bpm (The Journal of Experimental Biology, vol 214, p 887).
Elsewhere, they found data from a previous study showing that juveniles of a small trout species have a similar heart rate of 50 to 60 bpm. A mammal or bird of the same size would have a heart rate closer to 1000. 鈥淭his challenges a fairly fundamental law,鈥� says Farrell. 鈥淲e expected at least some relationship to body mass, but we found none.鈥� As yet, the pair have no explanation.
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