Most large creatures have much lower heart rates than smaller a nimals. In fact, it is even possible to find a correlation between body size and heart rate, which has led some scientists to propose a model that could explain this phenomenon. But do all animals conform to this scaling law? Mounting evidence suggests that this may not always be the case. Timothy Clark and Tony Farrell from the University of British Columbia, Canada, point out that little is known about the scaling relationship between body mass and heart rate in fish. Curious to find out whether fish heart rate patterns follow the same scaling trend as other vertebrates, Clark and Farrell caught nine male Chinook salmon, ranging from 2.5 to 15.9 kg, as they migrated to their spawning grounds and implanted data loggers that could record each fish's heart rate and activity patterns. Next, Clark and Farrell allowed the animals to swim freely in a holding channel for 9 days before retrieving the data loggers ().
However, when they analysed the data, there was no correlation between the fish's body size and heart rate. Instead of decreasing with size, the fish's resting heart rates were all between 30 and 43 beats min–1. And when the duo considered heart rate data from the literature for tiny 0.02–0.05 g trout, they found that even the smaller fish only had heart rates of 50–60 beats min–1, rather than the 1000 beats min–1 range you would expect if they followed the same size scaling pattern as birds and mammals.
Next, the duo analysed the relationship between the fish's body size and various blood parameters, such as haemoglobin and glucose levels. They found that all of the parameters varied, but in no consistent way relative to the fish's body masses. Meanwhile, the fish's heart chamber (ventricle), spleen and heart muscle (myocardium) masses all scaled in proportion to the fish's body masses. The duo suggest that the Chinook salmon's ability to transport oxygen and their mass-specific cardiac output and cardiac power may be maintained across all body masses.
Having shown that the scaling law for heart rate does not hold in Chinook salmon, Clark and Farrell point out that fish offer great potential for examining the effects of body mass on other physiological variables, as their body masses vary to a greater degree than any other vertebrates and they account for over half of all vertebrate species.
