Why do fish and amphibians have a lower relative heart mass

Why do fish and amphibians have a lower relative heart mass than that of mammals? And why do tetrapods have such a close relative heart mass to eachother but such a large difference between them and fish?

Solution

Answer:

The body mass of fishes & amphibians are lower so that rate of oxygen consumption (cold-blooded animals) is also lower and they do not possess lungs whereas mammals have relatively higher body mass & the rate of oxygen consumption (due to high metabolic rate) is higher. So that, heart mass with large arteries & capillary beds is also higher in mammals compared to fish and amphibians.

Tetrapods have such a close relative heart mass to each other but such a large difference between them and fish (0.06-0.20% of body mass) because tetrapods do possess relatively lower body size as fishes even though they are morphologically different in caecilians, snakes