I am doing a Biology lab on dissolved oxygen in water, etc. etc. Mammals use about 1-2% of their energy breathing, while fish must spend about 15% of their energy moving water through gills for THEIR energy. Why the huge gap?!

different mechanisms of breathing. we use sacks that we can stretch to draw air into our lungs using muscles and expel air by relaxing. fish use gills which are the equivalent of a hollow tube, there are no muscles capable of creating a current so they must swim. also, up here we have a higher concentration of oxygen.

Water is miuch more viscous and doesn't have much oxygen in it so there's a lot of energy needed to pull enough water through the gills to get enough oxygen.