Comment Re:A very unusual toad (Score 1) 121
I don't think you can argue that building dams is a requirement of survival. It is disconnected from evolution.
It is certainly not disconnected from evolution: they started doing for some reason, and continued to do it as they evolved. Maybe at some point it WAS a matter of survival, such as lowering down-stream water levels so larger water predators couldn't swim up-stream to get at them.