Ants. Ants are the most populous insects in the world, and it is believed, that the weight of all ants in the world is more than the weight of all humans in the world.
And it was not difficult to get to this conclusion since ants live pretty much everywhere and each colony can have many, many individuals.
Decreased surface-to-volume ratio. The dolphin's fusiform body shape and reduced limb size decrease the amount of surface area exposed to the external environment.
Increased insulation. Dolphins deposit most of their body fat into a thick layer of blubber.
