A frog is an amphibian, but adult frogs do not have gills. They absorb oxygen from the water through their skins, using special blood vessels. If the oxygen level in water is too low, frogs will move around to increase the water flow across the skin. Some frogs have creased skins that can increase their surface area. The larger the animal, the less surface area per volume that animal has, making cutaneous repiration not very effective. It also requires that skin be moist, which can bring about a few other problems, such as not being as effective against bacteria and fungi. Land animals, which includes mammal, have evolved to lessen the amount of water we loose to the environment. Having a large surface area of moist skin can be mal-adaptive. That is why cutaneous respiration is seen only in frogs.