- Thick skin: Mammals have a thick layer of skin that is covered in fur or hair. This thick skin provides insulation and protection, but it also makes it difficult for oxygen and carbon dioxide to pass through.
- High metabolic rate: Mammals have a high metabolic rate, which means that they need a lot of oxygen to produce energy. This high demand for oxygen makes it difficult for cutaneous respiration to meet the body's needs.
In contrast, small, thin-skinned animals such as frogs, salamanders, and earthworms can use cutaneous respiration because their skin is thin and they have a low metabolic rate.