Final answer:
Dolphins have evolved to have a minimal amount of hair to reduce drag and maintain high swimming speeds in aquatic environments where a streamlined body shape is advantageous.
Step-by-step explanation:
It is a disadvantage for organisms such as dolphins to possess hair in aquatic environments because hair increases the drag on their bodies as they move through the water. Aquatic mammals require a streamlined body shape to reduce drag and enable them to swim at high speeds efficiently. The streamlined, or fusiform shape which is tapered at both ends, helps dolphins achieve the necessary hydrodynamic efficiency. While some aquatic mammals like the river otter have hydrophobic lipids in their fur for protection from the elements, for dolphins that need to achieve high speeds, retaining hair would be contrary to their evolutionary adaptations for living in the sea.
Furthermore, maintaining any non-essential underwater structures in marine environments is challenging, and this includes hair which would create additional resistance and slow the dolphin down. Hence, over evolutionary time, dolphins have lost most of their hair to adapt better to their aquatic lifestyle, while retaining other mammalian traits like mammary glands and sweat glands.