Final answer:
Hair increases drag and could impair swimming efficiency in aquatic environments. Dolphins have adapted sleek, hairless bodies to minimize friction and increase speed. Additionally, maintaining hair underwater is impractical due to environmental damage and potential impact on buoyancy and temperature regulation.
Step-by-step explanation:
It would be disadvantageous for organisms such as dolphins to possess hair in aquatic environments because hair can increase drag while swimming. Due to the higher viscosity of water compared to air, aquatic organisms, like dolphins, are evolved to have streamlined bodies to minimize resistance and move quickly through the water. Dolphins have adapted to their environment by developing smooth, hairless skin which reduces friction and allows them to swim at high speeds. Hair could also absorb water, making the animals heavier and possibly interfering with their body temperature regulation. Lastly, maintaining hair underwater could be problematic due to the marine environment's potential to damage the structures of hair over time.
For instance, mammals such as dolphins and whales have evolved from land-dwelling creatures back to living in the ocean, shedding traits like fur that are not conducive to their aquatic lifestyle while retaining key mammalian traits like mammary glands and a lack of gills. By comparing their adaptations to those of land animals like deer, which are specialized for running with long legs and hard hooves, it becomes clearer why the presence of hair would be a maladaptation for an aquatic mammal whose survival depends on swift and efficient movement in the water.