Which adaptation in aquatic mammals allows them to dive effectively?

The adaptation that allows aquatic mammals to dive effectively is the presence of compact neck vertebrae. This anatomical feature provides enhanced stability and flexibility during swimming, enabling these animals to maneuver efficiently underwater. Compact vertebrae reduce the overall length of the neck, contributing to a streamlined body shape, which minimizes drag while diving. This design is crucial for maximizing propulsion and conserving energy as they navigate through water.

Other adaptations may play roles in the survival of aquatic mammals, but they do not directly contribute to their diving capabilities as significantly as compact neck vertebrae. For instance, lightweight bones might help with buoyancy, thick fur is primarily for insulation in cold waters, and elongated phalanges are often more relevant to locomotion on land or specific swimming adaptations rather than diving efficiency.