Heat conservation is a critical factor for sea-faring mammals, as they spend a significant amount of time in water, which has a higher thermal conductivity and specific heat capacity than air. This means that water can remove heat from the body more quickly than air. To counteract this heat loss, sea-faring mammals have evolved larger body sizes, as a larger body has a smaller surface-area-to-volume ratio compared to a smaller body. This means that a larger sea-faring mammal loses less heat relative to its body size compared to a smaller sea-faring mammal.

Buoyancy

Sea-faring mammals need to be buoyant in water to support their body weight and move efficiently. Buoyancy is the upward force exerted by a fluid (in this case, water) on an object submerged in it or floating on its surface. The larger the volume of an object, the greater the buoyant force it experiences. Since sea-faring mammals have larger body sizes, they have a greater volume and, therefore, experience greater buoyant force. This helps them stay afloat and reduces the energy required for swimming.

Energy storage

Sea-faring mammals need to store a large amount of energy to sustain themselves during long foraging trips and migrations. They obtain this energy from the food they consume, and larger sea-faring mammals can store more energy than smaller ones. This is because the total amount of energy stored in an animal's body is proportional to its body mass. With a larger body size, sea-faring mammals can carry more energy reserves, allowing them to travel longer distances and remain active for extended periods.

Diving and deep-sea adaptation

Many sea-faring mammals are capable of diving to great depths to search for food or avoid predators. Larger body sizes provide several advantages for deep diving. First, larger mammals can store more oxygen in their lungs and tissues, allowing them to stay submerged for longer periods. Second, larger mammals can withstand the immense pressure exerted by the water at great depths better. Third, larger mammals have a lower surface-area-to-volume ratio, which reduces heat loss and allows them to maintain their body temperature in cold deep waters.

In summary, sea-faring mammals need to be larger than land lubbers due to various factors related to heat conservation, buoyancy, energy storage, and diving adaptations. Their larger body sizes enable them to survive and thrive in the challenging marine environment.