Why are marine mammals so large?

The smallest toothed whale, the Vaquita porpoise, is still a hefty size compared to the smallest primate, the pygmy marmoset, which is a comfortable handful. Why is that?

This has to do with all mammals trying to maintain a body temperature of 98.6℉ (37℃), by having all cells in their bodies produce heat. Since water conducts heat much better than air, mammals living in water face much higher heat loss to the environment than land mammals. Everything else being equal, a larger body has a much smaller surface-to-volume ratio (S/V) than a smaller one. In addition, marine mammals also tend to be much more streamlined than land mammals, which further reduces the S/V ratio. This is important, since the volume - the number of body cells - is what produces body heat, while the skin surface area is where the heat is lost to the environment.

When a specific shape gets larger, the surface area will increase at a slower rate (by a factor squared) than the volume (by a factor cubed). This means that the Surface to Volume Ratio (S/V) gets smaller the larger the shape. In mammals, the volume (number of cells) produces the heat which is lost through the skin surface area.

So, the larger the body, the less heat it will lose through the skin surface relative to how much heat its cells produce. In fact, a blue whale is so large that it is struggling to dump enough heat, not to overheat.

S/V ratio can therefore explain some of the differences in both anatomy and behavior in nature. For example, humpback whale females migrate long distances from the feeding grounds in productive colder waters, such as off Alaska, to warmer waters, such as off Hawaiʻi, to give birth. The warmer waters will help the small, newborn calves to not lose too much heat to the water. The warmer waters are also where humpback whales mate. The males compete for access to females in what is called “surface-active groups”, where a receptive female is followed by a large number of males fighting over who is to escort the female and presumably mate with her. During these ‘bouts’ the animals generate a lot of extra heat that they need to get rid of, so as not to over heat. This is an extra challenge in tropical warmer waters, where heat does not dissipate as fast as in colder waters. Humpback whales have much longer pectoral flippers than other whales, just for this purpose. By directing their blood flow to their skin, just like we humans do when blushing while exercising, whales can lose heat to the water. The extra surface area of the humpback flippers makes it possible for these whales to compensate for the extra challenge posed by the warmer waters, to be able to compete aggressively in the warmer waters closer to the equator.

Another example, where variation in anatomy can be explained by S/V ratio is the dorsal fin of orcas. Here the males have much larger fins, both pectoral flippers and dorsal fins, than the females. This difference can be explained by the difference in body size between male and female orcas, males get up to 9m long, while females get up to 7.9m long. As a result, the adult males will generate more heat than adult females while performing the same task, thus necessitating the need for a larger surface area to lose the extra heat. The larger fins and flippers also help the males to maneuver their larger bodies. This difference in dorsal fin size is common among dolphin species, where adult males tend to have larger fins than the females.