No one is certain why marine mammals like whales, sea cows and leopard seals are so big. To date, the best guess is that without the great tug of gravity, marine mammals could simply swell up to massive weights that would buckle the legs of even the strongest land mammals. But as Laura Geggel at LiveScience reports, a new study suggests that their size is a delicate balance between the amount of food they can eat and how much body heat they lose in the chilly ocean.
To look into the size of marine mammals, researchers examined the body masses of 3,859 living mammal species and 2,999 from the fossil record. The results suggest that when mammal species move into the water, they begin to evolve into larger species. According to a press release, aquatic mammals reach a sizable sweet spot around 1,000 pounds. Overall, the study suggests that for water mammals, bigger is better, but only to a certain point. They detail their results in a study published in the journal Proceedings of the National Academy of Sciences.
“Many people have viewed going into the water as more freeing for mammals, but what we’re seeing is that it's actually more constraining,” co-author Jonathan Payne of Stanford says in the release. “It’s not that water allows you to be a big mammal, it’s that you have to be a big mammal in water—you don’t have any other options.”
As Carrie Arnold at National Geographic reports, two factors converged to make aquatic mammals so huge. The first is the need to conserve body heat. Smaller land mammals—including humans—quickly lose body heat to chilly water, making them susceptible to hypothermia. “When you're very small, you lose heat back into the water so fast, there's no way to eat enough food to keep up,” Payne says in the release. A larger animal, however, is able to maintain a warmer body temperature and limit heat loss.
The second factor is how much a critter can eat. There is only so much food an animal can consume to keep that fiery metabolism going, a factor that limits the maximum size of marine mammals. In fact, the study suggests that the viable sizes for mammals in the ocean are more limited than those on land, which range from inch-long shrews to African elephants.
There are a few exceptions to this small range in aquatic mammal size. Baleen whales are much larger than the magic 1,000 pounds sweet spot. “The sperm whale seems to be the largest you can get without a new adaptation,” William Gearty, a Stanford scientist and lead author of the study, says in the release. Baleen whales, however hacked the system by developing a new adaptation to the environment: filter feeding.
Instead of chasing down fish like toothed whales, they strain tiny zooplankton from the water column with their fringed baleen. Using this technique, they waste little energy while gathering massive amounts of krill.
As Gearty tells Geggel, there are also smaller species of seals and many species of dolphins that don’t reach 1,000 pounds. This is actually helpful since not all marine mammals are fighting to fill the same ecological niche. “Instead, they spread out across the range of possible sizes,” he says.
Then there are otters, which are small for a marine mammal. As the press release notes, this is likely due to the creatures spending a fair amount of time on land. But perhaps they’re just evolving along a different trajectory—one that maximizes cuteness, not size.