Why Do Whales Get So Big? Science May Have an Answer.

Land mammals can get plenty big, but to find the planet’s true
giants, you’ll have to take to the seas.

Previously, researchers believed that marine mammals could be so
large because the buoyancy of water frees them from the constraints
of gravity. Although this freedom may still be a factor, Gearty
says that his results show that marine mammals need their heft to
keep themselves warm in the often chilly oceans.

“These animals are big for very specific reasons. It’s not that
they could be big, it’s that they must be big,” he
says.

When Gearty and colleagues created a series of computer models
analyzing factors that influence size, they found two that
converged to determine body size in aquatic mammals.

But larger animals need more food to support their bulk, which
created the second factor in Gearty’s model. Big mammals may trap
heat better, but if they can’t get enough food to fuel their
metabolism, then it doesn’t matter. (See
National Geographic’s amazing whale pictures^[1].)

Body size is one of the most important traits to study in
animals, according to Chris
Venditti^[2], an evolutionary
biologist at the University of Reading in England who wasn’t
involved in the new study.

“If you’re going to measure one thing in an animal, it should be
body size because that one thing is related to so many others,”
Venditti says. “If you know how big an animal is, you probably know
something about how it moves and its metabolic rate.”

In the last five years, scientists have uncovered evidence
showing that, over time, families of mammals have tended to evolve
larger body sizes. Bulkier animals can better fight off rivals for
mating, food, and other resources, as well as access a wider
variety of foods.

Land mammals, however, are hemmed in by gravity: They need
massive bones and blood vessels to support their bulk while
maintaining mobility—no easy feat when you tip the scales at
several tons, like an elephant. (Read how
blue whales are mostly “left-handed^[3].”)

Initially, when Gearty started studying the factors that
affected body size in marine mammals, he thought that he would
simply see the elimination of gravity as a constraint.

Instead, his data told him that the minimum size of aquatic
mammals was a thousand times larger than the smallest terrestrial
mammals. The maximum size, however, was only 25 times larger, which
meant that something must be forcing marine mammals to get
large.

Scientists still haven’t entirely cracked the mystery of what
determines body size in animals, Venditti says, but that hasn’t
stopped life from evolving an array of shapes and sizes to fill
every niche.