Whales are getting even bigger
A study in the journal Science reports that evolution privileges large body size, especially for animals dwelling in the sea.
The blue whale is the largest living animal on Earth—and it may not be a coincidence that it lives in the ocean.
A new study in the journal Science reports that evolution privileges large body size, especially for animals dwelling in the sea. The idea is not a new one. More commonly known as Cope’s rule, the theory that animals tend to get larger over time was first proposed by the American paleontologist Edward Drinker Cope in the late 19 th century.
While scientists have held Cope’s law to be true generally across mammals, they haven’t been sure whether to attribute that to active natural selection or just neutral “drift” away from initial size. Now, though, Noel Heim of the University of Wisconsin-Madison and his colleagues have discovered that the mean body size of all marine animals has increased 150 times since the Cambrian period.
Here’s Jonathan Webb, writing for BBC News:
Today’s tiniest sea critter is less than 10 times smaller than its Cambrian counterpart, measured in terms of volume; both are minuscule crustaceans. But at the other end of the scale, the mighty blue whale is more than 100,000 times the size of the largest animal the Cambrian could offer: another crustacean with a clam-like, hinged shell.
To arrive at this conclusion, Heim and his team—which consisted of colleagues, undergraduates, and even high school students—pored over scientific records to find adult body size measurements for 17,208 groups of marine animal species, called genera (plural for genus), spanning the past 542 million years. This wide data set constitutes an impressive 60% of all animal genera that ever lived. What they found was a trend toward increasing body size in marine animals over the years. Though the trend is far from universal. Here’s Webb again:
But Dr. Heim and his colleagues say the consistent trend does not mean that every single genus of animals evolved to grow larger.
Instead, the branches of the family tree that were populated by larger animals divided many more times—diversifying and expanding so that the ocean gradually built up a greater variety of bigger and bigger beasts.
To see if evolution drove these changes or if the pattern was the result of mere chance, the scientists put their data into a computer model that mimicked how family trees might evolve. “The degree of increase in both mean and maximum body size just aren’t well explained by neutral drift,” Dr. Heim told the BBC. “It appears that you actually need some active evolutionary process that promotes larger sizes.” Why Cope’s rule is particularly applicable to marine mammals remains unexplained. Early marine mammals were on the smaller side relative to today’s, and Heim believes that as they adapted to breathing air while living under water, their more efficient use of oxygen may have spurred an increase in body size. On the other hand, it may be that the mammals that returned to the ocean were large to begin with, and that body size accommodated large lungs, which made it easier to breathe.
Cope’s law, of course, does not necessarily hold true for land animals, if only because they are burdened by having to carry their bodies around. Applications of Cope’s law vary under different circumstances—for example, a 2012 study showed that some dinosaurs’ sizes steadily increased but eventually plateaued; in mollusks from the Cretaceous period, increases in size are no more common than decreases (or no changes at all). And sometimes, the Cope’s rule operates on specific taxonomic levels only—for instance, an order of animals may abide by Cope’s law as whole, but families, genera, and species within it may not.
Marine mammals represent a small portion of the overall biomass and number of species living in the oceans, which makes the overall trend of a 150-fold increase in body size that much more revealing. What’s really been driving the trajectory toward larger masses are the smaller, more diverse schools of fish and other invertebrates. Though they’re not nearly as large as a whale, they are following the same path toward gigantism, just in their own way.
Photo Credit: potvisstaat / Flickr (CC BY-ND 2.0)