Ancient shifts in climate may have powered the baleen whale’s growth to such “ginormous” sizes, a new paper reports.

“Ginormous” seems rather fitting.

With some individuals growing to be the length of an average basketball court and weighing upwards of 200,000 kilograms (441,000 pounds), the blue whale is big fry indeed — in fact, they’re believed to be one of the largest animals that have ever lived on Earth. Which naturally begs the question of what led them, and their kin, to grow to such proportions.

[Turns out that a long time ago, a larger-than-whales dinosaur roamed the Earth. Why not read about it?]

Up to now, biologists have had (and debated over) two main theories in regards to why. The first one is that whales simply grew because they could, as water provides a lot of buoyancy for their bodies. So although they’d weigh a lot on dry land, way too much to be able to even move, they’re pretty nippy underwater and can still catch prey quite easily. The other theory is that the whales grew out of necessity, as their monumental size made them virtually immune to the attacks of giant sharks or other mega-predators.

Another point of interest is when they got so large. One paper published in 2010 under the lead of Graham Slater, an evolutionary biologist currently at the University of Chicago in Illinois, estimates that cetaceans (the whale’s extended family) split into size groups around 30 million years ago. It’s a lineage that still holds today, the paper argues — so the baleen whales trace their ancestry to the giant group, predatory whales (such as the beaked whale) hail from the middle-sized group, and dolphins from the runts of the litter, becoming the smallest of cetaceans.

Chubby cheeks

A new paper however could address both questions in one single swoop. Penned by Slater alongside Nicholas Pyenson, a whale expert at the Smithsonian Institution’s National Museum of Natural History in Washington D.C., and Jeremy Goldbogen at Stanford University in Palo Alto, California, the paper proposes that the whales’ size is a product of environmental stresses associated with global cooling in the Neogene some 4,5 million years ago.

The paper started taking shape a few years ago when Pyenson and Slater started working with the museum’s cetacean fossil collection to see if the diverging lineages theory holds water. Previously, Pyenson studied living whale populations to determine that a whale’s total size correlated well with the width of its cheekbones. So the duo gathered this numbers for 63 extinct whale species and 13 contemporary ones and plotted these values over the family’s timeline.

The trend showed that there weren’t any big whales early on, contradicting Slater’s earlier theory. There wasn’t any gradual increase in size over time, either — instead, what the team saw was that whales became moderately large and stayed so up until about 4.5 million years ago. After this, baleen whales suddenly grew “from relatively big to ginormous,” Slater says.

In case you’re not familiar with the ginormity scale, whales 4.5 million years ago clocked in at around 10 meters (about 32.5 feet) long — whereas today’s blue whales grow to around 30 meters (98.5 feet). So evolutionary speaking, the whales’ size is a pretty recent development.

Long road, big fins

“Laters haters!”

The next step was to look at the going-ons of the time to see what caused this very dramatic, 300% increase in size. The team found that the growth coincided with the beginning of the first ice ages. They explain that the colder climate lead to an increase in glacier cover which would melt during the warmer months of spring and summer, sending cold sediment (and nutrient) rich runoff into coastal waters which supported plankton and zooplankton (who like cold waters) blooms — which the whales were more than happy to dine on.

The problem was that until then, this krill was evenly distributed in the oceans and relatively plentiful, so the whales could go anywhere they pleased and dinner would be waiting for them. But climate change killed off most of the ocean biosphere at the time (ironic isn’t it) and severely weakened existing ecosystems, drastically lowering primary and secondary productivity (the rate at which plants turn sunlight into organic compounds, and the rate at which animals turn plant matter into their own biomass respectively).

Combined, this changed the pattern of food availability from “decent food pretty much anywhere” to “truckloads of food in far-apart areas at certain times during the year,” and the whales had to adapt. Goldbogen, who studies whale eating and diving behavior, helped explain the link between food availability and size. The more concentrated food becomes, larger whales with really big mouths gain a huge boost to feeding efficiency, he says. Moreover, larger whales could travel between feeding areas faster and with less effort than smaller ones.

Overall, these two factors put huge selective pressures on growing larger frames, so the bigger species thrived while smaller whales went extinct.

The paper, while not being the first to show how food and feeding habits shaped whale evolution, does offer a simple and pretty elegant explanation for the whales’ size. It also goes to show that evolution is powered by an interplay of factors, from climate to the way other species adapt to present conditions. And finally, it shows that a species’ adaptation to one particular constraint — in the whales’ case, food availability — can inadvertently address some of its other needs — such as safety from predators — or provide an unexpected boon to ecosystems.

The full paper “Independent evolution of baleen whale gigantism linked to Plio-Pleistocene ocean dynamics” has been published in the journal Proceedings of the Royal Society B.