Whales are huge. Even the smallest of them, the pygmy right whale, can reach 20 feet, while the blue whale, the largest of the baleen whales, can stretch to about 105 feet, and some will get a bit larger.

But why did these giants of the deep get so, well, giant? Was it to ward off predators, like massive megalodons, or was it caused by competition among whale species for food? Turns out we may have been asking the wrong question. Maybe the "why" isn't quite as important as the "when."

That's the tack taken by a trio of researchers in a study published in the Proceedings of the Royal Society B regarding baleen whales and their sizes. Instead of looking for the cause first, the researchers turned to fossils of extinct species and museum specimens for extant species to track the development of the whales' growth across groups.

What they found "suggests a recent emergence for gigantism" in whales. "Recent" is a relative term here, of course. When baleens first came onto the scene, it was about 35 million years ago, and they were already large. But they didn't get to be large and in charge until 4.5 million years ago, which, in evolutionary terms, is pretty darn recent. Another million years go by, and the planet is entering a new cycle of weather patterns during the late Pliocene epoch as glaciers expand and retreat, and suddenly we have variable weather over the year, meaning seasons, and that, the researchers contend, changed everything for whales.

It's all comes back to food

The view of a blue whale from above A blue whale is breathtakingly large — and researchers think receding glaciers may have played a role in that. (Photo: NOAA Photo Library/flickr)

So why does that matter to a whale's size? Well, as the glaciers receded, nutrients were released into the oceans along the coasts. Along those same lines, shifts in temperatures resulted in a new currents that drove up nutrients from the oceans' depths. Essentially, the oceans became the whale equivalent of a truly amazing buffet, but those buffets were very localized. That meant that were long stretches of ocean where there were no buffets at all. Depending on where the whales were, the ocean could provide a feast or near famine environment.

The result was that whales' prey — small fish and crustaceans — were more densely concentrated than before, and that made the whales' now trademark lunge feeding method of opening their mouths wide and sucking in huge amounts of water and then filtering out actual food, even more efficient energy-wise. It also meant, however, that they needed to feed more to survive those stretches of limited prey. So the whales needed a larger body to lower metabolic rates and to lower the energy costs of swimming long distances while still being able to eat large amounts of food.

The shifts that happened during the Pliocene all may have contributed to an increase in whale size. It also meant that smaller whales struggled to survive in this new ocean world, what with the energy costs of traveling between meals and larger whales gobbling up all the food in sight.

The researchers acknowledge that other factors may have played a part in whale size, and that their study is a hypothesis and not a definitive answer since the fossil record from that time isn't as complete as they'd like.