Overall, the modern world is a little lacking in megafauna. The entire planet used to have a diversity of terrestrial giants on par with modern Africa, with multiple giant herbivores and not quite as giant predators wandering every continent except for Antarctica. As such, when we think of large land animals, we often look to the past, with its mighty indricotheres and steppe mammoths, or more commonly even further back to the age of dinosaurs. However, in the oceans we have what are quite possibly the largest animals to ever exist; whales.
Antarctic blue whales outsize even other members of their species, reaching up to 110 feet (33.5 meters) and 330,000 pounds (149,685 kilograms or 165 short tons). While some of the sauropod dinosaurs, with their lengthy necks and tails, might have reached similar lengths, they were far lighter. So how did whales get so large, and more importantly, when did they get so huge? Do we live in an age of giants after all?
In modern times, we have two major groups of whales, the toothed whales and the baleen whales. Overall, toothed whales are significantly smaller. The world’s smallest whale, the vaquita, has a maximum size under 5 feet, which is probably significantly smaller than you are. The largest of the toothed whales is the sperm whale, which are usually about 52 feet (16 meters), with the largest individual ever physically measured being closer to 60 feet (18 meters) according to the Guinness Book of Animal Records. Despite being puny next to a blue whale, this is still pushing up against the very limits of what is possible for a large predatory vertebrate hunting in the deep.
If you’re a dinosaur nerd like me, you probably remember the gigantic Liopleurodon from Walking With Dinosaurs and think of its great size when you think of the sea reptiles of the Mesozoic. This, along with the reputation of the Mesozoic as a time of giants, has pushed the idea that the various groups of reptiles that ruled the oceans at the time were all as big as whales or bigger. However, this was never the case. Liopleurodon was what is known as a pliosaur, a group made of various plesiosaurs who traded in their long necks for larger heads, and none of them were so large. Kronosaurus was significantly bigger than Liopleurodon, but still only reached about 35 feet (10.5 meters) in length. This is still longer than your average fishing boat, but doesn’t come close to many of our whales. Many of the long-necked plesiosaurs were longer, but much more lightly built.
Mosasaurs were longer and more sinuous, putting the lengths of their largest species at near par with sperm whales. Members of the Mosasaurus and Tylosaurus genus could reach 56 feet (17 meters), with possible 60 foot lizards estimated from teeth! This would easily put them in the same range of size as modern sperm whales, and if they continued to grow throughout their lives like some of their modern relatives, the oldest members of the largest species might have been even bigger. Even they were beaten by the icthyosaurs, the first groups of reptiles to conquer the sea. One species from the Late Triassic may have reached blue whale sizes. However, this is an estimate from fragmentary remains. If it had different proportions to its relatives, it could have been quite a bit smaller.
So the question now is how our second group of whales breaks this limit. The question is how, because it wasn’t always that way. Before the evolution of baleen whales, the largest whale was Basilosaurus, a primitive toothed whale that again reached sizes similar to a sperm whale. From the extinction of Basilosaurus a good 30 million years ago to an incredibly recent 5 million years ago, most baleen whales were relatively small. A family of whales called the Cetotheriidae represented the common trend at the time. Today, only one species of this family survives, the pygmy right whale. They are the smallest of all baleen whales, measuring only 20 feet (6 meters) and weighing about 5 tons (4500 KG). It’s small now, but during the heyday of the Cetotheriidae, most whales didn’t get much longer than this. At the time, whales faced far more predators than they do today, including both massive sharks like Otodus megalodon and even other whales like Livyatan melvillei.
The tipping point was about 5 million years ago, a blink of an eye geologically. The flora and fauna of the world were largely recognizable to us at the time, and our ancestors had already split off from other apes, although they may not yet have left the safety of the trees. Exactly why this is when whales began to really take off in body mass is hard to say. One reason could have been to escape predation, a very real reason to get huge. No whale of the time was large enough to escape the predations of megalodon, however. Tooth marks on flipper bones and tail vertebrae suggest that megalodon hunted giant whales by destroying their fins and tail flukes first, letting them eat immobile prey.
The second reason is likely more important, because it is a major part of why they were able to grow so large. Filter-feeding is already an excellent way to grow large. Instead of spending energy actively seeking prey, a filter-feeder simply needs to keep water running through its mouth, straining out the small animals within. And at the tipping point of whale evolution, filter feeding was gaining an upgrade. The oceans used to be much the same as a modern El Niño event, with warmer temperatures and no cycle of colder water from the lower ocean. With the onset of the ice ages, the ocean took on this modern cycle, with the layers of the ocean exchanging their water. While colder, the ocean is actually far richer this way, with the nutrient-rich lower ocean replenishing the exhausted surface. This fuels the growth of plankton, and in turn the krill that baleen whales eat.
This doesn’t just mean there are more krill but that they existed in extremely dense numbers. This made filter-feeding all the more energy efficient. A whale now finds a thick clump of krill, opens wide, and suctions in thousands in a single gulp. The greater efficiency allows more energy to be put into growth, and the multiple areas in which krill now cluster encourages a larger body that can store more energy for trips between different feeding grounds. This may well be instrumental to the modern whale’s way of life; at the same time whales were breaking the barriers of size, they underwent a severe restriction in their diversity, with most of the small baleen whales that dominated the seas of the time vanishing. They gave way to a new world of giants.