The ancestors of baleen whales increased their sizes as a result of the Ice Age, adapting to the changed feeding conditions.
Scientists have long debated the reasons for the increase in whale sizes. According to one theory, they became large simply because it was possible: the aquatic environment supports mass and facilitates movement, allowing whales to find sufficient food, even if it consists only of small marine crustaceans.
According to another theory, the increase in whale sizes is related to the presence of large predators that inhabited the seas and oceans in the distant past: to avoid encounters with giant ancient sharks, it was necessary to grow as large as possible.
It is important to note that not all species of cetaceans reached gigantic sizes. In 2010, Graham Slater from the University of Chicago suggested that during their evolution, cetaceans became what we know them to be quite early: around 30 million years ago, the ancestors of modern dolphins appeared, which, like their modern relatives, were relatively small, as well as the ancestors of 'medium-sized' carnivorous toothed whales and the ancestors of baleen whales, which fed on plankton and quickly reached impressive sizes.
However, currently, Graham Slater and his colleagues, Jeremy Goldbogen from Stanford and Nicholas Pyenson from the National Museum of Natural History, in their article in Proceedings of the Royal Society B, argue that the situation was somewhat different. A few years ago, it was established that the body mass of cetaceans correlates well with the size of the zygomatic bone.
By examining the skulls of several dozen extinct and modern species, scientists concluded that, having split into different groups, whales initially did not grow very actively, and for a long time even the largest species did not exceed 10 meters in length. Only about 4.5 million years ago did a sharp increase occur, and baleen whales significantly increased their sizes. What caused this? The authors of the study suggest that it was the Ice Age: advancing glaciers regularly melted and washed numerous nutrients into the ocean, which served as food for small crustaceans and other small animals.
Baleen whales, as mentioned earlier, feed by filtering various small organisms from the water. It is extremely important to have a large mouth: the bigger it is, the more efficient the food filtration process. In other words, when feeding on plankton, it is better to be large than small. At the same time, plankton is distributed unevenly across the ocean — there are zones, a kind of plankton 'fields', where it is abundant, and moreover, due to climate changes, its quantity fluctuated in different seasons.
Whales need to swim a lot from one 'field' to another, and it is easier to do this if you are large. Thus, it can be argued that baleen whales became large due to food — species that had more modest sizes and fed on plankton went extinct, unable to adapt to the new feeding conditions.
