Q: As part of our second grade curriculum, we study adaptations. The
question came up about the size of whales. Do scientists feel that
whales were this large when they began as land animals or did they
become larger once they had evolved into marine mammals? If the latter
is the case, why did the "great whales" become so large. Many thanks
for your help. Jan Ford for Grade 2 at the Dallin School.
A: Interesting question. The ancestor of whales was actually a wolf-like animal (well, it looked like a wolf - a mesonychid condylarth), in size as a modern-day wolf or German Shepherd. Whales, however, are actually closely related to cows and pigs, so the wolf-like description refers to the body shape more so than feeding habits, etc.
An animal's size, as you've learned, is a reflection of a host of things, including its environment. When this ancestor made it's way back to the ocean, there was a breakout of three groups of whales - toothed whales (odontocetes) (includes dolphins, sperm whales, etc.), baleen whales (mysticetes), and 'ancient whales' (archaeocetes, which are extinct now). Toothed whales came to use echolocation, while baleen whales filter their food out of the water. Why did this happen - who knows, but it is likely a reflection of filling various niches, which is what all animals (and plants) do in current time as well. Something eats a different size or type of food, or prefers a certain soil type over another animal, etc.
Hope that helps. To make things more confusing, not all whales are huge. There are animals with the name 'whale' that are actually just large dolphins. The 'great whales' - the baleen whales and the sperm whale are very large. And one more interesting note - the sperm whale is actually a whale with teeth, and not with baleen, like a humpback or blue whale.