I understand, generally, *how* they got that big but not *why*. What was the evolutionary advantage to their massive size? Is there one? Or are they just big for the sake of being big?
In: Biology
This is ELI5 and not askscience, but anyone interested in a paper on the topic can find a good one here
[https://www.science.org/doi/full/10.1126/science.aax9044](https://www.science.org/doi/full/10.1126/science.aax9044)
To try to boil it down to ELI5 level….whales benefit from increased energy efficiency the larger they get. For example, oxygen storage gets better as size increases, and movement through the water gets more efficient. However, toothed whale size is limited by the size of prey they can find. Abundant large prey is needed to support large body sizes, because it’s just not efficient to have a big body and individually chase down large numbers of small prey.
Baleen whales avoid this problem by filterfeeding. Instead of eating one prey at a time, they scoop up a swarm of prey animals and eat them all at once. As such, their size isn’t constrained by abundance of large prey, but by abundance of swarms of small prey. And there’s a lot of krill in the ocean. So they could get bigger and bigger and benefit more and more from those size-based efficiencies in diving and movement.
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