Why are whales so big? One answer is simply that they can be.
The size of land animals is constrained in part by their need to support themselves against the force of gravity.
Marine creatures have that support provided free, by the medium they live in.
Even so, what is possible is not always sensible. Resources put into growth are unavailable for reproduction.
Given that whales can and do become big, however, a second question arises: what, if anything, stops them being even bigger?
Jeremy Goldbogen of Stanford University and his colleagues suspect that the answers to both questions are related to the animals' food supply.
And, as they describe in a paper in Science, they have gathered data that illuminate how this might work.
Broadly, big whales come in two varieties. Toothed whales, such as sperm whales, hunt individual prey.
Baleen whales suck in mouthfuls of water and extract small organisms such as krill, using fibrous buccal filters.
The biggest whales of all (blue, humpback and so on) are baleen whales.
This might be viewed as paradoxical, because on land, as predators get bigger, so do their individual prey.
Both toothed and baleen whales often hunt by diving deep—prey being more abundant at depth.
To do this they have to hold their breath, which limits how long they can stay underwater.
One explanation of giantism in whales is that because bigger whales can hold their breath longer, they can spend more time hunting.