Whales: big enough, too big, or bigger than big?

This picture can’t do them justice. No picture can.

That’s because this is a picture of a blue whale, the largest animal to live on this planet. Ever.

Goodness knows, people try to show you the size. They put up mounts of blue whale skeletons in museums, or life sized models. There’s a very cool online animation that shows images from the blue whale full sized, on your computer screen, as has it drift by lazily. But I suspect that even these clever things can’t do the trick of conveying what the size of the living, breathing animal must be.

But while the blue whale has the undisputed title of being the biggest, whales, dolphins, and their brethren in general are all very big compared to most mammals. In a new paper, Clauset tests a model that tries to explain why whales might so big.

Normally, when I think of limits to size, I think of biomechanical and physical constraints. “In a big animal, can you make the bone thick enough to move without breaking?”, for instance. This is a common sort of explanation for why you can’t make giant insects like in the old 1950s monster movies:

Strength increases as you make muscles bigger, but strength doesn’t increase as fast as mass does.

What’s interesting about Clauset’s approach is that he explains the sizes of whales without using too many of these sorts of arguments. He does invoke the physics to explain the limits to small sizes. Mammals (and birds) can only be so small because of how they regulate their body temperature. If you’re too small, you cannot eat enough food to make up for the heat flow away from your body.

This thermoregulation problem explains why there are no cat-sized dolphins that you keep in a backyard pool, or hamster-sized porpoises you can keep in a home aquarium. The smallest cetacean is the La Plata dolphin, which are around 35-50 kg as adults. Although the babies are pretty squee-worthy:

Clauset ignores all the details of biomechanical and physical and energetic limitations by rolling all that into “extinction risk.” Can make bones strong enough? Can’t eat enough food? All of those mean that those big species are more likely to go extinct.

Clauset assumes a species can either get bigger or smaller over evolutionary time, although Clauset assumes there are some fitness advantages to being bigger. You have a hard limit on how small you can get set by your ability to themoregulate. The limit to how big you can get is a soft limit set by the likelihood your lineage will go extinct. With only these facts, Clauset’s model fits the size distribution of cetaceans extremely well. Presumably, the same model could be used for terrestrial mammals or birds.

Even the massive blue whale, Clauset says, is not particularly unlikely according to his model. Clauset draws out the line from his model and suggests that it might be possible to have a whale species that is over three times bigger than blue whales; 3.7 times, to be exact. Clauset notes that such a massive whale could not just be the blue whale scaled up. To be bigger than the blues, a new whale species might have to evolve some innovation that would allow them to forage more efficiently than the blue whale’s lunge feeding.

The notion that even the blue what could be dwarfed by another sea creature is an awe inspiring thought.

Reference

Clauset A. 2013. How large should whales be? PLOS ONE 8(1): e53967. DOI: 10.1371/journal.pone.0053967

Blue whale photo by Seabass London on Flickr; used under a Creative Commons license. La Plata dolphin from Washington Post.