And for that matter, what’s to stop them being bigger? I like seemingly simple questions like these that we take for granted, because they often give way to a beautiful cascade of information by way of explanation. Discover magazine has a nice article called “The Origin of Big”, that plots the course of the whale’s evolution.
Simply put: In order to harvest enough microscopic krill from the sea to provide their energy, they have to eat a LOT. In order to take big enough sifting gulps, they need a big mouth. In order to have a big mouth, they need a big body. In order to power a big body, they need… more krill! So they just kept growing until they had reached the optimum level (if they got to require too much energy, they would become too vulnerable if food supplies should wane even temporarily).
In growing so large they face certain logistical problems. They are taking on enough water in each gulp to make their body weight twice as much (sometimes more) as usual. How do they cope? Discover on a recent report:
If a whale simply let the water come rushing in, there would be a tremendous collision–more than a whale could handle. Instead, the scientists argue, the whales actively cradle their titanic gulp. As the water rushes in, the whales contract muscles in their lower jaw. The water slows down and then reverses direction, so that it’s moving with the whale. (It just so happens that fin whales do have sheets of muscle and pressure-sensinging nerve endings in their lower jaw. Before now, nobody quite knew before what they were for.) Once the water is moving forward inside the whale it can then close its mouth and give an extra squeeze to filter the water through its baleen.
As their body increases in size, the energy their bodies demand rises faster than the extra energy they can get from their food.
This scaling may explain some of the weird diving patterns found in lunge-feeding whales. Blue whales are twice as big as humpback whales, for example, but both species dive for the same period of time (about eight minutes) and to the same depth (148 meters). All things being equal, you’d expect that blue whales would be able to dive deeper and longer, because they could store more oxygen in their bigger bodies. Blue whales also make fewer lunges than humpback whales (6 versus 15). It’s possible that the gigantic blue whales are hard up against a size limit. They need so much energy for their lunges that they can’t afford to hold their breath longer, and they can only manage to make a few lunges before they run out of reserves and have to head for the surface.
(Read More @Discover, via Neatorama)