Symmetry is beautiful, right? People with symmetric faces are considered more attractive. Almost all animals have symmetrical bodies. Self-help books always blather on about 'symmetry and balance'. But when you hunt under water, does symmetry become a hindrance?
Top image: Giant squid vs. sperm whale, by Ryan Somma
Baleen whales, like the Humpback or the Minke, that drift through the water gulping up krill by the swimming-pool-ful, have lovely symmetric skulls. They also cannot use echolocation. Why would they? They don't need to hunt. They just need to find big clouds of krill to hork down. Sperm whales, on the other hand, plunge into the depths, chasing after giant squid with sharp beaks and suction cups lined with chitinous teeth. They need to know where to go, and fast, and they use echolocation to get there. They also have strange, asymmetrical skulls.
For a time, scientists were sure that asymmetry was developed along with echolocation. But now, a paper to be published in The Proceedings of the National Academy of Sciences, has shown that the asymmetry came first. Underwater, to hear directionally, whales needed asymmetrical skulls.
The ancient whales' skulls were actually twisted around a line in from the tip of their snouts to the back of their heads. They have that in common with another predator that hunts by hearing; the owl. In normal creatures, the tiny differences in time it takes for a sound to hit each ear gives a sense of where the sound is coming from. If the sound is coming from anywhere on the line equidistant between the ears, or if scattered sounds are coming from many different places, the animal is unable to get a precise bead on the sound. An asymmetric skull will channel sounds completely differently, depending on where they come from, and will interpret different areas of a wall of sound differently. The animal is able to recognize the direction and quality of each sound precisely.
If the hearing difference is that important, why did the baleen whales, and most other animals, lose it? It seems that the cost of asymmetry is high. Only creatures that hunt in three dimensions — owls in air and whales in water — and in very low light will pay it, in order to find their next meal.