Among birds and mammals, symmetry is beauty. Females frequently choose mates, in part, on how balanced antlers, horns, facial features and the like are displayed. Symmetry is even a part of humans' perception of beauty. Left-right (bilateral) symmetry is the general rule in the body plan from the internal organs out: two kidneys, two wings, two eyes, etc., all symmetrically placed.
When asymmetry is encountered, there is usually an interesting story to be told. This story begins with a couple of recent early-morning walks with the sky brightening on the eastern horizon. Twice in the last couple of weeks, I heard the call of a northern pygmy owl. The call is a series of slowly delivered toots easily imitated by whistling. As the name suggests, pygmy owls are tiny, barely larger than a good-sized sparrow. They are residents of our conifer forests, but a few descend to the valley floor during winter. They have a long tail for an owl and two black patches on the back of the head and yellow eyes.
Unlike most owls, they lack the pronounced facial disk that helps collect faint sounds from rustling prey in the dark of night, much like a satellite dish. That is because pygmy owls hunt extensively during the daytime using sight at least as much as sound to locate prey. They hunt worms, lizards and insects as well as mice and small birds nearly as large as themselves.
OK, back to symmetry. Consider your ears. You have two. No surprise there. They are located on the left and right sides of your head, placed symmetrically. Because sound is relatively slow (compared to light) and the brain is quick, we use the time difference that a sound reaches each ear to locate a source. This is why we know whether to look left or right when we hear a noise. We aren't nearly so good at locating the source of a noise in the up or down direction. That is because the sound reaches the symmetrically placed ears at the same time.
This has been a long way around to consider the ears of owls. They are crooked. The left ear is placed higher than the right. This looks really weird if you examine the skull of a barn or western screech owl.
The asymmetry is jarring. But it makes sense if you consider their needs. An owl in a tree needs to know where a mouse is located left and right, but it also needs to know where the prey is located up and down, too. Otherwise it might miss by either aiming too high or too low. The asymmetrical placement of the ears allows them to judge location in both directions. Studies have shown that barn owls, and likely others, can hunt successfully in total darkness.
This brings us back to the northern pygmy owl. I recently had the opportunity to examine the skull of one. How do you think the ears are arranged on the face of this species? They are much more symmetrical than the ears of other owls. Of course. Pygmy owls hunt more by sight than sound.
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.