An earthquake generates two types of seismic waves. The first is the relatively weak, fast-moving P wave, or primary wave. Then comes the more powerful S wave, or secondary wave, which lumbers along at a leisurely pace and heaves the ground up and down.

A back-of-the-envelope calculation by Hough suggests that the first P waves would have reached Washington about 15 seconds before the S waves. That may explain a lot: Iris and the other animals may have been responding to the P waves before humans noticed the ground shaking.

That leaves the mystery of the red-ruffed lemurs. They began hollering about 15 minutes — not seconds — before the earthquake. But that could be a coincidence, an outcry unrelated to the temblor. Hindsight can be misleading, as selective memory creates illusions of cause and effect.