Her name is Iris, and with her straight, elegant, red-orange hair, she is beyond dispute the prettiest orangutan at the National Zoo. She’s calm, quiet, unflappable. “Iris lives the life of a queen,” says great-ape keeper Amanda Bania.

On Tuesday afternoon, the queen lost her cool.

It happened a little before 2 p.m. Primate keeper K.C. Braesch was standing just a few feet away when Iris emitted a loud, guttural cry, known to scientists as belch-vocalizing. Iris then scrambled to the top of her enclosure.

Braesch stepped back and scanned the enclosure to see what might have agitated the ape. Was it Kiko, the male? Although generally a lump, Kiko can turn into a hothead and throw things. But no, Kiko was lounging.

Then — all this happened within about five seconds — Braesch felt the earthquake.

“Animals seem to know,” she said Wednesday. “You always hear it anecdotally, but this is the first time I’ve seen it.”

Orangutans, gorillas, flamingos and red-ruffed lemurs acted strangely before humans registered the historic magnitude-5.8 earthquake. Now the question hovering over the zoo is: What did the animals know, and when did they know it?

Therein lies a scientific mystery, one in which hard facts and solid observations are entangled with lore and legend. There has been talk over the years about mysterious electromagnetic fields generated by rupturing faults. There has been speculation about sounds inaudible to humans, and subtle tilting in rock formations, and the release of vapors that people can’t smell.

But there also may be less to the mystery than meets the eye, with Tuesday’s zoo weirdness merely serving as a reminder that many wild animals are paying close attention to nature while humans are doing whatever it is that humans do.

The zoo documented a broad range of animal behavior before, during and after the tremor that began in central Virginia and shook much of the eastern United States. For example, a gorilla, Mandara, shrieked and grabbed her baby, Kibibi, racing to the top of a climbing structure just seconds before the ground began to shake dramatically. Two other apes — an orangutan, Kyle, and a gorilla, Kojo — already had dropped their food and skedaddled to higher turf.

The 64 flamingos seemed to sense the tumult a number of seconds in advance as well, clustering together in a nervous huddle before the quake hit. One of the zoo’s elephants made a low-pitched noise as if to communicate with two other elephants.

And red-ruffed lemurs emitted an alarm cry a full 15 minutes before the temblor, the zoo said.

The belief that strange animal behavior is a precursor to earthquakes goes back to antiquity. A recent scientific study suggested that toads fled to higher ground days before the 2009 earthquake in L’Aquila, Italy. In the most famous case of modern times, snakes and frogs emerged from their holes in 1975 in the dead of winter several weeks before a magnitude-7.3 earthquake in Haicheng, China. The odd animal behavior helped persuade officials to evacuate the city just before the tremor.

But scientists have struggled to convert anecdotal evidence into testable hypotheses and robust conclusions that can be published in peer-reviewed journals.

Susan Hough, a U.S. Geological Survey seismologist who has researched earthquake predictions, said the simplest explanation for what happened at the zoo on Tuesday involves what scientists call the P wave.

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.

This remains an unresolved issue, and the possibility that animals are sensitive to terrestrial phenomena not discovered by humans can’t be ruled out.

As Hough put it, “There’s more going on in the earth than we understand.”