Ahab had the great white whale. Rob Nelson has the great white squirrel.

Nelson — a biologist and creator of science videos who lives in Charlotte — did not set out to become the White Squirrel Guy. In fact, his obsession with the ghostly creatures has waned in recent years. But the mechanism Nelson set in place years ago — an online tool that allows people to post details on where they have seen white squirrels — continues to tick on.

When I consulted Nelson’s white squirrel tracker for a recent day — April 10, 2019 — I learned there were reports of 13 white squirrels, from Brownsboro, Ala., to St. Paul, Minn. There were 14 the day before, including two in Xenia, Ohio. Or was that the same white squirrel, spotted twice?

This white squirrel database isn’t the most scientific project in the world. The squirrels it tallies are self-reported. Well, not self-reported — squirrels lack Internet access — but reported only by people who see a squirrel and are moved to share the moment.

Nelson saw his first white squirrel in 2007, then set about visiting some of the towns in North America that take pride in their colonies of the pale rodents: Brevard, N.C.; Kenton, Tenn.; Marionville, Mo.; and Exeter in Ontario, Canada.

“They really are pretty rare,” he said.

Any white wild animal is, whether it’s a deer, an alligator or a giraffe.

“That’s why everybody looks at them,” Nelson said. “Not just because it’s unique in squirrels, but because it’s unique in everything.”

With the help of Roland Kays , a zoologist at North Carolina State University, Nelson set up an online Google Doc that people can use to report where they’ve seen a white squirrel: untamedscience.com/biodiversity/white-squirrel.

Nelson started with a theory that mapping white squirrel sightings would show that the creatures are more common around those hot spot cities he visited, places that celebrate their unique squirrels with festivals and bumper stickers.

But what he found was that white squirrels seem distributed fairly evenly across the range of the Eastern gray squirrel. Of course, that’s what a white squirrel is: a regular Sciurus carolinensis that, genetically speaking, has been bleached.

A white squirrel can either be a pigmentless albino — pink eyes are the clue — or what’s called a white morph. A morph has black eyes and often a touch of color on its fur: a black cap or dark stripe along the back. (It’s also known as a leucistic squirrel.)

What is life like for a white squirrel?

“You would assume that predation would be different,” Nelson said.

To a hawk, coyote or other hungry animal, a white squirrel might as well have a target painted on its back. That prompted another hypothesis.

“Our initial thought was that there would be a difference in abundance in cities versus rural areas,” Nelson said. “The implication is that being white is not very advantageous in the wild. That’s why they’re rare: They get picked off by carnivores. But in the city there’s a decreased abundance of predators, and maybe that’s allowing the white color pattern to become more abundant.”

But Nelson isn’t sure. Though reports continue to come in — the tally was up to 31,718 sightings when I last checked — the huge pile of data has never been scrutinized.

“I want a grad student to take this over,” Nelson said. “I have a big data set that could be a fun project for somebody. It’s never been studied, because there’s no money in looking at white squirrels.”

There may not be money, but there is pleasure. White squirrels are so unexpected and oddly alluring that they set off a frenzy of tweeting and Instagramming wherever they appear.

Last year, a white squirrel was spotted in Franklin Square in downtown Washington. Then another. Then another: a trio of white squirrels blocks from the White House.

“I was very excited,” said Wesley Morgan, a military affairs reporter at Politico. “When I pass by there, I keep an eye peeled for them.”

Carolyn Fiddler, communications director at Daily Kos, said that when she spied the first one, she thought: “Wow, there’s a white squirrel surviving in an ecosystem that literally does not reward sticking out.”

Now she finds her spirits lifted whenever she catches a glimpse of these alabaster beauties.

“I kind of think of it as a good omen, for no logical reason,” Fiddler said.

Squirrel Week

Today in KidsPost: Read the winners of the inaugural squirrel haiku contest. And tomorrow in this space: Squirrel Week continues with an electrifying tail, er, tale. (No, tail.)

Twitter: @johnkelly

For previous columns, visit washingtonpost.com/john-kelly.