In the vast majority of U.S. parks, gray squirrels are positively everywhere, burrowing, climbing, and scurrying from tree to tree.

For most Americans, spotting one of the fluffy-tailed rodents is such a common occurrence that it might even go totally unnoticed.

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For anyone who’s ever visited a park in the U.S., this likely isn’t news. What most of us don’t know, though, is that the population density of squirrels in parks throughout the nation isn’t just natural happenstance; it’s actually a thoroughly man-made phenomenon.

In 2013, University of Pennsylvania professor Etienne Benson published a paper entitled The Urbanization of the Eastern Gray Squirrel in the United States, which explained in great detail how the became fixtures in parks across the U.S.

According to Benson, the story begins in the early 1800s. While squirrels were plentiful in undeveloped woodlands—often serving as a source of nutrition for frontiersman—they were a rare sight in cities. In fact, an article from one of New York City’s newspapers in 1856 told the story of a crowd of hundreds gathering to gawk at an escaped pet squirrel, referring to the animal as an “unusual visitor.”

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In the middle of the 19th century, Americans’ relationship with the tiny creatures began to shift. In 1847, Philadelphia became the first American city to place squirrels in one of its parks. That year, the city released three of the tiny creatures in Franklin Square, giving them boxes for shelter and providing them with food.

They were a hit.

Residents were enamored of and fascinated by the twitchy little rodents, one visitor even remarking that “it was a wonder that [squirrels] are not in the public parks of all great cities.” Well, that person would certainly be pleased at the current state of affairs when it comes to squirrels in American parks.

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Within just a few years, cities all along the East Coast began to follow suit. In New Haven, Connecticut, and Boston, Massachusetts, there were reports of squirrels—fed by their human admirers—growing so fat that they fell out of the trees. In order to more properly nourish the parks’ new residents, cities began planting nut-bearing trees for them to munch on.

By the time that the 1870s rolled around, the squirrel craze in American cities was in full swing.

Thanks in large part to the rapid increase in sprawling city parks, such as those designed by landscape architect Frederick Law Olmsted, gray squirrels became and even more common sights in parks. According to Benson, the trend of filling urban parks with squirrels “was related to the idea that you want to have things of beauty in the city, but it was also part of a much broader ideology that says that nature in the city is essential to maintaining people’s health and sanity, and to providing leisure opportunities for workers who cannot travel outside the city.”

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This next wave of the American squirrel-craze was spearheaded by New York City’s Central Park, which introduced a small number of squirrels to the park in 1877.

By 1883—just six years later—Central Park’s squirrel population numbered approximately 1,500, prompting discussions among park authorities about thinning their numbers to combat potential imbalances in the ecosystem. It was also around this time that gray squirrel populations began to explode around the rest of the nation including the National Mall in Washington, D.C., and Boston’s Harvard Yard.

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Feeding the adorable little creatures soon became an enormously popular pastime throughout the nation, and was lauded by conservationists and naturalists as a way to teach humans to have better relationships with animals. One of the founders of the Boy Scouts advocated the feeding of squirrels as a strategy to “cure boys of their tendency toward cruelty.”

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Because of both their popularity among just about everyone in the nation and their natural virility, gray squirrel populations continued to grow, expanding beyond the boundaries of the urban parks where they’d initially been introduced. Numerous accounts from the late 19th century mention the popularity of feeding squirrels not only in city parks, but also in suburban communities.

And the squirrel’s spread wasn’t just limited to the U.S.

America’s squirrel craze eventually made its way across the globe, with gray squirrels being transported all the way to the United Kingdom and Italy. Some of this massive popularity surge owes to ideas like the Boy Scouts’, postulating that contact with nature was inherently good for people, and would lead to a more content, well-adjusted populace. Alternatively, some of it was simply because people thought the squirrels were cute.

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Unfortunately for some of the sites where gray squirrels were exported, the non-native species tends to be highly invasive. This was especially true in England, where the native red squirrels simply couldn’t measure up to the larger and more daring gray squirrels, resulting in a still-declining red squirrel population.

To make matters worse, scientists have since discovered that gray squirrels serve as carriers for a parapoxvirus that—despite leaving grays totally unaffected—has a 100 percent mortality rate among red squirrels.

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While their proliferation has caused problems elsewhere, the U.S.’s gray squirrel population remains a well-loved part of American outdoor life, even if we do sometimes take them for granted.