Brooklyn’s Prospect Park is dog heaven. On sunny Saturday mornings, the park’s open green space, Long Meadow, fills with hundreds of canines frolicking during off-leash hours. The dogs’ owners hover nearby like watchful parents who, when playtime ends, head over to the nearby farmers’ market or go out for brunch. Later in the day, they might make time for doggie yoga or the pet bakery before coming home to their pet-friendly apartment buildings, many featuring dog baths and groomers.

Roughly 600,000 dogs live in New York City, along with half a million cats. About half of U.S. households own a pet, which adds up to at least 77 million dogs and 54 million cats. Generationally, millennials are the most enthusiastic pet owners, with some 70 percent boasting of having at least one pet.

What you’re less likely to see, especially in America’s largest cities, are children. Pets are now more common than kids in many U.S. cities. San Francisco, for example, is home to nearly 150,000 dogs but just 115,000 children under age 18. Farther north, Seattle has more households with cats than with kids. Nationwide, pets outnumber children in apartment buildings. In New York neighborhoods like Long Island City and Williamsburg, wealthy singles have the highest number of pooches per capita.

In a recent Atlantic essay, Derek Thompson wrote about how “America’s urban rebirth is missing a key element: births.” Manhattan’s infant population is projected to halve in 30 years. High-density cities are losing families with children over age six, while growing their populations of college-educated residents without children. Indeed, the share of children under 20 living in big cities has been falling for 40 years.

Young professionals’ four-legged friends have replaced those babies. While statistics are spotty, the cultural signs of a shift toward the parenting of pets in major cities are evident in apartment ads, park design, retail mixes, and the explosion of services catering to the “fur-baby economy.” In the absence of kids, a dog or cat serves as something like a starter family. Young Americans dote on their pets with the care once reserved for children, with lavish birthday presents or “family portraits” on Instagram.

The cost of ownership over a medium-sized dog’s lifetime has grown at twice the rate of inflation since 2008, to $12,700. Americans spent $70 billion last year caring for and feeding pets; they spent $59 billion on child care. No wonder pet insurance is now “the hottest employment benefit,” especially when it runs north of $100 a month in New York for the most comprehensive plans. In my Chelsea neighborhood, the local pet boarder comes with a chef, chauffeur, and a private room larger than my own.

As markets and employers respond to these moves with fewer family-friendly amenities, childless cities are becoming the norm. When Zappos polled its employees on the amenities that they’d like in a new headquarters, the greatest vote-getter was doggie daycare. A growing number of companies even offer “fur-turnity” leave to their employees and allow pets to visit work.

Dogs and cats should be welcome in cities, of course, but their ever-increasing popularity among young professionals—and the attendant decline in children—portends a shift in urban America that we will have to reckon with in the coming decades. More Americans live alone than at any time in the country’s history. Singles make up 28 percent of households, up from 13 percent in 1960. Household size is declining in New York City, as it is in much of the country, from around 3.5 persons at mid-century to 2.67 today. We’re living longer and marrying later.

No wonder pet ownership among men and women living alone has grown by 25 percent since 2006, particularly among single women. Dogs and cats provide a ready source of companionship, possibly substituting for children and spouses. According to the Bureau of Labor Statistics, married couples without children spent the most on their pets. A millennial’s first pet is considered a major milestone.

In cities, dog parks and pet boutiques flourish, but families are thinning out. In New York, “purse dogs” ride subways in comfort, while a mother falls to her death carrying a baby stroller down flights of stairs to the platform’s edge. Children add much more to life than a morning feed and walk. Families need what cities are struggling to provide today: affordable housing, good schools, public order, and quality public spaces—but policymakers have yet to deliver a family-friendly urban agenda. Raising a family is hard, under the best of circumstances. American cities need more children—not just pets.

Photo: zhuyufang/iStock