Forget worrying about the “Rise of the Planet of the Apes.”

Demography suggests Earth is far more likely to become the “Orb of the Dogs,” as human fertility rates plummet for a slew of reasons, including canines substituting for kids in many households.

It’s not just Americans turning pets into “fuzzy, low-maintenance replacements for children,” says writer Jonathan Last. It’s happening in Italy, Japan and elsewhere.

American pets outnumbered children for the first time in 2006, said Last, a father who’s not kidding when he says people with only pets are happier and less stressed than parents of humans. That’s what surveys and studies find, he said in a telephone interview Thursday.

Colorado has more than 1.1 million dogs, according to the American Veterinary Medical Association, and not quite 350,000 children under 5 years old, according to the U.S. Census Bureau. In the U.S., there are 145 million dogs and cats, compared with 20.2 million kids under 5.

Yet more spoiled pooches is one of the least troublesome consequences of collapsing fertility rates, said Last, a writer with the conservative Weekly Standard and author of the new book “What to Expect When No One’s Expecting: America’s Coming Demographic Disaster.”

To those who think declining birth rates are a good thing for a crowded planet — and that “be fruitful and multiply” is so two millennia ago — Last says, “Think again.”

Pew Research Social & Demographic

Trends reported in December that the overall birth rate in the U.S. fell to 63.2 births per 1,000 in 2011, the lowest level since records began in 1920. By contrast, in 1957 — the height of the baby boom — the overall birth rate was 122.7

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“It isn’t the contraction in population that worries you so much,” Last said. “It’s that you end up with so many more old people than young people.”

Higher divorce rates, cohabitation outside marriage, effective birth control, legalization of abortion, women in the workplace, delayed marriage and motherhood and the exploding costs of raising children have pushed Americans into having fewer kids, Last said.

From stringent safety requirements for car seats to lost wages for stay-at-home parents and a 1,000 percent increase in college tuition in real dollars since 1960, Last said, having kids is a terrible financial investment.

It once made sense to have kids who would take care of you in your old age, Last wrote, but now there are government entitlements for that.

Yet, as the largest generation — 78 million baby boomers — progresses into retirement, paying higher medical costs and collecting Social Security payments, Pew analysts noted, the newest, smallest generation of workers will have to support it.

The Social Security Administration reports that in 1940, 159 workers supported each U.S. retiree. By 2010, just under three workers supported each retiree.

For a country or state to maintain a steady population, it needs a total fertility rate of 2.1. (The rate is the average number of babies a woman would have if she survived to the end of her reproductive years.) National Vital Statistics Reports indicate the U.S. rate in 2010 was 1.931. Coloradans’ fertility, which ranks 27th among states, was 1.924.

State demographer Elizabeth Garner said births have been below replacement rate for a while, but the drop is manageable as long as plans are made for a changing labor force and more retirees.

In 2010, Colorado households consisting of people ages 65 and older numbered fewer than 350,000. The state projects that by 2030, almost 900,000 households will consist of those 65 and older.

As baby boomers die, regional populations will decline. People might be cheered by the prospect of less crowded roads and malls, Last said, but it historically has meant economic decline.

“A lower birth rate for a while is not a major problem, but if you’re at the lower rates Western Europe has been experiencing for some time, it can create real problems,” said professor Jane Menken, director of the Institute of Behavioral Science at the University of Colorado at Boulder.

Yet there are opposing views. Harvard School of Public Health researchers David Bloom and David Canning find a “demographic dividend” in hardworking young adults who aren’t distracted by the responsibilities of parenthood.

These productive young workers and policy reforms, such as higher retirement ages, will offset effects of dwindling populations, they said. Declines in economic growth should be modest, not catastrophic, they wrote in a 2011 paper for the National Bureau of Economic Research.

Another way to maintain a youthful population is a continual influx of immigrants. America and Colorado so far have experienced mostly net migration gains, enough to keep populations growing.

“In America, we’ve been insulated from the worst effects by immigrants,” Last said. “It helps. But it doesn’t seem to have the same rejuvenating effect as everybody making babies.”

There are indications, too, he said, that immigrants won’t compensate indefinitely for low U.S. birth rates because their birth rates are also plunging.

And the U.S. can’t rely on steady immigration. People don’t move to a place with a bad economy. For example, Colorado experienced a net migration loss from 1985 to 1991 in the wake of an energy-and-banking bust, according to state statistics.

Religion doesn’t exert the influence on procreation it once did, Last said. While Americans believe Catholics, Mormons and evangelical Christians have big families, fertility rates no longer track along sectarian lines, he said.

“We actually don’t have real differences along religious lines,” he said. It’s a question now of “religiosity,” or how much one attends church. Those who regularly attend worship services have more children.

“I think those who believe they have a duty to the past and to the future also believe in the importance of having children,” Last said.