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Last updated on April 21, 2014 at 12:45 EDT

Declining Male Birth Rate Baffles Scientists

June 2, 2008

CHICAGO – Once there was a kids’ hockey team on the reservation of the Aamjiwnaang First Nation in Canada just across the border from Michigan.

No longer. There aren’t enough boys.

This community, surrounded by dozens of pollution-spewing chemical plants, is an especially extreme example of a puzzling phenomenon playing out across the world, in countries as diverse as the United States, Sweden and Japan.

Though more boys are being born than girls in most places, their numbers are falling. And no one is sure why.

The change is small, but real. In the U.S., the number of baby boys vs. girls has been declining since 1970, translating into 17 fewer males for every 10,000 births or an estimated 135,000 fewer boys born between 1970 and 2002, according to a study last year in Environmental Health Perspectives.

Some experts suggest the shift is part of a naturally occurring, cyclical pattern in population dynamics. But others think a notable change is under way, driven by factors such as environmental contaminants and various types of stress, such as economic hardship.

These issues could affect boys more because they’re actually the weaker sex _ more vulnerable than girls to illness and death from conception to grave.

Nature’s way of compensating is to produce more males than females, increasing the likelihood that the sexes will survive to reproductive age in equal numbers. But recent decades have eroded the gap between the sexes.

The difference may seem tiny, but “it’s important to look at the really big picture here, which is that there are global indications that something unusual is going on,” said Devra Davis, director of the Center for Environmental Oncology at the University of Pittsburgh and lead author of last year’s report.

The sex ratio is an indicator of population health, and unexpected changes could be an important signal that people are at risk biologically, she said.

Several Latin American nations have reported a similar shift in the sex ratio at birth, as have Finland, Norway, Wales and the Netherlands. Late last year, several Arctic communities documented a startling decline in the number of boys being born. Studies have shown changing sex ratios in Italian cities and among fish-eating women in the Great Lakes region.

None of these countries or areas has a tradition of sex selection, which in any case usually favors boys.

The puzzling phenomenon has inspired a flurry of research on what could be causing the population shifts. Davis’ hypothesis is that “there is something happening after conception that is making it harder for boys to exist in the maternal fetal environment.”

A growing body of research indicates that could include exposure to pollutants such as pesticides, mercury, lead and dioxin. More controversial is the idea that synthetic chemicals known as endocrine-disrupters may be damaging male fetuses during critical periods of development or affecting men’s sperm counts and testosterone levels.

That thesis is “very interesting and provocative” but hasn’t been proved, said Dr. Rebecca Sokol, past president of the Society of Male Reproduction and Urology.

The steepest sex ratio declines observed in the world have occurred on the 3,000-acre Aamjiwnaang (AH-jih-nahng) First Nation reservation in Canada.

The number of boys vs. girls there began dropping in the early 1990s, according to data published in 2005 in Environmental Health Perspectives. Between 1999 and 2003, researchers found, only 46 boys were born out of 132 recorded births.

“You get angry and you get worried, thinking what could be causing this,” said Ada Lockridge, a member of the tribe who compiled the data and has since become an activist. “And then you want to learn more.”

Dozens of petrochemical, polymer and chemical plants surround the reservation on three sides. Mercury and PCBs contaminate the creek that runs through the land, and air-quality studies show the highest toxic releases in all of Canada, said Jim Brophy, executive director of Occupational Health Clinics for Ontario Workers, based in Sarnia, the nearest city.

Several months ago, Brophy and co-worker Margaret Keith did additional calculations, finding that boys made up only 42 percent of the 171 babies born from 2001 to 2005 to Aamjiwnaang living on the reserve or nearby.

“A disruption in the sex ratio of this magnitude has to be taken seriously,” Brophy said.

Still, there is no proof that pollution is responsible, and data from surrounding Lambton County don’t show a similar impact. The findings represent a “short period of time and a small population” and require further study, said Dean Edwardson, general manager of the Sarnia-Lambton Environmental Association, which represents area industry.

Experts note that other factors might include diet, alcohol use, smoking and occupational exposures.

Indeed, there’s strong evidence from other areas that men exposed in the workplace to pesticides, lead and solvents and in industrial accidents to toxic substances such as dioxin end up fathering fewer boys.

When a July 1976 chemical plant explosion in Seveso, Italy sent a cloud of dioxin over the area, researchers discovered that no boys were born for seven years to parents who had the highest levels of the toxin in their blood.

In another study, men exposed to the pesticide dibromochloropropane fathered three times as many daughters as expected.

Some evidence also suggests stress can reduce the motility or viability of Y-bearing sperm, reducing the likelihood that boys will be conceived. This may help explain why fewer boys are born after natural disasters such as earthquakes and floods _ a finding well documented in the scientific literature.

Moms are thought to have a different set of responses to stress, which also could favor girls over boys.

When pregnant women struggle with adverse circumstances _ economic hardship, poor food supply _ a biological mechanism that “culls” weak male fetuses may be inadvertently deployed, said Ralph Catalano, a professor of public health at the University of California, Berkeley.

From an evolutionary standpoint, this would make sense, since boys require more parental effort to raise while also dying at a higher rate, Catalano explained. When times are tough, it’s more advantageous to give birth to a girl, he said.

Among Catalano’s thought-provoking findings: The number of boys born in New York City relative to girls fell significantly after the Sept. 11 terrorist attacks.

That result, reported in 2006 in the journal Human Reproduction, applied primarily to women in their second trimester at the time of the attacks. In the paper, Catalano suggests that “fetal response to maternal stressors appears strongest in the second half of gestation” and “mothers may use that response as a test of male fetal robustness.”

Separately, in 2003 Catalano reported that the proportion of boys born in East Germany dropped sharply in 1991, when that nation’s economy collapsed.

The world’s leading expert on the science of sex ratios, William H. James, who spent his career at University College in London, offers another possible explanation: Hormones in both parents at the time of conception affect the sex of offspring.

Higher levels of testosterone and estrogen are associated with the birth of sons, James says, while elevated levels of gonadotropins and progesterone are associated with daughters. These hormones are internally regulated but also are subject to external influences such as alcohol, cigarette smoke, radiation, chemicals, and illnesses in parents.

A spinoff of the theory is the notion that the timing of conception can help determine the sex of offspring because hormone levels fluctuate during a woman’s fertile period.

James observed that women who conceive early or late in their fertile periods are more likely to have boys. Couples who have lots of sex have a higher probability of bearing sons, he concluded, because they’re more likely to conceive early on, he concluded.

This ties in to the stress hypothesis: If adults have less sex when enduring adversity, then they’d be less likely to conceive male children.

Another notable finding conforms to James’ research: Women receiving ovary-stimulating drugs (including gonadotropins) during assisted reproduction give birth to more girls.

On the Aamjiwnaang reservation, it took people a while to recognize the trend toward fewer boys. Families were more concerned about how many babies they were losing: The miscarriage rate for women on the reservation is about 40 percent, much higher than Ontario’s average.

Even when there were three girls’ baseball teams and only one boys’ team, “people just thought there were girls running in their families,” Lockridge said.

It still isn’t a subject that people talk about much, said Stephanie Stone, 37, who lives on the reservation with her husband, Paul, and three young daughters.

Stone has an 18-year-old son from a previous marriage but said “it hurts my heart” that she hasn’t been able to have another. “We have tried and tried to have a baby boy,” she said.