April 1, 2009 -- Girls are born more often in the tropics than anywhere else on Earth, according to a new study.
Around the world, about 51.3 percent of all babies born are male. It's difficult to tease out the influences of culture and socio-economic status, but researchers generally believe this is because males are the real "weaker sex;" they are more likely to die young and are more prone to genetic defects and diseases.
Perhaps that's why a study of global sex ratios of babies born over the last 10 years has found that figure is more like 51.1 percent in between the equator and 32.5 degrees north and south latitude. It seems like a small change from the global average, but it translates to thousands more female babies born each year.
"We're talking about very large numbers of people here," Kristen Navara of the University of Georgia said. Her work was published today in the journal Biology Letters.
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The difference might have to do with the nearly constant amount of sunlight Earth's tropical belt receives. Day length varies a lot at higher latitudes in both hemispheres in unison with seasons, a prime indicator of whether food and other resources will soon be abundant or scarce.
"Length of day is a really good indicator of oncoming resource limitation," Navara said. "It all depends on the cost of producing sons versus daughters. If females require more energy to reproduce, you might want to produce males in an environment with limited resources."
Navara speculated that temperature may be a similar physical cue that could influence newborns' sex ratio.
While human males may be more likely to die young, females seem to require more resources to mature and reproduce. If true, it makes sense that they'd be born more often in the tropics, where steady daylight and warm weather tell our bodies that food should be readily available.
But John Martin of Arizona State University in Tempe said the study ignores trends in human reproduction that could confuse the findings.
"In places where birth control isn't practiced, women continue reproducing, and as they age they give birth to more girls," he said. "This is documented in the United States, Africa and the circum-Carribbean region."
He added: "That's not to say that climate and amount of sunlight don't affect human sex ratios. But there are important things they're just not considering here."
Navara admitted her finding is preliminary, and she's still searching for a solid biological mechanism to explain it. Still, she speculates that the result could be evidence of our evolutionary past; we are hard-wired to tweak the number of boys and girls we give birth to depending on cues like day length and temperature.
"My sense is that this is reminding us of our evolutionary roots," she said. "We still adjust our reproductive patterns as we were originally programmed to do."
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TreeHugger: Are Boys Disappearing Because of Gender Bender Chemicals?
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