Moms in Tropical Climates Have More Baby Girls

April 03, 2009 01:30 PM
by Rachel Balik
New research shows that women who live near the equator have more female children than males, but scientists have yet to explain why.

More Baby Girls Born Near Equator

New research conducted by Dr. Kristen Navara suggests that in tropical climates, women are more likely to have daughters than sons. She tracked populations in 202 countries over a 10-year period and found that regardless of socioeconomic and cultural factors, more baby girls were born in regions closer to the equator. Globally, the ratio of boys to girls favors boys; roughly 51.5 percent of babies are male, reported the BBC.

It is not clear whether temperature is the determining factor. The abstract published by the Royal Society journal Biology Letters, simply states that the skewed ratio could be the result of various “latitudinal variables.

But scientists do know that in times of stress or hardship, women are more likely to give birth to females. Dr. Bill James of University College London told the BBC that Dr. Navara’s findings were less significant than those studies, adding that there is an evolutionary explanation.
Most females will have at least one child, whereas males may have many, or none. Therefore, females have a greater chance of reproductive success than males. This means that if a woman is only feeling healthy enough to have one baby, her chances of continuing her line are more likely if the baby is female. The Register reported in 2006 that researchers had found evidence supporting a theory that weaker male fetuses spontaneously aborted during stressful times, called the “culled cohort” theory.

While the BBC suggested that warm climates might somehow encourage the birth of more girls, the Daily Mail had a different angle on the story. It referred to a theory that cold climates might encourage the birth of more boys. Because boys appear to be more sensitive to the cold than girls, more boys are born in those latitudes to ensure even numbers in the population even if some boys do not survive.

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Related Topic: “Sex and survival” in reptiles

It has been known for some time that the sex of many reptiles is determined by incubation temperature after the eggs are laid. In January 2008, scientists produced findings suggesting that there was an evolutionary advantage to what is called Temperature-Dependent Sex Determination (TSD).

The theory had been developed 30 years prior to the conclusive findings, the Australian Broadcasting Corporation says. Two American biologists had speculated that TSD allowed a species to have a optimal amount of offspring, but lacked concrete evidence.

The 2008 study, published in Nature, involved breeding, raising and monitoring a short-lived species of lizards, the jacky dragon. They compared the reproductive fitness of jacky dragons, whose gender was naturally determined, with lizards whose gender was artificially manipulated by hormones, and found that “the fitness of each sex was maximized by the incubation temperature that produces that sex.”  Males and females born at their optimal temperature produced more offspring than those lizards whose genders were artificially determined.

Reference: Sex ratios across the globe


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