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. Related Content:
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. Get More NewsSpiders, Scorpions Among World's Oldest CreaturesMany creepy crawlies have been on Earth much longer than previously believed.Blood-Sucking Vampire Bats Sing DuetsWhite-winged vampire bats "harmonize" with separated roost mates.Oldest Hebrew Writing Possibly FoundAncient inscriptions on a 3,000-year-old pottery shard could make history.Rare, Prehistoric-Age Reptile Found in N.Z.A tuatara has been spotted on the New Zealand mainland for the first time in 200 years.Iceman Has No Living RelativesOetzi, the 5,300 year-old frozen mummy, left no living genetic legacy.SLIDE SHOW: Landscapes of TerrorWhat makes a place feel scary? There are scientific explanations.It's Official: People Are Warming the PolesHumans are conclusively to blame for polar warming, say scientists.Eight-Armed Animal Preceded DinosaursWhat may be one of Earth's first animals was no bigger than a coaster and had eight arms.Phoenicians Live on in People's GenesOne in 17 Mediterranean men may be descended from ancient Phoenicians.Pesticides, Fertilizers Linked to Frog DeclineA pesticide is found to promote parasites among amphibians.Hubble Telescope Taking Photos AgainThe Hubble Space Telescope is once again snapping stunning photos of the universe.Andean Mummy Hairs Show Hallucinogen UseScientists find direct evidence of hallucinogenic drug use among ancient Andeans.Opals on Mars Reveal Planet's Long Wet PastOpals found on Mars suggest the planet has been wet for much longer. |
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