Because of these similarities, human biologist Aaron Blackwell of the University of California, Santa Barbara, and colleagues wondered whether parasitic infections could pave the way for pregnancy. The researchers tried to answer the question by analyzing data on the Tsimane people who live in the Amazon rainforest of Bolivia.
The roughly 16,000 Tsimane survive mainly by hunting, fishing, and raising crops such as rice and plantains. Their homeland is prime parasite country. About 15% to 20% of them harbor Ascaris, and 56% of them carry hookworms. Infected women in the study usually were unaware they were playing host to the parasites, Blackwell says.
The only health effects the researchers could detect among nearly 1000 Tsimane women were in individuals carrying hookworms. They had a slightly smaller body mass index and lower levels of the oxygen-carrying protein hemoglobin in their blood. Hookworms were also detrimental to fertility. They increased the age at which Tsimane women first gave birth and stretched the amount of time between pregnancies. As a result, the team calculated, a woman with hookworms would bear three fewer children in her lifetime than would a woman lacking the parasites. For Tsimane women overall, however, fertility is not a problem, as they give birth to an average of nine kids.
In contrast, the giant roundworm Ascaris was a boon for reproduction. It shortened the time between pregnancies and reduced the age at which women first give birth. A woman infected with Ascaris would bear on average two more children in her lifetime than would a woman free of the parasites, the researchers report online today in Science. “It’s a little bit counterintuitive,†Blackwell says.
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