Female frogs are nearly twice as abundant as males in the American suburbs, where manicured lawns, pesticides and other pollutants may be disrupting the amphibians' reproductive systems, researchers said Monday.

The study in the September 7 issue of the Proceedings of the National Academy of Sciences included 21 ponds -- some in the suburbs and others in the forest -- in southwestern Connecticut.

"In suburban ponds, the proportion of females born was almost twice that of frog populations in forested ponds," said lead author Max Lambert, a doctoral student at the Yale University School of Forestry and Environmental Studies.

"The fact that we saw such clear evidence was astonishing."

Past research has shown that agricultural pesticides and wastewater treatment facilities could disrupt the endocrine systems of fish and frogs.

The new study suggests other factors could also be at play when it comes to influencing the sex of frogs, including water runoff from paved driveways and roads, along with the mere act of keeping a trimmed lawn, thereby encouraging the growth of clover, which contains natural phytoestrogens.

Researchers did not probe the exact sources of the contamination, but said further study is needed to understand why there are higher levels of estrogen in areas where there are shrubs, gardens and lawns, and why there was such a strong association between landscaping and frog offspring sex ratio.

"Our work shows that, for a frog, the suburbs are very similar to farms and sewage treatment plants," Lambert said.

"Our study didn't look at the possible causes of this, partly because the potential relationship between lawns or ornamental plantings and endocrine disruption was unexpected."