Science

While other species emit scents or display other physical changes when they are ready to mate, researchers have traditionally assumed humans conceal their fertility, the paper's authors wrote. They added that their work suggests that ovulating women engage in "self-ornamentation through attentive personal grooming and attractive choice of dress."

"It's just showing us our evolution, our biology, is showing up in even the most modern of behaviors," said Haselton, a scientist at the Center for Behavior, Evolution and Culture at the University of California at Los Angeles.

The researchers were uncertain what was motivating these women, however: whether ovulating women may be trying to attract mates beyond their primary partners or simply reflecting a mood change, they wrote.

-- Juliet Eilperin

When Earth Tilts, Animals Fall Off

Paleontologists have long wondered why the fossil records they study often point to mass extinction of certain mammal species every 2 1/2 million years. Dutch climate experts and biologists think they have found the answer: Periodic fluctuations in the way Earth orbits, tilts and wobbles on its axis have led to climate changes that wiped the animals out.

The researchers, at the University of Utrecht in the Netherlands, looked at 80,000 fossilized rodent teeth from central Spain that had been collected over the past four decades, which came from animals that lived between 24.5 million and 2.5 million years ago.

The researchers determined which animals lived in specific time periods, and with that information they found evidence for two different cycles of die-offs that eliminated up to 30 percent of the species alive at that time. Those die-offs occurred most prominently every 2.5 million years, but also at million-year intervals.

With that information in hand, the team matched it to changes in the orbit, the tilt and the wobble of Earth. Those changes, together called the Milankovitch cycles, can greatly influence the amount of heat and light reaching Earth from the sun and cause significant global cooling when two or all three of the cycles peak together. In last week's issue of the journal Nature, the team reported that the rodent die-offs matched up to recently discovered variations in the Milankovitch cycles.

The phenomenon was not, however, quick and dramatic like the mass dinosaur extinction of 65 million years ago. It was rather a slow process by which certain species could no longer live in the increasingly cold climates of their area. Other species would then move in and take over their environmental niche.

-- Marc Kaufman