Modern medicine, a healthy lifestyle and a shot or two of Botox might leave a woman feeling youthful for decades, but by her early 30s, her reproductive ability is already in rapid decline.

Now, a study in the journal Cell suggests that one of the favorite animals for experiments in modern science, the tiny worm C. elegans, may provide new insight into female fertility. For the worm, as for women, it is diminished egg quality, not quantity, that marks the first sign of reproductive aging.

Coleen Murphy, a molecular biologist at Princeton University, and colleagues found that as C. elegans ages, its oocytes, or unfertilized eggs, start to degrade because of increased secretion of a protein called transforming growth factor beta, or TGF-beta. The same protein is found in humans and other mammals.