As mice age, their total egg reserve collapses around tenfold. Yet, according to a new study published in Nature Aging, the proportion of eggs in the brief window where a follicle is shifting from dormant to growing stays constant throughout life, at approximately 14%.

“The same percentage of oocytes are being activated regardless of how old the mouse is. That means the ovary has a sensing mechanism that knows how many oocytes are in there and only awakens a fixed proportion. No one has ever known that before. It is completely new biology,” says Dr. Elvan Böke, group leader at the Center for Genomic Regulation (CRG) in Barcelona and senior author of the study.

The finding challenges the perception that ovaries are a passive reservoir of eggs. The study’s authors reframe the female reproductive organ as an actively monitored system and suggest the ovary could be counting its own egg supply through a yet-to-be-identified hormone or neuronal signaling.