Researchers say they’ve discovered one cause of infertility in a rare species of rhinoceros, and how to fix it.

Gut microbes in the female southern white rhino metabolize phytoestrogens, estrogenlike plant compounds, in a way that reduces fertility. This knowledge may help preserve the rhino, said study co-author Candace Williams.

Moreover, the research may also help save a related species, the northern white rhino, which is nearly extinct.

Captive southern white rhino populations are declining because of the low fertility levels, said Williams, who is affiliated with the Institute for Conservation Research at San Diego Zoo Global. And wild rhinos are vulnerable to poaching.

Previous research by the zoo’s Christopher Tubbs has already shown a link between phytoestrogen consumption and reduced southern white rhino fertility, Williams said. The new research published in the journal mBio explains why.

The study compared captive southern white rhinos with another species, the greater one-horned rhino. Fertility in this rhino isn’t impaired by phytoestrogens, the study found. It also has a different gut population, or microbiome.

“This is first time we’ve looked at the microbiome and its contribution to metabolizing phytoestrogens,” she said.

Fertility of southern white rhinos at the zoo has been improved by reducing phytoestrogen content in feed, she said.

“Just by reducing the amount of phytoestrogens that were fed to the rhinos, we did see some calves born to individuals that never reproduced.

The discovery is good news for the northern white rhino, said Jeanne Loring, a stem cell biologist who is working with the zoo to save it. Just two northern white rhinos are living, both females too old to reproduce.