How the female baboon body has the final say in sperm selection

Paul Arnold

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Just because a female olive baboon has mated with a specific male doesn't mean he will be the father of her offspring. According to a new study published in PLOS Biology, mate selection continues long after copulation as the vaginal tract appears to "weed out" genetically incompatible suitors.

This process, known as cryptic female choice (CFC), had already been observed in small mammals such as rodents, but evidence for such internal screening in larger mammals, such as primates, was lacking.

Breeding pairs

Researchers studied a captive population of 13 olive baboons (nine females and four males) living at a primate center in France. First, they mapped the DNA of each animal, focusing on immune-related and other genes that influence genetic compatibility, which help the body recognize foreign and similar genetic material.

The animals were placed in groups to ensure the study included both closely related and unrelated mating pairs. Through positive reinforcement training, the females learned to present themselves for quick and noninvasive vaginal swabs and pH readings. Samples were taken before mating to establish a baseline and four hours after copulation to see how the body reacted to the presence of specific sperm.

Biological gatekeepers

The study found that the environment of the female reproductive tract changes in response to sperm, either hindering or helping it. When females mated with genetically similar males, the vaginal pH dropped significantly. This resulted in a more acidic environment, reducing the likelihood of fertilization. When mating with genetically different males, the vaginal pH remained stable or decreased only slightly, making it a more welcoming environment for sperm.

The research team also found that hundreds of genes, especially those involved in immune responses, behaved differently after mating. During a female's most fertile days, parts of her immune system become less active, allowing foreign sperm to survive.

However, after mating with genetically similar males, the researchers found a spike in the expression of genes related to inflammatory responses and immune signaling (how immune cells communicate to coordinate a defense). This suggests that the female body can detect "unwanted" genetic markers and mount a defense to block them.

"Our findings suggest post-copulatory differences in vaginal gene expression and pH, with the strongest immune responses and largest pH decreases, harmful to sperm, exhibited by females mating with genetically similar males," wrote the scientists in their paper.

"These findings are consistent with genetically based post-copulatory mate discrimination, offering new insights into how interactions between male gametes and the female reproductive tract may shape conception probability in primates."

Future studies

Although the research involved only a small number of baboons, it appears to show that the female body plays an active role in ensuring her offspring are genetically diverse. Larger studies will need to confirm whether this actually translates to higher rates of successful births or healthier infants.

Written for you by our author Paul Arnold, edited by Gaby Clark, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.

Publication details

Rachel M. Petersen et al, Evidence for genetically-based sperm discrimination in the vaginal tract of a primate species, PLOS Biology (2026). DOI: 10.1371/journal.pbio.3003699

Journal information: PLoS Biology

Key concepts

mammalogyBiological Variation, PopulationSpecies Specificity

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