In nature, it is not usual for animals to be deceptive, as evolution has typically favored communication that benefits both the sender and receiver by conveying reliable information. But, there are exceptions, particularly when the “lie” leads to beneficial mating or survival. For example, female poison frogs are known to prolong courtship to keep their mate around longer to care for young—increasing the survival of the young.
Gelada baboons from Ethiopia live in packs with a single male and up to 12 females, all of which mate with the male. Occasionally, new males come along and take over the group. The female gelada has been observed to display signals of fertility at times when new male geladas take over, even when they are still lactating from a recent birth—something that typically does not occur in the absence of a takeover.
On the other hand, new male geladas have been observed to kill infants fathered by their predecessors. This had researchers wondering if the two behaviors were connected and whether deception was involved.
Now, a new study, published in Current Biology, dives into 14 years worth of gelada observations and data to answer whether females are being deceptive or whether they are just trying to accelerate the weaning process and move on.
An outward appearance of fertility
The first question the researchers wanted to answer was whether or not takeovers actually hasten the onset of fertility indicators in the form of sexual swellings for lactating females. The neck and chest area becomes swollen when a gelada is fertile—and this is seen in many lactating females within three months of a takeover, regardless of the age of their young.
Statistical analysis revealed that this was a significant change. The team writes, “We observed an abrupt and obvious rise in the resumption of sexual swellings during the month after a takeover. Statistical models confirmed that lactating females resumed sexual swelling 6.5 months earlier if they experienced a takeover compared with those that did not, and this effect was robust across infant age. Even females with the youngest infants—infants far too young to survive without a lactating mother—resumed sexual swellings while visibly continuing to lactate.”
Is it real?
After it was clear that geladas were changing their outward appearance after takeover, the team wanted to know whether the geladas were actually fertile, willing to mate, and getting pregnant—leaving their infants to fend for themselves. Were they mating and conceiving on normal timelines again or was it just for show?
They found that the gelada mothers were, in fact, mating with the new males, but they weren’t getting pregnant any sooner than they would without faking their fertility.
“Specifically, once they resumed swelling, we tested whether takeover mothers took longer to conceive than non-takeover mothers. Indeed, they did; takeover mothers took nearly 57% (or ?4.0 months) longer to conceive than nontakeover mothers,” the study authors write.
They also compared the time to conception of takeover mothers with that of mothers whose infants died or were killed before lactational amenorrhea was complete. The takeover mothers also took significantly longer to conceive than mothers whose infants died. So, while the gelada mothers appeared to be fertile to the new males, they still seemed to be incapable of conception for quite a while.
Does it work?
To answer whether or not gelada mothers were actually faking their fertility, one would need to prove that they would have a motive to do so. Then, the question becomes, did the method work to prevent infanticide within these groups?
The study results indicate that it did. While it did not prevent 100% of deaths by infanticide, the deceptive signals did reduce the likelihood of infanticide by new males. The likelihood that the infant would survive depended on the infant’s age, with likelihoods increasing with age. However, all infants were more likely to live if their mother showed signs of fertility.
“Even the youngest infants always survived better when their mother resumed cycling than when she did not. For example, infants who were younger than 3 months when a takeover occurred were twice as likely to survive if their mothers resumed post-takeover swellings (66.7% survival) than if their mothers did not (31.6% survival),” the authors explain.
Ultimately, the study points to deceptive fertility as an effective counterstrategy to male infanticide for geladas. It is unclear whether other primates partake in such deception and researchers are still uncertain what stage reproduction failed.
The study authors write, “In this context of extreme sexual conflict, ‘faking it’ may lead to females ‘making it’—securing infant survival through functional deception.”
Written for you by our author Krystal Kasal, edited by Sadie Harley, —this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
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More information:
Alice Baniel et al, Evidence for deceptive fertility in a wild primate, Current Biology (2025). DOI: 10.1016/j.cub.2025.10.070
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