Bonobos Send the Noisiest Fertility Signal in Primates — Males Still Have to Decode It
Female bonobos send one of the most puzzling signals in the primate world: a bright pink genital swelling that remains fully swollen long before – and long after – they are actually fertile. This sounds like a terrible signal: vague, misleading and almost impossible to decode. But male bonobos still learned to read it.
A new study in Biology PLOS shows that males are not fooled by the ambiguity of swelling. Instead, they rely on additional clues — including the duration of swelling and the age of a woman’s youngest child — to identify the narrow window at which conception is actually possible. The strategy allows them to focus their efforts when the odds are highest.
“Our results help explain how visible but noisy ovulatory signals, like those of bonobos, can persist and shape mating strategies in complex social environments,” the study authors said in a press release.
Bonobo behavior, hormones and timing of ovulation
In many species, ovulation and sexual receptivity go hand in hand. This is not the case with bonobos. Their visible swellings last much longer than the fertile window itself, turning what should be a useful signal into a confusing one.
To understand how males manage to deal with such an unreliable signal, researchers followed a community of wild bonobos in the Wamba forest in the Democratic Republic of Congo. Each day, they tracked sexual behavior, recorded changes in swelling, and collected urine samples on filter paper to measure estrogen and progesterone – the hormones that reveal when ovulation actually takes place.
These hormonal profiles confirmed that ovulation does not occur when the swelling appears most pronounced. Instead, ovulation peaked 8 to 27 days after the women reached peak swelling – a window far too wide to rely on visually. Yet the males still managed to time their mating efforts with striking precision.
They focused more attention on females who had reached peak swelling earlier and on females with older infants, whose reproductive cycles were more likely to resume. Combining these two clues – the duration of swelling and the age of the infant – the men focused on the days when conception was most likely.
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Why bonobos never evolved a clearer fertility signal
The results suggest that male bonobos succeed not by searching for a perfect signal but by assembling a mosaic of information. By associating swelling patterns with a female’s reproductive history, they effectively target the fertile window even when the visible signal is noisy. Because this strategy works, the researchers note, there may have been little evolutionary pressure for females to evolve a more precise signal, which explains why this unusual system persisted for so long.
“Male bonobos weren’t the only ones paying close attention to sexual swelling – we spent countless days in the Wamba rainforest in the DRC doing exactly the same thing! All that observing, sweating, and scribbling in our notebooks finally paid off. By tracking these daily changes, we discovered how impressively bonobos can read the meaning of a signal that seems loud and confusing to us,” the authors said in the release. press.
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