Female bonobos send one of the most confusing signals in the primate world: a bright pink genital swelling that stays fully inflated long before — and long after — they’re actually fertile. It looks like a terrible cue: vague, misleading, and nearly impossible to decode. But male bonobos have learned to read it anyway.
A new study in PLOS Biology shows that males aren’t fooled by the swelling’s ambiguity. Instead, they rely on additional clues — including how long the swelling has lasted and how old a female’s youngest infant is — to pinpoint the narrow window when conception is actually possible. The strategy allows them to concentrate their efforts when the odds are highest.
“Our results help explain how conspicuous 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 Ovulation Timing
In many species, ovulation and sexual receptivity go hand in hand. Not so in bonobos. Their conspicuous swellings last far longer than the fertile window itself, turning what should be a useful cue into a confusing one.
To understand how males navigate such an unreliable signal, researchers followed a wild bonobo community in the Wamba forest of the Democratic Republic of the Congo. Each day, they tracked sexual behavior, recorded swelling changes, and collected urine samples on filter paper to measure estrogen and progesterone — the hormones that reveal when ovulation is actually taking place.
Those hormone profiles confirmed that ovulation doesn’t occur when swelling looks most pronounced. Instead, ovulation peaked 8 to 27 days after females reached maximum swelling — a window far too broad to rely on visually. Yet males still managed to time their mating efforts with striking accuracy.
They focused more attention on females who had reached maximum swelling earlier, and on females with older infants, whose reproductive cycles were more likely to resume. By combining these two cues — swelling duration and infant age — males zeroed in 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 seeking a perfect signal but by assembling a mosaic of information. By pairing swelling patterns with a female’s reproductive history, they effectively target the fertile window even when the visible cue is noisy. Because this strategy works, the researchers note, there may have been little evolutionary pressure for females to evolve a more precise signal — helping explain why this unusual system has persisted for so long.
“The male bonobos weren’t the only ones paying close attention to sexual swelling — we spent countless days in the rainforest at Wamba, DRC doing exactly the same thing! All that watching, sweating, and scribbling in our notebooks eventually paid off. By tracking these daily changes, we uncovered just how impressively bonobos can read meaning in a signal that seems noisy and confusing to us,” the authors said in the press release.
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Article Sources
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- This article references information from the recent study published in PLOS Biology: Male bonobo mating strategies target female fertile windows despite noisy ovulatory signals during sexual swelling
