Male bonobos are able to decipher confusing fertility signals from females, say scientists.
Most female mammals are only receptive to mating during ovulation, which provides males with precise signals to time their efforts accurately.
However, some primates which live in multi-male societies – such as bonobos (which are still dominated by females, however) – must rely on less obvious cues.
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Female bonobos display a noticeable pink swelling around the genitals for a prolonged period of time alongside sexual reception – but this stays inflated long before and after they’re actually fertile.
This is described as a “noisy” signal, as this is not a reliable indicator of fertility.
To investigate how males cope with this unreliable signalling, researchers at Kyoto University, Japan, studied a group of wild bonobos at Wamba in the Luo Scientific Reserve in the Democratic Republic of the Congo.
They carried out daily observations of the bonobos and recorded sexual behaviours, as well as visually estimated how exaggerated each female’s genital swelling was. They also collected urine samples from the females, allowing them to measure oestrogen and progesterone levels to estimate ovulation timings.
The team discovered that ovulation likely peaked between 8 and 27 days after females reached maximum swelling – which creates an unusually large window of time for the males to attempt mating.
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However, they found that the males ‘track’ this swelling by watching and checking the females for any changes.
The males then concentrated their mating efforts on females that had reached maximum swelling earlier on and who had older offspring – suggesting that these two points indicated a higher probability of ovulation in the females.
As the males can effectively estimate female fertility using this unreliable signal, this method has likely not changed over evolutionary time.
The authors said, “We found that bonobo males, instead of trying to predict precise ovulation timing, use a flexible strategy to fine-tune their mating efforts.
“Our results help explain how conspicuous but noisy ovulatory signals, like those of bonobos, can persist and shape mating strategies in complex social environments.”
“By tracking these daily changes, we uncovered just how impressively bonobos can read meaning in a signal that seems noisy and confusing to us.”
Read the full paper in PLOS Biology: Male bonobo mating strategies target female fertile windows despite noisy ovulatory signals during sexual swelling
