Male bonobos track females’ reproductive cycle to maximize mating success

Male bonobos can decipher females’ unreliable fertility signals, allowing them to focus their efforts on matings with the highest chance of conception, according to a study by Heungjin Ryu at Kyoto University, Japan, and colleagues publishing December 9^th in the open-access journal PLOS Biology.

In most mammals, females are only receptive to mating during ovulation, allowing males to time their mating efforts to maximize the chances of conception. But in some primates, such as bonobos (Pan paniscus), females become sexually receptive and display a conspicuous pink swelling around the genitals for a prolonged period of time.

To investigate how males cope with this unreliable fertility signal, researchers studied a group of wild bonobos at Wamba in the Luo Scientific Reserve in the Democratic Republic of the Congo. During daily observations, they recorded sexual behaviors and visually estimated the status of each female’s genital swelling. They also used filter paper to collect urine samples of the females, allowing them to measure estrogen and progesterone levels and estimate the timing of ovulation. They found that ovulation probability peaked between 8 and 27 days after females reached maximum swelling, making it difficult for males to predict. Despite this, males’ sexual advances were closely aligned with the timing of ovulation. Males concentrated their mating efforts on females that had reached maximum swelling earlier, and whose infant offspring were older — two key sources of information indicating a higher probability of ovulation.

The results show that males focus their mating efforts on the most fertile females by combining information about the timing of swelling and reproductive history. Because male bonobos can effectively estimate female fertility despite an unreliable signal, there has likely been little evolutionary pressure for the signal to become more precise. This may explain how this system has been maintained over evolutionary time, the authors say.

The authors add, “In this study, we found that bonobo males, instead of trying to predict precise ovulation timing, use a flexible strategy—paying attention to the end-signal cue of the sexual swelling along with infant age—to fine-tune their mating efforts. This finding reveals that even imprecise signals can remain evolutionarily functional when animals use them flexibly rather than expecting perfect accuracy. 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 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.”

In your coverage, please use this URL to provide access to the freely available paper in PLOS Biology: https://plos.io/4r584R0

Citation: Ryu H, Hashimoto C, Hill DA, Mouri K, Shimizu K, Furuichi T (2025) Male bonobo mating strategies target female fertile windows despite noisy ovulatory signals during sexual swelling. PLoS Biol 23(12): e3003503. https://doi.org/10.1371/journal.pbio.3003503

Author countries: Japan

Funding: This study was supported by the Global Environment Research Fund (D-1007 to TF) of the Japanese Ministry of the Environment, the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (22255007 to TF; https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-22255007/, and 25304019 to CH; https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-25304019/), and the JSPS Asia-Africa Science Platform Program (2012–2014 to TF). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.