Male bonobos in the wild use a combination of genital swelling patterns and female reproductive history to identify optimal mating times, despite unreliable visual signals. Researchers observed this behavior in a community at Wamba, Democratic Republic of the Congo, revealing how males maximize reproductive success. The findings, published in PLOS Biology, highlight flexible strategies in primate mating.
Researchers led by Heungjin Ryu from Kyoto University tracked a wild bonobo community in the Luo Scientific Reserve at Wamba, Democratic Republic of the Congo. Over months of field observations, the team documented daily sexual interactions, assessed the degree of female genital swelling, and analyzed urine samples for estrogen and progesterone levels to pinpoint ovulation.
The study found that female bonobos develop a bright pink genital swelling that lasts well beyond the fertile period, making visual cues misleading. Ovulation was most likely between 8 and 27 days after a female reached maximum swelling, complicating predictions for males. Despite this, males directed mating efforts toward females who had peaked in swelling earlier and those with older infants, indicators associated with higher fertility chances.
This approach allows males to estimate fertility effectively without precise signals, reducing evolutionary pressure for more accurate cues. The persistence of this system underscores how imprecise signals can endure when animals adapt flexibly.
As the authors noted, "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."
Fieldwork involved intense rainforest monitoring, with researchers enduring heat and humidity to record these behaviors. The research received support from Japan's Ministry of the Environment and the Japan Society for the Promotion of Science, with no influence on the study's design or outcomes. Published on December 9, 2025, in PLOS Biology, the work appears in volume 23, issue 12.
