Male bonobos track females’ reproductive cycle to maximize mating success

Female sexual swelling is an unreliable fertility signal, but males can time matings anyway

Peer-Reviewed Publication

PLOS

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Male–male agonistic interaction during a mating attempt in wild bonobos.

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Credit: Heungjin Ryu (CC-BY 4.0, https://creativecommons.org/licenses/by/4.0/)

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 9th 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.

Male bonobos track females’ reproductive cycle to maximize mating success [VIDEO] 

In this clip, the beta male Nobita carefully looks at the sexual swelling of the female Sala while they forage for fungi on the forest floor. Male bonobos take this task seriously—watching and checking swelling changes so they do not miss potential ovulatory periods.

Credit

Heungjin Ryu (CC-BY 4.0, https://creativecommons.org/licenses/by/4.0/)

Male bonobos track females’ reproductive cycle to maximize mating success [VIDEO] 

In this video, two male bonobos and one female engage in a mating-related conflict. The beta male, Nobita, solicits copulation from a female, Fuku. Once they begin copulating, the alpha male, Kitaro—Nobita’s younger brother—attempts to intervene. Despite his efforts, Nobita and Fuku continue and complete the copulation. Afterward, Fuku becomes upset and briefly chases Kitaro.

Credit

Heungjin Ryu (CC-BY 4.0, https://creativecommons.org/licenses/by/4.0/)

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Journal

PLOS Biology

DOI

10.1371/journal.pbio.3003503 

Method of Research

Observational study

Subject of Research

Animals

COI Statement

Competing interests: The authors have declared that no competing interests exist.

 

Humans rank between meerkats and beavers in monogamy ‘league table’

University of Cambridge

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Humans are far closer to meerkats and beavers for levels of exclusive mating than we are to most of our primate cousins, according to a new University of Cambridge study that includes a table ranking monogamy rates in various species of mammal.

Previous evolutionary research has used fossil records and anthropological fieldwork to infer human sexual selection. While in other species, researchers have conducted long-term observations of animal societies and used paternity tests to study mating systems.

Now, a new approach by Dr Mark Dyble from Cambridge’s Department of Archaeology analyses the proportions of full versus half-siblings in a host of species, as well as several human populations throughout history, as a measure for monogamy.

Species and societies with higher levels of monogamy are likely to produce more siblings that share both parents, says Dyble, while those with more polygamous or promiscuous mating patterns are likely to see more half-siblings.

He devised a computational model that maps sibling data collected from recent genetic studies onto known reproductive strategies to calculate an estimated monogamy rating.

While still a rough guide, Dyble argues this is a more direct and concrete way to gauge patterns of monogamy than many previous methods when looking at a spectrum of species, and human societies over thousands of years.    

“There is a premier league of monogamy, in which humans sit comfortably, while the vast majority of other mammals take a far more promiscuous approach to mating,” said Dyble, an evolutionary anthropologist at the University of Cambridge.

“The finding that human rates of full siblings overlap with the range seen in socially monogamous mammals lends further weight to the view that monogamy is the dominant mating pattern for our species.”

The question of human monogamy has been debated for centuries. It has long been hypothesised that monogamy is a cornerstone of the social cooperation that allowed humans to dominate the planet.

However, anthropologists find a wide range of mating norms among humans. For example, previous research shows that 85% of pre-industrial societies permitted polygynous marriage – where a man is married to several women at the same time.

To calculate human monogamy rates, Dyble used genetic data from archaeological sites, including Bronze Age burial grounds in Europe and Neolithic sites in Anatolia, and ethnographic data from 94 human societies around the world: from Tanzanian hunter-gatherers the Hadza, to the rice-farming Toraja of Indonesia.

“There is a huge amount of cross-cultural diversity in human mating and marriage practices, but even the extremes of the spectrum still sit above what we see in most non-monogamous species,” said Dyble.

The study, published in Proceedings of the Royal Society: Biological Sciences, has humans at an overall 66% rate for full siblings, placing us seventh of eleven species in the study considered socially monogamous and preferring long-term pair bonds.

Meerkats come in at a 60% full sibling rate while beavers just beat humans for monogamy with a 73% rate. As with humans, this suggests a significant trend towards monogamy for these species, but with a solid amount of flexibility.

The white-handed gibbon comes closest to humans in the study, with a monogamy rate of 63.5%. It’s the only other top-ranked “monotocous” species, meaning it usually has one offspring per pregnancy, unlike the litters had by other monogamous mammals.

The only other non-human primate in the top division is the moustached tamarin: a small Amazonian monkey that typically produces twins or triplets, and has a full sibling rate of almost 78%.   

All other primates in the study are known to have either polygynous or polygynandrous (where both males and females have multiple partners) mating systems, and rank way down the monogamy table.  

Mountain gorillas manage a 6% full sibling rate, while chimpanzees come in at just 4% – on a par with dolphins. Various macaque species, from Japanese (2.3%) to Rhesus (1%), sit almost at the bottom of the table.

“Based on the mating patterns of our closest living relatives, such as chimpanzees and gorillas, human monogamy probably evolved from non-monogamous group living, a transition that is highly unusual among mammals,” said Dyble.  

Among the few with a similar evolutionary shift are species of wolf and fox, which have a degree of social monogamy and cooperative care, whereas the ancestral canid was likely to have been group-living and polygynous.

The Grey Wolf and Red Fox sneak into the upper league with full sibling rates of almost half (46% and 45% respectively), while African species have much higher rates: the Ethiopian wolf comes in at 76.5%, and the African Wild dog is ranked second for monogamy with a rating of 85%.

Top of the table is the California deermouse that stays paired for life once mated, with a 100% rating. Ranked bottom is Scotland’s Soay sheep, with 0.6% full siblings, as each ewe mates with several rams.

“Almost all other monogamous mammals either live in tight family units of just a breeding pair and their offspring, or in groups where only one female breeds,” said Dyble. “Whereas humans live in strong social groups in which multiple females have children.”

The only other mammal believed to live in a stable, mixed-sex, multi-adult group with several exclusive pair bonds is a large rabbit-like rodent called the Patagonian mara, which inhabits warrens containing a number of long-term couples. 

Dyble added: “This study measures reproductive monogamy rather than sexual behaviour. In most mammals, mating and reproduction are tightly linked. In humans, birth control methods and cultural practices break that link.”

“Humans have a range of partnerships that create conditions for a mix of full and half-siblings with strong parental investment, from serial monogamy to stable polygamy.”

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Journal

Proceedings of the Royal Society B Biological Sciences

DOI

10.1098/rspb.2025.2163 

Method of Research

Data/statistical analysis

Article Title

Human monogamy in mammalian context

Article Publication Date

10-Dec-2025

 

US fossil reveals early mass-burial event and ancient microbial attack

Flinders University

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The fossil horseshoe crab (Euproops danae) was found at the Mazon Creek Lagerstätte in Illinois, USA. Courtesy Lauer Foundation. 

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Credit: Photo courtesy Gregory Lewbart (North Carolina State University)

A remarkably preserved horseshoe crab fossil from North America offers rare insight into some of the earliest known cases of animal disease in a Late Carboniferous swamp – more than 300 million years before the age of dinosaurs.

The specimen, uncovered from the mass-burial fossil deposit at the famous Mazon Creek Lagerstätte in Illinois in the US, shows more than 100 small pits across the front of its shell, representing one of the earliest documented examples of microbial or algal infection killing groups of these ancient aquatic animals.

“Ancient arthropods faced many of the same ecological pressures that modern species experience today, including microbial attacks and environmental stress,” says Dr Russell Bicknell, lead author of a new Biology Letters article, and recent appointee at Flinders University in South Australia.

“This fossil links a specific biological event – likely microbial or algal infestation – to a broader evolutionary picture, showing that interactions between animals and microbes were already well established long before dinosaurs evolved,” says Dr Bicknell, who previously worked at the American Museum of Natural History in New York.

“It also ties the specimen to the Late Carboniferous world, a time when Earth’s ecosystems were undergoing significant changes that shaped the future of animal evolution.

“From an evolutionary perspective, this discovery pushes back evidence of such infestations in horseshoe crabs by more than 300 million years. “

Dr Bicknell, an Australian Research Council DECRA fellow at Flinders University, says the fossil horseshoe crab (Euproops danae) regularly shed their shells while growing so the “heavy fouling” indicates the specimen had stopped moulting and reached maturity.

“The Late Carboniferous nutrient-rich environment at Mazon Creek, with its regular flooding and fluctuating salinity, would have promoted microbial growth and rapid burial. These conditions helped preserve organisms inside ironstone concretions with exceptional detail.”

At the time, vast forests covered the planet, oxygen levels were high, and land animals – including amphibians and reptile ancestors, were diversifying.

“This fossil adds to a new piece of the ecological puzzle, highlighting the pressures shaping ancient marine anthropods and their evolutionary responses to infestation.”  

Horseshoe crabs are related to spiders and scorpions today.

Acknowledgement: This research was funded by a MAT Program Postdoctoral Fellowship to Dr Bicknell. With thanks for access to the collection at the Lauer Foundation for Paleontology, Science and Education.

Images: https://www.dropbox.com/scl/fo/ihtxe571yvlz72icf6pz5/ANQG4cpm-WAxa29_ITV1LMo?rlkey=odle8a2n385leioiuc2oilts2&dl=0

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Journal

Biology Letters

DOI

10.1098/rsbl.2025.0565 

Method of Research

Observational study

Subject of Research

Animals

Article Title

Unique, dimple-like exoskeletal structures suggest syn-vivo infestations in Late Carboniferous horseshoe crabs

Article Publication Date

9-Dec-2025