Chimpanzee Mating Swelling: Why Female Chimps Swell
Chimpanzee mating swelling, also called sexual swelling, is a large, pink, fluid-filled swelling of the skin around a female chimpanzee’s genitals and bottom. It grows during each reproductive cycle as her oestrogen levels rise, stays at full size for about 10 to 12 days, and shrinks soon after she ovulates.
The swelling tells males that she may be fertile, and it is when almost all mating in a chimpanzee community takes place.
Chimpanzee swellings are among the largest of any primate. At full size, a swelling can measure well over 1 litre in volume and is easy to see from a distance, even in a dense forest.
This article explains what causes the swelling, how the cycle works, how it shapes chimpanzee mating behaviour, why scientists think it evolved, and how chimpanzees compare with gorillas, bonobos and humans.
Chimpanzee Mating Swelling: Quick Facts
| Fact | Detail |
|---|---|
| Scientific name | Pan troglodytes |
| Other names | Sexual swelling, oestrous swelling, perineal swelling |
| What swells | The skin around the genitals and anus (the perineum) |
| Cause | Rising oestrogen makes the skin hold extra water |
| Length of cycle | About 35 to 37 days |
| Time at full swelling | About 10 to 12 days per cycle |
| When ovulation happens | Usually in the last few days of full swelling |
| When swelling ends | Within a day or two after ovulation |
| First swellings | Around 8 to 10 years old, before a female can conceive |
| Pregnancy length | About 8 months (around 230 to 240 days) |
| Time between births | Usually 5 to 6 years in the wild |
What Causes the Swelling?
The swelling is caused by hormones. During the first half of the cycle, the ovaries release more and more oestrogen. Oestrogen makes the skin of the perineum take up water and fluid, so it puffs up and turns a bright pink.
When the female ovulates, her progesterone level rises and her oestrogen falls, and the tissue quickly releases the fluid. The swelling then shrinks back, usually within a day or two.
The swelling is not caused mainly by blood flow, and it is not painful. It does, however, make sitting a little awkward, and females sometimes pick their way carefully through thorny undergrowth.
Unlike many mammals, chimpanzees menstruate, much like humans. A light bleed marks the start of each new cycle, and the swelling begins to build again in the days that follow.
The Chimpanzee Swelling Cycle, Stage by Stage
| Stage | Approximate days of the cycle | What happens |
|---|---|---|
| Menstruation | Days 1 to 3 | A light bleed. No swelling. |
| Swelling builds | Days 4 to 12 | Oestrogen rises and the skin begins to swell. Males start to take an interest. |
| Full (maximal) swelling | Days 13 to 24 | The swelling is at its largest and firmest. Most mating happens now. |
| Ovulation | Near the end of full swelling | An egg is released. This is the most likely time for conception. |
| Swelling goes down | Days 24 to 26 | Fluid drains away within a day or two. Male interest drops sharply. |
| Flat (luteal) phase | Days 26 to 36 | No swelling. The body prepares for pregnancy or a new cycle. |
The exact timing varies between individuals and from one cycle to the next. What matters most is that ovulation falls near the end of the full-swelling phase, not in the middle of it. This gives the female a long window of attractiveness but only a short window of real fertility.
When Female Chimpanzees Do Not Swell
A female chimpanzee spends surprisingly little of her life with a swelling.
- Young females: swellings begin at about 8 to 10 years old, but there is usually a gap of a few years of “adolescent sterility” before a female can conceive.
- Pregnant females: most stop swelling, although some show small swellings in early pregnancy.
- Mothers with infants: a mother nurses her infant for about four to five years and usually does not cycle during most of that time. Her swellings return only as the infant is weaned.
Because of this, a female in the wild may only have a few dozen fully swollen cycles in her whole life. In a community at any one time, only a small share of the adult females will be swollen, which is why a swollen female draws so much attention.
How the Swelling Shapes Chimpanzee Mating
Chimpanzees live in communities of about 20 to 150 members, with many adult males and many adult females. Females mate with several males during a cycle, and the swelling is the signal that organises almost all of this activity. For an overview of courtship and behaviour, see our guide to chimpanzee mating.
Researchers describe three main mating patterns.
1. Group (Opportunistic) Mating
Early in the full-swelling phase, a swollen female may mate with many of the males in her party. Most of these matings are brief, and males rarely fight openly over her at this stage. This spreads the chance of fatherhood across many males.
2. Mate Guarding by High-Ranking Males
As the swelling nears its end, and ovulation gets closer, the alpha male or another high-ranking male often stays close to the female and tries to stop other males from mating with her.
This is called mate guarding or possessive mating. Genetic studies show that high-ranking males do father a large share of infants, especially the alpha male.
3. Consortships
Sometimes a male and a swollen female slip away from the rest of the community for days or even weeks, travelling quietly at the edge of their territory. These pairings are called consortships, and researchers sometimes say the pair has “gone on safari”. A consortship lets a lower-ranking male father offspring, but it only works if the female agrees to go with him.
Female Choice
Females are not passive. A female can ignore or move away from a male, solicit a male she prefers by presenting her swelling to him, or refuse to join a consortship. Males that groom a female often, share food with her or treat her well over time may have better mating success.
Male Competition and Aggression
The arrival of a swollen female tends to increase tension among males, not reduce it. Males display, chase and sometimes attack each other, and some males are aggressive towards the female herself. Rank, alliances and timing all affect which males get the most mating opportunities.
Chimpanzees are generally among the more aggressive great apes. Read more in are chimpanzees dangerous and how strong chimpanzees are.
Why Did Sexual Swellings Evolve?
Scientists still debate exactly why chimpanzee swellings are so large and last so long. Three main ideas are often discussed, and they may all be partly true.
1. The Reliable Signal Idea
A swelling shows males when a female is likely to be fertile, and a larger swelling may signal a healthier female. Males do pay most attention to females at full swelling, which fits this idea.
2. The Paternity Confusion Idea
Because the swelling lasts much longer than the fertile window, and a female mates with many males, no male can be sure which infant is his. In chimpanzees, males sometimes kill infants they did not father. Spreading the chance of fatherhood may make many males less likely to harm a female’s future infant, and more likely to tolerate or protect it.
3. The Graded Signal Idea
The swelling is a rough guide, not an exact one: it tells males that ovulation is likely around now, but not precisely when. This may let the female attract many males early in the swelling, then let the strongest, most dominant male compete hardest near ovulation. The female gets both benefits: protection from many males, and good genes from a high-quality father.
Chimpanzee Swelling Compared With Other Primates
| Species | Sexual swelling? | Notes |
|---|---|---|
| Chimpanzee | Yes, very large | Swells for about 10 to 12 days each cycle |
| Bonobo | Yes, very large | Swollen for much more of the cycle, even when not fertile, and while nursing |
| Gorilla | Only slight | A small, hard-to-see swelling. Gorilla groups usually have one breeding silverback, so there is less need to advertise. |
| Orangutan | No | Females live alone and meet males when they choose |
| Baboons and some macaques | Yes, large | Similar signal in multi-male groups |
| Humans | No | Ovulation is concealed, with no visible signal |
The pattern is clear. Large swellings appear mainly in species where females live in groups with many adult males, such as chimpanzees, bonobos and baboons. Where one male controls access to females, as in most gorilla groups, there is no large swelling. Compare chimpanzee behaviour with gorilla mating, whether gorillas mate like humans and whether gorillas have multiple partners.
Why Don’t Humans Have Swellings?
Humans and chimpanzees share about 98.8 percent of their DNA, but human ovulation is hidden. Many scientists think concealed ovulation evolved alongside long-term pair bonds, where it pays for a male to stay with one female throughout her cycle. That is very different from the chimpanzee system, where females mate with several males.
Seeing Chimpanzees in the Wild
On a chimpanzee trek you may well see a female with a swelling, and you may see mating or male displays nearby. It is part of everyday life in a chimpanzee community. Your guide will explain what you are seeing and keep the group at a safe distance, usually at least 8 metres.
The best places to track wild chimpanzees near Rwanda are:
- Nyungwe Forest National Park, Rwanda: Rwanda’s main chimpanzee destination, with a canopy walkway. See chimpanzee trekking in Rwanda and our 3 Days Nyungwe Chimps and Canopy Walk.
- Gishwati-Mukura National Park, Rwanda: a smaller forest with a recovering chimpanzee population. Read about Gishwati-Mukura National Park.
- Kibale Forest National Park, Uganda: one of the highest chimpanzee densities in Africa and some of the best sighting rates. Try our 3 Days Kibale Primate Safari or browse chimpanzee safari packages in Uganda.
You can also combine chimpanzees and gorillas on our 4 Days Bwindi Gorillas and Kibale Chimps or 5 Days Rwanda Gorilla and Chimp Tour.
Frequently Asked Questions About Chimpanzee Mating Swelling
Why do female chimpanzees have swollen bottoms?
Female chimpanzees develop a swollen bottom because rising oestrogen makes the skin around their genitals fill with fluid during each reproductive cycle. The swelling shows males that the female may be fertile.
How long does a chimpanzee swelling last?
A chimpanzee’s swelling stays at full size for about 10 to 12 days of a cycle that lasts about 35 to 37 days. It shrinks within a day or two after ovulation.
Does the swelling hurt the female?
No, the swelling is not thought to be painful. It is a normal part of the cycle, though it can make sitting and moving through thorny vegetation more awkward.
Do all female chimpanzees swell?
All healthy adult females swell when they are cycling. Young females start at about 8 to 10 years old, and pregnant or nursing females usually do not swell.
Do chimpanzees mate only when the female is swollen?
Almost all chimpanzee mating happens while the female is swollen. Bonobos are different: they mate throughout the cycle and use sex to ease social tension.
Do gorillas have mating swellings?
Gorillas have only a slight swelling that is hard to see. Because most gorilla groups have one breeding silverback, females have less need to advertise their fertility.
Is the swelling the same as a period?
No. The swelling builds in the first half of the cycle, before ovulation. Chimpanzees also have a light menstrual bleed, which comes at the start of each new cycle.
More Chimpanzee Guides
- Where do chimpanzees live?
- Chimpanzee diet
- Why are chimpanzees endangered?
- What do chimpanzees smell like?
- Chimpanzee strength
- Volunteering at Ngamba Chimpanzee Sanctuary
See Chimpanzees on Safari
The sexual swelling is just one part of the complex social lives of chimpanzees, and there is no better way to understand them than to watch a community in the wild.
Tell us your travel dates through our contact page, and we will plan a chimpanzee trek in Rwanda or Uganda, with gorilla trekking added if you wish.
