Menstruation is often considered a uniquely human experience, but science tells a far more fascinating story. While most mammals reabsorb their uterine lining instead of shedding it, a small and remarkable group of animals actually go through a true menstrual cycle—complete with hormonal shifts, endometrial buildup, and bleeding.
Researchers estimate that fewer than 2% of mammals menstruate, making it one of the rarest reproductive traits in the animal kingdom. Scientists believe menstruation evolved in species with highly invasive embryos, where the body prepares in advance for implantation rather than reacting afterward.
So which animals have periods? Let’s explore the surprising list—from our closest primate relatives to tiny rodents and even a few bats.
Post Contents
- What Is Menstruation?
- Primates That Menstruate
- Lesser Apes (Gibbons)
- Old World Monkeys
- New World Monkeys
- Tarsiers: A Weak Menstrual Presence
- What About Lemurs and Lorises?
- Non-Primate Mammals That Menstruate
- Elephant Shrew (Sengi)
- Cairo Spiny Mouse
- Why Is Menstruation So Rare?
- Animals That Do NOT Menstruate (But People Think They Do)
- Why Studying Menstruating Animals Matters
- Final Thoughts
What Is Menstruation?
Before diving into the species list, it helps to understand what “true menstruation” means.
Menstruation occurs when:
- The uterus builds a thick lining for potential pregnancy
- Fertilization does not occur
- Hormone levels drop
- The lining sheds through bleeding
Most mammals instead experience an estrous cycle, where the lining is quietly reabsorbed. Animals in estrus typically show visible signs of fertility (“heat”), whereas menstruating species may not.
Scientists associate menstruation with three main traits:
✅ Spontaneous ovulation
✅ Strong hormonal cycles
✅ Complex placental development
Now let’s look at the animals that share this unusual biological process.
Primates That Menstruate
Menstruation is most common among primates—especially simians (apes and monkeys). Because humans belong to this group, studying these animals helps researchers better understand fertility, pregnancy, and reproductive health.
Great Apes
Great apes are our closest evolutionary relatives, sharing complex social behavior, intelligence, and reproductive biology.
Humans
Human are the most well-known menstruating species. The average menstrual cycle lasts about 28 days, though variation is completely normal.
Key characteristics:
- Visible bleeding
- Hormone-driven cycle
- Concealed ovulation
- Thick uterine lining
Scientists believe menstruation in humans evolved to help the body screen embryos and protect against abnormal implantation.
Interestingly, humans shed more uterine tissue than most other menstruating animals—a topic still under scientific investigation.
Chimpanzees
Chimpanzee females experience menstrual cycles averaging 31–36 days.
Unlike humans:
- Bleeding is usually lighter
- Sexual swelling signals fertility
- Social hierarchy can influence reproduction
Chimpanzees are essential to reproductive research because their cycles closely mirror ours biologically.
Gorillas
Gorilla females menstruate quietly, with cycles typically lasting about 28 days.
Bleeding is rarely obvious, which is why many people don’t realize gorillas menstruate at all.
Notable traits:
- Subtle physical signs
- Strong maternal investment
- Long gestation periods
Their reproductive strategy focuses on fewer offspring with higher survival rates.
Orangutans
Orangutan females have one of the slowest reproductive rates among mammals.
Cycle facts:
- Roughly 28–30 days
- Minimal external bleeding
- Extremely long intervals between births
Some orangutans only reproduce once every 7–9 years, making each pregnancy critically important.
Lesser Apes (Gibbons)
Gibbon females also menstruate, though research is less extensive than with great apes.
Typical features:
- Cycles around 28 days
- Mild bleeding
- Strong pair bonding
Gibbons are largely monogamous—rare among primates—which adds another layer of interest for reproductive scientists.
Old World Monkeys
These monkeys live in Africa and Asia and share many reproductive traits with apes.
Baboons
Baboon females display dramatic swelling during their cycle.
Important notes:
- Cycles average about 30–35 days
- Menstruation occurs but is often subtle
- Social rank can affect fertility
Baboons are widely studied because their reproductive physiology is highly predictable.
Macaques
Macaque species—especially rhesus macaques—are central to medical research.
Why scientists study them:
- Cycles closely resemble humans
- Hormone patterns are similar
- Useful for fertility studies
Bleeding is typically light but biologically significant.
New World Monkeys
Menstruation is less common here but still present in some species.
Capuchins
Capuchin monkey females show menstrual-like cycles.
Traits include:
- Spontaneous ovulation
- Hormonal fluctuations
- Shorter cycles
Their intelligence makes them popular subjects in behavioral studies as well.
Marmosets (Some Species)
Marmoset reproductive biology is unusual.
Many species don’t menstruate clearly but still undergo uterine shedding internally—placing them in a gray area scientifically.
They are known for:
- Frequent twin births
- Cooperative parenting
- Rapid reproductive cycles
Tarsiers: A Weak Menstrual Presence
Tarsier species show evidence of menstrual cycling, though it is less pronounced.
Scientists consider them evolutionarily important because they sit between primitive primates and monkeys.
Their reproductive biology may offer clues about how menstruation evolved.
What About Lemurs and Lorises?
Lemur and Loris generally do NOT menstruate.
Instead, they reabsorb the uterine lining—highlighting just how rare menstruation really is.
Non-Primate Mammals That Menstruate
Outside primates, menstruation becomes exceptionally rare—but not nonexistent.
Menstruating Bats
Only a handful of bat species demonstrate true menstruation.
Black Mastiff Bat
Black mastiff bat females show uterine shedding similar to primates.
Why scientists care:
- Helps study implantation
- Offers insight into mammalian evolution
- Challenges assumptions about reproduction
Wild Fulvous Fruit Bat
Fulvous fruit bat is another confirmed menstruating species.
Researchers discovered:
- Visible endometrial shedding
- Hormonal cycling
- Spontaneous ovulation
These findings reshaped scientific understanding of bat reproduction.
Elephant Shrew (Sengi)
Elephant shrew—also called a sengi—is a small African insectivore with a surprisingly advanced reproductive system.
Key facts:
- Experiences spontaneous decidualization
- Sheds uterine lining
- Has complex placental development
Despite its mouse-like appearance, the elephant shrew is more closely related to elephants than rodents.
Evolution is full of surprises.
Cairo Spiny Mouse
Cairo spiny mouse may be the most scientifically exciting discovery in reproductive biology in decades.
Why?
It is the only known rodent that menstruates.
Cycle highlights:
- Lasts about 8–9 days
- Visible bleeding occurs for roughly 3 days
- Hormonal patterns mirror humans
Because mice are widely used in labs, this species offers enormous potential for studying:
- Endometriosis
- Fertility
- Pregnancy complications
Many researchers consider it a breakthrough model for women’s health research.
Why Is Menstruation So Rare?
Scientists are still debating the answer, but several strong theories exist.
1. Protection Against Aggressive Embryos
Species with deeply invasive embryos may menstruate to prepare the uterus in advance.
2. Energy Efficiency
If pregnancy is costly, the body benefits from strict biological screening.
3. Immune Defense
Shedding the lining may help remove pathogens.
4. Evolutionary Strategy
Menstruation tends to appear in species with:
- Long gestation
- Fewer offspring
- High parental investment
In other words, quality over quantity.
Animals That Do NOT Menstruate (But People Think They Do)
Many mammals experience bleeding or discharge during heat—but this is not menstruation.
Examples include:
- Dogs
- Cats
- Cows
- Horses
These animals reabsorb the uterine lining instead of shedding it.
Many people assume that if an animal shows bleeding during its reproductive cycle, it must be menstruating. However, true menstruation—where the uterine lining is shed through visible bleeding—is extremely rare in the animal kingdom. Most mammals instead experience an estrous cycle, meaning the body reabsorbs the uterine lining rather than expelling it.
For example, dogs often release a bloody discharge when in heat, but this fluid comes from hormonal changes that increase blood flow to the reproductive tract—not from shedding the uterus. Cats typically do not bleed at all; they are induced ovulators, meaning ovulation happens only after mating. If a cat is bleeding, it could signal a medical issue requiring veterinary attention.
Similarly, cows and horses go through heat cycles without menstruating. While mild spotting can occasionally occur in cows after ovulation, it is not the same as a human period. Horses rarely show visible discharge, making their cycle even less noticeable.
The key difference is biological efficiency. Instead of losing nutrients by shedding tissue, these animals conserve energy by reabsorbing the uterine lining. This reproductive strategy supports frequent breeding opportunities and highlights just how unusual true menstruation really is among mammals.
Why Studying Menstruating Animals Matters
Understanding which animals menstruate helps scientists:
✅ Develop fertility treatments
✅ Study miscarriage
✅ Improve pregnancy care
✅ Research reproductive disorders
✅ Advance regenerative medicine
The Cairo spiny mouse alone may revolutionize gynecological research.
Final Thoughts
Menstruation is far from a universal mammalian trait—it’s an evolutionary rarity shared by a surprisingly small circle of animals.
The confirmed menstruating group includes:
- Humans
- Great apes
- Gibbons
- Some monkeys
- Tarsiers (weakly)
- Certain bats
- Elephant shrews
- Cairo spiny mice
Meanwhile, most mammals follow an entirely different reproductive strategy.
The next time someone says humans are unusual for having periods, you can smile and point out that we’re part of a very exclusive biological club—one that spans rainforest canopies, African savannas, and even the nighttime skies filled with bats.
Nature, as always, proves to be far more complex and fascinating than we imagine.
94% of pet owners say their animal pal makes them smile more than once a day. In 2007, I realized that I was made for saving Animals. My father is a Vet, and I think every pet deserves one. I started this blog, “InPetCare”, in 2019 with my father to enlighten a wider audience.