Do other animals get periods?

Periods, formally called menstruation, happen to female mammals every menstrual cycle when an egg cell is not fertilised. Levels of the progesterone hormone decrease, causing the lining of the uterus to self-destruct and shed. This lining is called the endometrium and is flushed out of the body with blood during menstruation. Some primates, bats, the spiny mouse, and elephant shrews get periods (Figure 1). Since these groups are distantly related, menstruation likely evolved multiple times independently. Knowing which species menstruate lets us pick animal models which best reflect the human female reproductive system.

Why do we get periods?

Despite being painful and inconvenient, menstruation must have some benefit; otherwise, natural selection would not favour it on multiple separate occasions. Hypotheses put forward to explain menstruation include clearing the uterus of pathogens and saving energy compared to maintaining an endometrium all the time. A 2012 paper argues that neither of these hypotheses are true and that menstruation is an unfortunate byproduct of the way pregnancy occurs in certain animals. 

In non-menstruating animals, an embryo induces morphological and biological changes in the uterus, so those changes do not happen if they are not pregnant. The uterus of a menstruating animal undergoes regular changes even without an embryo, and one of those changes is shedding the endometrium. However, there is no consensus on the benefits of menstruation.

Non-human primates

Old World monkeys, apes, and humans menstruate conspicuously. This could be because their endometria have spiral arteries, which dilate and weaken in response to hormones. Eventually, the weakened arteries break and release blood, which carries dead and detached endometrial tissue out of the body. While chimpanzee menstruation is visible to the naked eye, menstrual blood in orangutans and gorillas is detected with a chemical urine strip. Gorillas bleed for 3 days, while orangutans bleed for 1-4 days. Humans have the most obvious, and possibly the most prolonged, menstruation out of the Old World primates. (Aren’t we unlucky?).

On the other hand, the very few New World monkey species which menstruate need a microscope to detect it. Pedro Mayor and colleagues sampled the endometria of various New World monkeys and viewed those samples under a microscope. They found that monkeys from the Aotus nancymaae and Sapajus macrocephalus species had weakened endometria with dilated blood vessels and blood clots (Figure 2). Combined with other context clues from those endometrium samples, they concluded that those monkeys must be menstruating.

Bats

Microscopy also identified menstruation in some bat species. In a 2011 study, uterus sections from Carollia perspicillata bats showed the endometrium getting thinner over a few days with associated bleeding. Some sections had endometrial debris in the lumen of the uterus – but unlike in Old World primates and humans, this debris was reabsorbed by the body rather than released. 

Menstruating Molossus ater bats had blood and endometrial cells in their cervix under a microscope, while one individual was visibly bleeding in its vagina. In contrast, a colony of female Rousettus leschenaulti bats all had visible vaginal bleeding on the same day. On that day, two-thirds of their endometria were shed, and they had low progesterone levels – meaning those bats were menstruating.

Bat menstruation differs from primates in at least two ways. Firstly, menstruation happens simultaneously with ovary development in Carollia perspicillata and before ovary development in primates. Secondly, some bat species only menstruate after an interrupted mating attempt – which scientists call coitus, and the public would call “pulling out”. Perhaps menstruation gives these bats a second chance at successful mating in that breeding season.

Conclusion

We rarely see other animals on their period because if the species does menstruate, they do not bleed as much as humans do. Evidence of menstruation in New World monkeys and bats usually came from microscopy, where the endometrium was seen to detach, and blood was seen in the uterine lumen. These monkeys and bats could be used as rudimentary animal models to study what happens in humans during a period.

Written by Simran Patel

Related article: Monkey see, monkey clone

REFERENCES

Catalini L, Fedder J. Characteristics of the endometrium in menstruating species: lessons learned from the animal kingdom. Biology of Reproduction [Internet]. 2020 May 26 [cited 2025 Jan 8];102(6):1160–9. Available from: https://doi.org/10.1093/biolre/ioaa029

Mayor P, Pereira W, Nacher V, Navarro M, Monteiro FOB, El Bizri HR, et al. Menstrual cycle in four New World primates: Poeppig’s woolly monkey (Lagothrix poeppigii), red uakari (Cacajao calvus), large-headed capuchin (Sapajus macrocephalus) and nocturnal monkey (Aotus nancymaae). Theriogenology [Internet]. 2019 Jan 1 [cited 2025 Jan 7];123:11–21. Available from: https://www.sciencedirect.com/science/article/pii/S0093691X18302796

Rasweiler IV JJ, Badwaik NK, Mechineni KV. Ovulation, Fertilization, and Early Embryonic Development in the Menstruating Fruit Bat, Carollia perspicillata. The Anatomical Record [Internet]. 2011 [cited 2025 Jan 8];294(3):506–19. Available from: https://onlinelibrary.wiley.com/doi/abs/10.1002/ar.21304

Graham C. Reproductive Biology of the Great Apes: Comparative and Biomedical Perspectives. Elsevier; 2012. 456 p.

Rasweiler IV JJ. Spontaneous decidual reactions and menstruation in the black mastiff bat, Molossus ater. American Journal of Anatomy [Internet]. 1991 [cited 2025 Jan 8];191(1):1–22. Available from: https://onlinelibrary.wiley.com/doi/abs/10.1002/aja.1001910102

Martin RD. The evolution of human reproduction: A primatological perspective. American Journal of Physical Anthropology [Internet]. 2007 [cited 2025 Jan 8];134(S45):59–84. Available from: https://onlinelibrary.wiley.com/doi/abs/10.1002/ajpa.20734

Emera D, Romero R, Wagner G. The evolution of menstruation: A new model for genetic assimilation. BioEssays [Internet]. 2012 [cited 2025 Jan 8];34(1):26–35. Available from: https://onlinelibrary.wiley.com/doi/abs/10.1002/bies.201100099

Zhang X, Zhu C, Lin H, Yang Q, Ou Q, Li Y, et al. Wild Fulvous Fruit Bats (Rousettus leschenaulti) Exhibit Human-Like Menstrual Cycle1. Biology of Reproduction [Internet]. 2007 Aug 1 [cited 2025 Jan 8];77(2):358–64. Available from: https://doi.org/10.1095/biolreprod.106.058958