Expert: Jonathan Wright - 8/29/2006

Question
I know that there are other animals other than humans that have menstrual bleeding such as old world monkeys, dogs etc. But how come we never notice it? What happens to the blood? And how much blood do they menstruate?

Answer
Dear Bill

Thanks for your question. I have included some information from an earlier answer and have looked up various books and websites and these contain interesting information about menstruation, but most of the information concerns humans. I wish to thank the authors of this information.

http://en.wikipedia.org/wiki/Menstrual_cycle says tht only humans and great apes show a true menstrual cycle. Eumenorrhea denotes normal, regular menstruation that lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7 days is considered normal.The average blood loss during menstruation is 35 millilitres with 10-80 mL considered normal; many women also notice shed endometrium lining that appears as tissue mixed with the blood. An enzyme called plasmin (contained in the endometrium) tends to inhibit the blood from clotting. Not all menstruations result from an ovulatory menstrual cycle. In some women, follicular development may start but not complete, but oestrogens will form and will stimulate the uterine lining, which will be shed later.

http://ajp.amjpathol.org/cgi/content/full/157/6/1759 contains information about menstruation and similar topics in various mammals. http://www.biologymad.com/master.html? http://www.biologymad.com/Hormones/Hormones.htm and http://www.howstuffworks.com/menstruation.htm may also be helpful for you.

The following mammals show menstrual flow: elephant shrews, Old World monkeys and apes, including humans. http://www.kband.com/reports/000116.html states that overt menstruation is recorded in a few species of new world monkeys and the Malayan flying lemur, as well as cat-like marsupials (dasyures), tree shrews, elephant shrews, hedgehogs and various species of free-tailed and American leaf-nosed bats and vampires.  In all of these, the loss of blood is a non-event, compared to the situation in humans. There are anecdotal reports for menstruation in the coyote and African elephant.

http://www.madsci.org/posts/archives/apr2001/986666565.An.r.html states that menstruation is the shedding of the uterine lining between ovulations. Many mammals  menstruate covertly, reabsorbing the uterine lining after it is shed so that bleeding is not externally visible. The mammals that menstruate are not all closely related to one another, so the site suggests that the process is probably widespread and that nearly all female mammals menstruate in some fashion.

http://psychcentral.com/psypsych/Menstrual_period#Menstruation_in_other_mamm als contains information with links to various scientific terms. It states that a regular menstrual cycle occurs in the great apes, varying from 29 days in orang-utans to 37 days in chimpanzees. Females of other mammal species go through episodes called "oestrus" or "heat" in each breeding season. During these times, ovulation occurs and females become receptive to mating, a fact advertised to males in some way. If no fertilization takes place, the uterus reabsorbs the endometrium: no menstrual bleeding occurs. Significant differences exist between the oestrous and the menstrual cycle.

I hope this helps. It is a bit confusing, as there is some seemingly conflicting information, but it seems that most species of mammals have not been studied and that menstruation is probably more widespread than the above information would indicate. As most menstruating mammals seem to show covert bleeding, this indicates why people don't notice it. I couldn't find any figures for how much blood is lost, but this seems to be minimal in most species.

All the best

Jonathan