Thursday, 31 January 2013

Do Greater Horseshoe Bats (and their relatives) Menstruate?

Greater Horseshoe bat in Devon Cave

In connection with my interests in human evolution I discovered – thanks to the blog “The Evolving Placenta,” that humans are one of the few mammals which menstruate – and that some bats do. I will, of course, be discussing the evolutionary advantages of menstruation in humans in the talk I am preparing – but I am intrigued by the mention of bats.

The reason for my interest was that in the late 50's I helped with the studies of the Greater Horseshoe Bats carried out in the Buckfastleigh area of Devon by John and Win Hooper. The young are born in communal nurseries between May and July, the earlier dates in mild springs but bats were occasionally observed mating throughout the colder months of the year when they were hibernating in caves – suggesting embryos of different ages might be born at about the same time. Because the bats only have one young a year, at a time when there will be plenty of food, it is important that they don't miss a year's breeding. The prolonged mating season is of interest and I am wondering if evolutionary pressures have caused them to menstruate for the same reason as is suggested for humans. If the womb is prepared at the time of ovulation the fertilized egg finds a ready prepared “bed” where it can get nourishment. It therefore starts growing more rapidly – and if it is defective it can be discarded – and because if the long mating season there is time to prepare another egg (and its ready made bed) in time to mate again. This approach could allow several ovulation, fertilization and mensuration cycles to take place during the lengthy mating season, until there is an acceptable embryo in the womb, in time for the young to be born when the food supply is good.

So why do humans, many other primates, and bats menstrurate? Could it be that if you only have one embryo at a time it is very important to avoid loosing an opportunity to breed. Many other mammals can "solve" the problem of missed pregnancy opportunities by having multiple embryos. 

I wonder if anyone interested in the sexual life of bats can throw any light on whether our U.K. species menstruate?. Due to their conservation status I am sure I would not be welcome to visit the Devon bat haunts to examine females bats to see if they were menstruating!!!

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