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Written By Elena Holmes

More to the point – will swimming with your Aunt Flo, when on the rag, surfing the crimson tide, when having the painters in, with your cousin Red (etcetera), make you more likely to be eaten during shark week?

Common myth suggests that sharks, whose apparent and oft-repeated superpower is their superior sense of smell, can detect blood from a mile away and will therefore smell your appetising menstrual blood and come zooming in to make a meal of you and teach you to dare to be a woman in their territory.

There are no definitive studies to tell us whether or not sharks attack menstruating humans more than other humans, and the figures we do have show that ninety percent of recorded shark attacks have involved men, according to Icthyologist George H. Burgess.  

However, this probably doesn’t mean sharks are gender-biased, but simply that, historically, more men have been engaged in marine aquatic activities. We must look, then, to science.

Whilst sharks can detect prey odours as minute as one part per billion, studies of shark behaviour suggests that human blood to sharks may not be as much of a catnip-to-cats as we believe it to be. Vice’s interview with Dr Tricia Meredith, who studied the olfactory responses of sharks, informed readers that human meat is ‘not a delicacy in shark circles’, to the extent that, when investigating shark sensitivity to blood, the scientist used amino acids because ‘they are prey-related odours and human blood is not.’ The fact, then, that a shark may be able to smell you – and your blood – does not mean it believes you to be a suitable food item.

Another myth-busting point to consider is the different properties of menstrual blood in comparison to other blood.  Menstrual ‘blood’ is a complex fluid that is chemically very different from other blood, and includes ‘old’ (hemolyzed) blood which, according to Richard Martin, a shark fisheries biologist, is uninteresting to sharks. Indeed, a 1992 study by Jacalyn Robert of Texas Tech University claimed that the ‘old’ blood may ‘instead act as a shark deterrent’, and concluded that ‘there is no evidence of increased shark interest in a menstruating female.’

As well as this evidence to suggest that sharks do not find periods particularly taste-bud-tingling, the complex makeup of menstrual fluid, which includes, according to The New Our Bodies, Ourselves, “cervical mucus, vaginal secretions, mucus and cells and endometrial particles as well as blood (sometimes clotted),” would mean that a shark, in order to detect menstrual blood, would need to sense a very weak concentration of blood surrounded by other substances. Dr Chris Lowe, head of California State University, Long Beach’s Shark Lab, told Outside magazine that:

‘The amount of blood that is produced during menstruation is so small that it becomes background with all of the other components that are in the water. It would be very difficult for a shark to localize that.’

Powerful as a shark’s sense of smell is, then, their nasal nosiness has been much exaggerated when it comes to their ability to track you down by your menstrual flow alone.  

To add to their lofty sensory repertoire, sharks are also very electro-receptive, meaning they can detect the electromagnetic fields in water that are emitted by any animal in the ocean. This means that for those of us worrying about how tantalising our menstruation is for sharks, we need to consider the electric conductivity of the blood we are releasing. The good news, however, is that red blood cells are not highly conductive, and, whilst plasma (also contained in blood) is, there are only very low plasma levels present in menstrual blood. As well as this, there is the fact that a tampon would, according to Dr Kajiura from Shark Lab, ‘be sufficient to short the electrical circuit between you and the water’, thereby removing the potential for any period-haters to caw triumphantly that menstruation in water transforms you into a floating battery broadcasting news of your flesh to sharks home and abroad.

It is also important to note that sharks only use their electro-receptive powers for close sensory perceptions approximately half a metre away. This means that the shark, if it is sensing you with that sneaky method, is already very close – and menstrual cramps are no longer your biggest problem.

All of this means that sharks, whilst able to detect blood, would find it very difficult to locate a human simply by following the scent of specifically menstrual blood.  

There are no studies to support the claim that menstruating humans are more attractive to sharks, and some studies even suggest that periods may actually repulse sharks!

Perhaps you all ought to be fitting in some shark-swimming when it’s that time of the month after all! Most reassuring of all is the fact that sharks are not specifically attracted to human blood as a prey indicator. If a shark is near you then it doesn’t matter what state your uterus is in or whether or not you have one, you just need to be a good swimmer and hope that s/he’s not particularly peckish!

And so, to all adventurers out there who menstruate, I call on you to get out there and swim if that’s what you want to do, and make sure you fear sharks only as much as all those non-menstruaters. 

 

References

‘Anatomy and Menstruation’, Our Bodies, Our Selves:

http://www.ourbodiesourselves.org

Bethy Squires, ‘Can Sharks Smell Period Blood and Will They Eat You Because of It?’ 22/2/16, Vice:

https://broadly.vice.com/en_us/article/can-sharks-smell-period-blood-and-will-they-eat-you-because-of-it

Erin Beresini, ‘Do Menstruating Women Attract Sharks?’, Outside Magazine, 3/9/12:

https://www.outsideonline.com/1784051/do-menstruating-women-attract-sharks

Rachel Feltman, ‘No, menstrual blood does not attract sharks’, Popsci, 26/5/17:

http://www.popsci.com/surfer-period-blood-shark-attack

‘Shark Lab’, Elasmobranch Research Laboratory

http://www.science.fau.edu/sharklab

‘Sharks and Menstruation’, Florida Museum:

https://www.floridamuseum.ufl.edu/fish/isaf/reducing-risk/menstruation

Shark Research Committee

http://www.sharkresearchcommittee.com/index.html

The Shark Research Institute

http://www.sharks.org