Women looking for a more productive sex life should beware of men with different size feet or one ear smaller than the other.
According to scientists who have measured almost everything on the male body that comes in twos, there is a definite association between semen quality and the degree of bilateral symmetry. The greater the difference between one side of the body and the other, the poorer the quality will be.
And that may explain why women are attracted to men with symmetrical faces, and why men who are considered to be attractive, such as Mel Gibson, often have big families.
The seemingly obscure link between sperm quality and relative ear size is the result of early events in the womb. These skeletal parts of the body are thought to develop at about the same time as the gonads and urogenital system.
The theory is that if something goes wrong at that time – for example, if the mother's diet is poor, or if she is exposed to disease or infection – this will affect the development of the foetus. As a result, say scientists, visible body asymmetry may be a marker for less obvious problems occurring during the development of sexual functioning.
Researchers at the University of Western Australia have looked in detail at the theory and concluded that it holds true. They report that: "Random deviations from symmetry can arise during development when factors interfere with the ability of an organism to [develop] equally on both sides of the body. This generates what is known as fluctuating asymmetry or FA.''
To test the theory, the scientists measured the height and weight, the ear length, wrist diameter, elbow diameter, ankle diameter, foot length and foot width of a group of volunteers aged 18 to 35. The right and left sides of each man were measured independently to the nearest 0.1mm, using precise digital callipers. For each area under investigation, an FA score was calculated by subtracting the size on the left side of the body from that on the right.
Three different measurements – ear length, the length of the fourth digit, and foot length – were then used to calculate an overall FA score. Finally, each man's score was compared with his semen value.
"We found significant negative relationships between FA and total numbers of sperm, and the motility [mobility] of sperm," the researchers continue. "The results show that developmental instability may indeed be associated with poor semen quality in the general population."
The researchers also believe that the link may explain why women are unconsciously attracted to symmetrical faces.
"Asymmetry is a marker for developmental stability,'' says Professor John Manning, professor of biological psychology at the University of Central Lancashire. "We all have small differences, but when you add them together you get a composite score. Fluctuating asymmetry seems to be a marker for a number of surprising things, and one of them appears to be how much sperm is produced and how efficient the sperm are. It may be the evolutionary reason why women are attracted to symmetrical-looking men.''
Another marker for poor sperm is the ratio between the ring and index fingers on a man's hand. Men usually have longer ring than index fingers, and the longer the ring finger, the more likely it is that the man has been exposed to higher levels of testosterone before birth.
There is also a link between finger length and sperm production. Researchers have found that very long ring fingers are associated with more sperm as well as more active sperm.
Other researchers have found links between FA and behavioural traits. Psychologists at Dalhousie University in Nova Scotia, Canada, discovered that FA is linked to feelings of romantic jealousy. Men whose bodies were asymmetrical were found to be significantly more jealous of their mate.
And the theory does not apply solely to humans. Scientists in Canada report that stags with lopsided antlers find it harder to attract a mate than their more symmetrical rivals.
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