Sperm Travels Faster Toward Attractive Females
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July 8, 2009 -- Females may be outwardly choosy when selecting sexual partners -- accepting or shunning mates in very public ways -- but males may get the last say in this battle of the sexes.
New research found that males can adjust the speed and effectiveness of their sperm by allocating more or less seminal fluid to copulations. The determining factor is whether the male finds the female attractive.
The study, conducted on red junglefowl, a director ancestor of chickens, adds to the growing body of evidence that males throughout many promiscuous species in the animal kingdom, including humans, can mate with many females, but chances of fertilization are greater when the female is deemed to be attractive.
Desirable female red junglefowl are easy to identify.
"Female attractiveness is determined by the expression of a sexual ornament -- the comb -- which is phenotypically and genetically correlated to the number and mass of eggs females lay," according to study co-authors Charlie Cornwallis of the University of Oxford and the Royal Veterinary College's Emily O'Connor.
For the study, published in the current Proceedings of the Royal Society B, the researchers collected natural ejaculates from dominate and subordinate red junglefowl males housed at the University of Stockholm. The males had either just mated with attractive or unattractive females.
The scientists next separated the sperm from the seminal fluid and analyzed the quantity and characteristics of both.
"There was a strong relationship between sperm velocity and the volume of the ejaculate sperm came from," Cornwallis and O'Connor determined, adding that males allocated "larger ejaculates to attractive females."
The mechanism behind this remains a mystery for now, but the scientists have an intriguing theory.
"Males may alter the velocity of sperm they allocate to copulations by strategically firing their left and right ejaculatory ducts, which can operate independently," they explained.
Stimulation from sexy, attractive females, therefore, leads to the double firing.
"Furthermore," they added, "differential firing of left and right ejaculatory ducts may contribute to how males strategically change the number of sperm in their ejaculates, a phenomenon that is widespread, but for which the mechanism remains unknown."
Similar findings were recently determined for African cichlid fish, whose males produce speedier sperm when females mate with many males in quick succession.
John Fitzpatrick of the University of Western Australia led the fish study while he was a graduate student at McMaster University.
"The first step in producing more competitive sperm was by influencing how much energy the sperm can produce," said Fitzpatrick. "Just like a mechanic could make a car drive faster by installing a better engine, evolution appears to act first on the engine that drives sperm movement."
While much of this process, including detection of attractive females, can occur on a subliminal basis, human males hoping to improve their fertility would be wise to not smoke marijuana, which University of Buffalo research shows reduces amounts of seminal fluid and lowers total sperm counts.
At the annual meeting of the European Society of Human Reproduction and Embryology, which recently took place in Amsterdam, Australian obstetrician and gynecologist David Greening announced his finding that daily sex, or daily ejaculation, for seven days improves men's sperm quality by reducing the amount of DNA damage, since lengthy exposure to oxygen in the testicular ducts may harm cells.
Both Greening and the British scientists hope future research will better identify how males adjust the sperm and seminal fluid in their ejaculates, and how this affects fertility rates.
Sauce
As if anyone needed an excuse to rub one out, science is at your side to help your argument. :wink:
