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.