Standing at the back of the line while several other men have sex with your wife would be more than human husbands could stand, but in the animal and insect kingdom he who stands at the end of the line is most likely to produce offspring.

From bonobo chimpanzees to fruit flies, many female animals mate with multiple partners that often queue up for the event. Studies have shown that the the last male to mate with a female is the most successful at impregnating her. Nobody has understood why.

Two researchers at the University of Exeter in the United Kingdom, David Hosken and David Hodgson, think they have discovered the answer.

The last male can take advantage of a more ��sperm-friendly�� environment created by males that have copulated before him, according to a new model put forth by David Hosken and David Hodgson.

Males ejaculate hundreds of millions of sperm into the female reproductive tract, but most don't make it to the egg for fertilization. In mammals, just .001 percent of the ejaculated sperm reach their goal.

��We know that the reproductive tract in females can be a nasty place for sperm," said Hodgson.

Hodgson said the acidity in the females reproductive tract can kill many sperm, and scientists think some females�� immune systems attack sperm as a ��foreign object."

The seminal fluid in a male��s ejaculate helps to buffer the acidity. By waiting in line, males could exploit the ejaculate from other males, giving their sperm a less hazardous path into the female's uterus to fertilize eggs.

Going last also means the male would not need as much sperm in his ejaculate because there's a higher probability of that sperm being successful.

��They may be able to adjust their behavior and their investment in ejaculate to maximize their probability of being a father and minimizing their expenditure of resources," Hodgson said.

The finding seems to apply to any organism in which females mate with multiple partners in rapid succession, out in the open where others can watch. The mating needs to be relatively rapid for the physical effects of a prior male's sperm to remain.

This includes females of many fruit-fly species, which re-mate within an hour. The male yellow dung fly will interrupt and take over a copulation, removing and replacing their rivals.

Hodgson said he couldn't think of any human examples.

Source:Xinhua/Agencies