Sperm 'clock' may help rape cases

It hoped the research could help rape investigations

Forensic scientists say they may be able to use sperm's "death rate" as evidence of when a rape took place.

They found that the chemical in spermicidal condoms - often used by rapists, causes sperm cells to die at a steady rate.

The team from the National University in La Jolla, California, says its findings could help corroborate the evidence of a victim of defendant.

The research is reported by New Scientist magazine.

In reality the condoms would not necessarily be found in an environment where they were at room temperature Forensic Science Service spokesman

Spermicidal condoms may be left behind at the scene of the crime, and the researchers from the National University wanted to look at whether the spermicide nonoxyl-9 might affect DNA which could be used to identify rapists.

They found that it did not, but a surprise finding was that when sperm were incubated with nonoxyl-9, their numbers appeared to fall at a steady rate.

'Not a crime scene'

The researchers then collected semen samples from five volunteers and put half into spermicidal condoms and the rest into regular condoms.

They then took samples from the condoms, which had been kept at room temperature for either 30 minutes, one hour, two hours and up to 72 hours.

Fluorescent staining was used to estimate the number of sperm cells which remained after each period.

It was found that the proportion of live sperm cells in normal condoms remained constant, at around 15%, over the 72 hours.

But the number of viable sperm in nonoxyl-9 began at 40% and fell to 6% by the end of the 72 hours.

The researchers, who presented their findings to the American Academy of Forensic Science meeting in New Orleans in February, suggest this could be because the spermicide damaged other cells in semen beyond recognition, so the live sperm cells appeared to make up a larger proportion of the total.

Robert Blackledge of the Naval Criminal Investigative Service in San Diego, California, told New Scientist that being able to determine the age of a semen sample could be invaluable in cases such as date rapes, where there may have been previous instances of consensual sex, or if several condoms are found at a suspect's home.

A spokesman for the UK's Forensic Science Service, which carries out work for police forces across England and Wales, told the BBC News Website: "The tests mentioned in the article were carried out in laboratory conditions, however in reality we do not know what may have happened to a condom at a crime scene.

"Weather conditions may have affected it so it is important to remember there are a lot of variables involved.

"It would be important to see the what effect temperature had on these experiments as in reality the condoms would not necessarily be found in an environment where they were at room temperature."