A series of 'sex videos' of worms have revealed how sex shapes sperm, according to evolutionary biologists Lukas Scharer and his wife Dita Vizoso at the University of Basel in Switzerland.
The worms are simultaneous hermaphrodites: each has both male and female genitalia.
"In the lab they mate like crazy. Once, we saw a pair mate 40 times in an hour," Nature quoted Scharer as saying.
Scharer found that species that mate reciprocally, and then suck at the female genital opening, carry ornate sperm with a pair of long bristles emerging at the mid-point and a tail resembling a paintbrush.
The two theorized that if the hair and bristles are a means of avoiding sperm being sucked out after sex, then flatworms that don't mate with stylet-antrum penetration. So subsequent sucking won't have extra appendages on their sperm.
By observing a range of species, the team discovered that species that engaged in 'traumatic' sex have smaller sized sperm. They have stylets shaped like hooks with which they jab other worms anywhere in their bodies, and inject the sperm.
"I like this story because it's very unusual for anyone to have any coherent idea about the functional significance of sperm morphology," says William Eberhard, a biologist at the Smithsonian Tropical Research Institute in Costa Rica.
"This is one story out of the many, many, many stories we don't understand - it's a first step in the right direction."
The findings are published today in the Proceedings of the National Academy of Sciences.