The question of why most animals have sex with other animals is answered today by a paper published in Nature (no snickering at the back).

This issue isn’t as simple as you might think. The real problem is that men are actually rubbish, note the paper’s authors Levi Morran, Michelle Parmenter and Patrick Phillips, who all work at the University of Oregon, Eugene.

Males “do not directly contribute offspring”, they say, so animals that self-fertilize should have numerical advantage over their sexually reproducing relatives. On the flip side reproducing sexually does mean you avoid inbreeding and you can adapt faster to environmental changes.

To get a better understanding of this, the researchers genetically tweaked C. elegans worms to be either sexually reproducing hermaphrodites (‘outcrossing’) or ‘selfing’ animals. They then exposed these animals to a chemical that upped their mutation rate. They also tested how well they adapted to a nasty bacterial pathogen.

Selfing animals showed a decline in fitness and were less good at adapting to the pathogen. The bottom line: animals that don’t do it on their own did better when forced to mutate or adapt to a new environment.

“Many scientists have argued that outcrossing has evolved to avoid the genetic consequences of inbreeding, while others have emphasized the role that outcrossing plays in generating the genetic variation necessary for evolutionary change,” says Morran (press release). “Our work shows that both of these factors are important.”

Image: hermaphrodite nematode C. elegans / Patrick Phillips, University of Oregon