Swedish scientists have found that female brown trout fake orgasms in about half of their spawnings.

Erik Peterrson of Sweden's National Board of Fisheries said out of 117 spawnings they observed, 69 were false orgasms.

During a normal spawning, the female digs a gravel pit for the eggs. When she prepares to mate, she crouches down to protect the nest, opens her mouth and starts to quiver intensely.

The male then swims alongside the female, assumes the same position,opens his mouth and starts to quiver as well.

After a few seconds, the female releases her eggs and the male fertilizes them.

But the researchers found that sometimes the female fakes it and doesn't release her eggs when the male releases his sperm.

Peterrson said the sperm have to be directly over the eggs or the fertilization rate is very low. He thinks the female fakes it if the male isn't in exactly the right position.

Confused males

"If she feels he is not in the right position or timing, she just stops the process," Peterrson said to CBC Radio's As It Happens. "But the male, he is so excited that he misinterprets the female's cues and goes the whole way. He's a little bit tricked there."

He added the male looks a little confused because when the female fakes orgasm, she doesn't cover the eggs as she would during a normal spawning.

Instead, she may dig some more to prepare the gravel pit for the next mating in about 30 minutes.

And it seems trout aren't alone. Researchers first saw this in Atlantic salmon in 1954. They now think it's common in all salmon.