London, December 8: Researchers at the University of Potsdam in Germany have discovered that male molly fish do not like being spied on by another male fish, when it comes to mating.

The researchers say that male molly fish of the species Poecilia Mexicana change their mind as to which females they prefer, when they come to know that another male fish is watching them.

Mate choice experiments have shown that males usually prefer to mate with large females who produce more eggs.

During the study, the researchers observed that a male fish would normally spend 80 per cent of its time near large females, and only 20 per cent near smaller ones.

However, upon induction of a glass container carrying another male fish to watch the show, the first molly began to pay equal attention to both large and small females.

The researchers also noticed that being watched by a green swordtail (Xiphophorus hellerii) did not faze the mollies at all, for they only slightly reduced their preference for large females.

"We think that the molly does this to avoid sperm competition, " New Scientist quoted lead researcher Martin Plath as saying.

"It's likely that the other male will share the preference for large females, so it makes sense for the molly to not invest all his sperm into one female, " he added.

Plath reckons that another possible explanation for this change in mate choice could be that the desire to fight a competitor distracts the very aggressive mollies from their true mate preference.

He, however, does not consider this explanation to be very strong.

"We have just repeated the experiment with a completely non-aggressive group of fish and got the same results, " he says.

Peter McGregor, a researcher at Cornwall College in the UK who studies communication networks in animals, says that Plath’s findings indicate how sensitive mate choice can be to even subtle interactions between individuals.

"The other male cannot get actively involved and still his passive presence has this dramatic effect", he says.

This is the first time that it has been shown that mate choice can be reversed by an observer.

Previous studies have shown that female guppies and humans prefer males that are popular with other females, and that human males adjust their sperm production to perceived competition.

"Mate preferences are often seen as drivers of evolutionary change. These results show how important it is to consider them in the appropriate social context, " says Plath.

The findings have been reported in the journal Animal Behaviour. (ANI)