We all act a bit differently when we know someone’s watching. When your boss walks into a room, perhaps you’re more likely to do a good job, (or if you’re anything like me, more likely to screw up). Maybe there’s a joke that you know is hilarious with one group of friends, but you don’t plan on telling at your parents’ dinner party. We constantly change our behaviour based on who is around us at the time, often without even knowing it ourselves.

A context in which you often hear about people changing their behaviour is with regards to sex, or rather, trying to obtain it. When I type into google ‘he acts differently…’ I get the top hit ‘… around his friends’. This phenomenon of a man or woman behaving differently towards their partner depending on who else is around seems to be fairly common.

Amazingly, it seems that fish do the same thing. Scientists recently found that live-bearing fish (so-called because they give birth to little free-swimming fish rather than eggs) also change their behaviour towards partners depending on who is watching 1.

Much like humans, males of this fish species (Poecilia mexicana) show that they like a female by spending time with her. However, males will spend less time around their female mate if there is another male watching who they don’t know. This is because the courting male doesn’t want to draw attention towards his female, as this could result in unwelcome competition from the new male.

However, from our own experience we know that not every audience affects your behaviour in the same way. You might be more enthusiastic about getting up for a karaoke Bon Jovi impression if you’ve got a group of close friends watching, rather than a group of work colleagues. The same is true for this fish. When the male knew the rival male watching him, and also knew that he was sexually active, he saw him as more of a threat. As a result he spent less time with the female. However, when the male knew that the other voyeur male was not sexually active (and therefore not a threat) he didn’t change his behaviour in his presence.

Whether or not the rival male was sexually active was not the only thing males paid attention to. If the watching male was larger, and therefore more of a threat, the male changed his behaviour more than if the new male was small and less threatening.

So, next time you hear someone say ‘I like them, but they just behave so differently when they’re around…’ you can smile and know there’s at least one fish who’s going through the same thing.

1 Bierbach, D., Girndt, A., Hamfler, S., Klein, M., M?cksch, F., Penshorn, M., Schwinn, M., Zimmer, C., Schlupp, I. & Streit, B. (2011) Male fish use prior knowledge about rivals to adjust their mate choice. Biology Letters. doi:10.1098/rsbl.2010.0982