Pro-social effects of oxytocin and its homologues are well-documented in birds 
and mammals. However, in fishes, the effect of isotocin, the homologue of 
oxytocin, on social behaviour is less clear. Studies in fishes have generally 
shown no effect of isotocin on social behaviours or even an anti-social effect. 
In our study, we measured association preference for conspecifics in 92 adult 
guppies (46 females and 46 males), half of which were injected with isotocin and 
the other half with an isotocin antagonist. We found that individuals injected 
with isotocin spent 29% more time associating with conspecifics than individuals 
injected with an isotocin antagonist. The effect of isotocin on association time 
did not differ between males and females. Our study provides some of the first 
evidence of a pro-social effects of isotocin in a fish and suggests that in 
fishes, isotocin may have a homologous role to oxytocin, at least in promoting 
shoaling behaviour.