a role in offensive aggression in agonistic behaviour, serotonin also plays a role in aggressive behaviour in rodents as well as similar effects in humans. Research has shown that increased levels of serotonin or stimulating serotonic receptors in rodents corresponds with decreased agonistic behavioural display, such as behaviours like attacking and biting. Male resident hamsters, which typically always display stereotypical offensive agonistic behaviours, display a significant decrease in bite attempts toward intruder males when treated with a serotonin reuptake inhibitor called fluoxetine. Vasopressin and serotonin both play significant roles in agonistic behavioural displays, and understanding the interaction of these two opposing neurotransmitters is important in fully understanding the neurobiology of agonistic behaviour. It is understood that vasopressin enhances aggression in agonistic displays due to increased activity in the neural pathways that are associated with increased flank marking and the offensive aggression demonstrated in resident hamsters in the presence of an intruder. This neural pathway that enhances aggression is subdued by the presence of serotonin. It is hypothesized that serotonin acts as an antagonist to vasopressin by eliciting its effects on vasopressin-sensitive neurons and therefore inhibiting these neurons. Steroid hormones are also associated with offensive aggression behaviour. Androgens in particular have well documented effects on enhancing aggression in male rodents, and testosterone injections into the septum and medial pre-optic area of castrated mice greatly increased offensive aggression. Glucocorticoids also have reported effects on agonistic behaviour in mice, though these effects are not as thoroughly understood as effects of androgens. Research has demonstrated that in mice that have been defeated in agonistic encounters have elevated levels of corticosterone, which appears to enhance submissive behaviour and therefore has opposing effects on agonistic aggressive behaviour. Prediction of winning The type of agonistic behaviour observed, whether it be aggressive or submissive, all depend on the likelihood of winning. For instance, size is usually a good predictor of fighting success, and many animals will display to flaunt their size. Animals are better able to assess their next form of agonistic action by judging the opponent's size and if they are likely to win a fight if a physical altercation were to occur. Example: Stalk-eyed flies (Diopsidae) In aggressive behaviour by male stalk-eyed flies the males "square off" by displaying their eyes. Females show a strong preference for mating with males with longer eyestalks. Due to the female preference, males have evolved to compete with each other