Prior experience and contest outcome: winner effects persist in absence of evident loser effects in a parasitoid wasp

The experience of a prior conflict can affect animals' performance during a later contest: a victory usually increases while a defeat decreases the probability of winning a subsequent conflict. These winner and loser effects could result from a reassessment by contestants of their perceived fighting abilities. Game-theoretic models based on this assumption predict that a loser effect can exist alone or in the presence of a winner effect, but a winner effect cannot persist alone, at least when contestants are young and without experience of contest. Moreover, when both effects coexist, the loser effect is expected to be of a greater magnitude and last longer than the winner effect. To date, these predictions have been supported by empirical evidence. Here, we show for the first time that a winner effect can exist in the absence of any evident loser effect, in a parasitoid wasp, Eupelmus vuilleti, when fighting for hosts. It consequently raises questions about the possible mechanisms invo...