Abstract: Remembering individual identities is part of our own everyday social life. Surprisingly, this ability has recently been shown in two social insects. While paper wasps recognize each other individually through their facial markings, the ant, Pachycondyla villosa, uses chemical cues. In both species, individual recognition is adaptive since it facilitates the maintenance of stable dominance hierarchies among individuals, and thus reduces the cost of conflict within these small societies. Here, we investigated individual recognition in Pachycondyla ants by quantifying the level of aggression between pairs of familiar or unfamiliar queens over time. We show that unrelated founding queens of P. villosa and Pachycondyla inversa store information on the individual identity of other queens and can retrieve it from memory after 24h of separation. Thus, we have documented for the first time that long-term memory of individual identity is present and functional in ants. This novel finding represents an advance in our understanding of the mechanism determining the evolution of cooperation among unrelated individuals.

Abstract: Domestic horses are kept in so-called “fate societies” where they have to deal with frequent mixing. Several studies have evaluated and discussed the aggression level and injury risk during the introduction of horses into new groups, but nothing is known about the endocrine responses and thus if horses experience stress during introduction.
In this study we analysed the efficiency of four approved introduction techniques and evaluated the introduction of 30 horses into 11 different groups. Horses were introduced: 1) immediately, 2) after observing the new group for several days, 3) together with an “integration horse” after several days of observation, or 4) with a mixed strategy. Aggressive as well as positive social behaviour between the introduced horses and the group members were analysed the two hours following the introduction event. In addition, we focussed on the glucocorticoid production of the newcomer horses by measuring faecal cortisol metabolites (FCM) on the day of the introduction as well as the following three days.
For the four introduction techniques we found significant differences in the horses’ aggressive and submissive behaviour as well as in their total interactions. The introduction together with an integration horse led to significantly lower levels of aggression and less total interactions than the immediate introduction of single horses.
Horses which were introduced immediately or after an observation period showed significantly elevated levels of FCM on the first, second and third day after the introduction. For horses introduced together with an integration horse FCM were already significantly higher on the day of the introduction, indicating a stressful event before the introduction itself. In contrast, FCM levels were always very low when using the mixed technique.
In sum, horses have the ability to deal with conflict when they are introduced to new group members. The introduction event itself appears not to be as stressful as previously assumed. Standing together with an “integration horse” on a separate paddock and not being able to integrate immediately into a new group appears to be stressful for the newcomer. Based on the findings of our study we suggest to introduce new horses in group management together with a new group mate, a so-called “integration horse”. This would reduce the number of total social interactions as well as the aggression level. While this technique may be stressful for the newcomer, it lowers aggressive behaviour between the introduced horse and the group members and consequently reduces injury risks.