Current Zoology 57 (4): 449/452, 2011 Received Dec. 03, 2010; accepted Feb. 08, 2011. , Corresponding author. Current address: Javier delBarco-Trillo; Duke University, Department of Evolutionary Anthropology, 128 Biological Sciences Building, Box 90383, Durham, NC, 27708, USA; delbarcotrillo@gmail.com; 919-660-7364 © 2011 Current Zoology Effect of losing a fight on later agonistic behavior toward unfamiliar conspecifics in male Syrian hamsters Javier DELBARCO-TRILLO * , Robert E. JOHNSTON Cornell University, Department of Psychology, Uris Hall, Ithaca, NY 14853, USA Abstract In many species, agonistic interactions result in social relationships that are stable over time. In Syrian hamsters, two unfamiliar males that are placed together will fight vigorously and a clear winner/loser relationship is usually established. In sub- sequent interactions, the loser will flee soon after detecting the familiar winner. Here we tested the hypothesis that losing a fight with a conspecific will affect future agonistic interactions not only toward that individual (i.e., the familiar winner) but also to- ward unfamiliar conspecifics. To test this hypothesis we paired two Syrian hamster males in three trials on one day in which the loser had the opportunity to escape the winner. The next day the loser was paired with an unfamiliar male, also for three trials. If he lost again, he was tested on a third day with a third unfamiliar male. Subjects were those males that were losers on all three days. The latency to escape on the first trial on Days 2 and 3 was significantly shorter than on the first trial on Day 1, indicating that losing against the first male affected the response toward unfamiliar males. However, the latency to escape on the first trial on Days 2 and 3 was significantly longer than that on the third trial on the preceding day, indicating that a loser treats unfamiliar males differently than a familiar winner. These results suggest that a defeat during an interaction with one male affects later ago- nistic behavior towards other, unfamiliar males [Current Zoology 57 (4): 449–452, 2011]. Keywords Agonistic behavior, Loser, Hamsters, Mesocricetus Among vertebrates, dominant-subordinate relation- ships between individual animals that are familiar with one another are generally stable over time (Hsu et al., 2006; Graham and Herberholz, 2009). For example, in Syrian hamsters, two unfamiliar males placed together in a neutral arena will fight vigorously during their first interaction and a clear winner/loser relationship will be established (Payne and Swanson, 1970; Bath and Johnston, 2007). In subsequent interactions, the loser of the first fight will flee soon after detecting the winner of the first fight (Petrulis et al., 2004; Lai et al., 2005; Bath and Johnston, 2007). Losers may show avoidance of the familiar winner for at least one week (Lai and Johnston, 2002). In addition, losers that are exposed to odors and other cues of the familiar winner one day after a fight showed greater neural activity in several relevant brain areas compared to losers exposed to an unfamiliar male (Lai et al., 2004). That is, losing to one particular male alters brain activity and behavior towards that familiar winner in later encounters. The studies cited above in- dicate (a) that the loser is able to recognize the winner on the second and subsequent interactions between the same two individuals, and (b) that such recognition in- fluences the outcome of another agonistic interaction with that individual (i.e., he would again be the loser). What is not apparent from the previous studies is whether losing to one male will affect the future agonis- tic behavior of the loser toward other unfamiliar con- specifics. Losing to a conspecific may either affect fu- ture interactions with only the familiar conspecific, or it may also affect interactions with other, unfamiliar con- specifics. In theory, there are three possible outcomes between a loser and an unfamiliar conspecific male. First, a loser may behave as a loser toward a familiar winner but act as a non-loser toward unfamiliar con- specifics. That is, the first interaction with an unfamiliar conspecific may be similar to the first interaction with the familiar winner in terms of duration of aggression and latency to escape. Second, a loser may behave as a loser toward the familiar winner and also show a sub- missive behavior toward unfamiliar conspecifics. For instance, the loser, when paired with a unfamiliar con- specific, may not escape as fast as it would in response to the familiar winner, but the loser may escape faster than a naïve individual would. Lastly, a loser may be- have as a loser toward any conspecific, including unfa-