initiations (16 out of 20 pairs have negative scores, i.e. non- random clumping, in a runs-test; binomial test: P = 0.012). Shy individuals who initiated a trip out of cover but failed to recruit their bold partner as a follower were subsequently less likely to initiate the next trip compared to when they were successful at recruiting (q2,9,q4,9, P,0.001). On the other hand, bold individuals were insensitive to failure in recruiting a follower (q1,5 = q3,5, P = 0.176). Interaction between Social Roles and Individual Temperament Including temperament scores of both the bold and the shy individuals as covariates in the model led to a significant improvement in fit (x248 = 1085.8, P,0.001; DAIC = 989.8), and was superior to a model with the temperament score of only one individual included as a covariate (bold temperament alone 2 x 24 = 364.5, P,0.001; DAIC = 316.5; shy temperament alone x224 = 839.4, P,0.001; DAIC = 791.4). Moreover, when the effects of covariates were constrained so that an individual’s temperament affected its own transition intensities but not its partner’s (i.e. the effects of an individual’s temperament score on the transition intensities of its partner were fixed at zero), there was a significant decrease in the model’s explanatory power (x224 = 362.0, P,0.001; DAIC = 314.0). As shown in Figure 3, bold individuals with higher temperament scores were more likely to be followed by their partner (95% CI: 0.270,b5,6,0.425, 0.425,b7,3,0.629, 0.178,b12,4,0.654, where bi,j denotes the log-linear effect of the temperament score of the bold individual on the transition intensity from state i to state j). Also, bold individuals with higher temperament scores were more likely to ignore their partner’s attempts to terminate a joint trip (0.196,b8,6,0.423, 0.058,b11,10,0.351). Finally, bold individuals whose partners had low temperament scores were more likely to initiate trips (21.079,s1,5,20.224, si,j denotes a log-linear effect of a temperament score of the shy individual from state i to state j). During trips initiated by their partner, shy individuals were less likely to return to cover if their partner had a high temperament score (20.201,b6,8,20.079), but no such effect was present during trips that they had initiated themselves (b10,11 = 0; P.0.05). On the other hand, shy fish were always more likely to follow their partner back to cover if the latter had a high temperament score, irrespective of who initiated the trip (0.425,b7,3,0.629; 0.178,b12,4,0.654). Bold fish showed a reduced tendency to return to cover if paired with individuals with low temperament scores only during trips they had initiated themselves (20.868,s6,7,20.117), but not during trips initiated by their partner (s10,12 = 0; P.0.05). Discussion Our results show that the bold fish in a pair generally had a greater propensity to leave cover than did the shy fish, confirming that individual variation in temperament is expressed consistently between asocial and social contexts (as also shown by [15–20]), and can be a predictor of leadership [17,21–24]. Moreover, our results indicate that the fish do not only respond to the current location of their partner, but also adopt different roles depending on which individual initiated the most recent emergence from cover, and whether or not its partner has yet joined it (Figure 2). Both members of a pair become more responsive to their partner’s behaviour when the latter has taken on the role of leader, and less responsive when they led the way out of cover themselves. An individual’s status as leader of the most recent trip out of cover can Initiative, Personality and Leadership in Pairs be retained over multiple changes in location within that trip (0–18 per joint trip, with mean 0.8 6 0.1), and social roles are reset only when both individuals return to safety and the former follower initiates a new trip out of cover. Our results also show how pairs exchange social roles between trips. Both members of a pair were more likely to initiate a new trip out of cover if they had initiated the previous trip and successfully recruited their partner as a follower, than they were if it was their partner who had initiated the previous joint trip. Successful individuals tend to initiate more trips, and this gives them even more chances to be a successful initiator, exaggerating any underlying difference in temperament. This pattern may help to explain the finding of Harcourt et al. [9] that differences in temperament between members of a pair are reinforced by positive social feedback. There was also an influence of relative temperament on an individual’s response to success or failure in recruiting their partner as a follower when they led a trip out of cover. Shy individuals were less likely to initiate the next trip when they failed to recruit their bold partner as a follower in the most recent trip, whereas bold individuals were insensitive to failure in recruiting a follower. Again, this effect may help to explain the finding of Harcourt et al. [9] that bold individuals were less sensitive to a partner’s presence and behaviour than were shy individuals, as also found in other studies [13,16,19,20,25,26]. Models of coordinated movement generally focus on the maintenance of cohesion while groups are on the move [27–30]; within moving shoals, cohesion arises from local responses to neighbours, with individuals in the front exerting stronger influence [31–33] (but see [34]). In our experiments, where we tracked pairs of individuals over multiple movements, it is clear that the act of initiating a joint trip confers greater influence on individuals’ future movements, and that this effect can persist through multiple changes in position, until both pair members return to the safety of cover (the full model yielded a significantly better fit than the ‘memory-free’ model and the ‘one-step memory’ model). In other words, influence is not only a consequence of current position, but depends on an individual’s role (during a particular joint trip) as initiator or follower, which in turn depends on which pair member was the first to leave cover. Temperamental differences exert a persistent influence on behaviour, but leadership changes dynamically on a much shorter time-scale, with individuals altering their responses to one another as they exchange roles between trips. Such alterations in behaviour are not attributable in this case to changes in energy reserves (as suggested in [8,14]), because no food was supplied during the experiments, and because there were negligible changes in the energy reserves of the fish over the short observation period of our study. In addition, fish were all of similar size (4565 mm in standard length), and had identical feeding histories over the pre- trial period, minimising possible differences in energy budget (see Materials and Methods). More generally, such alterations cannot be explained simply by fluctuations in any aspect of an individual’s motivational state (e.g. periods of higher or lower activity), because they depend upon success or failure in recruiting a partner during joint trips. Moreover, failure to recruit a partner influenced the subsequent behaviour of individuals differently depending on their temperaments. Many previous studies have reported short-term changes in the identity of the leader between trips within animal groups [35–39]. None that we are aware of, however, have shown that these shifts in social role between trips are associated with changes in the ways that individuals respond to one another. Harcourt et al. [11] conducted an experiment similar to the one described here, but in which the fish were offered a choice between two alternative, PLoS ONE | www.plosone.org 4 May 2012 | Volume 7 | Issue 5 | e36606