Initiative, Personality and Leadership in Pairs of Foraging Fish Shinnosuke Nakayama*, Jennifer L. Harcourt, Rufus A. Johnstone*, Andrea Manica* Department of Zoology, University of Cambridge, Cambridge, United Kingdom Abstract Studies of coordinated movement have found that, in many animal species, bolder individuals are more likely to initiate movement and shyer individuals to follow. Here, we show that in pairs of foraging stickleback fish, leadership is not merely a passive consequence of temperamental differences. Instead, the act of initiating a joint foraging trip out of cover itself brings about a change in the role that an individual plays throughout the subsequent trip, and success in recruiting a partner affects an individual’s tendency to initiate the next trip. On each joint trip, whichever fish took the initiative in leading out of cover gains greater influence over its partner’s behaviour, which persists even after several changes in position (i.e. termination attempts and re-joining). During any given trip, the initiator is less responsive to its partner’s movements than during trips initiated by the partner. An individual’s personality had an important effect on its response to failure to recruit a partner: while bold fish were unaffected by failures to initiate a joint trip, shy individuals were less likely to attempt another initiation after a failure. This difference provides a positive feedback mechanism that can partially stabilise social roles within the pair, but it is not strong enough to prevent occasional swaps, with individuals dynamically adjusting their responses to one another as they exchange roles. Citation: Nakayama S, Harcourt JL, Johnstone RA, Manica A (2012) Initiative, Personality and Leadership in Pairs of Foraging Fish. PLoS ONE 7(5): e36606. doi:10.1371/journal.pone.0036606 Editor: Gonzalo G. de Polavieja, Cajal Institute, Consejo Superior de Investigaciones Cient´ıficas, Spain Received March 15, 2012; Accepted April 10, 2012; Published May 2, 2012 Copyright: 2012 Nakayama et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study was funded by a grant from the Association for the Study of Animal Behaviour to AM, a NERC studentship to JLH, and a fellowship from Japan Society of the Promotion of Science to SN. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. No additional external funding was received for this study. Competing Interests: The authors have declared that no competing interests exist. * E-mail: sn407@cam.ac.uk (SN); raj1003@cam.ac.uk (RAJ); am315@cam.ac.uk (AM) Introduction Studies of leadership have shown that within animal groups, some individuals are consistently more likely than others to initiate and direct collective movement. Most recent work has focused on the characteristics of leaders, identifying the traits that distinguish these influential individuals from the rest of the group – those more likely to lead may be dominant [1,2], better informed [3–5], hungrier [6–8], or simply temperamentally less inclined to follow others [9,10]. Regardless of the particular characteristics in question, the assumption is that leadership is the outcome of pre-existing differences among individuals in how they tend to behave and react to one another. In this paper, we focus not only on the characteristics of leaders, but also on the consequences of their actions. Specifically, we ask whether an individual that initiates coordinated group movement gains influence thereby, and alters the way in which others respond to it for the duration of that group movement. We also test whether leadership status during the current group movement influences initiation of the next group movement. If so, leadership is not just a passive outcome of pre-existing differences in temperament or physiological state. Instead, the act of successfully taking the lead itself brings about a change in the role that an individual plays within the group, both in the present and in the future. When individuals occasionally change roles between trips, their behaviour should then alter accordingly, and this effect should be observable over and above any persistent differences in behaviour attributable to temperament or physiological state. To test this possibility, we use continuous-time Markov models to infer the rules underlying joint behaviour in pairs of foraging stickleback fish. In previous work [9], we showed that individuals in a pair of foraging stickleback fish respond to each other’s movements based on their personality. Overall, the bolder individual was more likely to initiate joint trips out of cover, but that there were occasional changes in who initiated and who followed from one joint trip to the next. Here, we extend the Markov approach to assess whether and how the members of a pair alter their behaviour, and the ways in which they respond to one another, depending on which individual initiated the current joint trip out of cover. We also consider whether successful initiation of joint trips affects an individual’s propensity to initiate in the future. Materials and Methods Acclimation and Training Procedures of animal collection and maintenance are found in Appendix S1. The experiments described below are the same as reported in Harcourt et al. [9]. All experiments were run in larger experimental tanks (90630630 cm; Figure 1), with walls covered by white plastic sheets and the bottom lined with gravel to create a slope (12 cm water depth at one end, 2 cm at the other end). Each tank was divided lengthwise into two lanes with a white opaque partition, and each lane contained two plastic plants at the deep 2 end and one white square plastic plate (about 1.5 cm ) at the shallow end as a feeding site. A white plastic sheet (868 cm) was PLoS ONE | www.plosone.org 1 May 2012 | Volume 7 | Issue 5 | e36606