Initiators, leaders, and recruitment mechanisms in the collective movements of damselfish.

Explaining how individual behavior and social interactions give rise to group-level outcomes and affect issues such as leadership is fundamental to the understanding of collective behavior. Here we examined individual and collective behavioral dynamics in groups of humbug damselfish both before and during a collective movement. During the predeparture phase, group activity increased until the collective movement occurred. Although such movements were precipitated by one individual, the success or failure of any attempt to instigate a collective movement was not solely dependent on this initiator's behavior but on the behavior of the group as a whole. Specifically, groups were more active and less cohesive before a successful initiation attempt than before a failed attempt. Individuals who made the most attempts to initiate a collective movement during each trial were ultimately most likely to lead the collective movement. Leadership was not related to dominance but was consistent between trials. The probability of fish recruiting to a group movement initiative was an approximately linear function of the number of fish already recruited. Overall, these results are consistent with nonselective local mimetism, with the decision to leave based on a group's, rather than any particular individual's, readiness to leave.

Assessment and assortment: how fishes use local and global cues to choose which school to go to.

Animals that live in groups are known preferentially to associate with phenotypically similar individuals. Despite this, groups of mixed phenotypic composition are the norm rather than the exception in several systems in the wild and this, combined with the large sizes of some animal groups, makes accurate global assessment by a choosing individual more difficult. In this study, we investigated the role of local and global information in mediating shoal-choice decisions in three-spined sticklebacks (Gasterosteus aculeatus) by manipulating the positions and phenotypes of stimulus fish in relation to a focal fish. Focal fish were able to assess globally mixed shoals composed of individuals of different body-length classes, preferring to associate with shoals where the majority phenotype matched their own. When local cues were manipulated this preference disappeared, although overall shoal composition remaining constant. Finally, if both stimulus shoals had the same overall composition but differed in their local cues, then the focus fish chose according to which local fish was of matching body length. These findings indicate that both local and global information play an important role in mediating assessment and shoal choice in fishes.