Fish shoal composition: mechanisms and constraints

An experimental study of social attraction and spacing between the sexes in sheep

Most ungulates are gregarious species and outside the mating season are typically observed in single-sex groups. However little is known about the mechanisms underlying social segregation between sexes. We investigated the effect of conspecific attraction on individual spacing between unrestrained merino sheep Ovis aries and confined conspecifics. We considered differences between males and females and whether attractiveness of the confined conspecifics depends on their sex. A series of binary choice experiments was conducted in a large outdoor arena, located in pastures. One or two stimulus animals were placed in small individual cages (1.5 m×1 m) on opposite sides of the arena. Sheep were tested with one fixed peer of the same or opposite sex vs an empty cage, and with two fixed peers of either the same sex as themselves, or one male and one female. Sheep in a control condition were exposed to two empty cages. In all of the test conditions, confined sheep were highly attractive. Males were more attracted by single stimulus peers of the same than the opposite sex, whereas females did not display such a preference. Sheep confronted with two restrained conspecifics tended to remain between the stimuli. This also occurred when the stimuli were of opposite sex, although the males tended to be located nearer the same-sex peer. Our findings can explain the strong aggregative behaviour of merino sheep, but also the social segregation previously observed in a mixed-sex group through higher attraction for same-sex than opposite-sex peers in males.

Consensus decision making in animals

Individual animals routinely face decisions that are crucial to their fitness. In social species, however, many of these decisions need to be made jointly with other group members because the group will split apart unless a consensus is reached. Here, we review empirical and theoretical studies of consensus decision making, and place them in a coherent framework. In particular, we classify consensus decisions according to the degree to which they involve conflict of interest between group members, and whether they involve either local or global communication; we ask, for different categories of consensus decision, who makes the decision, what are the underlying mechanisms, and what are the functional consequences. We conclude that consensus decision making is common in non-human animals, and that cooperation between group members in the decision-making process is likely to be the norm, even when the decision involves significant conflict of interest.

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.

Conspecific familiarity and shoaling preferences in a wild guppy population

In nature, social groups of animals are typically nonrandom associations of individuals. Such group associations may in part be based on social familiarity among individuals, acquired through repeated past encounters and behavioural experiences with each other. The factors that potentially affect the tendency to associate with familiar individuals remain poorly understood. Here, we investigated whether shoaling preferences based on presumed familiarity occur in wild adult guppies (Poecilia reticulata) originating from a Trinidadian population that experiences relatively low predation intensity from fishes and whether any such preferences differ between the sexes. When tested shortly after being collected in the field, neither males nor females clearly preferred to shoal with familiar same-sex conspecifics (collected from the same pool in the river) over unfamiliar ones (from different pools). Importantly, our results differ from those previously reported for guppies originating from the same population as ours and for a high predation risk population in Trinidad, suggesting that the development of social familiarity and its role in the formation of shoals in nature vary both within and among populations in the guppy. We provide plausible explanations for these observed populational differences in shoaling preferences in the guppy and discuss certain ecological factors that may affect the development and expression of shoaling preferences based on familiarity in nature.

Association patterns and shoal fidelity in the three-spined stickleback

We investigated pairwise association patterns and shoal fidelity in free-ranging, individual three-spine sticklebacks (Gasterosteus aculeatus) by capturing entire shoals of sticklebacks and tagging each shoal member with a unique individual mark before releasing the shoal at the point of capture. We recaptured tagged fishes in the study area on five subsequent days, noting their identity, their location and the individuals with which they were associated. Stable partner associations between fishes were observed which might provide the basis for shoal fidelity via social networks. These results suggest the potential for the kinds of inter-individual association patterns assumed by models of predator inspection and 'tit-for-tat' behaviours in free-ranging fishes.

Refuge use in a killifish: influence of body size and nutritional state

Refuge use by animals provides greater safety from predation. A refuging animal continuously must decide whether to stay in the refuge or to emerge into open habitat. This decision may depend on its energetic state and vulnerability to predation, both of which can vary with individual body size. We experimentally tested the concurrent effects of body size and nutritional (hunger) state on refuge use in the banded killifish (Fundulus diaphanus). Individual killifish were "attacked" in an open laboratory habitat (containing food) by either a trout predator model or a control model that did not resemble a trout. Frightened fish typically fled into a foodless refuge nearby. The amount of time that killifish spent in the refuge before emerging increased significantly with body length, but was not affected by their current nutritional state or the threatening stimulus (type of model). Smaller killifish were thus more risk-prone than larger conspecifics irrespective of their current nutritional state. Such size-mediated risk-taking behaviour has important consequences for individual fitness and implications for population and community composition and dynamics in nature.