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Allelomimetic synchronization in Merino sheep

Changes between the inactive (resting/ruminating) and active (grazing, walking) states in groups of Merino sheep were studied in the field for different group sizes (two, four, six or eight) of either male or female animals over 6-h periods. The amount of synchrony within groups was high (60–80%) and is attributed to the mutual adjustment of behaviour by group members. To quantify this process, changes in the number of active individuals were fitted by a time homogeneous continuous time Markov chain model. We found that the probability of an individual becoming active increased with the number of active conspecifics in the group and decreased with the number of inactive conspecifics. The reverse effect was found for the probability of becoming inactive. A model of this individual decision-making process is fitted to the data and predictions of the model are shown to account for the synchrony observed within the group. Group synchronization is thus presented as a self-organized dynamic system, where collective oscillations between activity and inactivity arise stochastically from the coupling between individual Markov processes.

Personality and collective decision-making in foraging herbivores.

The mechanisms by which group-living animals collectively exploit resources, and the role of individuals in group decisions, are central issues for understanding animal distribution patterns. We investigated the extent to which boldness and shyness affect the distribution of social herbivores across vegetation patches, using sheep as a model species. Using an experimental and a theoretical approach, we show that collective choices emerge through the nonlinear dynamics of interactions between individuals, at both short and long distances. Within a range of parameter values derived from the observation of homogeneous groups of each behavioural type, we propose a simple mechanism whereby the same interaction rules can result in different patterns of distribution across patches for bold and shy individuals. We present a mathematical model based on behavioural rules derived from experiments, in which crowding and conspecific attraction affect the probability of entering or leaving patches. Variation in the strength of social attraction is sufficient to account for differences in spatial distribution across patches. The model predicts that resource fragmentation more strongly affects the distribution patterns of shy groups, and suggests that the presence of both bold and shy individuals within groups would result in more flexible behaviour at the population level.

An experimental test of hypotheses explaining social segregation in dimorphic ungulates

In many sexually dimorphic ungulates, adults spend most of their time in single-sex groups outside the mating season. We tested two hypotheses proposed to explain this social segregation. The activity budget hypothesis predicts that females spend more time grazing than males, and that activity synchrony is higher within single-sex than mixed-sex groups. The social affinity hypothesis states that higher spatial proximity and aggregation within the sexes results from sexual differences in motivation to interact socially. Fifteen merino sheep, Ovis aries, 2 or more years old, of each sex were penned together in three 1-ha paddocks in succession during 7 winter weeks. Despite the high sexual dimorphism in body weight, both sexes were found together, and did not differ substantially in activity budget. Males grazed less than females but the difference was small. The degree of activity synchrony was similar within and between the sexes. When grazing, males walked more rapidly and tended to be at the front of the group more often than females. Ewes were rarely seen interacting with other sheep, in contrast to males, which interacted twice as often with females as with males. However, same-sex pairs of nearest neighbours were found more frequently than expected, whether or not the males were at the front of the group. These results indicate a social segregation at a small spatial scale, which can be explained by differences in speed of movement between males and females and/or higher social affinity between same-sex than opposite-sex peers. The lack of single-sex groups can be explained by the strong gregariousness of merino sheep, and possibly the young age of the adult males.

Moving together: Incidental leaders and naïve followers

Elucidating whether common general mechanisms govern collective movements in a wide range of species is a central issue in the study of social behaviour. In this paper, we describe a new experimental paradigm for studying the dynamic of collective movements. Some sheep (Ovis aries) were first trained to move towards a coloured panel, in response to a sound cue. We present data comparing the behaviour of test groups composed of one of the trained sheep and 3 naïve sheep, and control groups composed of 4 naïve sheep. In the tests, for both test and control groups, sheep were observed for 20 min before the sound cue was delivered and the panel made visible. Before the sound, trained and naïve sheep were similar in terms of activity budgets, spatial distribution, social behaviour and spontaneous movement initiation. After the sound, trained sheep moved toward the panel and systematically triggered a collective movement in all test groups. The results suggest that any individual moving away from the group can elicit a collective movement. Our experimental protocol provides an opportunity to quantify mechanisms involved in group movements, and to investigate differences between species and the effect of social context on collective decision-making.

Sexual dimorphism, activity budget and synchrony in groups of sheep

The activity budget hypothesis has been proposed to explain the social segregation commonly observed in ungulate populations. This hypothesis suggests that differences in body size – i.e. between dimorphic males and females – may account for differences in activity budget. In particular, if females spend more time grazing and less time resting than males, activity synchrony would be reduced. Increased costs of maintaining synchrony despite differences in activity budget would facilitate group fragmentation and instability of mixed-sex groups. In this paper two prerequisites of the activity budget hypothesis were tested: (1) that males should spend less time feeding and more time resting than females in single-sex groups and (2) that lower activity synchrony should be observed in mixed-sex compared to single-sex groups. The activity budget and synchrony in mixed and single-sex groups of merino sheep (Ovis aries) of different sizes (2, 4, 6, 8 individuals) were measured in three contiguous 491-m2 arenas located in a natural pasture. Three same-size groups, one of each category, were observed simultaneously. We found no sexual differences in the time spent inactive and active (i.e. grazing, standing, moving, interacting). Males spent significantly more time grazing and less time standing than females. These differences disappeared when yearling males were omitted from the group. Males and females had similar bite and step rates. Sheep of both sexes spent less time resting and more time grazing and moving and had lower bite rates when in mixed-sex groups than when in single-sex groups. The synchrony among visually isolated groups was near zero, indicating that they changed activities independently. On the contrary, within-group synchrony was high; however it was higher in single-sex groups, in particular for males, than in mixed-sex groups. Our results suggest that differences in activity budget and synchrony alone are insufficient to explain social segregation.

How group size affects vigilance dynamics and time allocation patterns: the key role of imitation and tempo.

In the context of social foraging, predator detection has been the subject of numerous studies, which acknowledge the adaptive response of the individual to the trade-off between feeding and vigilance. Typically, animals gain energy by increasing their feeding time and decreasing their vigilance effort with increasing group size, without increasing their risk of predation (‘group size effect’). Research on the biological utility of vigilance has prevailed over considerations of the mechanistic rules that link individual decisions to group behavior. With sheep as a model species, we identified how the behaviors of conspecifics affect the individual decisions to switch activity. We highlight a simple mechanism whereby the group size effect on collective vigilance dynamics is shaped by two key features: the magnitude of social amplification and intrinsic differences between foraging and scanning bout durations. Our results highlight a positive correlation between the duration of scanning and foraging bouts at the level of the group. This finding reveals the existence of groups with high and low rates of transition between activies, suggesting individual variations in the transition rate, or ‘tempo’. We present a mathematical model based on behavioral rules derived from experiments. Our theoretical predictions show that the system is robust in respect to variations in the propensity to imitate scanning and foraging, yet flexible in respect to differences in the duration of activity bouts. The model shows how individual decisions contribute to collective behavior patterns and how the group, in turn, facilitates individual-level adaptive responses.

Sexual Segregation in Vertebrates: Sexual segregation in ungulates: from individual mechanisms to collective patterns

OVERVIEW Sexual segregation is an integral aspect of the socio-spatial organization of ungulate populations. Very often, the social, spatial and ecological components have been confounded (Bon, 1992) and we have argued that it is necessary to define and distinguish between each of them (Bon & Campan, 1996; see also Chapter 2 by Larissa Conradt). In the present chapter, we point out that sexual segregation is a complex phenomenon that can be produced by distinct mechanisms. One of the main issues is to know whether segregation by habitats necessarily derives from sexual difference in habitat choice, or can derive from alternative causes, i.e. spatial and social mechanisms (see also Chapter 2). Habitat segregation implies heterogeneous habitat (Miquelle et al., 1992), which we assume not to be obligatory for social and spatial segregation to occur. We distinguish hypothetical mechanisms relevant only in a heterogeneous environment from those relevant in both heterogeneous and homogeneous environments. We focus on behavioural mechanisms that may generate social and spatial segregation/aggregation, and the problem of the scale at which segregation may occur. Finally, we suggest that segregation cannot only be considered as a result of individuals behaving independently of each another, but also as a result of interactions between individuals on a larger (population) scale. Habitat versus social segregation Miquelle et al. (1992) noted that differences in habitat selection often lead to sexual segregation and resource partitioning between the sexes.

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

SUMMARY 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.

Group size elicits specific physiological response in herbivores

With increasing group size, individuals commonly spend less time standing head-up (scanning) and more time feeding. In small groups, a higher predation risk is likely to increase stress, which will be reflected by behavioural and endocrine responses. However, without any predator cues, we ask how the predation risk is actually processed by animals as group size decreases. We hypothesize that group size on its own acts as a stressor. We studied undisturbed groups of sheep under controlled pasture conditions, and measured in situ the cortisol and vigilance responses of identified individuals in groups ranging from 2 to 100 sheep. Both vigilance and average cortisol concentration decreased as group size increased. However, the cortisol response varied markedly among individuals in small groups, resulting in a lack of correlation between cortisol and vigilance responses. Further experiments are required to explore the mechanisms that underlie both the decay and the convergence of individual stress in larger groups, and whether these mechanisms promote adaptive anti-predator responses.