Richard Bon

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.

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.

Condition‐dependent natal dispersal in a large herbivore: heavier animals show a greater propensity to disperse and travel further

Natal dispersal is defined as the movement between the natal range and the site of first breeding and is one of the most important processes in population dynamics. The choice an individual makes between dispersal and philopatry may be condition dependent, influenced by either phenotypic attributes and/or environmental factors. Interindividual variability in dispersal tactics has profound consequences for population dynamics, particularly with respect to metapopulation maintenance. A better understanding of the mechanisms underlying this variability is thus of primary interest. We investigated the ranging behaviour of 60 juvenile European roe deer, Capreolus capreolus, monitored with GPS collars for 1 year prior to their first reproduction, from 2003 to 2010 in South-West France. Dispersal occurs across a spatial continuum so that dividing individuals into two categories (dispersers vs. philopatric) may lead to information loss. Therefore, to investigate condition-dependent dispersal more accurately, we developed an individual-based measure of dispersal distance, which took into account interindividual variation in ranging behaviour. We assessed the influence of body mass, the degree of habitat heterogeneity and sex on dispersal initiation date, dispersal propensity and distance. The overall population dispersal rate was 0·34, with a mean ± SD linear distance between natal and post-dispersal home ranges of 12·3 ± 10·5 km. Dispersal distances followed a classical leptokurtic distribution. We found no sex bias in either dispersal rate or distance. Forest animals dispersed less than those living in more heterogeneous habitats. Heavier individuals dispersed with a higher probability, earlier and further than lighter individuals. Our individual-based standardised dispersal distance increased linearly with body mass, with some suggestion of a body mass threshold of 14 kg under which no individual dispersed. Natal dispersal in roe deer was thus dependent on both phenotypic attributes and environmental context. Our results suggest that population connectivity can be altered by a change in average body condition and is likely higher in the rich and heterogeneous habitats typical of modern day agricultural landscapes.

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.

The link between behavioural type and natal dispersal propensity reveals a dispersal syndrome in a large herbivore

When individuals disperse, they modify the physical and social composition of their reproductive environment, potentially impacting their fitness. The choice an individual makes between dispersal and philopatry is thus critical, hence a better understanding of the mechanisms involved in the decision to leave the natal area is crucial. We explored how combinations of behavioural (exploration, mobility, activity and stress response) and morphological (body mass) traits measured prior to dispersal were linked to the subsequent dispersal decision in 77 roe deer Capreolus capreolus fawns. Using an unusually detailed multi-trait approach, we identified two independent behavioural continuums related to dispersal. First, a continuum of energetic expenditure contrasted individuals of low mobility, low variability in head activity and low body temperature with those that displayed opposite traits. Second, a continuum of neophobia contrasted individuals that explored more prior to dispersal and were more tolerant of capture with those that displayed opposite traits. While accounting for possible confounding effects of condition-dependence (body mass), we showed that future dispersers were less neophobic and had higher energetic budgets than future philopatric individuals, providing strong support for a dispersal syndrome in this species.

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.

Congestion in a Macroscopic Model of Self-driven Particles Modeling Gregariousness

We analyze a macroscopic model with a maximal density constraint which describes short range repulsion in biological systems. This system aims at modeling finite-size particles which cannot overlap and repel each other when they are too close. The parts of the fluid where the maximal density is reached behave like incompressible fluids while lower density regions are compressible. This paper investigates the transition between the compressible and incompressible regions. To capture this transition, we study a one-dimensional Riemann problem and introduce a perturbation problem which regularizes the compressible-incompressible transition. Specific difficulties related to the non-conservativity of the problem are discussed.

Survival and spatial fidelity of moufl on (Ovis gmelini): A Bayesian analysis of an age-dependent capture-recapture model

We study the influence of age and sex on survival and spatial fidelity of moufl on (Ovis gmelini) in the Caroux-Espinouse massif. Survival and movement probabilities are estimated through a Bayesian analysis of an age-dependent capture-recapture model. Prior information is based on external data, namely on radio-tracked animals. Recapture rates differed between age, sexes, and areas. Whatever the area, survival of males and females less than 5 years old was high and decreased for older animals, particularly males. Female spatial fidelity was high; males were as faithful as females to their capture area in their first 2 years of life but gradually used distinct areas in two successive years as they were getting older.