A method for assessing the relative sociability of individuals within groups: an example with grazing sheep
Introduction
One of the main characteristics of the social behaviour of grazing animals is how they distribute themselves across the landscape. Sheep are highly social grazers, but different breeds vary in the degree of group cohesion that they display and lowland breeds generally graze closer together than hill breeds (Arnold and Dudzinski, 1978). Even within breeds, cohesion can vary with the terrain and type of vegetation (Dwyer and Lawrence, 1999), the type of behaviour, e.g. resting versus grazing (Lynch et al., 1985) and with the degree of familiarity within the group (Boissy and Dumont, 2002). Distances between grazing animals are also affected by the perceived risk of predation and most breeds of sheep will flock together in response to a fearful stimulus; a behaviour thought to have survival value since individuals near the centre of any group are less vulnerable to predation (e.g. Krause, 1994). Within groups of sheep, some individuals tend to stay closer to their neighbours than others (Lynch et al., 1985), which could be due to differences in the fear of predators and/or differences in social motivation.
The concept of using the spacing between individuals to describe their social relationships was first discussed by Hediger, 1950, Hediger, 1963 and later by McBride (1971). McBride (1971) used the term personal field to define the area around an individual that is normally not entered by other animals and determines the minimum distance found between nearest neighbours. The mechanism for this will vary, with dominant individuals tending to defend their personal fields and more subordinate ones tending to avoid entering the personal fields of others. Hediger (1963) referred to the maximum distance an animal will readily move away from the group, which determines group cohesion, as the social distance. It is useful to think of animals as normally staying somewhere between the personal fields of their neighbours and the social distance, moving in an area which has been referred to as a living space (McBride, 1971) or neutral zone (Gueron et al., 1996). The more sociable individuals are then those which tend to be closer to the limit of the personal field than the social distance. Although absolute distances between animals can be used to assess sociability (e.g. Lynch et al., 1985), we propose a method which requires only the identities of nearest neighbours to be recorded, since this will be less vulnerable to external factors which alter the distances between individuals (see above) and have less opportunity for observer bias. A method for quantifying sociability has valuable potential as a tool in other areas of behavioural research, where the effects of social motivation on other behaviours are of interest, or as a means of assessing the effects of a disease or drug on the social motivation and behaviour of individuals. In controlled experiments, the ranking of group members on the basis of their sociability can be used to allocate individuals to treatments.
The method, which has been developed from an earlier version reported briefly elsewhere (Sibbald et al., 1998), involves the calculation of a sociability index (SI) which is equivalent to the relative proportion of time that each individual is the nearest neighbour of any other animal in the group. Associations between pairs of individuals are also identified, wherever particular pairs of animals are nearest neighbours more often than would be expected by chance. The method is suitable for any size of group, provided it is discrete and all individuals are visible and identifiable during the observation periods. Differences between the relative sociability of individuals are tested for statistical significance, by determining whether they are more extreme than would be expected by chance. This is done by taking measurements over several independent observation periods and constructing statistical hypothesis tests by repeated simulations, in which animal identities are randomised between observation periods. An experiment is described in which the method is demonstrated for a number of small groups of sheep grazing in homogeneous grass plots. The data are used to calculate SIs and to identify pair-wise associations within the groups. The data are also used to compare results obtained from observations of one, two or three nearest neighbours and to examine the relationships between the SI and the mean distance and relative orientation of the animals.
Section snippets
Animals, plots and experimental design
Fifty-six, 1-year-old, female Scottish Blackface sheep were drawn from a single flock and allocated to eight groups of seven animals, which were balanced for live weight and condition score (Russel et al., 1969). In each group, the sheep were randomly allocated the numbers 1–7 and their fleeces marked to allow identification during field observations. The eight groups grazed in separate plots, each consisting of predominantly perennial ryegrass pasture and each measuring approximately 33 m × 60 m.
Sociability indices
Data for group 3 are presented as examples of the matrices used to calculate SIs, using the nearest neighbour only. Table 1a contains scan data from the first observation period and Table 1b shows one of the reordered matrices used for the simulations. Table 2 is the summary matrix used to calculate SI values and contains the percentages of all observations in which each potential neighbour was the nearest neighbour of each focal sheep. The mean of the standard deviations for the individual
Discussion
The motivation to be close to social companions has been assessed in tests which involve a response to social isolation (Faure et al., 1983, Syme, 1981) or measure the distance or speed that animals run towards conspecifics (Mills and Faure, 1990, Sibbald et al., 2000), but the interpretation of the results is inevitably complicated by the fear and stress experienced by the animals. Another approach to measuring social motivation is to use operant conditioning to obtain social contact with
Acknowledgements
This work was funded by the Scottish Executive Environment and Rural Affairs Department. The authors acknowledge the valuable contribution made by Trevor Smart during the initial stages of this work and wish to thank David Cope, Russell Hooper, Bertrand Dumont and Graham Horgan for helpful comments on an earlier version of the manuscript.
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