ESM – A
We considered a group as kangaroos whose most extreme associate was within 15 m of another group member. The low vegetation of the open pastures where individuals foraged gave individuals the opportunity to see all group members. Only groups whose predominant activity was foraging and that did not move far during recording were considered. We also only recorded groups whose size and composition did not change during the five minute samples. We began a video sequence when all animals displayed feeding behaviour. Data were collected on foot by one of us (FRF), keeping a minimal distance of 100 m between the focal group and the observer to minimize disturbance of the animals’ behaviour. The observer did not move during filming to avoid disturbing animals. To confirm that the presence of the observer did not trigger individuals’ vigilance, when we analysed video sequences, we paid special attention to identify whether the observer triggered group members’ scans, in particular by looking at the orientation of animals’ gazes. There was no evidence of this.
During the period of the study, the population contained very few young-at-foot (unweaned juveniles excluded from the pouch but still dependent on the mother) and large pouch young (juveniles still spending much of their mother’s foraging time in the pouch but able to leave it occasionally). Thus, this timing minimised the risk that female vigilance would have been much affected by attention to dependent out-of-pouch young.
Although animals were not marked, we avoided re-sampling individuals by studying groups from a track that crossed open paddocks, allowing us to ensure spatial independence between groups sampled in the same morning or evening session. On the rare occasions when two groups were filmed in the same paddock one after another, the observer ensured that no individuals had moved between the two studied groups during or between the video recordings. The observer changed the direction in which he walked along the sampling track every day. The observer usually filmed only four or five groups in a day and was confident that no group was filmed more than once during the day, and that sampled groups were independent of each other.
To investigate factors influencing time spent looking at group members or scanning the environment, we studied groups of at least three individuals. As a link might exist between the social space of an individual (i.e. space in which an individual can interact with its neighbours), group size and group geometry, such groups allowed us to define an area (hatched area in Fig. 1a, b) defined by the positions of the focal peripheral individual (1) and its two immediate neighbours (2 and 3). Thus, when group size increased, the number of group members increased in the hatched area (Fig. 1b). If the number of individuals in the hatched area (i.e. group size minus three individuals) influenced peripheral focal individuals' likelihoods of raising their heads, we expected that the time they would spend facing into the group (and thus assumed to be showing social vigilance) would increase with group size.
Fig. 1: Representation of the spatial framework that we used to distinguish whether individuals were exhibiting antipredator or social vigilance. In (a), individual 1 exhibits anti-predator vigilance, scanning the environment, while in (b) it exhibits social vigilance, looking at group members. We only collected data on individuals when all their group members were located in the hatched space in the figures (grey diagonal strips).