The social context of individual foraging behaviour in long-finned pilot whales (Globicephala melas)

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Long-finned pilot whales (Globicephala melas) are highly social cetaceans that live in matrilineal groups and acquire their prey during deep foraging dives. We tagged individual pilot whales to record their diving behaviour. To describe the social context of this individual behaviour, the tag data were matched with surface observations at the group level using a novel protocol. The protocol comprised two key components: a dynamic definition of the group centred around the tagged individual, and a set of behavioural parameters quantifying visually observable characteristics of the group. Our results revealed that the diving behaviour of tagged individuals was associated with distinct group-level behaviour at the water’s surface. During foraging, groups broke up into smaller and more widely spaced units with a higher degree of milling behaviour. These data formed the basis for a classification model, using random forest decision trees, which accurately distinguished between bouts of shallow diving and bouts of deep foraging dives based on group behaviour observed at the surface. The results also indicated that members of a group to a large degree synchronised the timing of their foraging periods. This was confirmed by pairs of tagged individuals that nearly always synchronized their diving bouts. Hence, our study illustrates that integration of individual-level and group-level observations can shed new light on the social context of the individual foraging behaviour of animals living in groups.

The social context of individual foraging behaviour in long-finned pilot whales (Globicephala melas)

in Behaviour

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Figures

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    Determination of the focal group. The focal group (grey area) is the group of individuals in closest proximity to the tagged individual (grey animal) and each other. The focal area (dashed circle, not to scale) encompasses the 200 m radius around the tagged whale. For example, a focal group of 7 individuals (left) changes its organisation. Top right: Two individuals become more distantly spaced (3–15 BL) from the tagged whale and other individuals in the focal group than the spacing within the focal group (<1 BL). The group splits up in two smaller groups, and the group with the tagged animal remains the focal group. Bottom right: The focal group becomes more widely spaced, but the relative spacing between individuals remains the same (3–15 BL). One animal leaves the focal group and the focal area.

  • View in gallery

    Example of group behaviour and individual diving behaviour of long-finned pilot whales during 8 h of observation. (a) Group size (filled circles) and number of individuals in the focal area (open circles), (b) individual spacing, (c) surfacing synchrony, (d) number of groups in the focal area, (e) distance of the focal group to the nearest other group (n.i.s. = none in sight), (f) surface behaviour events, (g) deep diving bouts (black) and shallow diving bouts (grey) recorded by the tagged individual (TAG), and classified by the Random Forest model (RF) based on surface behaviour of the group and (h) diving pattern of the tagged individual. The data were all recorded on 23 May 2010.

  • View in gallery

    Diving patterns of two tagged individuals within the same focal group. Horizontal dotted line: boundary between deep and shallow dives at 34 m depth. Vertical dotted lines: transitions between deep and shallow diving bouts for one or both individuals. The whale icons indicate whether the individuals performed deep diving bouts (icon below 34 m) or shallow diving bouts (icon above 34 m). The data were recorded on 17 May 2009.

  • View in gallery

    Comparison of group-level data observed at the surface during shallow versus deep diving bouts of the tagged individual. (a) Group size, (b) number of individuals in the focal area, (c) number of groups in the focal area, (d) distance of the focal group to the nearest other group (n.i.s. = none in sight), (e) individual spacing, (f) surfacing synchrony, (g) presence of calves, line swimming and milling and (h) surface behaviour events. Differences between deep and shallow diving bouts were tested with Generalised Estimating Equations:  marginally significant after Bonferroni correction at p<0.1012=0.0083; ∗∗ significant after Bonferroni correction (p∕number of hypotheses tested) at p<0.0512=0.0042; n.s. = not significant.

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    Relative importance of the behavioural parameters included in the final Random Forest model.

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