Aggression, segregation and stability in a dominance hierarchy

Abstract

Central to our understanding of social group formation and maintenance is the question of how withingroup conflict resolution is achieved in the face of asymmetrical competition over resources and reproduction. A crucial yet implicit assumption of many conflict resolution models dealing with reproductive skew is that subordinates have perfect knowledge of the extent of conflict between themselves and their dominants, enabling behavioural responses on an individual rather than evolutionary scale. However, a mechanism enabling subordinates to accurately assess their relative conflict levels has yet to be empirically demonstrated. Here, we show in the angelfish Centropyge bicolor that the rate of overt mild aggression from dominants to subordinates acts as a signal of increasing rank conflict. The clarity of this signal can be reduced by spatial segregation, causing subordinates to be less able to respond appropriately by regulation of their foraging rates. A reduced signal ultimately leads to a less well-defined dominance hierarchy and destabilization of the social group. Our study suggests that, contrary to previous suggestions, dominant aggression rates play a crucial role as an accurate information signal required for the evolutionary stability of skew models.

The four elements of within-group conflict in animal societies: an experimental test using the clown anemonefish, Amphiprion percula

“…This simple yet key result confirms the relationship between size difference and conflict intensity which previously had not been established despite Buston (2003a) and Buston and Cant (2006)'s suggestion that a precise size hierarchy is established to minimize conflict between individuals of adjacent rank. Direct evidence for a relationship between size similarity and intensity of conflict reflects that found for other social fishes (Hamilton et al 2005;Ang and Manica 2010a;Matthews and Wong 2014). For example, in the cooperatively breeding African cichlid fish, Neolamprologus pulcher, subordinate helper males showed more submissive and reduced affiliative behaviours -indicative of increased conflict -when there was a small size difference between themselves and breeder males (Hamilton et al 2005).…”

“…For example, in the cooperatively breeding African cichlid fish, Neolamprologus pulcher, subordinate helper males showed more submissive and reduced affiliative behaviours -indicative of increased conflict -when there was a small size difference between themselves and breeder males (Hamilton et al 2005). In addition, Ang and Manica (2010a) reported increases in aggression between group members as the size ratio between them increased to unity, although only for groups where there was substantial spatial overlap.…”

“…However, the regulation of growth and the creation of distinct size differences arising from growth regulation clearly took place over a much longer period. Therefore, the rapid decline in aggression is likely to reflect other mechanisms promoting more immediate conflict resolution, which could include processes such as winner or loser effects (Dugatkin 1997;Chase et al 2002;Dugatkin and Early 2003) or spatial segregation (Ang and Manica 2010a), and testing these possibilities would be an intriguing area of future research. Whatever the mechanism and its associated proximate cues, our results enable us to advance our current understanding of conflict resolution mechanisms by indicating that different mechanisms of conflict resolution are likely to prevail in terms of relative importance depending on the timeframe.…”

“…This hypothesis was based on three untested assumptions: i) actual conflict would be highest when individuals are similar in size, ii) the outcome of conflict would be dependent on relative size, and iii) that conflict would be resolved by subordinates regulating their growth to maintain the well-defined size differences. While there was plenty of indirect evidence for these assumptions (Buston 2003a;Buston and Cant 2006), and some of these assumptions were subsequently verified in other social fishes (Hamilton et al 2005;Wong et al 2007;Ang and Manica 2010a;Matthews and Wong 2014), there have still been no direct tests of these assumptions in clown anemonefish themselves.…”

“…Field et al 1999;Spinks et al 2000;Stiver et al 2007). It also increases the expression of actual conflict over reproduction, for example, when subordinates fight to become the new breeder following death of a dominant (Clarke and Faulkes, 1997;Wong and Balshine 2010), when dominants use aggression to suppress subordinate reproduction or evict them from the group (Clutton-Brock et al 1998;Hackländer et al 2003;Ang and Manica 2010a;Nichols et al 2012), or when infanticide occurs within the group (Clutton-Brock et al 1998;Cant 2000;Young and Clutton-Brock 2006). In vertebrate societies, the outcome of conflict is often dependent on traits correlated with competitive ability, such as relative body size (e.g.…”