Concede or clash? Solitary sharks competing for food assess rivals to decide

Abstract

To adapt to their environment, organisms can either directly interact with their surroundings or use social information (i.e. information provided by neighbouring individuals). Social information relates to the external features of surrounding peers, and little is known about its use by solitary species. Here, we investigated the use of social cues in a solitary marine predator by creating artificial aggregations of free-ranging sicklefin lemon sharks (Negaprion acutidens). Using a novel monitoring protocol, we analysed both dominance interactions and tolerance associations between sharks competing for food in relation with the number, the morphology and the behaviour of rivals. Sharks produced more agonistic displays and spent more time around the bait as competitors were more abundant. Moreover, the morphological attributes of competitors had very limited influence on the structure of shark social interactions. Instead, sharks appeared to establish tolerance relationships with competitors according to their individual behaviour. Furthermore, the more two sharks were observed together at a given study site, the fewer agonistic interactions they exchanged. We discuss these findings as evidence of the use of social cues in a non-gregarious predatory species and suggest directions for future research.

Keywords: dominance hierarchy; heterarchy; in situ experiment; shark; social information.

Figure 1.

Sicklefin lemon shark tolerance network and dominance hierarchy. (a) Tolerance network. Nodes represent sharks and individuals are labelled as follows: F (females) or M (males) + ID number. Edges indicate the SRI association index. Node size and colour are respectively proportional to shark length and dominance score. (b) Sharks had more connections in the tolerance network (degree) as they showed stronger fidelity to the respective study sites. (c) Sharks produced fewer agonistic behaviours as they showed higher fidelity to the respective study sites. (d) Sharks that showed intermediate submissive rates had more connections in the tolerance network (degree) than sharks that showed high or low submissive rates.

Figure 2.

Shark dominance hierarchy. (a) Relationship between the difference in rank and the probability for the higher-ranked individual to win the interaction. Value and significance of the hierarchy transitivity (Ttri) and the ratio of interactions to individuals are also indicated. (b) Dominance interaction matrix. Cell colour captures how often individual sharks (rows) dominate rivals (columns), while controlling for the co-occurrence of individuals.

Figure 3.

Behavioural shift within the shark aggregation. Green arrows show the direction of submissive displays pictured in three steps.