More aggressive sea turtles win fights over foraging resources independent of body size and years of presence

Abstract

Interactions among individuals of the same species inform us about resource value and sociality; however, for species that are difficult to observe directly, these behaviours are potentially missed or misinterpreted via remote monitoring approaches. Here, we investigated how agonistic interactions influenced temporal and spatial variation in the social structuring of loggerhead sea turtles, Caretta caretta , foraging on sponges at a reef (Zakynthos, Greece) over multiple years. We directly observed individuals via snorkel surveys (with photoidentification and videos) supported by aerial drone surveys. Most turtles were immature loggerheads, of which half were detected in more than 1 year. Individual loggerheads occupied distinct (very small) patches on the reef, with low overlap when foraging. Interactions generally occurred when loggerheads were adjacent to, or passing through, patches occupied by other individuals. Interactions between loggerheads represented an extremely small fraction of the activity budget, but potentially consumed two to three times more energy than swimming and foraging. Escalation of aggression, unexpectedly, skipped the passive (evaluation) phase in half of interactions (starting with biting and chasing), with contests being primarily won by more aggressive individuals, independent of body size and duration of presence (i.e. reef occupancy in years). Distinct hierarchical structuring was detected based on win outcomes and spatial area use within each year; however, the number and combination of turtles frequenting the reef varied across years, with individuals only dominating for 2–3 years before departing (or returning intermittently) and being replaced. The unexpected fight dynamics and regularly changing hierarchies indicate that individuals perceived the resource value of sponges differently, which was attributed to differences in their ability to assess resource abundance and/or availability of resources at alternative sites. Our results contribute to the emerging field of sociality in reptiles. • Interactions represented 1% of the activity budget of foraging loggerhead turtles. • Interactions appeared more energetically costly than swimming or foraging. • Interactions were immediately aggressive (no passive evaluation) in 48% of cases. • Winners were the most aggressive, independent of body size or years of presence. • Individuals remained dominant for 2–3 years only before leaving or, rarely, returning.