Agonistic behaviour in the long‐snout seahorse: A gentlemanly challenge

Abstract

AbstractTerritorial defence is a relatively common behaviour in reef fish. Aggression typically occurs between individuals of the same sex, given that members of the opposite sex may represent potential breeding partners. Seahorses are cryptic reef fish with typical sedentary behaviour, especially during the male pregnancy. In this context, we investigated the agonistic behaviour of pregnant seahorses, Hippocampus reidi, towards other males, pregnant or not, relating the duration of their interactions to the size and robustness of the resident and intruder. Robustness was measured as the circumference of the brood pouch divided by total body length. The study was based on four alternative treatments, which were monitored in situ: (i) a pregnant male in its home range was exposed to a pregnant intruder male; (ii) a non‐pregnant male in its home range was exposed to a pregnant intruder male; (iii) a pregnant male in its own home range was exposed to a non‐pregnant intruder male, and (iv) a non‐pregnant male in its home range was exposed to a non‐pregnant intruder male. The control to test for the effect of seahorse translocation on later displacement behaviour consisted of male seahorses translocated to an area of 60 m from the capture site and quantification of their displacement range. Low‐level aggressive behaviour was recorded in the trials, although the pregnant males presented dominance behaviour, both within and outside their own home ranges or territories, to expel intruders, while non‐pregnant males avoided interaction. No correlation was found between robustness and either the displacement of the intruder or the duration of the interaction.