Agonistic interactions in the male fiddler crab Uca leptodactyla Rathbun, 1898 at varying densities

Abstract

In this study we tested the effect of population density on agonistic interactions in male Uca leptodactyla Rathbun, 1898. We recorded the crab’s behaviour in pairs or in groups of five animals composed of conspecifics and heterospecifics living in the same area (sympatric) and distinct areas (allopatric) of the mangrove forest. Allopatric conspecific crabs showed higher approaching and signalling than those in other conditions. The higher the crab density, the lower the interaction intensity between animals. Low-level agonistic signals were mainly displayed in high density (groups), while claw touch mainly occurred in pairs. Allopatric conspecifics showed the more intense agonistic interactions. Therefore, Uca seems to decrease energy investment in unnecessary fights against sympatric and conspecific crabs. Population density is a major factor driving agonistic behaviour mainly when conspecific animals are kept together. This may occur due to the increased competition for the same resources.