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

in Crustaceana
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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.


International Journal of Crustacean Research



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  • Geographic location of the Ceará-Mirim River estuary: A, its general location in Brazil; B, near the city of Extremoz, Rio Grande do Norte State; C, detail, aerial image of the study site, 05°40′32.25″S 35°14′19.61″W; and, D, the local distribution of the various Uca species in the mangrove forest.

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  • Distance and agonistic interactions between pairs of fiddler crabs. Four paired Uca conditions were tested: pairs of Uca leptodactyla Rathbun, 1898 immediate neighbours (sympatric conspecifics) and from different areas (allopatric conspecifics) and pairs of Uca leptodactyla and Uca rapax (Smith, 1870) immediate neighbours (sympatric heterospecifics) and from different areas (allopatric heterospecifics). Bars are the mean distance ± SD between animals. The same letter means no significance and different letters mean significance value (ANOVA, p<0.05). Circles are the total number of agonistic interactions showed by the pairs. An asterisk indicates statistical difference in the agonistic interactions between pairs (Goodman test, p<0.05).

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  • a, Distance; and b, agonistic interactions between the focal Uca leptodactyla Rathbun, 1998 and the other fiddler crabs in groups of 5 animals. Four conditions were tested: groups of five Uca leptodactyla that were immediate neighbour (sympatric conspecifics) or inhabited different areas (allopatric conspecifics) and groups of three Uca leptodactyla and two Uca rapax (Smith, 1870) that were immediate neighbours (allopatric heterospecifics) or residents of distant areas (allopatric heterospecifics). The crabs were classified as a, b, c and d according to their distance from the focal Uca leptodactyla in the first minute of observation; a represents the closer animal to the focal Uca leptodactyla and d the farthest crab. Legends: a, bars show the mean distance ± SD between animals; an asterisk indicates statistical difference between animal a and the others in the same condition (ANOVA, p<0.05); b, circles show the total number of agonistic interactions initiated by each animal in the group; different Greek letters indicates statistical differences between the crabs in the same condition (allopatric conspecifics) and an asterisk indicates statistical difference between crabs from different conditions (Goodman test, p<0.05).

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  • Relationship between agonistic interactions (type 1, 2, 3 and 4) and conditions tested: SC2, sympatric conspecific pair; AC2, allopatric conspecific pair; SH2, sympatric heterospecific pair; AH2, allopatric heterospecific pair; SC5, sympatric conspecific group; AC5, allopatric conspecific group; SH5, sympatric heterospecific group; AH5, allopatric heterospecific group. Small symbols are the Eigenvectors of the LDA and large symbols are the corresponding centroid. Agonistic interactions 1 and 2 are marginally significant and highly significant, respectively (solid arrows). Agonistic interactions 3 and 4 do not show statistical significance (dashed arrows).

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