Density-dependent influence of male characters on mate-locating efficiency and pairing success in the waterlouse Asellus aquaticus: an experimental study

Authors

  • Angéline Bertin,

    Corresponding author
    1. Equipe Ecologie Evolutive, UMR CNRS 5561 Biogéosciences, Université de Bourgogne, 6 Boulevard Gabriel, 21 000 Dijon, France
    Search for more papers by this author
  • Frank Cézilly

    1. Equipe Ecologie Evolutive, UMR CNRS 5561 Biogéosciences, Université de Bourgogne, 6 Boulevard Gabriel, 21 000 Dijon, France
    Search for more papers by this author

*All correspondence to present address: A. Bertin, Institut des Sciences de l'Evolution (CNRS-UMR n5554) CC065, Université Montpellier 2 Bat 22, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France. E-mail: bertin@isem.univ-montp2.fr

Abstract

Population density is likely to determine the form of competition in which males are engaged for access to females. At low density, scramble competition should be of paramount importance because of the low probability of encounter between males and females. Consequently, sexual selection should favour characters that allow rapid detection of females. Conversely, at high population density, sexual selection should favour attributes that raise the fighting ability of males because of the more frequent contacts between males. These general predictions were tested in this study for the precopulatory mate-guarding isopod, Asellus aquaticus. In this species, male-biased sexual dimorphisms are reported for body size and antennae length and are, respectively, ascribed to contest and scramble competition over females. Therefore, the relative strength of sexual selection on male body size and antennae length at two different densities was experimentally assessed. Multivariate analyses indicated that density affected morphological correlates of the mating success of males, with body size being the main determinant of pairing success in males at a high density, whereas only antennae size significantly affected access to females at a low density. The antennae length of males was manipulated to examine how antennae length affects the ability of males to detect females in three experimental conditions varying in the probability of random contacts with receptive females. An advantage of having long antennae was only observed when there was an intermediate difficulty in finding females. Our results are discussed in relation to the influence of density on selective regimes in A. aquaticus.

Ancillary