Guarding success of the rarest sex in the amphipod Hyalella azteca Saussure, 1858 (Amphipoda: Hyalellidae)

Abstract

Abstract Traditional methods to study precopulatory mate guarding in Hyalella azteca (Saussure, 1858) have some limitations because precopulatory pairs are usually observed in isolation. This condition precludes researchers from testing the role of the social environment in the duration of precopulatory mate guarding. We present the use of a sampling method to investigate the influence of sex ratios on guarding success (the proportion of precopulatory pairs per trial) in mixed-sex groups of H. azteca. The presence/absence of precopulatory pairs over eight consecutive days, morning and afternoon samplings, was analysed as a series of successes (runs) to estimate the mean precopula duration. The non-random nature of the observed duration of the precopula was tested against a probabilistic distribution based on the set of all ways every possible duration may occur. While skewed sex ratios increased guarding success of the rarest sex, even sex ratios decreased it. The mean duration of observed precopulas (2.32 and 2.08 d for the morning and afternoon periods, respectively) is consistent with theoretical and empirical results, which confirms the accuracy and usefulness of the sampling method we used. The use of this method and the associated probabilistic analysis of the relevant data can enhance the study of the effect of biotic and abiotic factors on precopulatory mate guarding. It can also be used to test predictions derived from hypothesis concerned with mate guarding among crustaceans.