Energetically costly precopulatory mate guarding in the amphipod Gammarus pulex:: causes and consequences

Precopulatory mate guarding (PCMG) is thought to have evolved as a male mating strategy in species in which female receptivity is limited to a short time. It is common among crustaceans, and energetic costs associated with PCMG are thought to promote size-assortative pairing in such species, although direct evidence is lacking. Using both field surveys and laboratory experiments, we assessed the energetic costs of PCMG in Gammarus pulex and investigated their possible causes. Energetic costs were measured as differences in lipid and glycogen reserves. In field-collected samples, size-corrected lipid and glycogen reserves of paired males were both significantly higher than those of unpaired ones. In the laboratory, the energetic cost of PCMG was unrelated to its duration, but was strongly positively correlated with female size (relative to the size of the male). In addition, the increased energetic cost of guarding a larger female was independent of an induced starvation cost. Our results show for the first time an energetic cost of PCMG in male G. pulex, and indicate that such a cost is more likely to result from pair formation than from the cost of carrying the female, as has previously been assumed. We discuss our results in relation to sexual conflict over PCMG duration and the ability of males to overcome female resistance. (C) 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.