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Theory predicts that males that provide little or no parental investment can increase their reproductive success by fertilizing as many females as possible. By contrast, low male mating rate is often related to a large investment in a single mating. This view is challenged, however, by the occurrence of species in which males achieve only one or a few matings, yet invest little in mate guarding or paternity assurance. An explanation of this requires knowledge of the ecological factors affecting the mating system. When there is a female-biased sex ratio and a low probability of a female mating with additional males, the benefits of paternity protection are reduced. Under these conditions males are expected to search actively for females and invest little in each mating to maximize their mating rate. We investigated the influence of the sex ratio and male mortality on the male mate search strategy of the desert spider Stegodyphus lineatus (Eresidae), by following marked males in their natural habitat over the entire mating season. Males searching for mates suffered high mortality but encountered multiple females. During the mating season the sex ratio was highly female biased and females mated on average with a single male in their lifetime. We conclude that high costs of male mate search together with a female-biased sex ratio reduce the likelihood of multiple mating per female and the chances of male-male competition. Under these conditions males should invest in searching for more females and increasing their encounter rate. (C) 2011 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.