Alternative mating strategies are common in nature and are generally thought to increase the intensity of sexual selection. However, cuckoldry can theoretically decrease the opportunity for sexual selection, particularly in highly polygamous species. We address here the influence of sneaking (fertilization thievery) on the opportunity for sexual selection in the sand goby Pomatoschistus minutus, a marine fish species in which males build and defend nests. Our microsatellite-based analysis of the mating system in a natural sand goby population shows high rates of sneaking and multiple mating by males. Sneaker males had fertilized eggs in Ϸ50% of the assayed nests, and multiple sneakers sometimes fertilized eggs from a single female. Successful males had received eggs from 2 to 6 females per nest (mean ‫؍‬ 3.4). We developed a simple mathematical model showing that sneaking in this polygynous sand goby population almost certainly decreases the opportunity for sexual selection, an outcome that contrasts with the usual effects of cuckoldry in socially monogamous animals. These results highlight a more complex and interesting relationship between cuckoldry rates and the intensity of sexual selection than previously assumed in much of the literature on animal mating systems.microsatellites ͉ mating systems ͉ paternity ͉ polygyny

Abstract.-We used field observations and experiments to show that sexual selection in two populations of sand gobies, Pomatoschistus minutus (Pisces, Gobiidae), was affected by differences in resource availability. Male sand gobies rely on empty mussel shells for nest building and spawning. The two populations differed considerably in nest-site abundance and sexual-selection regimes. In one population nest sites were scarce, leading to stronger male-male competition over nests, a higher nest site colonization rate and reduced potential for female choice compared with the other population that had a surplus of nests. In the high-competition population, males were larger than females, perhaps as a response to selection, whereas the other population was not sexually size dimorphic. The results from the field were confirmed in a pool experiment that demonstrated the effect of nest abundance on nest occupancy and male reproductive success. Larger males were more successful in obtaining nest sites in both high and low nest availability treatments. Larger males were also favored by females as mating partners, but only in the treatment with surplus nest sites. Nest shortage was associated with an increased potential for intrasexual selection (measured as the coefficient of variation), whereas the potential for intersexual selection was increased when nests were common. In conclusion, nest-site abundance can influence the relative contribution of intrasexual competition and mate choice in a population. Hence, resource availability can contribute to within-species variation in mating patterns.Key words.-Gobiidae, nest site, Pomatoschistus minutus, population differences, resource abundance, sexual selection.Received December 8, 1994. Accepted April 25, 1995 Sexual selection results from nonrandom variance in mating success caused by intrasexual competition for access to mates, or intersexual mate choice, when certain individuals are preferred as mating partners by members of the opposite sex (Darwin 1871). There is growing evidence that mating systems are much less static than previously thought. For example, populations may differ in the intensity of sexual selection because of differences in predation pressure (Farr 1975;Knowlton 1980;Endler 1983;Luyten and Liley 1985), population density (Gwynne 1984;Thornhill 1984;Conner 1989;Arnqvist 1992), interspecific competition (McLain 1981) or operational sex ratio (Colwell and Gring 1988;Madsen and Shine 1993;McLain et al. 1993). In resource-based breeding systems, the spatial distribution of resources and resource availability may be important determinants of the mating system and the intensity of sexual selection (Emlen and Gring 1977). In the dunnock, Prunella modularis, for example, food abundance may cause the mating system to shift from polyandry to polygyny via its effect on female territory size (Davies and Lundberg 1984). In many animals, males defend resources that are necessary to attract females, for example, nesting territories in birds and nest sites in fish. Th...

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org..Wiley-Blackwell and Nordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend access to Oikos. Lindstrom, K. 1988. Male-male competition for nest sites in the sand goby, Pomatoschistus minutus. -Oikos 53: 67-73.Nest size and amount of eggs in sand goby nests show a positive correlation. Nest size explained 40 to 51% of the variation in egg mass size. This relationship remained the same for most of the breeding season. Sand goby males were allowed to choose between large and small artificial nest sites in the field. When the distance between alternative sites was 10 cm, males showed a clear preference for larger nest sites. However, when the distance between alternatives was increased to 50 cm males occupied sites randomly in relation to the size of the site. In the case where alternative sites were 50 cm apart, both sites in a pair were frequently occupied, whereas when they were 10 cm apart both very rarely were occupied. In a square of 10*6 artificial nest sites of three different size categories, larger males were occupying larger nests three weeks after the beginning of the experiment. However, two weeks earlier there was no relationship between male size and nest size category. The most likely explanation for the observed pattern is that large males gradually displace smaller males from large nests. It is concluded that nest size is an important factor determining a male's potential fitness and that sand goby males show the adaptive behaviour of preferring larger nest sites. Males compete for nest sites which are in short supply. This competition will lead to intra-sexual selection for larger male size.