Mode of Sexual Selection Determined by Resource Abundance in Two Sand Goby Populations

Abstract

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...

Female sand gobies prefer good fathers over dominant males

“…The males care for the eggs by guarding against egg predators, fanning and cleaning them until they hatch, but males may also cannibalize their own eggs. Thus, the hatching success of the eggs is largely dependent on the performance of the male parent (Forsgren et al 1996a). After hatching, there is no further parental care, and the fry have to manage on their own.…”

Sex discriminatory effect of the AcanthocephalanAcanthocephaloides propinquuson a gobiid fishGobius Bucchichii

“…In our study, male gobiids are signifi cantly larger and have significantly lower biological indices (condition, HIS and GSI) than females. It has been demonstrated that male hatching success (the number of eggs hatching after being cared for by the male) increases with initial brood size, that is with fish size because brood size increases with fish size (Forsgren et al, 1996). The parental investment hypothesis predicts that males should invest and allocate more energy in growth in order to reach sexual maturity during the early years of life (Sargent, 1988;Kraak & van den Berghe, 1992).…”

First information on age and growth ofPadogobius bonelli(Bonaparte, 1846) outside of its native range: River Aggia (Central Italy)

“…During the early stages of establishment, low density and relatively abundant resources lead to low male-male competition for good-quality nests. In these conditions, no selection for larger males occurs, and consequently the population will not be sexually size dimorphic (Forsgren et al, 1996). Nevertheless, P. bonelli from the River Aggia can be considered a long-established population, since it has been present in the river sector investigated for almost 20 years.…”

“…In resource-based breeding systems, the availability of resources may be important for both the mating system and intensity of sexual selection (Emlen and Orign, 1977). In nest-guarding species, nest-site abundance has been found to affect mating patterns, and consequently the males' dimensions (Forsgren et al, 1996). Different sexual selection patterns are likely to exist at different stages of invasions of a non-native species.…”

“…Although the maximum lengths of P. bonelli males from the River Aggia are similar to that found in the literature (7.7 cm), the females were found to be longer. Therefore, it is possible that the absence of sex size dimorphism could be due to other factors, such as a stronger selection for females' fecundity favouring larger female size (Forsgren et al, 1996), in order to maximise the reproductive success in a highly unpredictable environment such as the River Aggia, and facilitating the invasion process (Pompei et al, in press). Another cause for the absence of sexual size dimorphism could be selective mortality affecting the smaller females, as emerged from the analysis of back-calculated lengths, suggesting the presence of a reverse Lee's phenomenon.…”