Abstract
In the pipefish Syngnathus typhle, pregnant males provide all parental care. Females are able to produce more eggs than males can brood, and consequently females compete more intensely for mates than do males, a phenomenon defined as sex-role reversal. As the genetic mating system influences the operation of sexual selection, we investigate variation in one phenotypic component of mate quality, female body size, as a possible proximate influence on mating system variation in S. typhle. Breeding trials were employed, each consisting of a single receptive male with four adult females. In each replicate, a focal male was paired either with a set of small or with a set of large females. Males were allowed to mate freely, and after several weeks of brood development, maternity of the progeny was resolved using three microsatellite loci. Males with access either to small or to large females successfully mated with a mean of 2.1 or 1.3 females, respectively, a significant difference. Results indicate that variation in female size can affect the mating system and thereby influence sexual selection in pipefish. Thus, the high rate of multiple mating by S. typhle males in the wild may be explained in part by the extensive size variation in naturally occurring, sexually mature females.
Highlights
In the pipefish Syngnathus typhle, pregnant males provide all parental care
As the genetic mating system influences the operation of sexual selection, we investigate variation in one phenotypic component of mate quality, female body size, as a possible proximate influence on mating system variation in S. typhle
Mutations can be a concern for parentage analysis based on microsatellite markers
Summary
In the pipefish Syngnathus typhle, pregnant males provide all parental care. Females are able to produce more eggs than males can brood, and females compete more intensely for mates than do males, a phenomenon defined as sex-role reversal. Because the mating system is causally related to sexual selection, a logical goal is to assess proximate factors shaping the mating system One such factor is variation in the phenotypic quality of mates, which in addition to impacting sexual selection directly (by promoting competition for high quality mates in monogamous as well as in polygamous species; Andersson 1994) may have indirect effects via the genetic mating system. Paired to low quality mates are more likely to seek extra-pair copulations (Houtman 1992, Kempenaers et al 1992), or to switch mates (Lifjeld and Slagsvold 1988, Orell et al 1994, Otter and Ratcliffe 1996, Triefenbach and Itzkowitz 1998) In such cases, variation in male quality can promote genetic polygamy, presumably enhancing the opportunity for sexual selection. A female’s investment in progeny is complete after egg transfer, but a male carries offspring for several weeks, providing nutrients, aeration, osmoregulation, and protection for his developing brood (Haresign and Shumway 1981, Berglund et al 1986a)
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