Male pregnancy is a complex and energetically costly form of male parental care found exclusively in the fish family Syngathidae, which includes pipefishes, seahorses, and seadragons. The novelty of the male brood pouch raises questions about how this trait evolved and what role it plays in sexual selection. One possibility is that brood pouch evolution was partially shaped by postcopulatory sexual selection. We used the sexually dimorphic Gulf pipefish, Syngnathus scovelli, to test for postcopulatory sexual selection within broods and tradeoffs between successive male pregnancies as a function of female attractiveness. Offspring survivorship within a pregnancy was affected by the size of a male's mate, the number of eggs transferred, and the male's mating history. These results indicate the males invest more resources into broods from large, attractive mates to the detriment of future broods. Next, we investigated the effects of food limitation on male parental care strategies. Our data suggests that male Gulf pipefish sacrifice investment in future reproduction, via somatic growth, in favor of current reproduction. A positive relationship between number of failed eggs and male growth rate in our low-food treatments suggests that males may derive an energetic benefit from unsuccessful eggs in the brood pouch. Finally, we used a paired design, to investigate the effect of perceived female attractiveness on offspring survivorship. We found that, in general, males prefer the largest female available. Within a replicate, we found that the male that preferred their mate more, regardless of stimulus female size, also had higher offspring survivorship in the resulting brood. This result is exciting because it shows not only that cryptic male choice affects offspring survivorship in Gulf pipefish but also that mate preference can have a prolonged effect on an individual’s reproductive success. Ultimately, our data suggests that the evolution of the brood pouch has produced a trait that not only nurtures the offspring but also exert cryptic male choice.

The sex-role reversal and male pregnancy found in syngnathids are highly unusual traits in vertebrates. Reproductive hormones likely influence development and regulation of these traits. However, very few studies have examined the underlying hormonal mechanisms that mediate female competitive behavior and male pregnancy. New methodologies and better husbandry practices have made such studies more feasible in recent years. Research on a relatively small number of species has suggested that androgens are likely regulators of spermatogenesis and the development of the male brood pouch prior to pregnancy. Androgens are also potential candidates for mediating sex-role reversed behavior in female syngnathids. The goal of this dissertation was to examine the role of androgens in the male reproductive cycle and female intrasexual competitive behavior in the sex-role reversed Gulf pipefish, Syngnathus scovelli. From review of the literature, I developed a model for the hormonal regulation of the male reproductive cycle in seahorses. I predicted that androgens would be low during the early stages of pregnancy and increase during the end of pregnancy as males go through another cycle of spermatogenesis in preparation for the next mating event. My study of 11-ketotestosterone and testis mass across the reproductive cycle in male S. scovelli supported this model. I also conducted several studies on the role of androgens in female competitive behavior. I determined that treatment with 11-ketotestosterone the evening prior to an intrasexual interaction resulted in an increase in competitive behavior in large over small test females. Conversely, treatment with 11-ketotestosterone one hour prior to an intrasexual interaction resulted in a decrease in competitive behavior in large over small females when stimulus female behavior was controlled. A comparative study of competitive and courtship behavior in S. scovelli and the closely related S. floridae suggested that sexual selection has affected competitive and courtship behavior in both males and females of these species. The diversity of reproductive patterns exhibited by syngnathids suggests that they will provide a unique opportunity to assess how hormonal regulation of reproductive behavior and function has evolved within this lineage.

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 46-53).