Male pregnancy occurs in syngnathid fishes, a group that includes sea dragons, seahorses, and pipefishes. Females give eggs to their mates and males carry the developing offspring in a placenta-like structure called the brood pouch. The investigators recently showed that, in the Gulf pipefish, the male resources used during pregnancy are differentially invested, with more resources devoted to broods from attractive females. These results raise many questions regarding the mechanism underlying this phenomenon, which will be addressed in this project. Breeding experiments will be conducted to investigate male and female effects on offspring survivorship in the Gulf pipefish and to explore the interaction between pre-mating mate preference and post-mating resource investment by males. Offspring survivorship is expected to be determined mainly by males and that male perception of mate quality is the cue used to adjust resource investment in progeny. By utilizing a sex-role reversed species, in which females compete for mating access to males, basic principles of sexual selection theory, which has largely been developed in species with normal sex roles, will be tested. The results of this project will provide an in-depth understanding of the relative roles of pre- and post-mating sexual selection in the evolution of complex traits, such as male pregnancy. The project will involve undergraduates and outreach to elementary school children, as well as a collaboration with the Dallas World Aquarium to promote public awareness of scientific research on syngnathid fishes.
Pipefish, seahorses, and seadragons have often been called "nature’s best dads" because the males nurture their developing offspring in a placenta-like structure called the brood pouch. During the course of this "male pregnancy", the male invests resources, such as energy and nutrition, into the development of their brood. Our previous research in the Gulf pipefish has shown that the pregnant male is not as self-sacrificing as we had previously imagined. Males do not treat all broods equally; in fact, male resources are limited and invested in relation to the attractiveness of their mate. Simply put, males invested more in broods from attractive females and less from unattractive females. The goal of this grant was to (1) more closely investigate how males choose to invest their resources and (2) consider the context of this process in overall sexual selection and evolution. We addressed this goal by addressing a number of specific questions. During the lifetime of this award, we investigated: how a low-resource environment may affect the tradeoff between growth and reproduction in pregnant males, whether males or females control the investment of male resources in the brood pouch, and what the relative role of this process is in terms of overall sexual selection. As a system in which females compete for access to male mates, our research contributes a unique example to a growing body of scientific literature in the relatively new field of postcopulatory sexual selection. This research has significantly changed our understanding of the evolutionary history of male pregnancy. Our data suggests that the brood pouch is much more than a passive reproductive tissue. In fact, it is playing an active role in sexual selection and is an arena in which conflict between male and female reproductive strategies can be acted out. Our data also suggests that there may be a broader role for postcopulatory sexual selection and sexual conflict in the evolution of other complex reproductive tissues. By taking advantage of the transparent male brood pouch, we have been able to track the fate of developing embryos and answer questions that are not possible in other organisms with a more traditional pregnancy. For example, studies such as these would be difficult to perform in other animals with true placental pregnancies, such as mammals. Over the course of this project, we have uncovered many new and exciting results that provide the groundwork for using this species as a model system in studies of postcopulatory sexual selection and the evolution of complex traits.
