The use of an alternative male mating tactic is frequently discussed as an option that makes the best of a bad situation for disadvantaged males and not as part of an adaptive system that can produce equal average levels of reproductive success for individuals using each tactic. It is easy to develop scenarios in which some combinations of tactics produce greater success than others; the difficulty of testing hypotheses about these scenarios and the optimal distribution of tactics revolves around the difficulty of understanding how tactics are expressed. To be specific, it is unknown whether alternative behaviors are conditional and if conditional, how an individual assesses and ultimately chooses which tactic to use in any given context. Under conditions of limited mate availability, variable resource allocation or altered physiological response (e.g. temperature or poor food quality), one might expect variation in mating patterns, but the direction and extent of these variations is relatively unknown.
In the simultaneously hermaphroditic marine fish, Serranus subligarius , male role individuals are known to pair spawn, group spawn and streak spawn. These mating strategies are common among marine reef fish and their behavior has been well studied. What is unclear is how each behavior translates into reproductive success and how these competing strategies are maintained within the same population. This research will focus on the use of alternative mating tactics among simultaneous hermaphrodites. The majority of simultaneous hermaphrodites mate in pairs, often cross-fertilizing each other. Alternative tactics among external fertilizers include group spawning, sneak spawning (males mimic females), or streak spawning, in which a male releases sperm over a spawning event already in progress. In this dissertation I examined the ecological and social contexts under which alternative tactics are utilized and their resulting outcomes with respect to mating behavior and fertilization success.
Chapter one investigates the role of simultaneous hermaphrodites in sexual selection and reviews several studies in which sexual selection is revealed in systems with equal allocation to sex. I discuss ways in which sexual selection has been measured and suggest ways in which this may be modified in hermaphrodites. I also indicate ways in which males compete and females are chosen in these systems, despite their ability to mate with any other individual. Finally, I implicate some of the potential causes of sexual conflict in my study system; namely, that individuals vie for the male role and larger fish spawn more frequently in the male role.
In chapter two, I explored the ecological and physiological contexts under which streak spawning occurs and discuss their relative importance. Size structure was characterized at each of three sites within the field study site and was subsequently used as the impetus for the experiments performed in Chapter 3 to tease out its relevance. Size structure appears to play an important role in the occurrence of streak spawning, as more streaking appears in the sites, which have greater than 20% of individuals comprised of the small size class (measured by density transects at three sites). This chapter also aims to quantify the type of habitat over which a pair spawns and whether or not this differs when additional males participate. I found that at one of the sites, there was a preference for streak spawners when macroalgae was present than when the rocks were bare. In addition, this chapter demonstrates the peak spawning times to be July and August, which were confirmed by both field observations and investigation of the gonads of actively mating individuals.
In chapter three, I investigated the fertilization success of animals spawning with and without streakers participating, both in the field and in a controlled manipulation experiment. In the field I find that fertilization rates are lower in fish spawning with multiple males participating. I confirmed this finding under more controlled conditions in a field manipulation experiment. Rock rubble reef plots provided substrate on which I tested the role of density and size structure on the frequency of streak spawning and resulting fertilization success. Spawns with additional males resulted in lower fertilization success and the number of streak spawns was significantly greater when small size-class individuals were present. Thus, both field and experimental evidence suggest that small individuals predict the incidence of streak spawning and their participation lowers fertilization success. Reduced fertilization success means that both males and females incur a fitness cost. The mechanism for lowered success remains unclear but likely includes the physical disturbance created in the water column by the activity of additional males.
The results of this dissertation suggest that there may be a cost to certain mating systems and the contexts under which these different tactics are utilized are dependent on ecological, social and physiological factors. It points to the complexities of individual variation in behavior and the need for more rigorous experimental work to tease apart the factors associated with variation in mating tactic. Interactions among environment, social group, and ontogeny are not often addressed simultaneously in study systems and more empirical work in these areas would help elucidate the mechanisms associated with the use of alternative mating tactics.