The evolution of alternative mating tactics in the yellow dung fly
Date of Award
Doctor of Philosophy (PhD)
Scott S. Pitnick
alternative mating tactics, behavior, female preference, plasticity, sexual selection, sperm competition
Evolution is expected to favor the proliferation of one strategy in a population and eliminate less successful alternatives. However, in some species, males within a single population adopt discrete behavioral and life history strategies, but the processes responsible for their evolution and maintenance are poorly understood. Recent work suggests that many examples of alternative mating tactics may be expressed through a conditional evolutionarily stable strategy. In this framework, all males in a population are capable of adopting either tactic, but environmental cues such as resource availability during development and social context ultimately determine which tactic maximizes an individual’s reproductive success. Empirical support for key predictions of the conditional evolutionarily stable strategy, however, is currently lacking. Here, I use the yellow dung fly, Scathophaga stercoraria, whose males adopt size-dependent alternative mating tactics, to provide a novel test of these predictions, while also identifying pre- and post-copulatory processes driving variation in male reproductive success. In this species, large males aggregate on fresh cow dung (oviposition substrate) and engage in intense male-male competition for gravid females, whereas small males search for mating opportunities at off-pasture foraging sites (i.e., fallen fruit or flowers) where competition is absent. To accomplish the goals of my project, I employed various experimental techniques, including detailed behavioral observations of tactic expression and mating success, as well as molecular genetics approaches for distinguishing offspring sired by males adopting alternative mating roles. Specifically, in Chapter 1, I identified novel male and female behavioral repertoires specific in the off-pasture foraging environment and their implications for male reproductive success. Observations of mating interactions in the field revealed both that small males are more likely to attempt copula on foraging substrate than large males and that those attempts are more successful, in part due to a surprising female preference for small body size. I then used large experimental mesocosms to compare the behavioral expression of size-dependent alternative mating tactics across geographically widespread yellow dung fly populations in Chapter 2. These experimental trials demonstrate that distant populations do not differ in their expression patterns (i.e., switchpoint) when tested under common garden conditions, despite originating from ecologically variable environments. In Chapter 3, I confirmed the long-standing prediction that off-pasture matings (primarily associated with the subordinate male tactic) sire significantly fewer offspring than copulations on dung. Given this disparity in reproductive pay-off, theory predicts that male investment (i.e., ejaculate expenditure) per mating should differ between locations. Support for this prediction was found, though small and large males exhibited opposing patterns of expenditure between these alternative resource environments, indicating that male tactics also encompass distinct strategies for allocating ejaculate resources. Finally, in Chapter 4, I quantified size-dependent tactic expression and reproductive success for S. stercoraria males under low and high mate competition intensity treatments. I found that males adaptively alter the expression of behavioral tactics based on their size and relative dominance in the population. These findings provide strong evidence that S. stercoraria males express conditional mating tactics and demonstrate that subordinate male tactics can persist in a population despite conferring lower fitness on average than dominant male tactics.
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Gress, Brian, "The evolution of alternative mating tactics in the yellow dung fly" (2016). Dissertations - ALL. 977.