| This work integrates theory and empirical study to develop and support a new hypothesis of sexual selection. The Cost Avoidance Hypothesis (CAH) (Chapter 1) states that to achieve reproductive success, un-preferred males should coerce females by imposing pre-copulatory costs on those females. In this way, females choose to mate with these un-preferred males to avoid incurring further costs. The CAH additionally states that females and preferred males should cooperate to ensure low-cost reproduction. This cooperation between members of a mating pair is predicted to minimize pre-copulatory costs, even in the face of coercion. Thus, the simultaneous occurrence of coercion and cooperation in mating systems is likely to be an important force in sexual selection.; The CAH is examined in coho salmon (Oncorhynchus kisutch). This species is well known for the occurrence of two male phenotypes. These phenotypes, known as “hooknoses” and “jacks,” represent males who have taken alternative life-history paths. The large hooknose is three years of age at maturity while the small jack matures at only two years of age. Mature hooknoses are fighters, gaining access to females with force, while jacks draw little attention to themselves by hiding and moving in towards females when the opportunity arises. But the behavior of adult males belies the circumstances that carry a growing juvenile along a given path to maturity. Other researchers have shown that large juveniles are those most likely to mature early, as jacks. Chapter 2 shows that the distribution of resources in the natal habitat plays an important role in directing the growth rate of juveniles and the subsequent expression of alternative reproductive phenotypes. When resources are clumped, the growth rates of individuals vying for those resources vary. Conversely, when resources are evenly spread, individuals grow at similar rates. Under clumped resources, the expression of alternative phenotypes is higher than under evenly spread resources. Chapter 3 tests the predictions that female coho salmon prefer to mate with early maturing jacks, and given this mating preference, hooknoses coerce females to mate by increasing pre-copulatory costs. Data collected in a field study of wild spawning coho salmon support these predictions. |
