The Role of Sex and Sexual Selection

Most mechanistically based growth models, including the Roff and Lester et al. models, describe the lifetime growth patterns of female fish. Models of male fish growth are more infrequent. For example, in the Roff and Lester et al.

models, reproductive investment is defined by the gonado-somatic index which measures investment in gonad tissue. This is thought to accurately depict the reproductive investment of females because most of their reproductive energy is allocated to development of ovaries, in contrast to males that often invest less energy in testes but more energy in reproductive behavior. Such reproductive activities include display, defending territories, competing for females, and guarding offspring. Most models have focused on female growth because it is easy to measure the mass of ovaries, whereas it can be difficult to quantify the energy expended on aggression or courtship behavior. The downside of focusing only on females is that many fish display sexual dimorphism in growth, and we are missing important information when not explicitly considering the growth and investment of males. It is likely that future models incorporating male reproductive investment and growth will reveal important insights into sexual selection in fish and its consequences for behavior, population dynamics, and fisheries yield.

A common assumption when using growth models in life-history theory is that larger size equates to higher fitness (see Fitness); this is true for females where fecundity is often limited by body size. When males are considered, however, this picture may change due to sexual selection and female choice. During the reproductive season, investments in secondary sexual characters, display behavior, territory defense, or aggression toward competitors may compromise growth but lead to increases in fitness through components that are not directly related to size.

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