Age-specific forced polymorphism: implications of ontogenetic changes in morphology for male mating tactics

Age-specific forced polymorphism is the presence of two or more distinct phenotypes (here we consider only males) that occur in separate sexually mature age groups (e.g., horns in older males but not younger males). The life-stage morph maturation hypothesis posits that all younger males that possess a particular structure can transform into older males with a different structure, most likely via the influence of hormones. The life-stage morph selection hypothesis posits that polymorphism is due to intense selection resulting in a highly nonrandom sample of younger males surviving to become older males, thus leading to different mean phenotypes in different age groups. We conducted an extensive review of literature from the past 20 years (1983-2003) for cases of age-specific forced polymorphism. Overall, we found only a few cases that fit our criteria of age-specific forced polymorphism, and we argue that most (e.g., orangutans, elephant seals) have likely arisen via the life-stage morph maturation mechanism, but we also present several examples (e.g., green anole lizards) that appear to be candidates for life-stage morph selection. However, none of the reviewed studies provided enough information (e.g., age of morphs, growth patterns of the morphological structure) to definitively invoke either of the two mechanisms. We suggest that age-specific forced polymorphism is more common than reflected in this review and that future studies should gather demographic and laboratory data that will directly compare the life-stage morph maturation and life-stage morph selection hypotheses.