Sexual size dimorphism and assortative mating in Carolina Wrens

ABSTRACT.   Sexual size dimorphism (SSD) may be due to sexual and natural selection, but identifying specific mechanisms that generate such dimorphism in a species is difficult. I examined SSD in Carolina Wrens ( Thryothorus ludovicianus ) by examining (1) the degree of SSD in the population and between pairs using five morphometrics, (2) assortative mating patterns based on size and age, and (3) relationships between size and longevity. Analysis revealed that males were significantly larger than females in all body measurements. For example, mass, bill, and wing measurements yielded a canonical variable that permitted separation of the sexes and linear classification functions correctly determined the sex of 95% (238/250) of all wrens measured. No evidence was found to suggest that SSD was related to resource partitioning. However, assortative mating trends based on morphometrics (e.g., wing length), positive associations between longevity and morphometrics (e.g., wing length in females and body size in males), and intense male-male contests for territorial resources year-round provide evidence that sexual selection may contribute to SSD in Carolina Wrens.