Demographic consequences of sexual selection in the long-tailed manakin

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Abstract
Demographic divergence between the sexes is a major consequence of sexual selection. Matrix-based demographic measures, including the sensitivity and elasticity of λ (population growth rate, fitness) to survival and fertility rates are powerful indexes of intersexual divergence. Many morphological, behavioral, and ecological differences distinguish males and females in lekking long-tailed manakins (Chiroxiphia linearis), and none is more dramatic than the demographic divergence. Only 16 of 142 (8%) banded males copulated during an 8-year period. The mean estimated age of male copulators was 10.1 years (SD = 2.2), and only 5 of 166 copulations were by males ≦8 years old. Females probably begin reproduction at age 1 or 2. The reproductive value curve reached a peak of 15.0 for 12th-year males, versus 2.7 for sixth-year females. The matrix-based elasticity of X to survival rates was greater in males (91% of total elasticity) than in females (80% of total). In a literature-based, interspecific comparison, the difference in elasticity to survival between the male and female manakins (91–80=11; ranks 2 and 9 of 16 species/sex combinations) was greater than that between the sexes in northern elephant seals (90–84=6; ranks 3 and 8), which have the highest variance of male mating success documented for mammals; red deer (88–87=1; ranks 4 and 5); Galapagos cactus finches (79–74=5; ranks 10 and 12); and acorn woodpeckers (76–74=2; ranks 11 and 13). In the face of continuing debate over appropriate measures of sexual selection, matrix-based demographic techniques facilitate quantitative, comparative analyses of the life-history consequences of sexual selection. Measures of intersexual demographic divergence may provide insights into heretofore puzzling instances of sexual selection in species with little dimorphism in size or ornament.

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