DEMOGRAPHIC CONSEQUENCES OF SEXUAL SELECTION IN THE LONG-TAILED MANAKIN

Demographic divergence between the sexes is a major consequence of sexual selection. Matrix-based demographic measures, including the sensitivity and elasticity of lambda (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 1 66 copulations were by males less-than-or-equal-to 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.