Demographic consequences of sexual selection in the long-tailed manakin

Demographic divergence between the sexes is a major consequenceof sexual selection. Matrix-based demographic measures, includingthe sensitivity and elasticity of (population growth rate,fitness) to survival and fertility rates are powerful indexesof intersexual divergence. Many morphological, behavioral, andecological differences distinguish males and females in lekkinglong-tailed manakins (Chiroxiphia linearis), and none is moredramatic than the demographic divergence. Only 16 of 142 (8%)banded males copulated during an 8-year period. The mean estimatedage of male copulators was 10.1 years (SD = 2.2), and only 5of 166 copulations were by males 8 years old. Females probablybegin reproduction at age 1 or 2. The reproductive value curvereached a peak of 15.0 for 12th-year males, versus 2.7 for sixth-yearfemales. The matrix-based elasticity of X to survival rateswas 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 andfemale manakins (91–80=11; ranks 2 and 9 of 16 species/sexcombinations) was greater than that between the sexes in northernelephant seals (90–84=6; ranks 3 and 8), which have thehighest 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 appropriatemeasures of sexual selection, matrix-based demographic techniquesfacilitate quantitative, comparative analyses of the life-historyconsequences of sexual selection. Measures of intersexual demographicdivergence may provide insights into heretofore puzzling instancesof sexual selection in species with little dimorphism in sizeor ornament.