Demographic drivers of age-dependent sexual selection

Area of Science:

• Evolutionary biology
• Behavioral ecology
• Population dynamics

Background:

• Sexual selection is a key evolutionary driver, but its ecological influences are complex.
• Understanding long-term ecological correlates of sexual selection in natural populations is challenging.

Purpose of the Study:

• To investigate how demographic changes affect the intensity, direction, and form of sexual selection.
• To determine if age structure and mating competition influence sexual selection on bighorn ram horn length.

Main Methods:

• Utilized 21 years of data on wild bighorn rams (Ovis canadensis) from Ram Mountain, Alberta.
• Analyzed demographic parameters, pedigree, and repeated horn length measurements.
• Assessed the influence of breeder sex ratio, competitor numbers, and age structure on selection differentials.

Main Results:

• Sexual selection on male bighorn ram horn length was consistently directional and positive across all ages.
• Selection intensity increased with the number of competitors, correlating with population size and male-male encounters during the rut.
• Breeder sex ratio and age structure did not significantly influence selection intensity.

Conclusions:

• Demographic factors, especially competitor density, significantly shape sexual selection in wild bighorn rams.
• Age-dependent mating tactics contribute to varying selection pressures, with younger males experiencing stronger selection.
• This research offers rare insights into the long-term demographic determinants of sexual selection in a natural setting.