DENSITY-DEPENDENT SEXUAL SELECTION IN THE FUNGUS BEETLE, BOLITOTHERUS CORNUTUS.

The hypothesis that population density can affect sexual selection on male horn size was tested in a three-year study of a fungus beetle, Bolitotherus cornutus. Males of this species have horns that vary greatly in length. These horns are used in fights over females; longer-horned males win the majority of fights, regardless of population density. However, density does affect the relationship between horn length and access to females. In six populations of naturally and experimentally varying densities, longer-horned males gained a greater advantage in access to females in low-density populations than at high density. This increase in access to females causes an increase in the number of females inseminated by longer-horned males; thus, sexual selection for longer horns is stronger at lower densities.