Darwinian balancing selection: predation counters sexual selection in a wild insect

Area of Science:

• Evolutionary Biology
• Behavioral Ecology
• Animal Communication

Background:

• Sexually selected traits can incur viability costs, influencing the evolution of exaggerated features.
• Selection analyses integrating multiple fitness components are crucial for estimating these costs in natural populations.

Purpose of the Study:

• To investigate the trade-offs between sexual selection and viability selection on male traits in tree crickets (Oecanthus nigricornis).
• To determine if exaggerated sexually selected traits impose survival costs in this species.

Main Methods:

• Compared survival rates of tree crickets preyed upon by solitary wasps versus those that survived.
• Analyzed sexual selection by comparing mating males to solitary males.
• Measured forewing width, head width, pronotum length, and hind jumping leg size as traits under selection.

Main Results:

• Found opposing selection pressures: directional sexual selection favored wider heads, while viability selection favored narrower heads.
• Nonlinear selection indicated that males with wide heads and small legs were most attractive, but narrow heads, large legs, and intermediate pronotum length maximized survival.
• Suggests that mating may increase predation risk, potentially due to copulation itself.

Conclusions:

• Balancing selection on male traits in tree crickets may constrain the evolution of sexual dimorphism in head size.
• These findings support the hypothesis that sexually selected traits can have significant viability costs.
• The results offer insights into the evolutionary dynamics of exaggerated traits in insects and other animals.