Genital behaviour in crane flies (Diptera: Tipuloidea): testing hypotheses for genital evolution.

Area of Science:

• Evolutionary biology
• Animal morphology
• Behavioral ecology

Background:

• Genital morphology often diverges faster than other animal traits.
• Observing male genital behavior during copulation is challenging, hindering evolutionary explanations.
• Tipuloid crane flies offer a unique model due to external male genital structures during mating.

Purpose of the Study:

• To investigate the evolutionary pressures driving rapid genital divergence in animals.
• To test the cryptic female choice hypothesis against other explanations for rapid genital evolution.
• To document male and female genital behaviors during copulation in tipuloid crane flies.

Main Methods:

• Observed copulatory behavior in 45 species across 21 genera and subgenera of tipuloid crane flies.
• Recorded male genital movements (e.g., brushing, tapping) and female responses.
• Analyzed patterns of male-female genital interaction and morphological coevolution.

Main Results:

• Male crane flies exhibit sustained, rhythmic genital movements; females frequently facilitate stimulation.
• Lack of consistent morphological coevolution between male and female genital structures was observed.
• No forceful male manipulation of females, female defenses, or direct male sperm interference occurred.
• Tipuloids generally lack rhythmic, forceful genital thrusting and squeezing typical of other Diptera.

Conclusions:

• Observed behaviors strongly support the stimulation-based cryptic female choice hypothesis for rapid genital evolution in tipuloids.
• This contrasts with hypotheses involving forceful male manipulation or female defense mechanisms.
• Tipuloid crane flies provide a valuable model for understanding the evolution of animal genitalia.