Sexual Selection in Penises and Clitori

In species that do not use copulatory thrusting, especially insects, penises evolve more obvious tactile stimulators: nubs, spikes, ridges, curls, barbs, hooks, and flagella. Male insects often try to push each other off during copulation, so copulatory thrusting would risk disengagement. Better to lock the genitals together and have internal flagella to excite the female. With primates, it is not so common for male rivals to swarm over females knocking each other off. This allows couples a ...