Movements and morphology under sexual selection: tsetse fly genitalia

Abstract

Shon (2009, Ethology Ecology Evolution 21: 161-172) pointed out that in order to understand the functional morphology of sexually selected structures that are used as signaling devices in birds, it is crucial to understand how these structures move during sexual interactions. This insight applies not only to bird feathers, but also to many other types of possible signaling devices, including male genitalia. This note highlights the need for studies of the behavior of genitalia, and describes two promising techniques, using a tsetse fly as an example. Observations of this species revealed otherwise cryptic, highly rhythmic and forceful thrusting, pinching, pressing, and scraping movements by the male's genitalia within the female's body that have no obvious relation to sperm transfer. Thus even though on the outside the male`s body is nearly motionless during long lapses during copulation, the female is subject to a barrage of possible stimulation from his genitalia during copulation. Similar studies are needed in other groups to understand the functional significance of genital morphology.