"The genital organs are, far more than any other external member of the body, subject merely to the will, and not at all to knowledge....According to all this, the genitals are properly the focus of will." -Arthur Schopenhauer, The World as Will and Representation (1818)
"They sedulously display their various ornaments, exert their voices, and perform strange antics in the presence of the females."
-Charles Darwin, The Decent of Man, and Selection in Relation to Sex (1871)
We are not reflex machines and we suspect that flies are not either.
Most of our behaviors are flexible. Perception of an object in the world (say, food) only elicits a behavioral response (eating) when conditions are appropriate (we're hungry, food is good, eating companions have arrived).
Flies show an impressive amount of behavioral flexibility, in their feeding behavior, for example, or in determining of whether or not to continue mating in the face of dangerous external conditions. We are trying to understand how this kind of flexibility is worked into sensory-motor circuitry.
Whereas there is not much known about the circuitry underlying copulation duration, male courtship behavior has been studied for 100 years and the circuitry is among the best understood in neuroscience. We are using this system to study how the male's fertility status determines whether or not he will court a virgin female. Because mating behaviors are sexually dimorphic, we are able to focus on the ~2% of the neurons that expresses sexually dimorphic transcription factors. The predominant instructor of sexual dimorphism in the fly brain is Fruitless, which is expressed in what appears to be a continuous circuit stretching from sensory input neurons through to motor output neurons. We want to know how internal state information controls the flow of information through the Frutiless circuit so that sensory perception of a female is only translated into courtship behavior when internal conditions are appropriate for mating.
We found a small group of dopaminergic neurons whose activity is a functional neuronal correlate of mating drive: their activity reflects and instructs mating drive (see picture above). The activity of these dopaminergic neurons is decreased following matings, because the male temporararially uses up his supply of sperm and seminal fluid. These dopaminergic neurons signal to a cluster of neurons called P1 that receive sensory input from the female. So, in order to get courtship behavior, you need two inputs onto P1: high quality sensory input from the female and high drive input that reflects (via dopamine) the reproductive capacity of the male.
This circuit logic makes a lot of sense and we expect it to be broadly conserved. We continue to work to understand how dopamine levels are set to reflect reproductive capacity and how these levels gate the flow of activity through the P1 node at the transition between sensory inputs and motor outputs.