“ The snake in the story of Eden is doubtless a phallic symbol…suggesting sexual awakening as the beginning of the knowledge of good and evil.”
Our motivations change throughout life, but three of the four fundamental drives are with us from birth: to consume, to sleep and to avoid danger. The fourth drive, to reproduce, takes much longer to arrive…where does it come from?
Juvenile flies show no courtship behavior. Mating drive increases over the first 3 days of life, as measured by the fraction of time males spend courting virgin females.
Since there are so many physical changes that occur around the same time, the dominant hypothesis has been that structural changes in the brain wire up the motivational and behavioral circuitries required for reproduction. But that hypothesis is hard to test directly. The only experiment you can really do is to manipulate the hormones that instruct the behavioral changes and see if they also induce structural changes in the brain. And they do. But does that mean that the structural changes drive the behavioral changes? It looks like it, but it also looks like the sun goes around the earth, right?
In flies we find strong evidence against the new wiring hypothesis for the onset of mating drive. Though very young flies can do most things older flies do, they don’t engage in reproductive behaviors. We had previously (Zhang 2016, Zhang 2018, Zhang 2019) found dopaminergic circuitry that instructs the onset and recovery from mating satiety in mature flies. What is going on with this motivational circuitry in juveniles, is it not set up yet?
We found that if you stimulate any of the courtship-motivating neurons, even the youngest males start chasing around females and singing to them. So the brains of juvenile males can perform the courtship ritual—they just don’t.
Thermogenetic stimulation of P1 neurons in juvenile males causes them to court, showing that the circuitry for courtship is intact in even the youngest males.
So…at least in flies, the circuitry for reproductive behavior seems to be all wired-up and functional in juveniles, it’s the motivation that’s lacking. We found a hormonal change that activates this circuitry as flies age, similar to the hormonal changes that happen in mammals. So has everyone had it backwards until now? Could it be that the structural changes we see in mammalian neurons during puberty are a consequence of increased activity in reproductive circuits, instead of the cause? It wouldn’t be the first time that simple experiments in flies led to important realizations about human biology.
Here’s a link to the paper: Zhang 2021.