The females storing sperm makes the genetics especially difficult because you have to make sure that there are no males in a cage of virgin females. So you have to separate them as pupae and to most novices they look very similar. I am not quite there yet.
Serotonin is an incredible molecule. Older than nervous systems. It is involved in communicating nutritional states and evokes behaviors in response. In human pop psychology, we associate it with mood. Specifically we associate higher levels with good mood because drugs that amp it up seem to reduce symptoms of depression. YET drugs that reduce serotonin sometimes also reduce depression. It also tends to involve tilting the brain into making decisions, i.e. increasing impulse tolerance. That is why in depression pharmacotherapy you always have to worry about the side effects of suicide. Before the serotonin levels adjust to the new baseline, you end up with severely depressed patients motivated to do something.
So, our understanding of serotonin is really spotty. It is everywhere in biology but no one really knows its fundamental role. In humans it is responsible for blood vessel tone as digestive movement, but also mood, memory, sex, disgust, fear, compulsion, and classical psychedelic experiences.
In terms of why they become dumber -- I should add a caveat. Dumber in certain ways. I think it is likely just a product of having fewer "dials" in the brain and so their entire behavioral apparatus is somewhat blunted. The social aggression stuff is the most well documented and to me, interesting. Serotonin seems to play an important role in determining where you "feel" you belong is a social hierarchy.
I have my pet hypotheses, but it is a fascinating molecule. Much more than the "mood molecule" is it portrayed.
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