According to the Metro today the production of sperm and brain cells are both very energy demanding. Thus males cannot generate large numbers of both.
The study found that in bat species where the females were unusally promiscuous the males had much larger testicles, up to 8.5% of their body mass, as opposed to 0.02-0.75% in apes, however these males also had much smaller brains.
In species where the females were monogamous have relatively large brains.
However male promiscuity doesn’t seem to affect this at all.
And some more cute bats