“An extraordinary range of testes mass was documented across bat species – from 0.12% to 8.4% of body mass. That exceeds the range of any other mammalian order.” Even the dumbest 12 pound monkey wouldn’t sport 1 pound testicles.

Likely, this balance between brain and testes size is related to flight.  The routine defeat of gravity requires strict maintenance within an aerodynimically restricted window of mass to volume ratio and strength to weight ratio.  Therefore, a flying mammal will likely show a seasonal variance is testes mass not found in earthbound mammals.  Also, nonessential organs, such as a brain, are also competing for space on the flight platform on a permenant basis within each individual.