Descent of testicles.

Another crucial question, the answer to which I think would help inform this discussion, is whether bats and possums, which gestate and brood their offspring while hanging upside down, produce males with ascending testicles. Also, there is a pretty obvious explanation as to why birds would evolve a solution to gonadal temperature control other than descendable testicles. Can you imagine the problems involved in flying with all that baggage dangling in the wind? Not to mention the inconvenience of snagging those appendages on tree branches.