Well, that explains a lot- Evolution catches up to Donald

"Howler monkeys can be found in the forests of Central and South America and weigh roughly 15 pounds, but their growl can reach 128 decibels, which is equivalent to the roar of a tiger.
The researchers speculate that this is because howler monkeys want to give the impression that they are bigger than they actually are. Dr. Dunn proposed:
“It may be that investment in developing a large vocal organ and roaring is so costly that there is simply not enough energy left to invest in testes.
“Alternatively, using a large vocal organ for roaring may be so effective at deterring rival males that there is no need to invest in large testes.”

http://nextshark.com/howler-monkey-testicles-study/