A new study in Current Biology has found an inverse correlation between the volume of howler monkeys' notoriously loud hoots and the size of their testicles.
I can attest to the remarkable volume of howler monkeys' howls; in my twenties, I volunteered on a school-building project in a swamp on the Nicaragua/Costa Rica border, and the local howler monkeys were our dawn chorus (they also stole our peanut butter and contaminated our food, giving me a naasty bout of typhus).
Male howler monkeys use their howls to attract mates; louder howls attract more mates, but the apparatus to produce them comes at the expense of testicle size and sperm production. The quieter monkeys have bigger balls and more sperm.
The two strategies have different payoffs. Loud monkeys end up with an exclusive group of mates; quiet monkeys live in free-love monkey communes, where mating takes place with multiple partners, and where increased sperm production improves the chances of a given big testicle/small voice monkey's genes.
Despite the researchers' caution against anthropomorphizing the monkeys' behavior, it's certainly tempting to see parallels in certain types/loud as a motorbike/couldn't bust a grape in a food-fight.
As Knapp mentions above, a louder, small-balled monkey is more likely to develop a harem of females with whom he has exclusive breeding access. The quieter, well-endowed monkeys, on the other hand, tend to end up in larger groups containing many males and females that copulate freely with each other. In this non-exclusive group, males compete for paternity quite literally with their balls. The bigger a male's sperm count, the more he is to edge out all the other males that are mating with the same females.
In this way, howler monkeys have evolved two sexual strategies—calls versus balls—to the exclusion of each other. Loud monkeys need to secure exclusive access to females because they would have trouble keeping up with their big-balled counterparts, who are more likely to fertilize females due to larger sperm production. Quieter monkeys have less success attracting females with their bellows, but they are locked-and-loaded where it counts.
Naturally, Knapp warns against anthropomorphizing these findings, and she is completely right. There is a world of difference between howler monkeys and humans, especially with regard to sexual selection.
The Louder the Monkey, the Smaller Its Balls, Study Finds
Evolutionary Trade-Off between Vocal Tract and Testes Dimensions in Howler Monkeys
[Jacob C. Dunn, Lauren B. Halenar, Thomas G. Davies, Jurgi Cristobal-Azkarate, David Reby, Dan Sykes, Sabine Dengg, W. Tecumseh Fitch, Leslie A. Knapp10/Current Biology]e