Howler Monkeys Can Either Be Really Loud or Well-Endowed

Dating is tough. We all know that. To attract the right mate, or any mate at all, you have to stand out from the crowd. Maybe your best quality is your sense of humor, or your twinkling eyes, or your sweet dance moves. Maybe—if you’re a howler monkey—it’s your enormous testicles, or your ability to yell louder than anyone else.

To ensure the continuation of their genes, male howler monkeys invest energy in making noise or making sperm. But scientists say they can’t do both. New analysis from the University of Cambridge finds that for howler monkeys, louder calls equal smaller balls.

The howler monkey is a curious creature, a terrier-sized primate with a tiger-sized roar. The monkeys can only reach about 15 pounds, but their calls are among the loudest on the planet. The males’ bellowing, belching calls serve two purposes: to catch lady monkeys’ attention, and to tell other males to take a hike.

The howler monkeys’ larger-than-life noises are possible thanks to a specialized bone called a hyoid. This hollow, pouch-shaped bone acts as an echo chamber, amplifying the calls before they ever leave the monkey’s mouth. The bigger the hyoid bone, the more noise a monkey can make.

This is more important for some species than others, and it all comes down to sex. Some howler monkeys have harem-type setups, where one male monkey mates with several female monkeys. In those arrangements, the challenge lies in appealing to the ladies and keeping other dudes away, but once mating has taken place, a male monkey can be pretty sure his sperm are safe.

Other species have a more relaxed sex scene in which female monkeys are free to take as many mates as they want. For these species, the males with the biggest cojones—and thus the most sperm—are the ones most likely to succeed in making monkey babies.

But yelling and making sperm each require a lot of energy. The monkeys can’t have their yelling and big balls, too.

These are the conclusions of a new paper published this week in the journal Current Biology. Researchers used 3-D laser scans to measure the hyoid bones and testes of 10 species. They found a distinct inverse relationship between the two. Harem-type species had larger hyoid bones and smaller testes, and the reverse was true for the free-love-type species. Exactly how this trade-off happened is not totally clear, lead author Jacob Dunn said in a press release.

“In evolutionary terms,” Dunn said, “all males strive to have as many offspring as they can, but when it comes to reproduction you can’t have everything.”