A new study reveals that howler monkeys with the loudest calls produce less sperm than their quieter counterparts.
The next time a male calls out to you on the street, you can rest easy knowing that he’s just trying to compensate for something. According to a report from Phys.org, catcalling may not just be a human trait. A recent study shows that howler monkeys, which weigh roughly seven kilograms, produce a loud, guttural wail to attract mates. The loudest ones, however, are the ones that produce the least sperm.
Howler monkeys are some of the loudest land-animals on the planet, and their tiny bodies can produce a roar that is similar in frequency to that of a tiger. For male howler monkeys, one of the main uses for their complex and robust set of vocal cords is to attract mates and scar off other rival males.
A new study compared the volume and intensity of a howler monkey’s roars with their overall reproductive success. Scientists found that the ape made a tradeoff, sacrificing testicular size and sperm production for a stronger set of pipes.
The key to the monkey’s ability to roar so loud is in a bone called the hyoid, which is bulbous, hollow, and located in the throat. This bone allows the monkey’s roar to resonate and multiply in intensity. The bigger this bone, the researchers found, the smaller the monkey’s testes would be.
The trade-off between testicular size and roaring intensity is linked to the different mating systems of a number of howler monkey species. Males with big hyoids and loud roars tend to stay in small social groups, with a male dominating a number of females at once.
Males with larger testes and smaller hyoids live in groups of five or six, and females mate with each male in the group. The competition for reproduction in these groups centers around the quality and quantity of sperm, and not so much an individual’s ability to “out-roar” another.
The study’s findings were published in the journal Current Biology, and explain why sexual selection hinges on a number of different factors. According to Charles Darwin’s research in 1871, these “pre-and-post-copulatory reproductive strategies” can help specialized species have the best chance of passing on their genetic information to a subsequent generation.
According to Dr. Jacob Dunn, a professor of Biological Anthropology at the University of Cambridge and the study’s lead author, each species of howler monkey takes its own strategy for reproduction.
When males produce large bodies, bright colors, and outwardly-aggressive body features like horns or big teeth, they are unable to create other traits that would help them successfully reproduce. Conversely, when males invest in their ability to produce higher quantities of sperm, they lose out on the outward signifiers of attraction and must spend more time looking for a mate on their own.
Dr. Dunn isn’t positive, but he and his research team believe that the tradeoff faced by male howler monkeys relates to the amount of energy required to produce such loud screams. Louder monkeys have less energy to invest in the testes, but they may not need to do so as their howls are effective at deterring rival males.
The researchers measured the testes size across numerous different howler monkey species and used 3D laser scans to measure over 250 hyoid bones. They also measured the volume of a number of different howler monkey roars.