Howler monkeys’ weird evolutionary tradeoff

Recent research reveals that the volume of a howler monkey’s cry and its testicular size are inversely correlated.

Animals have developed a diverse list of different reproductive strategies. Some of them are based on strength, some are based on outward appearance, and some are purely a game of numbers. One strategy, most commonly used by birds, is using sounds to attract mates from afar. Birds aren’t the only animals that call out to prospective partners. The howler monkey is known for its loud, bass-heavy scream. And a new study suggests that it could affect the strange mammal’s morphology.

It’s actually kind of similar to the way male humans call out to women on the street, in that the louder ones are often the more “insecure” individuals. According to a report from Phys.org, a recent study shows that howler monkeys, which weigh roughly seven kilograms, create a loud, guttural cry to attract  their potential mates. The loudest ones, however, are the ones that have the smallest testes.

Howler monkeys are some of the loudest land-animals in the world, and their small bodies can produce a cry that is similar in frequency a tiger’s. In male howler monkeys, one of the biggest reasons for their complex vocal apparatus is to attract mates and scare off other rival males.

The study compared the volume and intensity of a howler monkey’s screams with their overall reproductive capabilities. Researchers found that the primate made a tradeoff, sacrificing testes size and sperm capacity for a louder call.

The key to the monkey’s capability to roar so loud is in a throat bone called the hyoid, which is bulbous and hollow. This bone vibrates and resonates, causing the monkey’s roar to resound throughout its habitat. The larger this bone, the scientists found, the smaller the monkey’s testicles would be.

The trade-off between testicular size and scream volume is indicative of the different mating strategies of a range of howler monkey species. Males with big hyoids and loud cries live in small social groups, with a single male dominating a number of females at once.

Males with larger testes and softer cries, however, live in groups of five or six, and females mate with each male in the group. The competition for reproduction in these groups is higher, thus it’s more important for a male to share his DNA with as many potential mates as possible than it is to attract one from afar with a loud roar.

The study’s findings were published in the journal Current Biology, and explain why sexual selection is dependent on a number of different factors. According to Charles Darwin’s research in 1871, these “pre-and-post-copulatory reproductive strategies” can help highly specialized animals get the best odds for passing on their genetic information to the next 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 has its own unique strategy for reproduction.

Dr. Dunn isn’t completely sure, but he and his research team think that the tradeoff faced by male howler monkeys relates to the amount of energy needed to create such loud screams. Louder monkeys have less energy to invest in sperm production, but their screams have a better overall chance of attracting a mate in the first place.

When males create large bodies, bright colors, and outwardly-aggressive body features like horns or sharp, scary teeth, they have little energy to devote to other facets of the reproductive process. On the other hand, when males invest in their ability to produce more sperm, they miss out on the energy needed to produce outward signifiers of attraction and need to focus on mating as much as possible.

The scientists measured the testes size of numerous different howler monkey species and used 3D laser imaging to measure over 250 howler monkey hyoid bones. They also measured the volume level of a range of different howler monkey species’ roars.

A press release detailing the study’s findings can be found here.

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