women find deep male voices attractive...

No offense to tenors, but outside of opera, a high male voice is seldom, if ever, considered seductive. Scientific research has shown that women find deep male voices attractive, and the same is true in other species, like howler monkeys.

But evolution is often stingy in its gifts, and researchers investigating male competition to reproduce have discovered an intriguing trade-off in some species of howler monkeys: the deeper the call, the smaller the testicles.

Jacob Dunn of Cambridge University, one of the leaders of the research, said that species evolved either to make lower-frequency sounds, or have larger testicles, but none had both a very low sound and very large testicles.

“It’s a great study,” said Stuart Semple, an evolutionary anthropologist at the University of Roehampton in London who was not involved in the research. “It shows this really clear trade-off.”


Vervet monkeys relinquished their dislike of a colored corn when they changed location and saw other monkeys eating it.Study in Science Shows Monkeys Pick Up Social CuesAPRIL 25, 2013
ScienceTake: Monkeys Provide Clues to How Tool Use DevelopedMAY 18, 2015
Dr. Dunn and other researchers, including W. Tecumseh Fitch, of the University of Vienna, and Leslie A. Knapp, of the University of Utah, studied the size of a bone in the vocal apparatus, which is directly related to how deep the calls are, and the size of testicles, to come up for averages in nine species of howlers.

They had been intrigued by great variations in both the size of the howlers’ hyoid bones in museum collections and in the size of the monkeys’ testicles as seen in the field. Dr. Knapp said that some of them are large enough that they are quite obvious “when you look up into the trees.”

They used the museum samples of the bone and living monkeys in zoos for testicle measurements, and reported their findings Thursday in the journal Current Biology.

The differences in anatomy were directly related to differences in monkey lifestyle. In some species of howlers — monkeys that live in Central and South America — males compete to gain control of mating access to several females. A basso profundo helps attract mating partners and scare off rivals, so a deep call is important. Testicle size, which is directly related to sperm production, is not so important.

In other species, females copulate with several males, and the more sperm a male produces, the better the odds he has of reproducing because the sperm from several males all compete to fertilize the female’s eggs. In those species, sperm trumps sound. Dr. Dunn said that similar trade-offs are seen in other animals, like gorillas and chimps.

One male dominates a gorilla group, he said, and “They invest in body size. Males are huge.” But, he said, their testicles are “tiny.” In chimpanzees, mating is more of a free-for-all and sperm from several males may compete for success in fertilizing a female’s eggs. Chimpanzee testicles, he said, are “massive.”

The reason howlers do not have both large testicles and a very deep call, said Dr. Dunn, seems to be that both options impose a cost in terms of energy expended, either as the animal is growing or during its life.

And there are other costs. Testosterone, which makes for larger testicles, can suppress immune function, he said. Another possibility: “Bigger testicles are also vulnerable to being caught on a branch or scratched by another monkey.”

It may also be, he said, that once a species started down one evolutionary road, sound or testicles, evolutionary pressures kept them on that path.

The researchers studied nine howler species. They created 3-D scans of 255 hyoid bones in museum collections. In howlers, the hyoid bone creates a resonating chamber for sound and its size is directly related to sound frequency. The bones were saved in museums before the era of genetics as a way to identify species.

There are hints in other studies of possible similar evolutionary trade-offs in humans. On the one hand, some research has suggested that men with deep voices have more sex partners, and therefore more opportunities to reproduce. But another study showed lower sperm quality in deeper-voiced men.

Nonetheless, the recent research has no practical application to human mate choice. The monkey research was not about individual monkeys, but different species, while humans are all one species. The hyoid bone does not function the same way in human beings. And, as Dr. Knapp pointed out, human females have many more important criteria for mates other than testicle size.

The research on the live zoo animals posed some challenges. Their testicles were measured when the animals were lightly sedated for other purposes. Still, Dr. Knapp said, “It’s not the easiest thing to do.”

As for extending the research, perhaps into the monkeys’ natural habitat, she said, “We have to come up with another way to measure testes size if we’re working in the field.”