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.