Deep males voices have that distinct power to attract females, possibly leading to a relationship. For male animals, hooting and hollering are used to get the attention of females and ward off other suitors. However, deeper voice may not make howler monkeys manly enough as researchers found the species to have smaller testicles.
International researchers have been interested in the significant discrepancies between the size of monkeys' testicles seen in the field and the size of museum-displayed hyoid bones, which support the tongue and are found in the neck. Leslie A. Knapp, one of the authors from the University of Utah commented that some of the monkeys' testicles are big enough that they become too apparent when monkeys look up at trees.
The researchers obtained samples of hyoid bones from museums in the U.S. and Europe and measured testicles of monkeys from zoos in Germany and Brazil. The monkeys were sedated for health exams prior to measuring its testicles through calipers. Data on testicles sizes of 66 more monkeys were collated from published writings.
Male howler monkeys are said to be unique from other primates because of its bigger hyoid bone that acts as an echo machine to amp up voice. While all the species howl to interact with each other, some howl louder and more deeply than others. Species with larger hyoid bones are able to generate louder and deeper sounds thereby giving the impression of larger body size.
As hyoid bones are quite "expensive" organs, the researchers thought that male howlers may need to compensate by exuding smaller organs in the other systems. To see if this hypothesis is correct, the researchers studied information on hyoid volume in nine of 10 species, groups of social circles in eight species and volumes of testes in five species.
Knapp said that males with smaller hyoid bones have bigger testes and that it settle with a group composed of numerous males and few females.
The question now is, why does evolution result in such a tradeoff? Why not compensate with a smaller liver, a decreased heart size or shorter limbs? "Your liver or kidney or toe doesn't contribute to your having a mate or the possibility of more offspring," explained Knapp.
The authors are making further investigations involving the measurement of hyoid and volume of testes in individual male species to know if the link they discovered between species is also applicable between individuals, instead of groups.
The study was published in the journal Current Biology on Thursday, Oct. 22.
Photo: Deanna Keahey | Flickr
