A recent study published in the journal of Current Biology has found that deep-voiced howler monkeys have smaller testicles. Females tend to take this aspect into consideration when choosing to mate with a partner or not, scientists have explained.
Jacob Dunn, a co-author of the study and professor at the Cambridge University, has recently stated that the idea of a study on howler monkeys’ mating voices came to him after noticing there is a wide variety of monkey bones at museum collections. They were particularly interested in a special bone in the vocal apparatus and its capabilities.
Due to the current research they have conducted, scientists have discovered that these sizes have a direct relation with the places where the howler monkeys live and their physical construction. It appears that males living in Central and South America need deep voices to attract partners and scare potential rivals.
Other howler monkeys, living in different regions tend to have high-pitched voices because female monkeys don’t care about this feature when looking for a potential partner. Females in these regions copulate with different partners; therefore, the testicle size is more important than the voice because it proves males are capable of producing larger amounts of sperm and fertilizing women’s eggs.
When they compared the bone in the vocal apparatus with the testicle size of each and every species, scientists have found out that the two items go hand in hand. To be more specific, the deeper the voice of a male is, the smaller his testicles are.
Researchers have also discovered that few howlers rarely have large testicles and this, because the two assets would require many efforts on behalf of the animals. Large amounts of testosterone, which are specific for males with larger testicles can suppress immune functions; therefore, howler monkeys cannot have deep voices and large testicles at the same time.
These findings have been made after comparing the bones of nine different species. Investigators have performed 3D scans for 255 hyoid bones that were exposed in museum collections. In the future, scientists plan to recreate the same study on monkeys living in their natural habitats. In addition, they plan to make a comparison with human bones and identify possible similarities.
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