Howler Monkeys

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A new study found that “Howler Monkeys” scream “louder” when they have “smaller” testicles.

The study published in the journal “Current Biology” said that howler monkeys with “deep” roars, used to “attract” females, deter love “rivals” and scare off “predators,” tend to also have smaller “testicles” and a shortage of “sperm” compared to their peers.

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Charles Darwin, writing on how species’ “evolutionary” choices impact each other in “On the Origin of Species” said:

“The whole organism is so tied together that when slight variations in one part occur, and are accumulated through natural selection, other parts become modified.”

Biological “anthropologist” Jacob Dunn and his team from Cambridge University wanted to see if there was a “negative” correlation between the “size” of a howler monkeys “vocal” tract and the size of his “testicles.”

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The team used 3D laser scanners to “analyze” a cup-shaped chamber in the howler monkeys “throat” called a “hyoid” that acts an “echo” chamber. Dr. Dunn said:

“The results of our acoustic analyses show that howler monkeys produce roars at a similar frequency as tigers, which is far lower than we would have predicted from their body size, yet exactly what would be predicted from measuring their giant vocal folds’ which are three times bigger than in a human.”

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Howler Monkeys can be found in the “forests” of Central and South America and “weigh” roughly 15 pounds, but their “howl” can reach 128 decibels, which is equivalent to the “roar” of a tiger.

The researchers “speculate” that this is because howler monkeys want to give the “impression” that they are “bigger” than they actually are.

Dr. Dunn proposed: “It may be that investment in developing a large vocal organ and roaring is so costly that there is simply not enough energy left to invest in testes. Alternatively, using a large vocal organ for roaring may be so effective at deterring rival males that there is no need to invest in large testes.”

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The study further “revealed” that males with bigger “hyoids” had smaller “testicles” and lived in smaller “social” groups where there was a single male “dominating” a number of females.

Male howler monkeys with bigger “testes” and smaller “hyoids” most likely lived in “bigger” social groups and “share” partners.

Dunn said: “In evolutionary terms, all males strive to have as many offspring as they can, but when it comes to reproduction you can’t have everything.”

Howler Monkeys living in the West…

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