It takes a lot of energy for an animal to “invest” in traits that’ll attract a mate including large teeth, colorful feathers, or in the case of the howler monkey, a loud and boisterous mating call. Sometimes it can be worth it, as impressive males are more likely to find the animal bride of their dreams. Other times these animals are losing some potency at the cost of some flair.
Research published in the journal Current Biology found an inverse correlation between the loudness of a male howler monkey’s calls and the size of his balls. Smaller monkey testes produce less sperm, meaning the tiny-testacled primates are less fertile.
According to CNET, the correlation is tied with the hyoid bone, which is a bone in the howler’s throat that allows for loud lower-pitch calls. Larger calls would be associated with larger monkeys, both scaring off competing males and attracting fertile females.
The discrepancy between organs was pretty huge at times—the largest hyoid bone found was ten times bigger than the smallest specimen. We really don’t want to see what kind of heat the smallest-hyoid monkey was packing!
Reasons unknown
Researchers commented that the study was very interesting, but the reason for the inverse relationship is not yet known.
“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,” said Jacob Dunn, the study’s leading researcher from the University of Cambridge.
Although further testing is needed, this research might also provide an understanding as to why certain human males drive such large pickup trucks.
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Feature Image: Thinkstock
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