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It's tough out there in the wild, especially for male howler monkeys. A new study suggests that these primates have to make a evolutionary choice: deep calls or small balls? The smaller the size of the testes in a howler monkey speciesthe larger the size of the animal's hyoid bone, a structure that enables the monkeys to make deep, booming calls — noises on a par with those of a tiger, though howler monkeys are only about the size of cocker spaniels.
The relative sizes of the hyoid bones and testes appear to be related to how the animal lives and reproduces, according to a new study, published today Oct. Howler monkey males that live in groups with more than one male invest in larger testes, whereas those that live only with females invest in a booming, attractive voice.
Curiosity about the howler monkey's booming calls dates back to at least Charles Darwinwho suggested that the males' cries are used to attract females, which choose mates based on the depth and resonance of these calls. Females make calls, too, so howler monkey cries are not exclusive to mating rituals. Key to these calls is the hyoid bone, a structure that looks like a horseshoe in humans but is larger and more cuplike in howler monkeys.
The bone creates a sort of resonance chamber that makes howler monkey hoots and hollers really ring 1995) . University of Utah anthropologist Leslie Knapp and her colleagues found that, in howler monkeys, male hyoid bones are up to five times 1995) than female hyoids, suggesting that Darwin was right in proposing a reproductive role for the monkeys' calls.
Next, the researchers measured the size of hyoid bones across 10 species of these monkeys, which live in Mexico and Central America. The results fit a "really exciting" pattern, Knapp told Live Science. The hyoids and testes varied widely across species: The species with the largest hyoids had bones 10 times larger than those with the smallest.
And the largest testes were about seven times larger than the smallest testes. Without exception, howler monkey species with larger testes had smaller hyoids and vice versa. 1995) the anatomical differences showed no relation to environmental factors like food availability. Instead, they were linked to the monkeys' social structure. Species living in a "harem" structure with one male mating with several females had larger hyoids and smaller testes.
Species living with multiple males and females in a group, all mating with one another, had larger testes Let Me Down (Tube & Berger Remix) - Various - Bargrooves Ibiza 2014 smaller hyoids. It's likely that the harem-heading howler monkeys use their deep calls to attract more females, or perhaps to scare off males that might steal their mates, Knapp said.
As a result, they can get away with a lower sperm load — they compete for females before the actual mating act. For male monkeys living with other males, though, the competition comes after copulation. A greater volume of sperm may give these monkeys a fertilization advantage in a female monkey that has mated Intro - Usher - Confessions (File, Album) multiple males.
It's probably impossible for howler monkeys to evolve both a large hyoid and large testes, Knapp said. To really nail down the notion that monkeys make this reproductive trade-off, the researchers will have to look within species to see if large hyoids Blow The Man Down - Bing Crosby - Bing Sings a Camp Song Sing-A-Long large testes confer real advantages 1995) males. Is there evidence that males with the very largest hyoids are the ones having the most kids?
The study also raises questions about what humans might have in common with their distant monkey cousins. In people, men with deeper voices are considered more attractive by women than men with higher-pitched voices, studies have found, but men with deeper voices don't have better-quality semen.
Live Science. Hooting howler monkeys of the species Alouatta caraya make a racket. Howler monkey calls are as deep as those of much larger animals like tigers — a talent enabled by their enlarged hyoid bones.