Cambridge – A recent study from the University of Cambridge suggests that males from the howler monkey species tend to have smaller reproductive organs as bigger their howls are.
Their scrotum’s smaller and makes less semen, according to a study released Thursday by the University of Cambridge. Tecumseh Fitch, an evolutionary biologist and cognitive scientist at the University of Vienna, and Leslie Knapp, a biological anthropologist at the University of Utah, were among the other co-authors.
“The bigger a male howler’s vocal organ, and the deeper and more imposing roar they possess, the smaller their testes and the less sperm they produce,” the study authors found.
The study, published in the journal Current Biology was based on data from museums, zoos and research in many countries. To obtain the data, which Knapp collected with researchers in the United States and Europe, researchers calculated the volumes of 255 howler monkeys. Data on testicle sizes for 66 howler monkeys were collected from published works, but Knapp said researchers also partnered with several zoos to measure the testes of 21 monkeys while the animals were sedated during health exams.
The reason howlers do not have both large testicles and a very deep call, said Dr. Dunn, seems to be that both options impose a cost in terms of energy expended, either as the animal is growing or during its life. As explained by Jacob Dunn of Cambridge University, one of the leaders of the research, species evolve either to make lower-frequency sounds, or have larger testicles, but none had both a very low sound and very large testicles.
But that’s not the only difference, males with big howls live in small social groups where a single dominant male mates with a number of females lured by his sound and fury. By contrast, the other animals with bigger unmentionables live in larger communities of about six males and lots of females.
“There is evidence in other animals that when males invest in large bodies, bright colors or weaponry such as horns or long canines, they are unable to invest in other reproductive traits […] However, this is the first evidence in any species for a trade-off between vocal investment and sperm production,” said Dunn, who has worked with howler monkeys for a decade.
Source: NY Times