Promiscuous ape species have bigger testicles, and the latest discovery finally provides evidence that they also produce more sperm.
Scientists previously proposed that chimps have large testicles because several males mate with a single female, and so have to produce more sperm in order to compete.
Our data indicated that a chimpanzee usually produces about two hundred times more sperm than a gorilla
For their research, published in the American Journal of Primatology, scientists studied chimpanzees, orangutans and gorillas from zoos in Japan and Indonesia.
Analysing samples of testicular tissues at a microscopic level, researchers found remarkable variation between the apes.
They found that the sperm-producing tissue lining gorillas' testes was much thinner than that of orangutans and chimpanzees.
Chimpanzees were found to produce 14 times more sperm than orangutans and even more than the world's largest primates.
"Our data indicated that a chimpanzee usually produces about two hundred times more sperm than a gorilla," explained researcher Hideko Fujii-Hanamoto.
For these three species of ape, the scientists have now proven that testes size is proportionate to sperm production.
The researchers claim that these findings also support theories that sperm production relates directly to reproductive competition and mating behaviour.
Previous studies proposed that testes are smaller in polygynous species such as gorillas where one alpha male monopolises mating with multiple females.
In promiscuous species such as chimps however, there is greater competition between males as several copulate with one female.
APE SEX FACTS
Dominant gorillas are known as "silverbacks" because their coats change colour to highlight their status as alpha male
Adult male orangutans develop facial flanges and issue long calls to attract females
This competition is thought to be the driving factor for sperm production and larger testes are thought to produce more sperm.
However, practical limitations meant sperm production in apes was difficult to accurately measure.
"It is generally difficult to get semen from the animals even if they [are] kept in zoological gardens," said Ms Fujii-Hanamoto.
"Therefore, the testis weight or the ratio of testis weight [to] body weight was used to estimate the ability of sperm production."
Visual observations confirmed that chimpanzees have larger testes compared to their body size than gorillas but it was not clear whether they actually produced more sperm.
Story Credit Here