One vital part of the male reproductive system is linked with the word "courage." Thus, when a man is brave and would stand by his convictions despite the consequences, he is said to have "balls" which is also called nuts or family jewels.
Reuters Portugal's soccer star Cristiano Ronaldo poses during the launch ceremony of his CR7 by Cristiano Ronaldo underwear line at Madrid's city hall October 31, 2013
However, a new study links the testicles with a more negative connotation - infidelity, at least among primates.
The research led by Peter Bockman of the University of Oslo found that female primates are faithful to their partners if the male has large testicles.
Mr Bockman explained, quoted by Mail Online, "If the male will only fertilise one female and has no competitors, he only needs sufficient sperms to reach the egg ... If the female mates on the side, it is smart to have as many cars as possible in the race."
"Then, the male must have testicles that are as large as possible. In gorilla troops there is only one male," he added.
But he noted that gorillas actually have no need for large testicles because its balls are small. However, one primate species, the Bonobos, have huge testicles, resulting in an abundant flow of semen and the animal mating with everybody.
But grasshoppers and other animals with short lifespans, they have unusually large testicles, according to the researchers. Among grasshoppers, the balls occupy half of their body mass, while among sea urchins, it is considered as a sea creature with a huge testicle and a little shell around it.
Mr Brockman said since the human testicles are one and a half times bigger than gorillas, it is an indicator that humans are unfaithful by nature. He added that having very large balls could be a disadvantage because it increases the risk of testicular cancer.