How simian monkeys lose their sex glands to human monkeys…
Those who are simply interested in sex are actually no better than monkeys. Monkeys are very expert in sexual enjoyment, and sometimes sex glands are taken from monkeys and placed in the human body so that a human being can enjoy sex in old age. In this way modern civilization has advanced. Many monkeys in India were caught and sent to Europe so that their sex glands could serve as replacements for those of old people.
Srimad Bhagavatam 5.14.30 purport