"Reproductive isolation"

The local authorities in Japan subsequently conducted a DNA survey of all the monkeys in the zoo and confirmed that 57 of the 167 monkeys were hybrids of macaques crossed with a specific exotic creature, the rhesus macaque, accounting for about one-third of the monkey population

The existence of reproductive isolation, so that these situations can not occur, even if there are some individuals of different species sometimes complete the mating, but they generally can not produce offspring, or produce offspring can not continue to reproduce