Monogamy

Paternity tests given to birds, monkeys and foxes reveal that just because those species had developed a social relationship similar to marriage does not mean that they had been faithful to each other.

In 70% of cases, offspring turned out to have been fathered by males other than their partners.

The only species that does not commit adultery and which there is 100% monogamy is the flatworm, where the male and female meet as adolescents and their bodies fuse together.