Of the roughly 5,000 species of mammals, only 3 to 5 percent are known to form lifelong pair bonds. This select group includes beavers, otters, wolves, some bats and foxes and a few hoofed animals.
And even the creatures that do pair and mate for life occasionally have flings on the side and some, like the wolf, waste little time finding a new mate if their old one dies or can no longer sexually perform.
Staying faithful can be a struggle for most animals. For one, males are hardwired to spread their genes and females try to seek the best dad for their young. Also, monogamy is costly because it requires an individual to place their entire reproductive investment on the fitness of their mate. Putting all their eggs in one basket means there’s a lot of pressure on each animal to pick the perfect mate, which, as humans knows, can be tricky.
Because of recent revelations from animal studies, scientists now distinguish between three different types of monogamy:
• Sexual monogamy is the practice of having sex only with one mate at a time.
• Social monogamy is when animals form pairs to mate and raise offspring but still have flings—or "extra-pair copulations" in science lingo—on the side.
• Genetic monogamy is used when DNA tests can confirm that a female's offspring were sired by only one father.
For humans, social and sexual monogamy usually go together, but this isn't always the case with other animals. For example, an estimated 90 percent of all birds are socially monogamous, living and raising young together, but many frequently have sex with other partners. One famous experiment found that female blackbirds paired with sterilized males were still laying eggs that hatched. The females couldn't chirp their way out of that one.
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