News of politicians' extramarital affairs seems to be in no short supply lately, but if humans were cut from exactly the same cloth as other mammals, a faithful spouse would be an unusual phenomenon.

Only 3 percent to 5 percent of the roughly 5,000 species of mammals (including humans) are known to form lifelong, monogamous bonds, with the loyal superstars including beavers, wolves and some bats.

Social monogamy is a term referring to creatures that pair up to mate and raise offspring but still have flings. Sexually monogamous pairs mate with only with one partner. So a cheating husband who detours for a romantic romp yet returns home in time to tuck in the kids at night would be considered socially monogamous.

Beyond that, scientists' definitions for monogamy vary.