Today, we have an especially tantalizing Moment of Science. You see, scientists have discovered that swans and many other species that were believed to be monogamous actually cheat on each other.
Now that scientists are able to analyze DNA and do animal paternity tests, they've realized that swans, geese, lovebirds, and so on just aren't as faithful as they appear to be. Some species of animals were simply more discreet about their cheating than others.
Let's take a step back here. Words like "cheating" and "faithfulness" imply moral judgment, but at this point there's no evidence that indicates animals operate along these lines. Swans don't necessarily sit there and consciously decide to cheat on their partners. Most people know that some male animals are programmed to spread their sperm in order to pass their genes along and be more successful from an evolutionary perspective.
Females mate with more than one partner as well. Apparently, doing this enables females to find partners with better genes or who are better providers. But because male animals are more likely to get violent if they sense intruders, the females have also learned to hide their illicit trysts.