Think your family is dysfunctional? In the wild, males will sometimes kill the young of another male and replace them with his own. Lions do it. Even dolphins. As cruel as it sounds, it makes genetic sense to erase a rival’s DNA.

But occasionally, a male will not only let his rival's offspring live—but also care for them. These surrogate dads are pretty rare, raising the question: Why would any male spend energy on babies not his own? (Read about five benefits of fathers that may surprise you.)

First, not all species are monogamous, and father-like behavior can accompany polyandry—a phenomenon in which a female takes multiple mates, leaving paternity a mystery.

“If there is some chance the infant might be his,"