There have been many studies debating whether it's "normal" for human beings to remain with one partner for a lifetime. Certainly, many people have done so and continue to do so. But when it comes to evolution, monogamy hasn't always been the rule of thumb. Most animals are programmed to mate with as many partners as possible, in fact. Only about 3-5 percent of mammals form monogamous bonds in the animal world. Over time, the monogamous relationship evolved in order to protect offspring from being killed by other males, scientists theorize. Humans are most closely related to chimpanzees and the bonobo ape, both species that do not restrict themselves to monogamy. The female bonobo is receptive to multiple male partners when ovulating, and males fight one another, as they do in other species, to mate with females. Males also produce more sperm than other primate species to increase their chances of fatherhood.