mark mullin wondered if monday og mi is natural in the animal world or just a human fraility. he went to san diego zoo for answers. >> reporter: who says a tiger cheats? in the animal kingdom they choose one partner, they hook up a few days before mating and the females aren't picky. >> i am ready to go, putting the flag up for whoever is that willing receptor. >> reporter: go to the san diego zoo and all animal relationships are on display. take the red cheeked gibbons. they stick with one partner for life and raise babies together. gibbons are the flip side of their cousins who prefer to spread their charm, like most mammals. monogamy among animals is rare, less than 10% stay with one partner for life. this started a debate. >> men animals keep the girls in the group and do their business with all the ladies. i don't think girls really have that option. >> reporter: sometimes they do. elephant herds are ruled by a female who prefer multiple partners. >> women cheat more because they have more options but it goes to lover versus provider. >> reporter: when female birds like t