Basic instinct What makes some species more faithful than others?

In 2011 a shock celebrity break-up garnered headlines around the world.

- Donna Lu

Not the separation of Ashton Kutcher and Demi Moore, nor Jennifer Lopez and Marc Anthony, but the sudden, inexplicable rupture between Bibi and Poldi, two 115-year-old Galapagos tortoises at Happ reptile zoo in Austria.

After nearly a century as a couple, the female, Bibi, had had enough: one day, she bit a chunk off Poldi's shell, drawing blood, and continued to attack him until zoo staff moved him to a separate enclosure.

In the wild, Galapagos tortoises are not monogamous, so it's no small feat that Bibi and Poldi's liaison lasted as long as it did, though their coupling never produced any offspring. Attempts at reconciliation were not successful. "We get the feeling they can't stand the sight of each other any more," the zoo's director, Helga Happ, said at the time.

To break up, of course, you have to be together in the first place. In social monogamy, animals live together and form strong ties known as pair bonds - though sexual faithfulness is a separate question. In mammals, humans are among the exceptions: social monogamy has been observed in fewer than 10% of mammal species.