From what I've read it just showed up in the golden lanceheads because they're so inbred from being on the island. Islands do funny things to animal populations. I've heard two theories, one that the inbreeding and the hermaphrodism gene will drive them to extinction and another (ok, it was from Jeff Corwin) that they're evolving into something new and cool. In terms of energy budgets and such, it doesn't 'pay' animals to have to deal with being both sexes. Generally, it's only 'worth it' to creatures that have trouble finding each other, like flowers and snails. So it's probably just a fluke in lanceheads. Incidentally, there's a snake called the Brahminy blind snake with only one gender- female. Also, if one person did happen to have a snake that was a hermaphrodite odds are they'd never find out.