I've always loved opossums. Those creepy, lizard-like semi-mammals evolved in the Pacific Northwest back when it was an island, then spread to Europe, South America, Antarctica [it was hot then], Australia and Asia. Then they were driven out of their niches in every place when placentals evolved except for Australasia - and South America. From South America, the opossum form migrated back through Mesoamerica into North America, where it continues to flourish.
Now my favourite wild mammalian has another use: research on gender and sex. Opossum Study May Illuminate Gender Puzzle is brief but interesting. Take a gander.
EDIT: the original text is no longer available online, so here's a snippet:
Opossum Study may Illuminate Gender Puzzle
Stephen Stewart, Newsquest (Herald & Times), 25 Nov 2003Glasgow, Scotland — Transsexuals and people with sexual abnormalities could benefit from scientific research at a Scots university into the humble opossum.
Work at the University of Glasgow, with the UK's only breeding colony of the South American marsupial, could shed more light on the enigmatic mechanisms involved in sex determination and the development of reproductive organs in humans.
Academics believe the work may give greater insight into intersex conditions (where the appearance of the genitals is not consistent with the person's chromosomal sex) and could be used to treat people suffering the trauma it creates, as well as offering hope to males suffering from one of the most common birth defects, cryptorchidism or the failure of the testes to descend.
