|Opossum- MDC Photograph|
According to MDC, our common opossum, Didelphis virginiana, is an omnivore which eats insects, reptiles, frogs, crayfish, bird eggs and earthworms as well as browsing garbage cans. Oh, and apparently, the odd rattlesnake and copperhead. They have co-evolved with these pit vipers and thus developed a defense against their venom. It has previously been thought that snake venom evolved just as a tool to capture prey, but it also seems to be a defensive mechanism against predators.
A key clotting factor is called von Willebrand's factor and our opossum is one of a few related species that has a gene that has rapidly evolved to affect it, further blocking the effect of the toxin. Studies indicate that it is undergoing much more rapid selection than usual. To quote Robert Voss, Curator of Mammalogy at the American Museum of Natural History, "We've known for years that the venom genes evolve rapidly in snakes, but the partner in this arms race was unknown until now. Opossums eat snakes because they can." This means that the snakes are prey rather than predator in this circumstance.
The full study with further details is at plosone.org and information on the opossum antivenom is at this site.