Nature's Non-Existent Rulebook

 "Rara avis in terris nigroque simillima cygno"

For over a millennium, that saying, Latin for "no rarer bird on earth than a black swan," was commonly used in Europe to denote something that was impossible  Just like we might say "when pigs fly" to indicate something that will never happen, so did Europeans up until the 1600's use the black swan to indicate that something couldn't possibly exist.  All of that changed, of course, when Europeans first arrived in Australia and found that the swans there were, improbably, black.

A teacher told me once that Nature never read a textbook, and that there are no rules.  He was exaggerating, slightly - there are some rules.  No animal can photosynthesize or live off air.  No mammal can breathe underwater.  There are plenty of others we could probably come up with... but in so many cases, what we think we now about animals is offset by glaring exceptions.

There is there is the giant panda - a bear that lives off of grass - and the Gray's monitor lizard, a member of the same family of hypercarnivore lizards that spawned the Komodo dragon, but one that feeds on fruit.  Some mammals, such as the echidna, lay eggs, while some reptiles give livebirth... or give birth without a male present at all.  There are pythons which feed on prey that seems much too large for them, giant whales which feed on the tiniest of creatures, and vampire bats, leeches, and other species which feed on blood.  Speaking of bats, the very fact that there are mammals that fly is bizarre, offset by the fact that there are birds that can't.  Seals live in freshwater lakes far removed from the ocean and penguins swim on the equator.  Some species which we expect to be small develop into giants, like the Japanese giant salamander.  Some that we expect to be giants develop miniature forms.

It used to be that the one, unarguable rule in nature was that no animal used tools, apart from man.  Jane Goodall threw that one out the window with her work on chimpanzees.  Since then, we've seen tool usage in a host of other species, from mongooses to vultures, including relatively "primitive" species such as crocodilians.  Language and altruism were also considered the private properties of humans - but those are awkwardly showing up as the domains of other species as well in varying degrees.

When visitors come to the zoo, I love to show them the animal oddities, the ones that, according to the "rules," shouldn't exist, or at least shouldn't behave the way that they do.  They serve as a living reminder that so much that we thought we knew about the natural world is wrong, and we have so much more to learn.  500 years ago, everyone in Europe knew that there was no such thing as a black swan.  It makes you wonder what else we know is wrong, too.