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Thursday, February 12, 2015

Subspecies Soup

"When it comes to subspecies, things get even murkier.  Ask ten scientists to define subspecies and you'll get ten different answers.  It's a hazy concept that has never been satisfactorily pinned down.  The best way to think of it may be this: A subspecies is evolution in action."

Let's talk about two tigers.

The tiger rose from his bed of pine needles  and stretched, his breath fogging up in the frozen air.  He was a full grown male, heavy and pale and shaggy, weighing in at over 650 pounds.  He strolled silently through the taiga, leaving only paw prints in the snow as evidence of his passing.  Finally, he reached the frozen river, walking along its edge until he found a spot clear enough to drink from.  Dipping down his massive head, he lapped up the icy water eagerly.

The tiger crept through the shadows of the jungle, sleek and dark and silent.  Its coat was a tangle of black stripes, crowded together like creeping vines.  Around his face was a massive white ruff, as if from an abortive attempt to grow a mane like a lion.  He was a full grown male, a little over 300 pounds in weight.  Picking silently through the steamy marshes, he glanced upward and about, noticing monkeys and tropical birds in the trees above him.  

Two tigers - one small and dark and sleek, at home in the tropical rainforests, another huge, pale, and shaggy, living in the boreal forests of northeastern Asia.  Which one is the real tiger?  Well, they both are - both are members of the species Panthera tigris, or "tiger."  The difference is that they belong to two different subspecies.

A subspecies (usually abbreviated as ssp) is a division used in taxonomy, the study of the classification of living things.  A species, the basic unit of biology, (what most of us would think of as simply a type of plant or animal) is a group of living organisms which are capable of - naturally - interbreeding with one another to create more living organisms that are like the parents.  Giraffes beget giraffes, tigers breed tigers, etc.  Species are identified with a two word Latin name, or scientific name; Panthera tigris, again, in the case of tigers.

A subspecies, then, is a step of classification even finer, more specific than that.  It describes a geographic race of a species which tends to breed within its own subspecies, capable perhaps of interbreeding with other subspecies of the same species, but unlikely to do so due to natural barriers - a mountain range, an uncrossable river, or something like that.  In some cases, the differences between them can be obvious (tigers, for instance).  In others, only genetic analysis can reveal the division between the two.

If left isolated for long enough, two subspecies may grow so different from one another that they may eventually become separate species.  This is what is believed to have happened not-so-long-ago (evolutionarily speaking) when a population of coastal brown bears became isolated during the last ice age.  They specialized in their new arctic home and became what we now know as the polar bear.

Whereas the species name has two words, the subspecies name as three names - the Bengal tiger, for instance, is Panthera tigris tigris (the first subspecies, called the nominate, always has the same second and third name), while the Amur tiger is Panthera tigris altaica.

Lovely.  Fascinating.  Breath-taking scientific commentary.  What on earth does this have to do with zoos?

I'm getting there...

Not every species has subspecies - there is only one kind of giant panda, for instance, and only one kind of platypus.  For other species, especially those spread across a large geographic area - again, tigers come readily to mind -  there may be many.  Here is where it becomes tricky for zoos.

Zoos love to use the analogy of Noah's Ark, so we'll use it here, too.  According to the Bible, Noah was said to have taken two of every animal onto the Ark... two lions, two cheetahs, two tigers... what a second.  Did he take two tigers, or two Amur tigers, two Bengal tigers, two Sumatran tigers, and so on?  And what about leopards?  Gray wolves?  Brown bears?  Good heavens, we haven't even made it past the large carnivores, and the Ark is already overloaded and threatening to capsize.

Whatever anyone may believe about the Bible or Noah, the "Ark" that zoos are working with definitely has finite space, and the guest list has to be monitored sometimes.  Do you try to save a sustainable captive breeding population of every subspecies of tiger?  Because if you do, you're pretty quickly going to start eating into spaces that other big cats need (some of which have their own army of subspecies).  Do you forget about subspecies, breed them all together freely as one population?  Do that and loose the specialization and variety that made each of those subspecies special; you'll also be putting the animals at a disadvantage should the unlikely day come when you can ever try to release them into the wild.  Or do you pick one or two to save, and ignore the rest?

With some subspecies, it is the last approach that many zoos have taken.  The AZA has chosen to focus its efforts on three subspecies of tiger (there are nine, three of which are now extinct) - the Amur, the Sumatran, and the Malayan.  For gray wolves, it has focused on the Mexican subspecies.  For giraffes, the reticulated and Masai.  Hybridization of subspecies is strongly discouraged, and doing so (or the suspicion of having done so) can lead to an animal's exclusion from breeding programs.

Sometimes one subspecies remains common while another teeters on the brink, as in the case of the white rhinoceros.  The southern subspecies is reasonably abundant (I mean, for a rhino).  The northern population's days are probably numbered... and the number isn't a very high one.

A species is more than the populations of animals that make it up; it's also the genetic variety that make up those animals and those populations.  By preserving subspecies as integral population units, zoos and aquariums help to preserve the genetic diversity of those species for the future.  An uncertain future, to be sure, but hopefully a future nonetheless.

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