Sunday, July 15, 2012

Propleopus, the Carnivorous Kangaroo

Imagine you are on a safari to the Masai Mara National Reserve in Kenya.  If you timed your visit right, then the Mara is inundated with wildebeest and zebra, as well as numerous other herbivores.  You will probably see a number of carnivores as well: cheetahs, leopards, lions, and especially hyenas.  What you probably don’t realize is that most places aren’t like this.  The Masai Mara has one of the highest predator populations seen in Africa, due in large to the great number of prey animals available for a meal.  Regardless, you are forced to acknowledge that there are a great many more prey animals than there are animals to prey upon them. 
                This is known as the predator/prey ratio, and it can be seen all over the world, and can also be seen in the past.  While hundreds, sometimes thousands, of fossils of a single herbivorous dinosaur can be discovered, it is far more rare to unearth the remains of a predatory dinosaur.  But why is there this unbalance between predators and their prey?  Well, let’s take a look at a continuous, cyclical event that takes place in a period of 9.6 years in the wilderness of Canada, that should be able to shed some light on the situation.
                At the beginning of the almost-ten year cycle, we see a sharp increase in the number of hares.  When food is plentiful, these hares often produce two to three litters of around 12 leverets (baby hare) each.  After their numbers reach their limit, their population density reaches around eight hare per football field.  At this time, the hares have eaten all of the edible material they can reach.  Not only are they out of food, but the plants that they feed upon begin to create bitter chemicals in their leaves and edible parts, that keep the hare from digesting the plant material very efficiently. 
                While the hare population is flourishing so, the hare’s predators find an especially easy time of it.  Owls, wolves, and foxes all flourish, but one predator does particularly well: the Canadian lynx.  Hares consist between 40 and 85 percent of the average lynx’s diet, and they generally kill two hares every three days.  With such an explosion in hare numbers, all of these predators flourish, successfully raising more owlets, kits, and cubs.  However, shortly after the plants begin to release the anti-digestion chemicals, the hare populations tank.  With not enough food, many of the animals die off.  Then, a year or two after the hare die-off, the lynx also experiences a massive fall in numbers.  With the lynx no longer over hunting the hare, and the predator/prey ratio returned to normal, the hare starts the cycle all over again. 
                While this cycle seems to be a healthy part of the Canadian ecosystem, in other places, a rapid rise in the number of predators or prey could be disastrous.  Therefore, nature has made it so that, in a healthy ecosystem, the prey animals vastly outnumber the predatory animals.  But how does all of this tie into kangaroos?  We will get back to that in a minute.  First, let’s take a look at an extinct, 26,000 year old relative of today’s rat kangaroo. 
                First described by Australian zoologist Charles De Vis around the turn of the century, the holotype of Propleopus at first remained unique.  It wasn’t until the year 1967 that more remains belonging to the genus Propleopus appeared.  More was discovered in the following years, but very few remains have been discovered even to this day.  With millions of fossilized mammalian bones discovered in Australia, Propleopus are “known from teeth and jaws attributable to less than 20 individuals.” 
                There are multiple reasons why an animal does not appear with a great amount of frequency in the fossil record.  Perhaps it is because the animal was small; smaller bones are much more delicate, and therefore less likely to survive the fossilization process.  (They are also a lot smaller, and therefore usually harder to find then, say the humerus of an Brachiosaurus!)  Perhaps the animal lived in an environment where fossilization is unlikely.  For example, in millions of years, when intelligent life again evolves on this planet, or we are visited by intelligent life from elsewhere in the galaxy, they would find very little or no evidence of mountainous animals, like bighorn sheep or the snow leopard.  This is because mountains are in a constant state of geologic flux.  At times they are being pushed up; but even when they are rising, they are already eroding.  Given enough time, entire mountain ranges can disappear, or almost disappear, like in the Australian Outback.  Sometimes, it is because paleontologists are simply looking in the wrong places, and there are treasure troves of these animals just waiting to be discovered elsewhere.  It could also be because the rocks where the animal was deposited simply don’t exist anymore, something that is called an unconformity in geologist’s terms.  Maybe the animal was just not very successful, and went extinct after only a short amount of time.  The fossilized animal could also represent a transitional fossil, so creatures with those characteristics would have only been around for a few thousand years.  Or, of course, it could be because the animal was a predator, and there were fewer individuals to begin with.
                Now you are probably thinking that this is quite a leap.  Just because this kangaroo didn’t appear all that often in the fossil record doesn’t necessarily mean that it was carnivorous.  And you are right, as if this was the only evidence of Propleopus being a carnivorous kangaroo, I would be laughed right out of the Neolithic Age: and they, at best, had only a very, very primitive form of language, and probably would, at best, barely understand what I was saying.  However, there is more evidence in favor of a carnivorous Propleopus.  As I am fond of saying, “the teeth tell the tale.” 
                Studies of the dentition of Propleopus show a close resemblance to small, extant insectivores or omnivores, i.e. the mountain pygmy possum and the musky rat kangaroo.  Where the teeth of Propleopus differ from the mountain pygmy possum and the musky rat kangaroo, however, the differences “could be interpreted as adaptations to meat-eating.”  To sum up, the incisors are short, stout, and appear great for stabbing, the premolars are strongly serrated, which is perfect for tearing into very tough stuff (perhaps tendon), and the molars are greatly reduced in size, as sometimes seen in the genus Wakaleo, one of the genera of marsupial lion, like Thylacoleo.  Furthermore, the molars, although reduced in size, share similar features to the largest extant marsupial carnivore today, the Tasmanian devil.  These features in the Tasmanian devil serve to keep bone splinters from penetrating the gums, and clearly would not be needed to serve that purpose in an herbivore. 
                The most revealing tale of the teeth can only be revealed by a microscope.  Studies have shown that, when you compare the microscopic wear patterns on the teeth of a herbivore and a carnivore, you can see obvious, and distinguishable, differences.  The teeth tell the tale of the diet of an animal, and help to show us what Propleopus might have eaten.  When compared with the wear patterns of closely related herbivores, like the musky rat kangaroo, and marsupial and placental carnivores, like the thylacine, Thylacoleo, and dogs, the wear patterns seen on Propleopus resembles that of the carnivores more so than that of the herbivores. 
                So was Propleopus a carnivore, or a herbivore, or both?  One hypothesis that has been put forward is that Propleopus was situated in a similar ecological niche as the modern day African baboon, eating whatever came it way, be it plants, eggs, insects, or meat.  Whatever the answer, is is doubtful that it could be answered now, and it is likely that only further research, and more discoveries, will ever hope to unravel the mystery surrounding Propleopus, the seemingly killer kangaroo.  

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