Have you ever wondered why the number of extant mammal species is so imprecise?
Consensus seems to have the number at around 4500, others say 4000 is about right while some would have the true total at somewhat more than 4500.
Why is it that after nearly 300 years of a binomial system that strives for exactness and order, just providing a total species count of such obvious animals is so hard?
Well science has been out there examining, recording and classifying and has done an extremely good job to the point that it is rare that a new previously unknown species is found. Over the last decade or so truly new species have turned up at the rate of about one a year and are mainly quite small mammals that have previously been missed. Some fairly remote areas have however turned up two large surprises – the Sao Lao a bovine like antelope and a new species of deer. Both of these came from wild forest areas of south east Asia and though known well to local people they were quite unknown to science.
Australia has provided a few too with the stand out being the Pilliga mouse but that’s nearly 40 years ago. New Guinea has been far more productive but mainly in rodents especially the giant rats described by Australian scientist Tim Flannery.
So with maybe 10 new species over the last decade it is not going to explain a greater than 10 per cent variation in the total mammalian species count.
Some species previously thought to be extinct have been rediscovered and Australia has several recent examples, recent being the last 50 years. Burramys, the mountain pygmy possum was rediscovered in a Snowy Mountains ski hut by a Melbourne University biology faculty excursion group, Leadbeater’s possum in the tall forests of the Victorian divide and the Parma wallaby in the forests north of the Hunter Valley. Interestingly though a colony of Parma wallabies had just previously been discovered on an island near Auckland, New Zealand where it had been introduced in the 19th century. While these add to the overall total they are few and far between.
What about extinctions? Australia unfortunately leads the way with around 20 over the last 200 years, including some spectacular species such as the Toolache wallaby that went early and the Tasmanian tiger in the 1930s. Australia with nearly 50 per cent of all extinctions over the last 400 years gives a total that is not nearly enough to explain the discrepancy.
In the end what it comes down to is indecision, disagreement, the adversarial nature of science and dare I say, especially in the past, some degree of frailty of the human ego.
The first definition I was ever given of species differentiation (too many years ago to remember exactly when!) was that they were groups of animals that would not interbreed. This rapidly became problematic for me as it was obvious that donkeys and horses where different species and they interbred. The definition was modified to those groups of animals that interbred naturally and produced fertile offspring.
Obviously the real definition is far more complicated than this and an International Panel of Taxonomists has the task of sorting it all out. As our technology and understanding improves so the task should become easier.
Just looking at domestic animals provides interesting examples of the classification conundrum notwithstanding the fact that we as humans have muddied the field.
In the case of cattle there are currently four domestic species – European cattle Bos taurus, Indian cattle Bos indicus, Banteng Bos javanicus and the yak Bos gunniens these will all easily interbreed and produce fertile offspring yet they are very different in temperament social order and habitat. Should they be considered different species or just geographic variants and possibly closely related subspecies? The interesting thing is that Bison bison the American bison and Bison bonasus the European bison or wisent are very different but will also interbreed with domestic cattle and produce fertile female offspring although most of the males are not.
With horses it is probable that Przewalski’s horse Equus przewalskii is just a geographic variant and is more reasonably classified as a subspecies Equus ferus przewalskii of the tarpan Equus ferus which presumably produced the domestic horse Equus cabalus. Possibly therefore it should, as some say, be Equus cabalus przewalskii and yet it has 2 more chromosomes than its domestic relative. But that’s not so unusual because the water buffalo Bubalis bubalis has three different geographically based chromosome counts and they are not even described as sub-species. Taxonomists are still arguing this through!
The domestic dog Canis familiaris is now usually noted as a wolf subspecies Canis lupus familiaris, so what about the dingo? It is now accepted as a wild subspecies Canis lupus dingo. Then there is Hallstrom’s singing dog of New Guinea is it just a tropical rainforest variant of the dingo or a true subspecies Canis lupus hallstromi.
So it is obvious that we as humans have added to the confusion, removal of species to new habitats and selective breeding has added new variations and even it would seem new chromosome counts.
All this aside it is the Taxonomists – the “lumpers” and “splitters” that produce the wide variation in total species numbers. Lumpers are those that see widely varying physical, social and behavioural features as normal geographic variations within one species but accept often multiple sub-speciation. A good example would be the red deer of Europe Cervus elephas with geographic variation becoming more extreme through northern Asia until we arrive in North America where we find the Canadian wapiti or North American elk that while wildly different to the European type species certainly “belongs”, albeit as a sub-species Cervus elephas canadiensis.
Then there are the “splitters” – those that see it as important to record variation within mammal groups and ensure that those differences are exactly defined within the type species.
So the conflict goes on but the end result will be a better and better definition of the differences and indeed better and better science. The advent of DNA testing and other new advances will allow for more exact classification so that the uncertainty of the exact number of mammalian species will reduce and our understanding of this fascinating class of animals will increase accordingly.
Just think however that as Class Mammalia is relatively small at just 4000 odd species what are the classification problems faced by the other classes of vertebrates which in themselves fade into insignificance when we think about invertebrates.