This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
There are certain animal species where – for reasons related to the charisma of the animal concerned, and the distinct nature of its various populations – we tend to learn about the various subspecies. Giraffes are one good example. Another is the Tiger Panthera tigris.
Not all tigers are the same. Small size and a prominent facial ruff shows here that we're dealing with a Sumatran tiger (this one is in the Welsh Mountain Zoo, Snowdonia). Credit: Darren Naish
Most people interested in animals know that tigers vary enough across their extensive (historical) range that the naming of several different forms is warranted. There’s the ‘typical’ tiger of India (the Bengal tiger P. t. tigris), the comparatively gigantic Siberian or Amur tiger in far north-eastern Asia (P. t. altaica), the west Asian Caspian tiger (P. t. virgata), a Chinese form with distinctive stripes (P. t. amoyensis), some poorly known forms from mainland south-east Asia (P. t. corbetti and P. t. jacksoni), and the island-dwellers of south-east Asia (P. t. sondaica of Java, P. t. balica of Bali and P. t. sumatrae of Sumatra). Several of these forms were unable to avoid the persecution and destruction wrought upon them by our own species...
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Several books on tiger illustrate all (or most) of the subspecies recognised at the time. This montage is from Kailash Sankhala's 1978 book Tiger! Credit: Collins
It’s well known that ‘subspecies’ in a great many vertebrate animals are poorly founded, often being named for convenience or when little was known about variation across a species. Furthermore, which lineages or populations should be recognised as taxa – and which rank those taxa should be awarded – remains one of the greatest areas of disagreement and argument in zoology. If there are multiple tiger lineages, do we recognise a bunch of subspecies, a bunch of different species, or merely a bunch of different forms or variants or ecotypes within a variable entity? Yes, that debate again (see the recent white rhino article). One recent study (Luo et al. 2004) argued for the recognition of six tiger subspecies within the same one species, and another supported the view that we should split tigers and recognise more than one species (Mazák & Groves 2006).
Phylogenetic relationships among tigers as recovered by Luo et al. (2004). There are a few things to note here: one is the poorly resolved nature of the branching order, another is that P. t. corbetti is not monophyletic. Credit: Darren Naish
The reason this article is being published now is that a revision of tiger subspecies has just appeared, specifically as part of the IUCN Cat Specialist Group’s revised taxonomy for cats, as led by cat specialist Andrew Kitchener (Kitchener et al. 2017). This work incorporates thoughts from numerous recent studies, is conservative where data is lacking or poor, and provides a taxonomy that will be formally accepted by the IUCN until the next revision appears. Some people may not like various of the decisions concerned.
Captive Siberian tiger. Supposedly, record-breaking individuals of this form can exceed 300 kg and even 360 kg, but there's some reasonable doubt about the accuracy of these weights. Credit: Darren Naish
As discussed by Kitchener et al. (2017), a list of post-1997 publications have sought to better establish tiger phylogeny and taxonomy. One problem is that good information is often worryingly absent: there’s all too little of it when it comes to Javan and Balinese tigers, for example. Be that as it may, several of the results are surprising in view of the traditional subspecies system familiar to many of us: the (extinct) Caspian tiger might be similar enough to the Siberian/Amur tiger to warrant the taxonomic merging of the two (Driscoll et al. 2009: I wrote about this study when it appeared… go to ‘Revising’ the Siberian tiger), P. t. jacksoni is non-monophyletic and appears to be formed of different populations of P. t. corbetti (Kitchener et al. 2017), and the island-dwelling tigers are highly similar anatomically – sometimes overlapping in various skull traits – and have scarcely any geographical structure in their genetics (Kitchener 1999, Wilting et al. 2015).
The biggest and mightiest of the tigers: the Siberian or Amur tiger. These individuals were photographed at Marwell Wildlife, England. Credit: Darren Naish
Furthermore, modern tiger populations are far more distinct and inbred than they were just decades ago because of local extinction, the result being that the distinctive nature of some populations today is not reflective of the situation back when the subspecies were first named (Mondol et al. 2013).
In view of all of this, what do Kitchener et al. (2017) propose as goes tiger subspecies? Drumroll… they propose that we should recognise just two tiger subspecies: the nominal P. t. trigris of mainland Asia, and P. t. sondaica of Sumatra, Java and Bali. Notably, however, the authors themselves do not wholly agree on this proposal, two of them wishing for the continued recognition of P. t. amoyensis, P. t. corbetti and P. t. jacksoni in mainland Asia, and for distinction of P. t. balica and P. t. sumatrae from P. t. sondaica among island-dwelling forms. Science is said not to work on the basis of number of votes… but it often does, since we tend to work on the idea that a conclusion is more robustly supported the more workers agree on it. Clearly, doubt and differences of opinion remain but the primary conclusion of Kitchener et al. (2017) is that a two subspecies only taxonomy might be the best. Further study will, no doubt, support or refute this view.
Sumatran tiger yawning. I'm sure I've said this before, but -- it isn't difficult to get a photograph of any given cat yawning, you just have to watch it for long enough. Credit: Darren Naish
One final thought on this issue: there’s sometimes the worrying implication that a population is only ‘worth saving’ when it’s taxonomically distinct, and that – in synonymising tiger subspecies – there might be the concern that we are downgrading their value and significance. This is, of course, not accurate.
For previous articles on mega-mammal taxonomy and phylogeny...
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Kitchener A. C. 1999. Tiger distribution, phenotypic variation and conservation issues. In Seidensticker J., Jackson P. & Christie S. (eds) Riding the Tiger. Tiger Conservation in Human-Dominated Landscapes. Cambridge University Press, Cambridge, pp. 19-39.
Mazák, J. H. & Groves, C. P. 2006. A taxonomic revision of the tigers (Panthera tigris) of southeast Asia. Mammalian Biology 71, 268-287.
Mondol S., Bruford M. W. & Ramakrishnan U. 2013. Demographic loss, genetic structure and the conservation implications for Indian tigers. Proceedings of the Royal Society B: Biological Sciences 280, 20130496.
Sankhala, K. 1978. Tiger! The Story of the Indian Tiger. Collins, London.
Wilting, A., Courtiol, A., Christiansen, P., Niedballa, J., Scharf, A. K., Orlando, L., Balkenhol, N., Hofer, H., Kramer-Schadt, S., Fickel, J. & Kitchener, A. C. 2015. Planning tiger recovery: understanding intraspecific variation for effective conservation. Science Advances 1, e1400175.