It’s official: there’s a new, living species of African rhino, bringing the recognised number of living rhino species to six.
It’s the Northern or Cotton’s white rhinoceros (aka Grass rhino or Square-lipped rhino) Ceratotherium cottoni [captive individual shown above; photo by Jan Robovsky, from Groves et al. (2010)], previously classified as Ceratotherium simum cottoni, and first named by Richard Lydekker following its discovery in 1900* (surprisingly recently for such a large, terrestrial mammal**). In a new paper published in PLoS ONE, Colin Groves and colleagues argue that northern and southern white rhinos differ substantially in morphology and genetics, and should hence be recognised as separate species. As discussed below, this has implications for the conservation priority of the critically endangered Northern white rhino population [map below, from wikipedia, shows historical distribution of the two Ceratotherium taxa. C. cottoni's range is shown in orange and C. simum's in green].
* Percy H. G. Powell-Cotton was the first person to bring Northern white rhinos to scientific attention: in 1900 he collected a specimen from the Lado District of South Sudan. Oldfield Thomas published on these discoveries in 1900, but it was Lydekker (1908) – well known for being a consummate splitter, though don’t let that put you off – who made the decision to name the northern rhino as a new subspecies. Lydekker used the name Rhinoceros simum cottoni: the generic name Ceratotherium had been coined by Gray in 1868, but the taxonomy of African rhinos was confused at this time (Rookmaaker 2005a).
** Note that it’s far from being the most recently named living rhino: there are also the recently named supposed subspecies Rhinoceros unicornis sinensis Laufer, 1914, Diceros bicornis occidentalis Zukowsky, 1922, C. s. scotti Hopwood, 1926, C. s. efficax Dietrich, 1945, Dicerorhinus sumatrensis eugenei Sody, 1946, Diceros b. palustris Benzon, 1947, Diceros b. punyana Potter, 1947, Diceros b. longipes Zukowsky, 1949, R. u. kagavena Deraniyagala, 1958, Diceros b. michaeli Zukowsky, 1964, Diceros b. angolensis Zukowsky, 1965, Diceros b. atbarensis Zukowsky, 1965, Diceros b. chobiensis Zukowsky, 1965, Diceros b. ladoensis Zukowsky, 1965, Diceros b. nyasae Zukowsky, 1965, Diceros b. rendilis Zukowsky, 1965, Diceros b. rowumae Zukowsky, 1965, Dicerorhinus s. harrissoni Groves, 1965, R. u. bengalensis Kourist, 1970 and R. u. barinagalensis Srivastava & Verma, 1972. However – the majority of these proposed taxa have not stood the test of time, or are considered of dubious validity (e.g., Rookmaaker 2005b).
Northern and Southern white rhinos are clearly different in appearance and can be differentiated on all of their measurable components, including tooth measurements, skull lengths, widths and depths, and limb bone lengths. Southern white rhinos are, on average, larger, with adult males weighing 2000-2400 kg, compared to 1400-1600 kg for adult male Northern white rhinos. One interesting point that stands out from the article is that height and length measurements of white rhinos are not abundant at all (to put it mildly). Ok, maybe it’s not a surprise that live rhinos have been measured relatively infrequently, but it’s always worth noting how little data often exists on living animals. White rhinos of both taxa have shoulder heights of about 1.6 m – they’re huge. While the data on height and length isn’t great, Groves et al. (2010) note their impression that Northern white rhinos are taller than Southern white rhinos, and that Southern white rhinos seem to be longer-bodied.
Among the other differences, some stand out as easy to spot. Northern white rhinos have a rather straight back while the dorsal profile of the Southern white rhino is obviously concave and the shoulder hump is more prominent. In the southern form, the palate ends at a point that is approximately level with the junction between the second and third molars, while it ends level with the mid-point of the second molar in the northern form. Northern white rhinos have teeth that are proportionally smaller than those of southern rhinos, and their teeth are also lower-crowned. The two taxa also differ in skull profile. The dorsal margin of the white rhino skull is concave when the skull is seen from the side, but the degree of concavity differs quite strikingly between northern and southern rhinos: the dorsal surface of a northern skull is nearly flat, while that of a southern one is deeply concave (Groves et al. 2010) [the adjacent image, from Groves et al. (2010), shows Southern C. simum above and Northern C. cottoni below].
Various integumentary differences – concerning skin folds and hairiness – might also help distinguish the two rhinos, but they’re variable and (in my opinion) not altogether convincing. Southern white rhinos sometimes have distinct vertical grooves in between their ribs, while northern ones generally don’t, and the skin folds around the top of the foreleg are supposedly more prominent in southern rhinos. Southern white rhinos are also supposed to be hairier on the body, while Northern white rhinos have hairier ears and tails than Southern white rhinos according to some (Groves et al. 2010). These differences are not newly recognised: the two white rhino taxa have always been regarded as obviously distinct (Groves 1972).
Groves et al. (2010) reviewed genetic data as well, and this also shows that Northern and Southern white rhinos are highly distinct. The level of divergence observed between analysed segments of DNA is high, and a separation date of about 1 million years is hypothesised [two C. simum individuals from Namibia shown below; from wikipedia].
So, the two white rhino taxa are morphologically and genetically distinct, and have been separate for about a million years. It’s well known that Groves is a proponent of the phylogenetic species concept (or PSC), and – by applying this concept to the two taxa – Groves et al. (2010) conclude that “we have no option but to consider them specifically distinct” (p. 12). It will be interesting to see whether this proposal is accepted: it’s inevitable and predictable that some people will regard this act as an unwarranted taxonomic splitting. The proposal means that white rhinos join the long list of extant mega-mammal taxa where the ‘subspecies’ of tradition have recently been elevated to specific status: you might remember that we looked at this subject very recently in Laissez-faire lumping under fire.
Brief distraction from Killer whales
You may have heard that a new study (Morin et al. 2010) has also supported the idea that Orcinus orca should be split up into several distinct species: this is not a new idea, as the genetic and morphological differences observed between resident, transient and Antarctic orcas have long led some experts to make these sorts of suggestions (e.g., Berzin & Vladimirov 1983, LeDuc et al. 2008). Indeed, two additional species – O. nanus Mikhalev et al., 1981* and O. glacialis Berzin & Vladimirov, 1983 – were named in the 1980s.
* No type specimen was allocated, making O. nanus a nomen nudum.
Back to the rhinos… one argument that could (and will) be used to support the new taxonomy is that it should help bring attention to the Northern white rhino as an entity desperately in need of conservation priority. The Northern white rhino now seems extinct in the wild (the remaining handful of individuals – those in Garamba National Park in Democratic Republic of the Congo – seem to have died prior to 2003), and captive breeding could well bring the taxon back from the brink (as it has with Southern white rhinos). As is well known, ‘species’ get more attention than ‘subspecies’ [captive C. simum shown here; photo by Eric Harty, from wikipedia].
The danger in making this sort of suggestion, however, is that changing the taxonomy to suit conservation priority could eventually backfire: it would not look good if zoologists were thought to be tweaking their conclusions in order to suit their favoured conservation projects.
Having said all that, Groves et al. (2010) have done a good job of emphasising the point that the two white rhino taxa really are highly distinct, and have been so for a very long time. In fact, one might argue that the recognition of the Northern white rhino as a distinct species is probably long overdue, and some rhino experts are already saying exactly this.
More neat perissodactyl news is due very soon! If you’re interested in rhinos, be sure to check out the amazing resource that is the Rhino Resource Center [captive C. simum show below; photo by Dick Mudde, from wikipedia].
And for previous Tet Zoo articles on rhinos see…
- Stuffed megamammal week, day 4: Sumatran rhino
- How did the White rhino get its name? Not how you think (even if you’re very clever)
- Chinese black rhinos and deinotheres, giant sengis, and yet more new lemurs
- Tet Zoo picture of the day # 5 (on Ceratotherium and the 2007 Rhino May Day event)
- Tet Zoo picture of the day # 3 (on Elasmotherium)
- War rhinos
And if newly recognised species and taxonomic reshuffling interests you, check out…
- The first new European mammal in 100 years? You must be joking
- Monster hunting? Well, no. No.
- Multiple new species of large, living mammal (part I)
- Belated welcome to a ‘new’ clouded leopard… named in 1823
- Tetrapods of 2007 (happy birthday Tet Zoo part II)
- Chinese black rhinos and deinotheres, giant sengis, and yet more new lemurs
- New, obscure, and nearly extinct rodents of South America, and… when fossils come alive
- Giant furry pets of the Incas
- Over 400 new mammal species have been named since 1993
Refs – –
Berzin, A. A. & Vladimirov, V. L. 1983. Novyi vid kosatki (Cetacea, Delphinidae) iz vod Antarktiki. Zool. Zh. 62, 287-95. ['A new species of killer whale (Cetacea, Delphinidae) from Antarctic waters'].
Groves, C. P. 1972. Ceratotherium simum. Mammalian Species 8, 1-6.
Groves CP, Fernando P, & Robovský J (2010). The sixth rhino: a taxonomic re-assessment of the critically endangered northern white rhinoceros. PloS one, 5 (4) PMID: 20383328
LeDuc, R. G., Robertson, K. M. Pitman, R. L. 2008. Mitochondrial sequence divergence among Antarctic killer whale ecotypes is consistent with multiple species. Biology Letters 4, 426-429.
Lydekker, R. 1908. The white rhinoceros. Field 22nd February 1908, 319.
Morin, P. A., Archer, F. I., Foote, A. D., Vilstrup, J., Allen, E. E., Wade, P., Durban, J., Parsons, K., Pitman, R., Li, L., Buffard, P., Nielsen, S. C. A., Rasmussen, M., Willerslev, E., Gilbert, M. T. P. & Harkins, T. 2010. Complete mitochondrial genome phylogeographic analysis of killer whales (Orcinus orca) indicates multiple species. Genome Research [Epub ahead of print] doi: 10.1101/gr.102954.109
Rookmaaker, L. C. 2005a. Review of the European perception of the African rhinoceros. Journal of Zoology 265, 365-376.
– . 2005b. The Black rhino needs a taxonomic revision for sound conservation. International Zoo News 52, 280-282