The sixth rhino: a taxonomic re-assessment of the critically endangered northern white rhinoceros - PubMed (original) (raw)

The sixth rhino: a taxonomic re-assessment of the critically endangered northern white rhinoceros

Colin P Groves et al. PLoS One. 2010.

Abstract

Background: The two forms of white rhinoceros; northern and southern, have had contrasting conservation histories. The Northern form, once fairly numerous is now critically endangered, while the southern form has recovered from a few individuals to a population of a few thousand. Since their last taxonomic assessment over three decades ago, new material and analytical techniques have become available, necessitating a review of available information and re-assessment of the taxonomy.

Results: Dental morphology and cranial anatomy clearly diagnosed the southern and northern forms. The differentiation was well supported by dental metrics, cranial growth and craniometry, and corresponded with differences in post-cranial skeleton, external measurements and external features. No distinctive differences were found in the limited descriptions of their behavior and ecology. Fossil history indicated the antiquity of the genus, dating back at least to early Pliocene and evolution into a number of diagnosable forms. The fossil skulls examined fell outside the two extant forms in the craniometric analysis. Genetic divergence between the two forms was consistent across both nuclear and mitochondrial genomes, and indicated a separation of over a million years.

Conclusions: On re-assessing the taxonomy of the two forms we find them to be morphologically and genetically distinct, warranting the recognition of the taxa formerly designated as subspecies; Ceratotherium simum simum the southern form and Ceratotherium simum cottoni the northern form, as two distinct species Ceratotherium simum and Ceratotherium cottoni respectively. The recognition of the northern form as a distinct species has profound implications for its conservation.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1

Figure 1. Skull growth in males: a, basal length; b, occipitonasal length; c, nasal boss breadth; d, depth of dorsal concavity.

Age stages are as follows: Stage 2, first molar in process or erupting; 3, second molar in process of erupting, second and third premolars in process of replacement; 4, second molar in wear; fourth premolar in process of replacement; 5, third molar in process of eruption; 6, third molar in occlusion.

Figure 2

Figure 2. Skull growth in females: a, basal length; b, occipitonasal length; c, nasal boss breadth; d, depth of dorsal concavity.

Age stages as in Figure 1.

Figure 3

Figure 3. Nasal boss breadth.

Figure 4

Figure 4. Univariate comparisons of living samples, stages 5–6: a, depth of dorsal concavity; b, maxillary toothrow length.

Figure 5

Figure 5. Univariate comparisons of living and fossil samples; a, basal length; b, occipitonasal length; c, occipital crest breadth; d, occipital height; e, depth of dorsal concavity; f, maxillary toothrow length.

Figure 6

Figure 6. Bivariate plot: a, occipitonasal length relative to basal length; b, occipital height relative to occipital crest breadth; c, dorsal concavity relative to occipitonasal length.

Figure 7

Figure 7. Discriminant analysis using 7 variables.

Ungrouped fossil specimens, with their DF values, are: Ileret (3.12, 2.28), with simum males; Arambourg (2.59, −2.81), with simum females. DF1 accounts for 68.4% of total variance, DF2 for 31.1%.

Figure 8

Figure 8. Ceratotherium simum.

Left, Dan, male aged 40 years. Right, Zamba, female aged 37 years. Both, Usti nad Labem. Photos, Jan Robovsky.

Figure 9

Figure 9. Ceratotherium cottoni.

Left, Suni, male aged 27 years. Right, Nabire, female aged 24 years. Both, Dvur Kralove. Photos, Jan Robovsky.

Figure 10

Figure 10. Skulls of adult males of Ceratotherium simum (above) and C. cottoni (below).

Upper photo from Heller (1914, plate 17, fig. 3), lower photo by E. Trumler of skull in Zoologische Staatssammlung, Munich.

Figure 11

Figure 11. Maxillary toothrows of adult male Ceratotherium simum (left) and C. cottoni (second from left), and adult female C.simum (centre) and C.cottoni (second from right and right), from Heller (1913, plates 21:4, 21:2, 22:4, 21:1 and 22:3, respectively).

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