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Nile monitor taxonomy 2015 Print E-mail

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Two groundbreaking papers examining genetic diversity in Varanus niloticus have turned the taxonomy of the group on its head and raised important questions about the conservation status of some populations (Dowell et all 2015a, b). In contrast to many other molecular works that aim to investigate phylogeny of Varanus lizards, these studies rely on large samples that include both old museum material (59) and recently collected (66) specimens, and used microsatellite loci, mitochondrial and nuclear markers in analyses. This thorough approach allows researchers to measure diversity both within and between populations and to find evidence of past and present population declines.



Firstly, "Varanus ornatus" is not a valid species, it is a phenotypic morph of at least two species associated with closed canopy forest. Secondly, what was previously considered "Varanus niloticus" includes three very distinct populations. A West African group (Varanus stellatus DAUDIN 1802) split from a common ancestor towards the end of the Miocene (estimated at 7.7. million years ago) when cooler, drier climates caused grasslands to shrink the vast African forest to to isolated pockets. Around 4.7 million years ago, when forests were expanding and reconnecting, Varanus niloticus spilt into northern and southern forms. More recently a costal form split from the southern race of V. niloticus around 3.2 million years ago, V. stellatus split into two races around 1.7 million years ago and the northern race split around 500,000 years ago. The splits have parallels recorded in genetic analysis of  dwarf crocodiles and chimpanzees and follow historical changes in African forests and river systems.

 Varanus stellatus, Bui National Park, Ghana

Genetically, Varanus stellatus is a very distinct species, with over 8% uncorrected sequence divergence from either of the V. niloticus races. Although the case for V. stellatus is very strong, the taxonomic identity of the races of V. niloticus is less certain. Based on mitochondrial evidence alone, the northern and southern forms were more genetically distinct than many of the currently recognised species of water monitor (e.g. Welton et al. 2014), but mitochondrial mixtures were found along contact zones and analysis of nuclear DNA indicated mixed haplotypes. It was not possible to tell if this represents ongoing gene flow between the populations or the retention of ancestral polymorphisms. If the northern and southern populations warranted subspecific status they would probably be Varanus niloticus niloticus and Varanus niloticus capensis (SPARRMAN 1783). The authors note that in 1844 Schlegel suggested three forms; one from West Africa, one from the Nile and one from South Africa, but caution that more data is needed before the taxonomic identity of the northern and southern groups can be understood.

The case against Varanus ornatus looks conclusive. Specimens previously attributed to V. ornatus turned out to belong to all three races. Therefore an animal that was Varanus ornatus yesterday could be V. stellatus, V. niloticus niloticus or V. niloticus capensis tomorrow. Why animals in closed canopy forests benefit from the adaptations that were considered diagnostic of this species (stout build and skull, reduced number (3-5) of occelli rows and whitish tongue - Bohme & Ziegler 1997) is unknown.

In many ways it's not at all surprising that the Varanus niloticus group is very diverse. Despite the wide distribution of the group even large adults (with low surface to volume ratios) dehydrate very quickly without humidity (Cloudsley-Thompson 1967, personal observations) and movement between different river systems is very unlikely as a result. In contrast to species of the Varanus salvator group, where individuals regularly move between islands and thus reduce the effects of isolation (e.g. Rawlinson et al. 1990, personal observations), Nile monitors are more or less stuck in their drainage basins. An earlier paper reported evidence of four distinct populations occurring in different drainage basins within Sahelian Africa (primarily Mali, Chad and Niger), and found evidence of a bottleneck in the population between 1800 and 1000 years ago that they they attribute to a population collapse caused by human predation associated with the Djenné-Djenno, a trading centre that reached a vast size before being abandoned around 900AD .

The findings have important implications for the conservation of the group because whilst some races are intensively exploited for leather others are not. A global assessment of extinction risk for Varanus niloticus has not been carried out yet, but would likely result in a "least concern" rating based on a continental distribution and lack of exploitation in many areas.  Greater taxonomic resolution allows identification of populations at risk of over exploitation. However at present there seems to be no way of identifying members of any race based on appearance. Schlegel (1844) was the last to attempt such a scheme, suggesting that the ground colour of stellatus was dark brown, niloticus was olive brown and capensis black. Previously Daudin (1802) had suggested that the double keel on the tail of stellatus was absent from niloticus. A comprehensive study of morphology within the group may provide cues that can be used to tell these animals apart without genetic analysis.

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Böhme, W., & Ziegler, T. 1997. A taxonomic review of the Varanus (Polydaedalus) niloticus (Linnaeus, 1766) species complex. The Herpetological Journal 7: 155-162.


Cloudsley-Thompson, J.L. 1967. Water relations and diurnal rhythm of activity in the young Nile monitor. Brit. J. Herp. 3 (12):296-300.

Daudin, F.M., 1802. Histoire naturelle, générale et particulière des reptiles. Paris, France.

Dowell, S.A., de Buffrénil, V., Kolokotronis, S.-O., Hekkala, E.R., 2015. Fine-scale genetic analysis of the exploited Nile monitor (Varanus niloticus) in Sahelian Africa. BMC Genet. 16, 32.

Dowell, S.A, D.M. Portik, V. de Buffrenil, I Ineich, E Greenbaum, S.O. Kolokotronis and E.R. Hekkala. 2015. Molecular data from contemporary and historical collections reveal a complex story of cryptic diversification in the Varanus (Polydaedalus) niloticus Species Group. Molecular Phylogenetics and Evolution. doi:10.1016/j.ympev.2015.10.004

Rawlinson,P.A. Widjoya, A.H.T., Hutchinson,M.N. & Brown,G.W. 1990. The terrestrial vertebrates of Krakatau Islands, Sunda Strait, 1883-1986. Phil. Trans. Royal Soc. Lond B328:3-28.

Schlegel, H. 1844. Abbildungen neuer oder unvollständig bekannter Amphibien: nach der Natur oder dem Leben entworfen. Verlag von Arnz & Comp, Düsseldorf, Germany

Sparrman, A. 1783. Resa Till Goda Hopps-Udden, Soedra Pol-kretsen Och Omkring Jordklotet, Samt till Hottentott-och Caffer-Landen, Aren 1772-76. Anders J. Nordstroem, Stockholm.

Welton, L.J., Travers, S.L., Siler, C.D., Brown, R.M., 2014. Integrative taxonomy and phylogeny-based species delimitation of Philippine water monitor lizards (Varanus salvator Complex) with descriptions of two new cryptic species. Zootaxa 3881, 201–227.

 
 

 

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