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.

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.
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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