Giraffes, Giraffa camelopardalis,
have traditionally been split into nine subspecies based upon their coat
patterns, details of the skeleton and geographic distribution. Unlike many
other large Mammals they have not been the subject of extensive genetic
studies. The limited studies that have previously been undertaken have
suggested that the Angolan Giraffe Giraffa camelopardalis angolensis,
South African Giraffe, Giraffa camelopardalis giraffa,
West African Giraffe, Giraffa camelopardalis peralta,
Reticulated Giraffe, Giraffa camelopardalis reticulata,
Rothschild’s Giraffe, Giraffa camelopardalis rothschildi,
and Masai Giraffe, Giraffa camelopardalis tippelskirchi,
may in fact be separate species, while Thornicroft’s Giraffe, Giraffa camelopardalis thornicrofti may in
fact represent a population of the Masai Giraffe, Giraffa camelopardalis tippelskirchi. The Kordofan Giraffe of Central
Africa, Giraffa camelopardalis antiquorum
is closely related to the West African Giraffe, Giraffa camelopardalis peralta, and the Nubian Giraffe,Giraffa camelopardalis camelopardalis, has
never been genetically sampled at all. This lack of understanding of the
genetics of Giraffes has the potential to hamper conservation efforts, as
Giraffes are currently frequently relocated by Humans, more often with the
intention of providing visible Giraffes for the tourism industry than of moving
the animals to promote their survival.
In a paper published in the journal BMC Evolutionary Biology on 23
October 2014, Friederike Bock of the Biodiversity and Climate Research Centre
at the Senckenberg Gesellschaftfür Naturforschung, Julian Fennessy of the Giraffe Conservation Foundation and the School of Biological Earth and EnvironmentalStudies at the University of New South Wales, Tobias Bidon, also of the Biodiversity
and Climate Research Centre at the Senckenberg Gesellschaftfür Naturforschung, Andy Tutchings and Andri Marais, also of the Giraffe Conservation Foundation, Francois Deacon of the Giraffe Conservation Foundation and of the Department Animal,Wildlife & Grassland Science at the University of Free State and Axel Janke
of the Biodiversity and Climate Research Centre at the Senckenberg Gesellschaftfür Naturforschung
and of the Institute for Ecology, Evolution & Diversity at Goethe University Frankfurt, describe the results of a population genetic study
covering seven of the nine currently recognised Giraffe subspecies, primarily
from regions of southern Africa where Giraffes have not previously been
sampled.
Bock et al. used a
molecular clock method to estimate the divergence times of the Giraffe
subspecies sampled (i.e. work out when the most recent common ancestor of each
pairing of Giraffes lived). From this they estimate that the Giraffe subspecies
divided into two distinct groups, the Angolan Giraffe, South African Giraffe,
and Masai Giraffe to the south and the West African Giraffe, Reticulated
Giraffe, Rothschild’s Giraffe and Kordofan Giraffe to the north, about 2.02
million years ago.
Within the southern group the Angolan Giraffe split from the other
two species about 1.45 million years ago, and the South African Giraffe and
Masai Giraffe split about 610 000 years ago.
In the southern group the Kordofan Giraffe split from the other
three species about 780 000 years ago and the Reticulated Giraffe split from
the West African Giraffeand Rothschild’s Giraffe about 650 000 years ago. The
Reticulated Giraffe was recovered as the closest relative of the West African
Giraffe, with the two subspecies having split about 510 000 years ago.
Maximum clade credibility tree of the major Giraffe populations
as reconstructed by Bayesian analysis conductedin BEAST. Blue bars indicate 95%
highest posterior density intervalsfor node ages, asterisks denote posterior
probability >0.95. Scale onthe bottom represents divergence time (million
years ago). Bock et al. (2014).
This strongly supports the idea that some Giraffe subspecies should
in fact be considered to be species. In other Mammal groups species which
diverged over a million years ago, and many which are much more recent are
seldom questioned, for example the Chimpanzee and Bonobo are considered to have
diverged about 420 000 years ago. Interestingly this also hints that some
unrecognised diversity may exist within current subspecies populations, with
some Kordofan Giraffes having shared a most recent common ancestor around 430
000 years ago and some Masai Giraffes around 410 000 years ago.
The cause of divergence between African animal populations during
the Pleistocene is notoriously hard to unravel. The continent did not suffer
extensive glaciations during the period, but did undergo major climatic
changes, with cooler drier periods leading to expansions of grasslands and
deserts and warmer wetter periods leading to expansions of forests and lakes.
In each instance this led to the shrinking and fragmentation of some habitats
at as other expanded, driving speciation among African animals.
One surprising pattern that emerged among the Giraffe populations
studied was the separation of subspecies in Botswana and Namibia. It has
generally been accepted that the Giraffes of Botswana and Namibia are Angolan
Giraffes, while the South African Giraffe is found further to the south and
east in Zimbabwe and South Africa. However Bock et al. found that only the Giraffes of the Central Kalahari Game Reserve in Botswana and Etosha National Park in Namibia grouped with Angolan
Giraffes, while the Giraffes of the Chobe National Park, Moremi Game Reserve,
Nxai Pans Park, and Vumbura Concession in northern Botswana and the Bwabwata National
Park population in northeastern Namibia are all grouped with South African
Giraffes.
(A) Distribution range of Giraffe (yellow patches) and
sampling locations. Colours show genetically identified subspecies. (B)
Depiction of southern African Giraffe populations and location of geographic
boundaries. O-K-Z: Owambo-Kalahari-Zimbabwe epigeiric axis, O-B:
Okavango-Bangweulu axis. Bock et al. (2014).
These are populations of particular concern, as the population of
Giraffes in northern Botswana is thought to have declined from over 10 000
individuals to under 4000 individuals during the past 10 years, and the
Giraffes of the Bwabwata National Park were almost wiped out by poachers in the
1970s and 1980s, with the population having recovered somewhat but still only
standing at slightly over 150 individuals.
These populations are separated by less than 500 km in places,
distances which should in theory present little obstacle to Giraffe movements,
but these populations have apparently remained isolated for over a million
years. While there is no good explanation for this line of separation, it has
previously been recorded in other animal populations, with separate species of
African Elephants (Loxodonta spp.)
and subspecies of Damara Dikdik (Madoquada marensis)
and Impala (Aepyceros petersi) on
either side of the boundary.
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