Wednesday 4 February 2015

The genetic diversity and distribution of modern Giraffes.

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.

A West African Giraffe, Giraffa camelopardalis peralta, near Koure in Niger. Wikipedia.

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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