Friday 4 April 2014

The origin and diversification of modern Lions.

Lions (Panthera leo) are large, charismatic predators currently found in Africa and India, with a fossil record that includes areas of northern Eurasia and North America. Lions outside their current range persisted in North Africa (Barbary Lions) and the Middle East (Persian Lions) into historic times, but are now extinct. Modern Lions are divided into two subspecies, the Indian Lion (Panthera leo persica), which is considered to be 'Endangered' under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species, with only about 400 individuals thought to be surviving in the wild, and the African Lion (Panthera leo leo) considered to be 'Vulnerable', having suffered a roughly 30% decline in numbers in the last two decades.

Lion Frieze from the Palace of Darius I, dating from circa 510 B.C., currently in the Louvre Museum. Lions are no longer found in the Middle East. John Malyon/Artcyclopedia.

In a paper published in the journal BMC Evolutionary Biology on 2 April 2014, a team of scientist led by Ross Barnett of the Department of Archaeology at Durham University describe the results of a study into relationships between modern and historic Lion populations, based upon the analysis of mitochondrial DNA, and the implications of the results of this analysis for the conservation of Lions.

Traditionally modern Lions have been split into two subspecies, with Indian and Persian Lions being described as Panthera leo persica and all African Lions, including the Barbary Lions of North Africa, being described as Panthera leo leo, while other subspecific names were assigned to various fossil Lions.

Barnett et al. could find no evidence to support this classification system, with modern Lions falling into five distinct groupings (fossil Lions were not examined in the study). Persian and Indian Lions did form a single group, but this group also included the Barbary Lions of North Africa, while the Lions of sub-Saharan Africa could be divided into four groups; West African, Central African, East African and Southern African. Furthermore these five groups could be grouped into two larger groups, one comprising the Barbary and Asian Lions plus West and Central African Lions, and the other comprising Southeastern and Southern African Lions.

Furthermore the study suggested that all the modern Lion groups shared a common ancestor in the Late Pleistocene, around 124 200 years ago, and that the Lions of Asia are the result of a much later migration, in the Late Pleistocene to early Holocene, 24 000 to 10 000 years ago. The fossil Lions of Eurasia and North America are therefore likely to have been the result of one or more earlier migrations, which subsequently became extinct.

An Indian Lion at Arignar Anna Zoological Park. Vincent Paul/Wikimedia Commons.

Barnett et al. suggest that the divergence of the different groups of Lions can be dated using a molecular clock method. The oldest split occurred between Southern and Eastern African Lions and all other lions around the calculated time of the most recent common ancestor, so about 124 200 years ago. Southern and East African Lions then split around 81 900 years ago. The Central African Lions split from the West and North African/Asian Lions around 61 500 years ago, and the West African Lions split from the North African and Asian Lions around 51 000 years ago.

Phylogenetic tree of modern Lions, based upon mDNA analysis. Estimates of divergence times: (a) 124,200 years; (b) 61,500 years; (c) 51,000 years; (d) 81,900 years; (e) 57,800 years; (f) 21,100 years. Branch colours correspond to reconstructed ancestral geographic states (Purple, South Africa; Yellow, East Africa; Orange, West Africa; Red, Central Africa; Teal, North Africa; Blue, South Asia; Green, Near-East). Barnett et al. (2014).

Barnett et al. suggest that the separation of the different Lion groups represents shifting climatic patterns during the Pleistocene, with lions expanding to populate most of Africa during the dryer period prior to about 130 000 years ago,  when the continent was covered by savanna and scrub woodland, but subsequently becoming divided into two populations by the expansion of rainforests in the subsequent more humid period. Around 74 000 years ago the climate began to dry again, enabling some Lions to colonize Central Africa from the West, then eventually leading to the expansion of the Sahara Desert, which separated the Lions of North and West Africa. Finally some North African Lions expanded into southwest Asia in the warming climate at the end of the Pleistocene.

Reconstructed distribution of the modern lion at different times. Estimates of spatial diffusion pathways at Marine Isotope Stage (MIS) time points: (A) MIS5 (B) MIS4-MIS3 (C) MIS2-MIS1 (D) Estimated natural distribution prior to anthropogenic disturbance. Black arrows show
estimated spatial diffusions, with thicknesses proportional to Bayes factors. Movement from East Africa to South Africa (4.83), from South Africa to East Africa (4.66), from West Africa to Central Africa (3.00), from North Africa to South Asia (4.37), from South Asia to North Africa (4.50), from North Africa to Middle East (21.03). Tropical rainforest is shown in light grey (present distribution), maximal extent during humid periods (black dashed line), and minimal extent during arid periods (white dashed line). The Great Rift Valley is shown in dark grey. African rivers are shown in blue. Co, Congo; Ng, Niger; Ni, Nile; Se, Senegal. Barnett et al. (2014).

This has strong implications for the conservation of Lions. All African Lions have previously been regarded as members of a single sub-species, Panthera leo leo, which while stressed across parts of its range, was thriving in others, giving the species the potential to recover should the opportunity arise or be created. However if African Lions comprise four separate sub-species, then two of these subspecies should be regarded as being at a much higher level of risk; there are thought to be about 800 West African Lions and about 900 Central African Lions surviving in the wild, making these groups almost as vulnerable as the Lions of India. Furthermore almost all Lions in captivity in zoos are descended from East or Southern African Lions, severely limiting the potential for restoring West or Central African Lion populations with captive breeding programs.

There has also been considerable talk of re-introducing Barbary Lions to North Africa in recent years. This idea has been based upon the idea that these Lions belonged to the same subspecies as the Lions of sub-Saharan Africa, making fairly large, healthy populations available from which to recruit Lions for the re-colonization of North Africa. However if the Barbary Lions were in fact the same sub-species as the endangered Lions of India, and re-introduction methods wish to repopulate North Africa with the same subspecies that formerly colonized the region, then this project becomes correspondingly more difficult.

See also...

 A new species of Fox from the Pleisticene of Gauteng Province, South Africa.

 The diet of the Langebaanweg Hyaenids.

 Fossil Pandas from the Middle Miocene of Spain.

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