The impacts of lions' evolutionary history on current conservation status and threats

By David Youldon
Last updated 16 Nov 2013

Populations of lions have decreased from 400,000 in the 1950’s [1] to an estimated 32,000 today with “abundant evidence of widespread declines and local extinctions” even in protected areas [2].  This report aims to assess the impact of the species’ evolutionary history on its current conservation status.

Origin of a species

The earliest lion-like cat (Panthera leo fossilis) was discovered at Laetoli in Tanzania and dated to the late Pliocene, 5.0 – 1.8 million years ago (mya) [3, 4].  The first undisputed record of a lion was discovered at Olduvai Bed I, in Tanzania, dating to 1.87 – 1.7 mya [5].  During the late Pliocene and early Pleistocene lions become increasingly common, coinciding with large scale vegetation change from forests and woodlands towards savannah-type grasslands, that heralded the evolution of many ungulates and ground-living primates [6-8].

Broadly mimicking human dispersal, and corresponding to climatic changes and subsequent vegetation change towards grasslands, influencing prey base distribution, lions migrated out of Africa 800 – 100,000 years ago during the middle Pleistocene into Europe, Asia, and the America’s, as far south as Peru [9-12].  The lion became the most widespread large terrestrial mammal on Earth [12], its success possibly as a result of a more developed brain compared to other large predators of the time, such as Smilodon, the sabre-toothed cat [13-15] as well as the possibility that their social structure had already evolved to include group-living [12].

A population bottleneck c. 320 – 190,000 years ago created a single population of the modern lion that dispersed throughout Africa and south-west Eurasia, replacing earlier lion populations, although the geographical origin of the modern lion remains undetermined [12].  This single origin replacement model mimics the ‘recent African origin’ model of human evolution and dispersal to replace hominids elsewhere during the same time period [3].

Taxonomic Classification

Lions were first classified, using a specimen from Constantine in Algeria, as Felis Leo (Linnaeus, 1758).  Today, they are classified as; family: Felidae, subfamily: Pantherinae, genus: Panthera, species: Panthera leo (P. leo).  Although up to 24 sub-species classifications have been suggested based on external morphological differences [16, 17], recently, mitochondrial DNA analysis suggests that all African lions should be considered one sub-species; P. l. leo [18, 19]  Further analysis has revealed regional genetic differences, and that west and central African lions are genetically closer to Asiatic lions than to those of eastern and southern Africa [20]. The IUCN [21] recognize two extant sub-species of lions; the African lion; P.l. leo, and the Asiatic lion; P.l. persica, although the distinction may be to permit differing regional red list conservation status classifications rather than being based on significant genetic difference, which at only 1.1% [17] is less than between human racial groups [16].

The Evolution of Group Living in Lions

Analysis of the food intake needs of eight extant felids suggests that parasitism benefits lions more than any other species and that it was in the interest of early lions to form groups to increase opportunities to obtain food by parasitism [12].  The parasitized lion gains advantages from this association by reducing intra- and interspecific competition through joint defence of kills it made [22, 23].  Additional advantages of group living may have then become apparent, fixing the behaviour in the species.  These advantages include; that in areas / times of prey scarcity hunting cooperation increases food intake per capita [24, 25] - although this advantage may not apply in all locations [23]; defence of carcasses against competitors [26, 27]; increased reproductive success [23, 26]; for cooperative defence of cubs [23]; and, maintaining a territory within which to forage [23, 26].  Male grouping behaviour likely evolved in response to female grouping behaviour as such grouping increases success in pride take-over and length of pride tenure [26].

The Evolution of Mane’s in Male Lions

Lions are the only felid to maintain a mane, although the length, density and colour of this trait are highly variable [28].  Several hypotheses of the purpose of manes were tested in a long-term study within Tanzania’s Serengeti National Park; those being that: the mane provides protection during fights, or as a signal of genetic and/or phenotypic condition.  The study concluded that males with darker manes were more likely to achieve pride tenure, have a higher proportion of surviving offspring, are less likely to be wounded and have greater chance of surviving injury [28].  Analysis of lion cave paintings suggests that the evolution of the mane occurred later in the species’ evolutionary history than the evolution of group-living, suggesting that this trait may have developed in response to the reproductive advantages of holding tenure over a group of reproductively active females [12, 28-30]. 

Ecology of Lions

The ecology of lions differs both between and within populations.  They have a wide habitat tolerance but with a preference for vegetation complexes comprising grasslands, open woodlands and scrub.  They are absent only from rainforest habitats and the centre of deserts [21, 31].  Population densities, ranging from 1.5 – 55 adults per 100km2 [32] are highest in grassland habitats with the highest biomass and diversity of hoofed ungulates [e.g. 33-35]. 

Lions live in fission-fusion social groups [23] comprising related females, unrelated males, and their dependent offspring.  Pride size is highly variable from 2 – 37 [e.g. 34, 36-37].  Prides maintain exclusive core home ranges within overlapping territories.  Territory size is influenced by food availability in the leanest season of a year and range from 20 – 2075km2 [38].

Due to their opportunistic nature, lions prey on a diversity of species (from fish to elephants, Loxodonta africana), however each pride has distinct prey preferences that constitute 80 – 90% of their diet [25, 34, 39-44].  Lions scavenge from other predators, as well as from natural mortalities, although the amount of scavenging as a proportion of their diet is highly variable, from 0 – 81% [23, 25, 34, 45].  Lions are able to survive without water for long periods, obtaining their needs from prey, and some plants (e.g. tsama melon, Citrullus lanatus) [21, 31].     

Conservation Status & Threats

Since 1996, and as of the most recent assessment (1st July 2008) the African lion has been classified by the IUCN as ‘vulnerable’ under criteria A2abcd [21] due to an estimated population reduction of c. 30% during the previous two decades, and where the causes of this reduction are unlikely to have ceased.  The principle threats to lions include: indiscriminate killing of lions, primarily in response to actual or potential predation by lions of livestock [21]; loss of habitat and increasing fragmentation of the remaining suitable habitat that closes natural routes of dispersal and immigration and increases risks of inbreeding [46-48]; loss of suitable prey-base through poaching activities [49], improperly managed trophy hunting [e.g. 50]; and, actual / probable disease threats in combination with the influence of climate change on disease transmission & virulence [e.g. 51-53].

The Impact of the Species’ Evolutionary History on its Current Conservation Status and Threats

As a territorial, group living species that requires large areas to support sufficient prey species to provide their dietary needs, as well as for unrestricted dispersal of males, and some females, it has been estimated that a “continuous population of at least 50 prides, but preferably 100 prides, with no limits to dispersal” [46] is required to avoid risks of inbreeding depression and ensure evolutionary potential; a minimum of 10 prides is suggested as necessary to avoid these risks in the short term.  Björklund [46] uses an average pride size of 11.1 individuals, and therefore assumes 1,110 lions are needed in a population, although this model is based on Tanzania and likely does not directly apply to populations in other areas / habitats.  Of 67 identified lion habitat patches, only 8 contain sufficient numbers to avoid the risks of inbreeding long term: East-Central African Republic, Ruaha-Rungwa, Serengeti-Mara, Selous, South-East Angola, Okavango-Hwange, Niassa, and Great Limpopo [2] – although the study suggests the estimates for South-East Angola are highly hopeful and likely does not contain sufficient numbers.  The converse is that 59 lion habitat patches contain below the threshold for long term evolutionary viability, and 35 contain insufficient to avoid inbreeding risks in the short term.  Accelerating land conversion and suitable lion habitat fragmentation in response to the needs of increasing human and associated livestock populations will exacerbate the risks of inbreeding, further reducing the species’ evolutionary potential.  Interventionist management to artificially replicate natural gene flow through translocations may be one solution to alleviate these risks, however there are also risks in such actions with the potential for creating a problem of outbreeding depression in destination populations, as well as the introduction of disease into naïve populations [54]. 

The extent and impact of endemic disease infection in African lions, and the consequences of transmission between now isolated populations remains little understood [55, 57].  The most profound negative epidemic disease impacts on lion populations have originated in domestic animals and include canine distemper [53] and bovine tuberculosis [58].  Increasing human populations with their associated livestock, and more frequent mixing of wild and domestic animals, can provide additional opportunities for disease transmission with negative consequences for lions.  Changes in climate regimes can have significant impacts on disease virulence, as has been identified in a period of high lion mortality in the Serengeti ecosystem [52].  Lions are considered to have evolved to survive FIVple infection with only limited immunological cost [59], however, with 100% of lions likely infected by at least one pathogen, and many by multiple pathogens, and multiple sub-types of those pathogens, the opportunities for recombination of infectious diseases causing more virulent strains exist [60]. 

Changing climate regimes can also impact lions by making habitat less suitable for them and their prey species.  As a territorial species with altricial young incapable of long movements, lions are susceptible to alterations in their habitat that reduces prey availability.  With increasing habitat fragmentation as a result of human induced land conversion, opportunities for lions to move to new areas that may become suitable as a result of the same climate change will require emigration through an increasingly hostile habitat matrix.  Additionally, rising temperatures may place heat stress on male lions unable to dispel heat through their skin and that maintain large and dark manes necessary for maximising reproductive success. 

Conclusion

The African lion is subject to increasing anthropomorphic threats that threaten its survival.  The species has evolved social and reproductive behaviours that require space, the greatest threat therefore being an increasing human population and the subsequent land conversion to meet the needs of people.  Humans however have induced additional threats by introducing disease and the impacts of climate change to lions, whose populations continue to decline.

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