Elephant Evolution of the Past and Conservation Today

Over the past few decades, genetic methods have greatly improved, with costs lessening, allowing scientists to evaluate species in a new light. For elephants, this has sparked a debate. Genetic evidence now suggests that the African elephants, which have been traditionally characterized as the subspecies forest and savanna (or “bush”) can be upgraded to species level distinction of Loxondonta cyclotis and L. africana, respectively. These two species live in Africa, but mostly on opposite sides of the continent. Savanna elephants are typical of what people think of when they hear “African elephant,” as they are often seen on African safaris and are well represented in nature documentaries. As their name implies, they live in savanna habitats of eastern and southern Africa, although there are some populations in forested habitat, but within the same east-south region of Africa. Forest elephants live in central and western African rainforests, with a potential small hybrid zone existing between the two regions of the species in the middle of Africa (eastern Democratic Republic of Congo).

The idea that African elephants could be different species was originated through morphological evidence on skull measurements, and first supported genetic evidence by Roca et al. in 2001 [1]. However, the genetic species distinction was somewhat controversial because although the nuclear markers showed a lack of gene flow between forest and savanna species, savanna species were found with forest elephant mitochondrial DNA, which is passed directly from the mother to her offspring. Later, Roca et al. [2] argued this cyto-nuclear genomic disassociation could occur because in historic hybrid zones, savanna males would outcompete forest males for mates (larger males in general are more successful at mating, and savanna elephants are larger than forest elephants). Therefore a forest elephant female would have a hybrid calf with a mix of forest-savanna nuclear DNA, but only forest mitochondrial DNA. Over generations, this would continue to occur with savanna males being more successful with mating as they continue to dilute the nuclear forest elephant DNA with the hybrids that they mate with. Therefore, the mitochondrial forest elephant haplotype would still exist, while the nuclear genes would become dominated by the savanna species.

Figure from Roca et al. 2007 which shows how through a series of back-crossings African savanna elephants can have forest elephant mitochondrial DNA.
Figure from Roca et al. 2007 which shows how through a series of back-crossings African savanna elephants can have forest elephant mitochondrial DNA.

However, in 2010, the species distinction became clearer, as Rohland et al. [3] included genomic evidence from the extinct mastodon and woolly mammoth species, which are from Mammut and Mammuthus genera. The genomic data were much larger than what was used in previous studies (39,763 bp sequence across 375 loci), and it also included two extinct species to allow for deeper comparisons between the three extant species. The results of the study found that Asian elephants are the closest living relative to the extinct woolly mammoth, and that surprisingly, savanna and forest elephants are actually as more or more divergent in the nuclear genome as mammoths are from Asian elephants. Mammoths and Asian elephants are actually considered to be different genera, Mammuthus and Elephas, respectively, while forest and savanna elephants belong to the same genus. In regard to cyto-nuclear disassociation, the authors of the study hypothesize that the Loxodonta species was separated in range by the presence of the Elephas genus in Africa, creating a barrier that prevented admixture between the now forest and savanna species. Later when the Elephas genus disappeared from Africa, gene flow could occur between the forest and savanna species, hence the mitochondrial DNA from forest elephants in savanna elephants.

Figure 2 from the Rohland et al. 2010 study showing the relationships between African forest, savanna, Asian, mammoth, and mastodon elephants.
Figure 2 from the Rohland et al. 2010 study showing the relationships between African forest, savanna, Asian, mammoth, and mastodon elephants.

Given the very strong genetic evidence that African forest and savanna elephants are different species (as different as Asian elephants and mammoths), it’s hard to believe this species distinction has not been designated in terms of conservation. If you go to the IUCN Red List website and look up African elephants, you will see this: The African Elephant Specialist Group believes that more extensive research is required to support the proposed re-classification. Premature allocation into more than one species may leave hybrids in an uncertain conservation status (IUCN SSC African Elephant Specialist Group 2003). For this reason, this assessment was conducted for the single species as currently described, encompassing all populations.” However, leaving the species separate and ignoring genetic evidence will likely have strong implications for forest elephants.

If the two species are kept together for conservation, and if conservation status is evaluated by numbers of individuals ignoring forest and savanna status, than forest elephants will be at a greater risk of extinction. As forest elephants live in forests, much of the poaching that occurs goes undetected. You cannot fly planes over areas to look for carcasses due to thick canopy cover and it is slow and unwieldy to navigate forests on foot. In general, forest elephant populations are difficult to estimate because of the aforementioned reasons. Population estimates often rely on smaller sampling efforts that have to be scaled to large areas, greatly increasing error. Some populations are even just best guesses. This makes it incredibly difficult to establish baseline data, declines in populations, and recovery.

In contrast, for savanna elephants, populations are easier to estimate by vehicle or aircraft and therefore we have more reliable data. Although savanna elephants are being poached heavily more recently, there have been and are larger populations, and in some cases, so dense that countries have culled, proposed culling populations, or used birth control. African elephants are listed as vulnerable on the IUCN Red List, but forest elephants, according to regional assessments, are endangered. If poaching declines and elephants populations recover, I worry that decisions about the conservation status of African elephants will be based on numbers, which may be buttressed by savanna elephants, and are not reflective of what is happening to forest elephants.

Before the current poaching epidemic that started in 2011-2012, forest elephants suffered their own, quiet epidemic. Approximately 60% of the population declined in less than a decade. It wasn’t until the recent poaching threat, when more well-known savanna elephant populations became under attack, that elephants gained worldwide media and political attention. While I am grateful that elephants are receiving this attention and funding is going towards their protection, if savanna elephants do recover and stabilize, I worry that the forest elephants may go extinct in silence.

1. Roca AL, Georgiadis N, Pecon-Slattery J, O’Brien SJ (2001) Genetic evidence for two species of elephant in Africa. Science 293: 1473-1477.
2. Roca AL, Georgiadis N, O’Brien SJ (2007) Cyto-nuclear dissociation and the African elephant species question. Quarternary International 169-170.
3. Rohland N, Reich D, Mallick S, Meyer M, Green RE, et al. (2010) Genomic DNA sequences from mastodon and woolly mammoth reveal deep speciation of forest and savanna elephants. PLoS Biol 8.

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