Evolution

A species of their own

Portrait of the Aida family in 2005 © Andrea Turkalo

Only in 2013 had sufficient evidence accumulated to consider the forest elephant its own distinct species (Loxodonta cyclotis). Before this, the savannah or bush elephant and the forest elephant were considered subspecies of the inclusive taxon Loxodonta africana. Recent analysis of DNA from fossils reveals that forest elephants are more closely related to the now-extinct straight-tusked elephant (Paleoloxodon antiquus) of European forests than to any other living elephant (1, 2).

Despite their clear genetic and morphological differences, the International Union for the Conservation of Nature (IUCN) has not yet acknowledged cyclotis as a species. It has now been fifteen years since the IUCN released a statement citing the “uncertain” taxonomy of elephants, and calling for further evidence before determining the relationships among existing elephant populations (3). This delay, which is mostly due to politics, has far-reaching consequences for elephant conservation (read more below).

The relationships among the extant (living) elephants shown above would have been controversial several years ago. It takes the scientific community a long time to debate and accept changes in the species designation of an organism, particularly mammals. Changes in bird taxonomy are often decided more efficiently because a specific organization has been vested with the authority to do so. In the mammal world there is no parallel structure.

For example, an analysis of elephant skulls collected across Africa proposed that forest elephants “deserve to be ranked as full species.” This study, done by Peter Grubb and colleagues (4), concluded that “living bush and forest African elephants are evolutionarily and ecologically distinct forms.” What the discussion lacked at the time was genetic evidence. With the appearance of elephant research that included genetic sequences, we have now learned “that little or no nuclear gene flow occurs between forest elephant and savanna elephant populations” (5). In fact, forest and savannah elephants may be as genetically distinct as mammoths (Mammuthus) and Asian elephants (Elephas maximus), which are considered to be different genera (6).  More on elephant evolution below.

Evolutionary relationships of the Elephantidae as proposed from recent analysis of nuclear and ancient DNA. The two extant genera, Loxodonta and Elephas, are shown in parentheses because, if accepted, this proposed tree would require a separate genera for the two African species as well as perhaps for Asian elephants or the now extinct mammoth.

A Conservation Dilemma

These taxonomic issues are more than an evolutionary debate within the scientific community. Classification of the species is important for the conservation of forest elephants. It might have been easier to ignore the decline of forest elephants if they were seen as populations within the African elephant species (in fact, the relatively poor data on forest elephant populations before 2013 resulted in conservation decisions by the IUCN and CITES that essentially ignored forest elephants). However, once it is acknowledged that the forest elephant is a unique species, the importance of their conservation rises greatly. The threat of biodiversity loss is increased even further because forest elephants have the highest within-species genetic diversity of all elephantid taxa (4). Hopefully, as the new taxonomic order enters mainstream scientific thinking as well as the imaginations of the public and of policymakers, it will facilitate the effort to study and conserve forest elephants.

Forest Elephants

Dig Deeper: Forest Elephant Evolution

A mitochondrial DNA (mtDNA) study was published in 2012, conducted by Adam Brandt, Yasuko Ishida, Nicholas Georgiadis, and Alfred Loca (5). It proved to be the final piece of the puzzle. Previous studies had shown that evolutionary trees based on nuclear DNA samples did not match trees based on mtDNA. The mtDNA from savannah elephants showed significant overlap with mtDNA of forest elephants; the same overlap was not present in the comparison of nuclear DNA samples. In the 2012 study, Brandt and his colleagues analyzed specific genetic markers taken from different elephant lineages. The results supported the hypothesis that the aforementioned incongruence between mtDNA and nuclear DNA studies was caused by a combination of the elephant social system and differences in reproductive success between males. Females of both savannah and forest elephant species remain close to their places of origin, while males disperse. Additionally, where populations of the two species mix, male savannah elephants, because of their larger size, are reproductively more successful than forest elephant males. Incongruence between mtDNA — which is passed down only through the maternal lineage — and nuclear DNA is attributed to these differences in male reproductive success.

The evolutionary tree of proboscidea, a taxonomic group that unites all elephant lineages as well as mammoth and mastodon species, can now be constructed with higher certainty than ever before. Two distinct clades seem to have formed 6 million years ago. The first clade included the hypothetical ancestor of savanna and forest elephants. The two seem to have diverged sometime during the Miocene-Pliocene transition 5 million years ago. The second clade includes Asian elephants and the now extinct woolly mammoth. It is important to note that these two diverged from their common clade later than savanna and forest elephants diverged from theirs, making them more genetically similar than to the two species of Loxodonta. Africa has been shown to be the cradle of all elephantid species (5). Asian elephants migrated from Africa to Asia and the African savannah elephant began to dominate the expanding grasslands of East Africa. Forest elephants followed an independent evolutionary path in the dense Central African forests. Important differences between the forest and savannah habitats, in particular the abundance of grasses and relatively lower abundance of fruits and trees in the savannah, resulted in very different diets for the two species. Savannah elephants now faced large, group-hunting predators (lions and hyenas) not present in the rainforest, as well as seasonal shortages of open water, which together might explain some of the differences in the social system between savannah and forest elephants.

 

References:

1. Callaway, E. 2016. Elephant history rewritten by ancient genomes. Nature News 16 September 2016.
2. Meyer, M., et al. (2017). Palaeogenomes of Eurasian straight-tusked elephants challenge the current view of elephant evolution. eLIFE 6:e25413.
3. IUCN African Elephant Specialist Group (2003). Elephant Genetics: Statement on the taxonomy of extant Loxodonta.
4. Grubb, P., et al. (2000). Living African elephants belong to two species: Loxodonta africana (Bumenbach, 1797) and Loxodonta cyclotis (Matschie, 1900). Elephant 2(4):1-4.
5. Brandt, A. L., et al. (2012). Forest elephant mitochondrial genomes reveal that elephantid evolution in Africa tracked climate transitions. Molecular Ecology 21(5):1175-1189.
6. Rohland, N. et al. (2010). Genomic DNA sequences from mastodon and wooly mammoth reveal deep speciation of forest and savannah elephants. PLOS Biology 8(12):e1000564