- The loss of more than 60 percent of the world’s forest elephants to poaching has led to calls for its official recognition as a separate species worthy of international conservation support.
- Scientists examined the nuclear DNA of forest elephants across their range to assess the species’ genetic diversity. They found that the elephants’ nuclear DNA, as opposed to mitochondrial DNA (mtDNA), is diverse but consistent among populations across Central Africa.
- Adult male elephants that wander great distances in search of females promote gene flow among populations and maintain the species’ genetic diversity. The authors suggest conservation measures that retain three major forest elephant populations representing existing genetic variation.
- The importance of forest elephants for dispersing the seeds of most large trees in the Congo Basin makes their conservation critical to maintaining the health of Central African rainforests.
Male elephants wander. As they mature and prepare to breed, they roam great distances in search of estrous females.
These movements are key to elephant population dynamics: by breeding with females far from their family, males promote gene flow and ensure a genetically healthy population. In Africa, males of both forest and savanna elephants travel and compete for access to females to the extent of hybridizing, but what does this mean for elephant conservation?
Over the last decade, research has shown that the DNA of forest elephants (Loxodonta cyclotis) differs sufficiently from that of savanna elephants (L. africana) to be considered a separate species. A recent study examined the within-species genetic structure of the forest elephant across its range to better understand how genetically distinctive regional populations of the species are and how that might affect its conservation.
The species debate
African forest elephants have roamed the forests of West Africa and Central Africa for millions of years. They stand up to a meter (3 feet) shorter than savanna elephants and weigh about 2,700 kilograms (6,000 pounds), roughly half that of savanna elephants, which inhabit southern and East Africa. Forest elephants also have more rounded ears and longer, straighter tusks.
More importantly for conservation, female forest elephants first give birth as late as 23 years, with longer birth intervals than their savanna cousins. Their slow birth rate means forest elephant populations take a long time to recover from poaching.
Forest elephants differ on the inside as well. Genetic research indicates that the forest elephant is a species distinct from the savanna elephant, having separated some 2–7 million years ago. In fact, analyses have found that African forest and savanna elephants differ from each other as much as modern Asian elephants do from ancient mammoths. Nevertheless, the International Union for the Conservation of Nature (IUCN) does not yet recognize the distinction.
Scientists see the distinction as important to bring species-specific conservation recognition and action to bear for African forest elephants. Industrial-scale poaching for ivory has caused forest elephant populations to decline much faster than those of savanna elephant populations. Lumping the two groups together underestimates the vulnerability of the forest elephant.
“More than two-thirds of the remaining forest elephants in Africa have been killed over the last 15 years or so,” said study co-author Alfred Roca, professor of animal sciences at the University of Illinois’s Carl R. Woese Institute for Genomic Biology and College of Agricultural, Consumer and Environmental Sciences (ACES). “Some conservation agencies don’t recognize African forest elephants as a distinct species, and these animals’ conservation needs have been neglected.”
Males go, females stay
The researchers analyzed nuclear DNA of 94 forest elephants from six locations in West and Central Africa to better understand the potential genetic diversity within this species. They also examined the DNA of 15 savanna elephants, from 15 different locations, to help them understand patterns associated with hybridization between forest and savanna elephants.
The scientists used genetic markers called microsatellites to measure genetic variation in specific segments of nuclear DNA. They examined the markers to determine how genetically distinct the elephants from various sample locations were from each other. They also compared the geographic patterns in nuclear DNA from the microsatellite markers to patterns previously reported for forest elephant mitochondrial DNA (mtDNA) across the same locations.
“In general, forest elephants have greater nuclear DNA diversity [than savanna elephants],” Roca said in an email to Mongabay-Wildtech. The team’s analysis of nuclear DNA revealed that the relatively high diversity of forest elephant nuclear DNA was consistent across populations throughout Central Africa, suggesting a single species separate from the savanna elephant.
A previous study of the mtDNA across the range of forest elephants had detected five genetic groupings, each with a different geographically restricted distribution. MtDNA is identical among maternal relatives because it passes only from mothers to their offspring with few mutations. Moreover, female elephants generally remain with their mother and other close relatives even after they mature, so their mtDNA does not disperse geographically.
In contrast, male elephants disperse from their natal herds when they reach maturity and wander in search of females. Female elephants come into estrous for only a few days every few years, so roving males use their keen hearing and smell to find them. Because both males and females contribute to DNA from each cell’s nucleus, males that breed with females far from their kin promote gene flow.
“Since the [forest elephant] population was historically contiguous, it is better to think of males as dispersing and mating with nearby herds,” Roca said. “When this happens generation after generation, gene flow occurs across the range of the elephants.”
The study found that patterns of nuclear DNA across Central African forests differed slightly from east to west, though genetic profiles of animals from eastern and western Central Africa overlapped, suggesting continuous gene flow, at least until recently.
The contrasting dispersal behaviors of male and female elephants creates discord between the spatial distribution patterns of mtDNA and nuclear DNA, even among populations of a single species. In fact, the phylogenetic patterns, which reflect the evolutionary history and relationships among groups of animals, of nuclear DNA in forest elephants differ from the patterns of their mtDNA, a situation found in other species in which only males disperse.
“Forest elephant’s seemingly discordant DNA can be easily explained by their behavior,” said lead author Yasuko Ishida, a research scientist in ACES. “Their mitochondrial DNA is a relic preserved by their matriarchal society.”
In their paper, the researchers suggest that past shifts in climate and habitat have likely influenced the current genetic spatial patterns of forest elephants. They write that forest elephant populations likely retreated into discrete glacial refugia, zones where forest remained during the last Ice Age, and expanded outward across their current range as more habitat became available.
“Male-mediated gene flow erases the nuclear genetic signatures of past climate and habitat changes,” the researchers write, “but these continue to persist as patterns in mtDNA because females do not disperse.”
“It looks [from the data] like ancient forest refugia affected the elephants,” Roca said, “and the female-transmitted mitochondrial DNA still reflects this pattern.”
The researchers state that the diverse yet consistent genetic makeup of forest elephants at the various sample sites demonstrates a single distribution of forest elephants across Central Africa until the mid- to late 1900s. However, they write, “Forest elephant numbers declined by ca. 62% between 2002 and 2011, to <10% of their estimated historical population size, mainly due to illegal poaching for their tusks. There is thus a strong need to examine fine-scale population substructure within forest elephants using nuclear markers, for proper conservation management of the species.”
Where species meet
Mitochondrial DNA from African elephants has provided evidence of interbreeding between forest and savanna elephants around 500,000 years ago, the main reason IUCN and others maintain the two groups as a single species. Research has documented hybrids between forest and savanna elephants, but only within relatively narrow transition zones between forest and savanna habitats.
“The evidence suggests that any hybrids between the two [species] are less successful reproductively,” Roca said, “and this has kept the two species apart genetically for many hundreds of thousands of years.”
The authors suggest that their findings–that forest elephants are a distinct species with some geographic differences below the species level–suggest conservation measures that retain the major populations representing this genetic variation. These could include managing West African, western Congolian, and eastern Congolian forest elephant populations separately, to promote habitat conservation within these three units needed for gene flow to continue within populations or to allow recolonizing of areas where elephants have been extirpated.
Affecting the forest elephant population will alter the composition and diversity of the region’s forest, say scientists. “Because they are very large animals, they can eat fruits and disperse seeds too big for other animals to digest. And because they are highly mobile, they help disperse these seeds far and wide through their dung,” said John Poulsen, assistant professor of tropical ecology at Duke University, in a statement.
“Forest elephants are the heart of these ecosystems,” Roca said in a statement, “without them, the system falls apart, and many other species are jeopardized.”
The scientists write that targeting conservation effort specifically to forest elephants is particularly urgent in the face of strong poaching pressure that has decimated forest elephant populations. The intensity of human activity across West Africa has fragmented elephants’ forest habitats; fragmentation reduces genetic connectivity, which can lead to loss of genetic variation, inbreeding, or genetic drift.
“Human activity is too recent to have greatly affected genetic patterns,” Roca said, “although eventually it will.”
Citations
Ishida, Y., Gugala, N. A., Georgiadis, N. J., & Roca, A. L. (2018). Evolutionary and demographic processes shaping geographic patterns of genetic diversity in a keystone species, the African forest elephant (Loxodonta cyclotis). Ecology and evolution, 8(10), 4919-4931.
Ishida, Y., Oleksyk, T. K., Georgiadis, N. J., David, V. A., Zhao, K., Stephens, R. M., … & Roca, A. L. (2011). Reconciling apparent conflicts between mitochondrial and nuclear phylogenies in African elephants. PloS one, 6(6), e20642.
Banner image of forest elephants at the Dzanga bai in Central African Republic by Sue Palminteri/Mongabay.
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