After performing the most detailed genomic sequence analysis to date of the world’s tallest land animal, researchers argue for the existence of four distinct giraffe species. But their report, published yesterday (May 5) in Current Biology, appears not to have settled the long-standing debate among giraffe experts on precise species numbers, with some still arguing there are likely more species and others fewer.
“This is really state-of-the-art genetic data [and] a tremendous contribution to science,” says evolutionary geneticist Rasmus Heller of the University of Copenhagen who was not involved in the research. “It’s really nice that we finally have whole genome data on this scale for giraffes,” he adds, noting that having numerous genomes representing so many giraffe populations is “not easy to get.” As to whether he thinks the data confirm the existence of four and only four species, he says, “it’s been kind of a contentious issue for a number of years and . . . I prefer to stay a little bit agnostic. . . . I really don’t know, to be honest.”
Since humans started classifying species, the iconic giraffe, which roams the savannahs of Africa munching on trees and towering above all other animals, had been considered a single species. With the advent of genetic sequencing, suggestions of six, eight, four and three species of giraffe, with varying numbers of subspecies, have been proposed.
And the debate has been “surprisingly heated,” says Heller. “It’s a problem of life being messy and difficult to pigeonhole, and humans having brains that compulsively put things into pigeonholes,” adds wildlife biologist Derek Lee of Penn State University who did not participate in the study.
“[We wanted] to tackle this problem once and for all,” says evolutionary geneticist Axel Janke of the Senckenberg Biodiversity and Climate Research Centre in Germany.
Janke’s team created a reference genome by de novo sequencing a newly acquired giraffe DNA sample, and used this to align 50 more whole genomes obtained from high-quality re-sequencing of existing giraffe samples and two publically available giraffe sequences. Forty-three of the samples came from wild populations in 17 locations across the African continent, collected by members of the Giraffe Conservation Foundation (GCF). The remaining eight individuals came from three European zoos. The samples represent members of all suspected species or subspecies of the mammal.
Comparative sequence analyses, examining nearly 200,000 single nucleotide polymorphisms in the animals’ genomes, confirmed Janke’s previous finding that the sequences cluster into four distinct groups, or species. The previous study had been based on only seven genomic loci together with mitochondrial sequences. The team’s whole-genome analysis further suggested that the four lineages had been evolving separately with no significant evidence of hybridization. According to the team, the species are: Giraffa camelopardalis (including the subspecies G. c. antiquorum, G. c. camelopardalis, and G. c. peralta); G. tippelskirchi (including the subspecies G. t. tippelskirchi and G. t. thornicrofti); G. giraffa (including the subspecies G. g. angolensis and G. g. giraffa); and G. reticulata.
While hybridization between these species can occur in captivity, hence the previous argument for a single species of giraffe, “we don’t see signs of hybridization in the genomes, and by inference we say it does not occur in the wild,” says Janke, and that supports the biological definition of separate species, he says.
Evolutionary biologist Alexandre Hassanin of Sorbonne University, whose previous analysis had argued for the existence of only three species, disagrees. He writes in an email to The Scientist that hybridization actually does occur between G. reticulata and G. camelopardalis, making them members of the same species. The current paper, he says, “is limited in scope because the authors choose to include only a single wild population of the subspecies reticulata in their genomic analyses.” If they had included “populations of reticulata found in the western and northern parts of its distribution, I am pretty sure that the conclusions would be different,” he adds.
In contrast, molecular biologist Douglas Cavener of Penn State University argues there may be more than just four species. He says that because the team does not disclose the precise locations of the animals sampled, it is possible that some may be members of the same family and therefore genetically very similar to each other. If that is the case, he says, the natural populations may actually be more diverse, with more species and subspecies than this study indicates.
Janke writes in a follow-up email to The Scientist, “The samples are from dart biopsies taken by GCF over the course of a few days, I think. Of course, one can never exclude involving related individuals, but I am very confident by knowing how professionally the GCF team works firsthand that they did their best to avoid sampling from related individuals.”
So, why does knowing the precise number of giraffe species even matter?
“Whether we like it or not, the species is still the basic currency . . . to measure biodiversity,” says Heller, and “the way in which conservation attention and resources are allocated is based on species delimitation. . . . It is the species unit that we care about protecting.”
If there are four species of giraffe instead of one, Heller explains, then “they will each have their own status,” which may strengthen conservation efforts.
Ultimately, says Cavener, it’s a pity there has been so much controversy over this question of species because, when it comes down to it, “everybody, I think, has the same interest in mind, and that is giraffe conservation.”
R.T.F. Coimbra et al., “Whole-genome analysis of giraffe supports four distinct species,” Curr Biol, doi:10.1016/j.cub.2021.04.033, 2021.