Anyone can tell you that zebras have distinctive black and white stripes. But in some cases, these African equines sport unusual color patterns, such as large, black splotches or golden coats with light-colored stripes. Spotted zebras are appearing as well. In 2019 in Kenya’s Masai Mara National Reserve, scientists recorded a polka-dotted foal, with white spots covering its dark-brown body.
Such aberrations—often caused by genetic mutations that alter the production of melanin, a natural pigment—are generally rare among mammals. So biologist Brenda Larison found it striking that an unusually high number—an estimated 5 percent—of plains zebra living near Uganda’s Lake Mburo were abnormally striped.
Though plains zebras are the least threatened of the three species, their numbers have dropped by 25 percent since 2002, with around 500,000 animals ranging from Ethiopia to South Africa. Habitat fragmentation caused by fences, roads, and human development have squeezed zebra populations, like the one in Lake Mburu, into small pockets of land, preventing some of the animals from migrating between herds.
Migrating infuses populations with new genes, making it key to a species’ long-term survival. A lack of gene flow can lead to inbreeding and ultimately infertility, disease, and other genetic defects.
“The observation [of the oddly patterned zebras] led me to wonder: Is part of the reason that I’m seeing so many is because this population is inbred?” says Larison, who studies the evolution of zebra stripes at the University of California, Los Angeles. (Read more about Larison’s research in her own words.)
To find out, Larison and colleagues ran genetic analyses on 140 individual plains zebras—including seven animals with unusual coat patterns—from nine locations in Africa, including Nambia’s Etosha National Park and South Africa’s Kruger National Park.
Their study, published recently in the journal Molecular Ecology, found that smaller, more isolated populations of zebras had lower genetic diversity—not a surprise. But the study also revealed these isolated groups were more likely to produce abnormally striped zebras, suggesting these genetic mutations are caused by their poor genetic diversity.
While the study only looked at seven animals with odd patterns, the results could be a visual warning about the plains zebra’s future, says Larison.
“Even though plains zebras aren’t highly threatened, these genetic issues often show up before really problematic things start happening,” she says.
It’s possible odd stripes could make the zebras more obvious to predators; for instance, most recorded instances of polka-dotted zebras are as foals, not adults. Within their family groups, however, zebras don’t much seem to mind who’s striped and who’s spotted, notes Larison, whose latest research suggests zebra stripes help the animals avoid biting flies.
The more immediate concern, she says, is the plains zebra’s genetic health. For their analysis, Larison and her colleagues used advanced genetic sequencing techniques to closely study differences between not only inbred zebras, but also the zebra populations in distinct locations. (See pictures of a white giraffe and other unusually pale animals.)
“We found that there are populations that are possibly diverging more than they would under normal circumstances, because of human population pressure,” says Larison, whose work is supported by the National Geographic Society.
In other words, zebras are becoming genetically closer within their populations, but these populations are growing more distant genetically—mirroring their physical separation. This could eventually lead to new subspecies of plains zebra.
A conservation complication
That’s worrisome, says Desire Dalton, who studies wildlife genetics at the South African National Biodiversity Institute in Pretoria, because one of zebra conservationists’ main tools is translocation—moving individual members of one population to breed in another population.
If the populations are too genetically different from each other, though, the opposite of inbreeding can occur. Outbreeding, as it’s called, causes abnormalities from genes being too dissimilar.
There’s conflicting research on which plains zebra populations might be on their way to becoming genetically distinct, or subspecies. Scientists have not yet arrived at a consensus about how to define and group these subspecies.
But she agreed with Larison’s team that defining these groups is critical for managing the species. (Read how people are helping Grevy’s zebra, a rare species, survive drought.)
“You must be really sure what populations you can mix, and what you have to keep separate,” Dalton says.
The new study is also a reminder to keep an eye on other African species that might not currently seem in dire straits, says Philip Muruthi, vice president for species conservation at the African Wildlife Foundation in Nairobi, Kenya.
For instance, Muruthi is concerned that the plains zebra could follow in the footsteps of another emblematic African species, the giraffe.
Due largely to habitat loss and poaching, giraffes have experienced a 30 percent population decline over the past 30 years; the International Union for Conservation of Nature now considers the animal vulnerable to extinction. Yet the phenomenon is still so little known it’s been dubbed a “silent extinction.”
That’s why the zebra study is crucial: By “highlighting the possibility that common species already have conservation issues,” Muruthi says, “it’s saying, Here is the issue. Don’t wait.”
nationalgeographic.com, 21 January 2021