In most African elephant populations, as few as 2 percent of the cows lack tusks. But among Addo’s 300-odd females, the rate is 90 percent to 95 percent, a trait that has evolved rapidly over the last century, in part as a result of poaching.

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ADDO, South Africa —

Through the narrow slit of the underground hide in front of the water hole, an African morning revealed itself. The sun painted the earth orange. A lion stepped out of the bush, and a small herd of perfectly camouflaged kudus, a large antelope-like animal, started and bolted away.

Soon a single bull elephant appeared where the lion had been, shaking his head as if scanning the bush. After a while, five female elephants descended the orange hillside to drink.

Even from a distance it was easy to tell they were females; in South Africa’s Addo Elephant National Park, they are almost always the ones without tusks.

In most African elephant populations, as few as 2 percent of the cows lack tusks. But among Addo’s 300-odd females, the rate is 90 percent to 95 percent, a trait that has evolved rapidly over the last century.

And at least partly as a result, Addo’s elephants have also been spared something else: poaching.

“Addo elephants might be the biggest success story anywhere,” said the park’s conservation manager, John Adendorff. “So maybe it’s not a bad thing that they don’t have tusks. Tusklessness has helped protect them.”

Addo is the most dramatic example of the increase in the numbers of African elephants without tusks but not the only one.

In Mozambique’s Gorongosa National Park, widespread poaching during that country’s civil war in the 1970s to 1990s killed off disproportionately large numbers of elephants with tusks.

The result is that in Gorongosa, 53 percent of adult females and 35 percent of newborn females have no tusks, said Joyce Poole, an elephant biologist with the research and conservation organization Elephant Voices who has studied the animals for 43 years.

“Among females then, the poachers were preferentially killing animals with tusks and leaving tuskless ones to survive, so they were breeding and producing more tuskless offspring,” Poole said.

An increase in females without tusks has also been seen in Zambia, Tanzania and Uganda in recent years.

Although scientists have not worked out the genetics, the absence of tusks appears to be a sex-linked trait and rarely occurs among males, except through injury.

This is why the unnatural selection brought about by poaching has not affected bull elephants much. Even in Addo, nearly all bulls have tusks, although they tend to be smaller than those of bulls elsewhere — another disincentive to poachers.

A 50-year-old bull can grow tusks as heavy as 108 pounds each. That’s a nearly $100,000 payday for poachers.

The Addo park on South Africa’s Eastern Cape is almost as far south as any of the world’s wild elephants go.

Although the lack of tusks on the female population has discouraged poachers, the park is taking no chances.

Its 80 rangers are armed and ready with military training and weaponry, a small air wing, and high-tech infrared and motion-detecting sensors planted throughout the park.

The rangers stake out water holes and game trails, regularly camping out overnight in the thickets.

When one of the sensors picks up something that may be human or a vehicle, the rangers’ smartphones trill with alerts.

“In order to catch a thief, you have to think like a thief,” said Michael Paxton, a ranger who is a veteran of poacher wars in South Africa’s Kruger National Park along the Mozambique border.

So far, Addo officials and scientists say, there hasn’t been a single instance of poaching of protected species in the park.

That seems odd considering that in addition to elephants, Addo also has more than 200 black rhino, the rarest rhino species, as well as Cape buffalo herds and other endangered species.

But it’s not just the security that keeps the poachers at bay, said Graham Kerley, an expert on the Addo elephants from South Africa’s Nelson Mandela University.

Another factor is the nearly impenetrable landscape, known as valley thicket.

Nearly everything in the thicket is edible to the mega-herbivores such as elephants and so nearly everything has evolved thorns and spikes in a kind of vegetarian arms race. The plants’ names reflect that: mother-in-law’s tongue, needle plant, spike thorn, pig’s ear and the elephants’ favorite snack — the speckboom or bacon bush.

“The dense bush makes it difficult and dangerous to be on foot as well as unlikely to encounter poachable animals,” Kerley said. “The prospect of getting good ivory is also low, and the risks of gunshots being detected is high.”

The Eastern Cape’s “great white hunter,” Maj. P.J. Pretorius, attested to that in his autobiography, calling Addo “a hunter’s hell.” Pretorius wiped out nearly the entire Addo elephant population in the early 1900s, killing more than 100 of them.

When he finished, there were only 11 surviving Addo elephants, and of the eight cows, four or more were tuskless.

Anna Whitehouse, an elephant expert who has studied the Addo population for many years, said the number of tuskless elephants increased steadily after the park was founded in 1931, reaching 98 percent by the early 2000s.

All of them descended from those original 11.

But Whitehouse attributed that to inbreeding among the small number of animals, a phenomenon known as “genetic drift,” rather than because of natural selection due to poaching.

Poole, the elephant biologist, acknowledged that likelihood as well.

But while Pretorius was not a poacher — local farmers hired him to kill off the elephants — the effect was the same.

“It was still guys with guns killing elephants,” she said, “and obviously they were most interested in elephants with tusks, who they would have shot first. People were into ivory then, too.”

Male elephants use tusks to fight other males for access to females, and to guard their family herds. Protecting the tuskless females seems to be the reason bull elephants often come first to the water holes in Addo.

But tusks are also tools for gathering food, digging for water and fending off predators, so cows need them as well.

Nonetheless, the absence of tusks does not seem to have hurt the Addo elephant population much. It has been doubling once every 13 years and now numbers more than 600.

“Maybe tusklessness is the future,” said Paxton, the ranger. “Our cows have gone a hundred years without tusks, and they’ve done OK.”

Many scientists say something similar happened to the Asian elephant, possibly before modern times.

Females of that species seldom have tusks, and the male tusks are much smaller than those of African elephants.

Ivory has been coveted in Asia throughout history, and the demand in China and other Far Eastern countries remains the biggest driver in elephant poaching. Recently, though, demand has dropped somewhat in China.

But there is a new problem for Asian elephants that may yet reach African ones — the desire in China for elephant leather accessories and traditional remedies made from their hides.

“Now the poachers are starting to come after elephant hides,” said Adendorff, the park’s conservation manager.

Addo’s rangers tend to get emotional about their elephants. Paxton took visitors to the final resting place of a big tuskless cow who was accidentally injured in March trying to force her 12-year-old son out of the herd, a normal social behavior.

The young bull resisted forcefully, breaking his mother’s leg so badly a bone protruded and rangers had to put her down. As soon as that happened, elephants that were scattered around the park — every one of them probably a relative of some degree — started coming to the body.

Some came from as far as 20 miles away until scores were standing nearby, their heads hanging, either quietly or making a low rumbling noise, in what some zoologists have interpreted as a display of mourning.

Paxton pointed to the elephant’s bleached skeleton, picked clean by scavengers.

“They still come to visit here,” he said.

“They’re so incredibly intelligent,” Adendorff said. “They cuddle their young and spank them when they misbehave. But I hate to say that they’re close to humans, because we’re the scourge of this planet. They’re not.”