This article was created in partnership with the National Geographic Society.

When he saw the pictures, Shane Gero froze.

A thick strand of rope, a wayward piece of quarter-inch cord from a fishing net, dangled off the tail of a young sperm whale. To the untrained eye, the line looked harmless—a lasso cinched near the base of the animal's fluke. But Gero knew the rope was a killer.

The photographs emailed from a colleague showed the heavy rope weighing down the animal's tail. That could prevent her from diving, which is how sperm whales hunt food. As she grew, the constriction would also slice through her flesh, strangling tissue like a garrote. The line might even amputate her fluke, though infection or starvation would probably do her in first.

At home in Ottawa, Gero pushed back from the computer. He called his wife and tried not to cry.

View Images Digit almost died from her ordeal. Many other marine mammals don't survive their encounters with ocean trash. Photograph by Arun Madisetti, Images Dominica

Digit the sperm whale was not quite four, but Gero had known her family for years. Each spring for a decade the Canadian behavioral ecologist had abandoned his own brood to spend months with these whales in the Caribbean Sea near the tiny West Indies island nation of Dominica. Though not yet 40, the assistant professor at Aarhus University in Denmark was rapidly becoming the world's foremost expert on baby sperm whales. Digit and her relatives were his star subjects.

Digit's very existence was significant. Thousands of sperm whales traverse the world's oceans. But 12 of the 16 whale families that returned each year to this stretch of the Caribbean were dying off. Each family could be down to a single whale in just 15 years.

Also, sperm whale families are matrilineal. Adult males eventually get cast out, and females bear the exclusive burden of rearing the young. For years, the family had produced a string of males. Three of them—Thumb, Tweak, and Enigma—had died already. Scar would soon disappear.

The family needed a female calf.

So Digit's arrival in 2011 left Gero's research team ecstatic. The crew watched Digit wean herself from her mother, Fingers. They cheered when she flipped her fluke up for her first deep dive. With Digit's arrival, the most-studied sperm whale family in the world seemed poised to carry on.

Then, in 2015, Gero received the images.

Sociable leviathans

In literature, sperm whales are ship-splintering beasts—monsters of "inscrutable malice," as Ahab seethed in Herman Melville's Moby Dick. In reality, that is far from true.

The world's largest toothed whales have the animal kingdom's biggest brains. The deep-diving nomads share membership in clans that can number in the thousands. Each clan chatters in its own dialect using a unique set of click patterns. These whales are social and playful. They roll and rub against each other near the surface. Some engage in hide-and-seek games, swimming circles around scientists' research boats and rolling sideways to eye the inhabitants. Sperm whales also are quite curious, especially when spying unfamiliar debris.

View Images Digit (left) swims with the calf Corkscrew and the mother Canopener. The adult whale often seems to toy with researchers, swimming next to their boat and rolling one eye up to glimpse those inside. Canopener even pretends to dive, waiting just below the surface until scientists move their boat to where she was last seen. Then she'll pop from the water again.

Photograph by Brian Skerry

Gero, a National Geographic Explorer, could guess what had happened to Digit. Caribbean fishermen anchor nets to the sea floor to lure marlin, tuna, and mahi mahi. Whales rarely disturb that fixed gear, but container and cruise ships often accidentally shred it. Flapping ghost nets draw inquisitive creatures, and those loose lines are to whales what spider webs are to flies. While there are no reliable global statistics, at least 76 large whales, including humpbacks, blues, and minkes, got trapped in nets, lines, or debris in 2017—just in United States waters. And the vast majority of entanglements go unseen.

Gero suspects Digit simply snagged a loop of loose line. Three other whales in the region had recently been snared by fishing gear. One, a mother with a broken jaw, was forced to drag her dead calf for days after both got trapped in the same nest of lines. (The mother's injured mouth suggested she'd tried gnawing the calf free.)

Gero and colleagues reached out to Michael Moore, senior scientist at Woods Hole Oceanographic Institution in Massachusetts. He had helped untangle endangered North Atlantic right whales.

Moore's assessment was bleak. The pictures showed Digit's noose was tight. Only a dozen feet of line trailed her—too little to attach buoys to keep a deep-diving sperm whale near the surface. That was essential for rescuers to work. Without more line, a team would struggle to get close.

"There was so little gear on her," Moore says. "It was not going to be a slam dunk."

There were other complications. Dominica isn't New England. There wasn't a trained disentanglement crew for hundreds of miles. Getting one would take money and time. No one knew how much time they had.

"We were confronted with the long-term, chronic, slow death of an animal we see every day—one we thought we'd know forever," Gero says.

It felt personal.

Getting to know them

Gero had studied under sperm whale guru Hal Whitehead, at Halifax's Dalhousie University. Whitehead believed these sophisticated leviathans deserved the same respect as primates. Whitehead mostly studied adults. As a student, Gero wanted to learn about the young: Which family members raised them? When did they first dive deep? How did they learn their dialect—and from whom?

So in 2005, the young scientist arrived in Dominica aboard Whitehead's 40-foot research sailboat, Balaena. There he found a gathering of whales he would dub the Group of Seven, named for a collective of famed Canadian painters.

The Group of Seven tended to spend weeks near this coast. They were spotted more often than other whales. That first year, Gero's team spent an astonishing 40 days cataloging this one family's behavior.

View Images Caribbean sperm whales face many threats—pollution, climate change, entanglement in fishing gear, ship strikes. So many members of Digit's family have died in recent years that her family eventually merged with another that included the highly social Canopener, seen here. Photograph by Brian Skerry

"We'd go into shore and get groceries and come back and we'd still see the same animals just offshore," Gero says. "That's unheard of."

It's why their names seem flip—Gero needed to tell them apart, but had never expected he'd see them again.

As with Jane Goodall's chimpanzees and Dian Fossey's mountain gorillas, intimate access revealed each animal's distinct habits and personality. Over time, Gero began to see these whales as individuals.

Fingers appeared to be in charge. She usually broadcast the "coda," four clicks that identified the family to other whales, like a surname. When her offspring, Thumb, died, Fingers helped watch over others' young. She steered clear of people and was known for spectacular dives, muscling her fluke high before plunging straight down.

"It's hard to describe how beautifully she flukes to someone who doesn’t watch hundreds of whales do it," says Gero, whose research is conducted through Aarhus University's Marine Bioacoustics Laboratory. "It feels like she is demonstrating to others the way."

Fingers' niece, Pinchy, was mother to Scar, who was so comfortable with humans he'd become a star in Dominica's swim-with-whales tourism industry. There was sickly Quasimodo, and Mysterio, so named because she appeared rarely.

Gero felt a growing kinship with the cetaceans. "The whales were becoming a part of my life," he says. "My kids knew these animals by name even though they'd never met them."

Related: Dead whale Found with 13 pounds of plastic in stomach

The whales prompted him to rethink his views on conservation. In many sperm whale families, calves get milk from other calves' mothers. The Group of Seven's young only got milk from their actual mothers. If behaviors and communication were unique to clans or families, didn't that suggest conservation should be about more than total population numbers? Wasn't each clan special in its own right?

In 2011 a documentary crew arrived, led by a filmmaker who'd co-produced the Fossey biopic, Gorillas in the Mist. When Fingers gave birth to a new calf that week, Gero knew what to call her.

He named the new calf Digit, after Fossey's favorite silverback. Only later would he recall what humans did to Fossey's gorilla.

Wasting away

Before arriving in Dominica for the 2015 research season, Gero had only seen Digit's injuries in emailed pictures. In person things looked even worse. Before she got tangled, the young whale had just started swimming and diving alone. Now she only appeared with adults. She was reserved instead of curious. She kept her distance from boats and people.

"It was like she was trying to say, 'This is all your fault, you humans,'" says Pernell Francis, who has worked with Gero.

Gero could see the rope gouging her flesh. More troubling still: Digit couldn't raise her fluke. The rope was creating too much drag. As he'd feared, she could not dive deep, which was hampering her hunt for squid.

Word of Digit's condition spread. Ted Cheeseman, who took clients swimming with whales, raised money to hire a professional disentanglement team. Whale advocates whispered about cutting the rope themselves. Gero knew that was too dangerous.

"There are Internet videos where people have done it, but they're bloody lucky they didn't kill themselves," Moore says. A trained rescuer would die in 2017, after being struck by a whale he'd just freed.

Eventually someone took the plunge anyway. The diver shortened Digit's rope, but could not cut the noose. The shorter line reduced the drag on Digit's tail, but now even less cord remained for pros to work with.

Ultimately, no rescue crew would be coming. Cheeseman ended up using the money he'd raised to buy and stash equipment for future rescues. He paid to assemble and train a future Dominica disentanglement crew.

Digit, meanwhile, grew ever thinner. No longer able to catch her own food, she returned to being nursed by Fingers.

"It was like watching your child go back to crawling," Gero says.

People care

One afternoon in Dominica a boat zipped by and a woman shouted: "Hey Shane, how can I help Digit?" Gero was taken aback. Even strangers were worried.

That night Gero ate on the deck of his research boat, beneath a dangling headlamp. The Group of Seven was in trouble. That family was now on the brink, down to just three whales: Fingers, Pinchy, and Digit. But the stranger's query was a reminder that Digit's story held real power.

While humans are attached to dolphins and orcas, many can't even identify a sperm whale. Fewer still understand the gantlet of threats these nomads face: pollution, climate change, ship strikes, fishing gear.

"But people can understand a mom caring for a kid who is suddenly facing a chronic injury," Gero says.

Gero vowed something useful would come of Digit's wounds.

Over the next several years, Gero expanded his research's focus on conservation. He wrote and lectured more. He spoke at museums and even mentioned Digit's quandary during a TEDx Talk. He and a team mapped whale and vessel movements and urged the government to restrict ship traffic to areas that whales avoided. Gero hoped that might help fishing-boat operators find ship-free places to set nets.

"Digit changed the whole perspective of our project," Gero says. No longer was whale behavior his sole interest. Now he asked: "What can we do to ensure we all co-exist?"

Still he couldn't help Digit. She had not resumed fluking. Her flesh began to grow around the rope, closing over it. Gero suspected he was watching Digit die.

Miracle

Then last spring he saw her again from the bow of Balaena. Days into the 2018 field season, Digit popped to the surface. Gero knew immediately that everything had changed.

The outline of Digit's spine was no longer so visible. She had gotten plump. Looking closely, Gero could see abrasions and marks where the rope had rubbed. The line itself was simply gone.

A few months earlier, a colleague in Dominica had emailed to say he'd heard Digit had lost her rope. Gero had been hopeful, but skeptical. Now as Digit moved to slip below the surface, Gero's entire team fell silent. Digit flipped her fluke and dove. A cheer erupted from the boat. After three years, Digit was free.

View Images For years, the heavy rope weighed on Digit's tail, preventing her from diving deep for squid. The noose scarred her flesh and she lost so much weight that her mother, Fingers, started nursing her again. Now that the rope is off, Digit (seen here at bottom) is socializing again with Canopener (top) and Canopener's offspring, Corkscrew (with fluke high in the water). Photograph by Brian Skerry

Later, the scientist and his team would attach a tracking device to Digit's back. When they eventually reviewed the tag's data, Gero was overwhelmed. Digit was diving more than 3,000 feet. She was slurping up squid. Digit was behaving like a healthy seven-year-old whale.

No one knows how she got free. Cheeseman suspects that sunlight, time, and pressure weakened Digit's line until it finally broke. Moore says if Digit swam near a sharp rock or crevice, she might have scraped the deteriorating rope off. Other whales might even have helped.

"If someone told me two sperm whales had a tug of war over the line and it broke, I'd believe it," Moore says.

Gero has another idea. He saw fresh scars on Digit's fluke. He suspects predators may have attacked her and unwittingly ripped off the gear.

But Gero knows he'll never be certain. He almost prefers it that way.

"It's easy to forget that that there are thousands of species right next to us with rich and complicated worlds, living their lives in parallel with our own," Gero says.

A new generation of Eastern Caribbean sperm whale was swimming free. Knowing that would be enough.