"He might actually amp it up," said James Gibbs, a professor of environmental and forest biology at the State University of New York in Syracuse who has assisted in the Galapagos programme. "I don't know - we shall see."
Little is known about Diego's early life. The long necked, yellow-faced tortoise is thought to have been picked up in Española during a scientific expedition sometime between 1900 and 1959. At some point, he landed at the zoo. When scientists discovered that Chelonoidis hoodensis was its own species, and that Diego was one of the few still in existence, he was brought back to the Galapagos for the breeding effort.
The tortoises had nearly died out, Gibbs said, after serving as food for whalers, colonists and others who passed through the islands. It did not help that settlers introduced goats, which depleted the cacti the massive turtles relied upon for food, water and shade.
Through captive breeding, which the Galapagos began in 1965, scientists have worked to restore the population of threatened tortoise species. The Giant Tortoise Restoration Initiative has an ambitious goal: to bring tortoise populations back to their historical numbers.
Despite the programme's successes, there was a setback in the case of Lonesome George, who died in 2012 as the last known member of the Chelonoidis abingdoni species. Over decades of mating attempts, he failed to produce any descendants.
"Only when George had died did an autopsy reveal it wasn't lack of potency that impeded his reproduction," the New York Times reported in 2017, "but a more anatomical ailment affecting his reproductive organ."
Diego had no such difficulty. At the breeding facility, he lived in a corral with his mates, a kind of reptilian Hugh Hefner. He won over the female tortoises who, Gibbs noted, choose whether or not to mate.
His popularity was not limited to his own species: Galapagos tourists became fond of him, too. Unlike most tortoises, he isn't reclusive, Gibbs said, and was often seen exploring his home or mating. He has been known to stare out at each of his visitors. Sometimes, he even hisses at them.
"It's not just that he has a name and a story," Gibbs said. "He is a very distinctive tortoise. He has this very bold personality, kind of broken scutes, a big yellow neck. He's just an out and about tortoise."
With Diego responsible for about 40 per cent of the tortoises produced in the breeding programme, there is debate among scientists over the consequences of having such a small gene pool. Gibbs said that this "bottleneck effect" is a concern, though the true impact won't be known until it is tested over time.
The Galapagos National Park said in its statement that researchers had decided to end the breeding programme and release the tortoises after using mathematical models to predict population changes over the next hundred years.
"The conclusion was that the island has sufficient conditions to maintain the tortoise population, which will continue to grow normally - even without any new repatriation of juveniles," said Washington Tapia, director of the Giant Tortoise Restoration Initiative.
That result, Gibbs said, offers a hopeful message for conservation efforts across the world.
"I don't know what's been the most successful species restoration campaign; there are many," he said. "But this one is very, very inspirational, that we can truly take something written off as extinct to fully recovered, and let them carry on."
Diego is set to be released within the next few months, after being quarantined to avoid carrying seeds from plants that are not native to Española. Then, Gibbs said, he'll likely be released along with the other captive tortoises to a cactus-rich area in the middle of the island.
Given his "spryness," he could live another 20 years.
