First published January 14, 2020.
Diego is a giant tortoise from Española, one of the Galápagos Islands, who became famous in January 2020 as the "playboy" tortoise credited with fathering hundreds of offspring. He was one of only three males in a captive breeding programme that rebuilt his species from 15 survivors to roughly 2,000–2,300 animals. In June 2020, aged well over 100, he was released back on Española, the island he had been taken from more than 80 years earlier.
The story is real, but the viral version left out some important details, including the second male who did most of the work. Here is what actually happened.
Who Diego is
Diego belongs to the Española giant tortoise, Chelonoidis hoodensis (often written Chelonoidis niger hoodensis), one of the Galápagos tortoise lineages that evolved separately on different islands. Española, also called Hood Island, is a dry, low island in the south-east of the archipelago, and its tortoises have a saddle-shaped shell that lets them stretch their necks up to reach cactus pads.
He is believed to have hatched before 1920 and was taken off Española as a young adult some time between 1928 and 1933, when many tortoises were shipped abroad to zoos. He spent around 30 years at the San Diego Zoo, apparently without anyone being sure which island he came from. He weighs about 80 kg and, at full stretch, stands around 1.5 m tall.
This 100-year-old tortoise was really good at one thing... and @SaveGalapagos says it saved his species.
— QuickTake by Bloomberg (@QuickTake) January 14, 2020
After fathering 800 offspring, Diego is retiring @parquegalapagos pic.twitter.com/He2o0sTgFk
A species down to 15 animals
Galápagos tortoises were devastated in the 18th and 19th centuries. Whalers and sealers took them by the thousands as living food stores, because the animals could survive months in a ship's hold without food or water. Ship logs record more than 13,000 taken between 1831 and 1868 alone. Introduced goats then stripped islands of the vegetation tortoises relied on.
By the 1960s, only 14 adult tortoises could be found on Española: two males and 12 females. They were so scattered across the island that they were no longer meeting to breed. Between 1963 and 1974, all of them were moved to a breeding centre run by the Galápagos National Park on Santa Cruz Island. (That is the Santa Cruz in the Galápagos, Ecuador, not the island of the same name off California, a mix-up that appeared in some reports.)
Diego was identified as an Española tortoise and sent from San Diego to join them, in 1976 according to UNEP and 1977 according to other accounts. That made three breeding males and twelve females.
How the breeding programme worked
Eggs laid at the centre were incubated, and the young tortoises were raised there for several years until they were big enough to survive on their own, then released on Española. Meanwhile, goats were removed from the island in the 1970s and cactus was replanted, so the young tortoises would have something to eat.
Over roughly five decades, the programme returned around 2,000 young tortoises to Española. A 2019 census found the island could now sustain its population without further releases, and the programme was closed.
Did Diego really save his species?
He played a big part, but not alone. Genetic studies of the young tortoises found that Diego fathered roughly 40% of them. A much quieter male, known only as E5, fathered most of the remaining 60%. The third male contributed very little.
Estimates of Diego's total offspring range from about 800 to 900, and his age is usually given as "over 100" rather than a precise number. Starting from so few founders also leaves a genetic question. With just three fathers and twelve mothers behind almost the entire population, Española's tortoises carry limited genetic diversity, which can make a population more vulnerable to disease or environmental change. That is one reason scientists continue to monitor the island's tortoises long after the releases ended, and why the near-equal contributions of Diego and E5 were good news rather than a footnote.
The "legendary sex drive" headlines made for a memorable story, but the real credit belongs to a long, patient effort involving 15 founding tortoises, park rangers, scientists, and the removal of goats that let Española's vegetation recover.
Going home in 2020
On 15 June 2020, Diego and 14 other tortoises from the programme were released on Española. For Diego, it was a return after more than 80 years away.
Galápagos tortoises are among the longest-lived vertebrates. Wild animals regularly live beyond 100 years, and captive individuals are reported to have reached 170 or more. Diego could, in principle, still have decades ahead of him.
Why the Galápagos matter
The Galápagos sit in the eastern Pacific, roughly 1,000 km west of mainland Ecuador. Their isolation produced species found nowhere else, from marine iguanas to the finches and tortoises that helped shape Charles Darwin's thinking about evolution. The islands were among the first places listed as UNESCO World Heritage Sites.
Of the 14 or 15 Galápagos tortoise lineages described by scientists, a few are extinct, the most famous being the Pinta Island tortoise, whose last known member, Lonesome George, died in 2012. Diego's story is the hopeful counterpart: proof that, with time and protection, even a population of 15 can come back.
Sources: UNEP, June 2020: Diego, key to saving his species, returns home after more than 80 years; Wikipedia: Diego (tortoise); Wikipedia: Galápagos tortoise; CNN, January 2020.
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