They mooch, bask and munch in their sunshine-soaked, island idyll off the east coast of Africa. But while the famously long-lived Aldabra giant tortoise seems to be doing well, an invisible crisis is looming.
A new study in Ecology and Evolution reveals that hatching rates have plummeted, and that huge numbers of embryos are dying in their shells. Now researchers are calling for more monitoring and research to help save the iconic species.
“We’re not saying the species is doomed,” says Alessia Lavigne from the University of Sheffield. “Instead, our study raises important questions and highlights the need to act before it’s too late.”
Aldabra giant tortoises are often heralded as a conservation success story. Rightly so. On the brink of extinction in the mid-19th century, the species was saved through a range of measures, including protecting Aldabra Atoll – part of the Outer Islands of the Seychelles in the Indian Ocean – and translocating individuals to other islands for safekeeping. Now, an estimated 100,000 of the gentle giants dwell on the atoll.
Individuals can live for a century or more, but this longevity masks a problem. While adult populations may appear stable, the number of young tortoises surviving to replace them has declined dramatically.
Lavigne and colleagues studied data on 317 eggs from 24 clutches laid over two breeding seasons. Fifty years ago, the hatching rate was 60–80%. Today, the figure is 46% for eggs laid on Aldabra, and 0–26% for the eggs of translocated populations on other islands. The tortoises, it seems, are not doing well, and do less well when they have been moved.
“We don’t yet know exactly why,” says Lavigne, “but it’s likely a combination of environmental conditions and habitat-related factors.” Differences in habitat quality, food and water availability and other factors could be influencing reproductive success, with the added problem of climate change.
Using recently developed microscopic techniques, the team found that low hatching success is mainly being driven by embryos dying very early in development, rather than by eggs failing to be fertilised. This matters because researchers can now investigate the causes of embryo failure and find ways to help the embryos survive.
“I’m both concerned and hopeful,” says Lavigne. While it’s worrying that these reproductive problems have been overlooked for so long, there is hope that things will improve. With the right monitoring and research, including new high-tech methods for studying development, Lavigne believes there’s every chance that Aldabra giant tortoises can remain one of the world’s greatest conservation stories.
Top image credit: Alessia Lavigne
More wildlife stories from around the world
- A carnivore has moved into Florida's Everglades – and it's threatening native crocodiles and alligators
- "I get goosebumps thinking about it.” Giant tortoises return to remote Galápagos island after more than 150-year absence
- Incredibly rare giant turtle with a face like a frog found in India's tropics
- There is only one place on Earth where crocodiles and alligators live together in the wild – and it's here
