The collaborative project led by University of Sheffield postgraduate researcher Alessia Lavigne in collaboration with 8 international partner organisations (Indian Ocean Tortoise Alliance, Nature Seychelles, Cousine Island Company Limited, School of Life Sciences University of KwaZulu-Natal, Seychelles Islands Foundation, Save Our Seas Foundation D'Arros Research Centre (SOSF-DRC), North Island Company Limited, and Island Conservation Society), published in Ecology and Evolution, warns that a traditional reliance on adult census data can mask reproductive failure for decades before a population decline becomes visible. By tracking 317 eggs across 24 clutches over two recent breeding seasons, the researchers discovered that on average only 16% of eggs successfully hatched and 67% of all clutches experienced total failure.
For conservationists, the challenge of establishing thriving relocated populations for ecosystem restoration has long been hindered due to insufficient nest monitoring and a lack of clarity surrounding why tortoise nests fail. While low hatching success is widely recognised as a threat, standard monitoring methods have been unable to separate fertilisation failure from the death of an embryo at a microscopic level in undeveloped eggs. This lack of data has left researchers unable to identify the distinct biological causes behind nest failure across different island environments.
The research team, based in Dr Nicola Hemmings’ lab in Sheffield, used advanced microscopic techniques to examine failed, undeveloped eggs (97% of all failed eggs were undeveloped), providing the first clear window into the fertilisation and hatching outcomes of the species. The microscopic analysis suggests that the primary driver of hatching failure in the studied populations was early embryo death rather than fertilisation failure.
The team found that this embryo mortality was particularly severe on islands altered by human activity, where three relocated populations suffered lower hatching success rates between 0%-26% . In contrast, a wild sub-population living in the natural habitat of the Aldabra Atoll had a 46% success rate with all hatching rates falling drastically below historical estimates of 60% to 80% recorded five decades ago.
“This study should be seen as a warning” says Alessia “Our results may only be a small snapshot of a potential invisible problem in these populations, however, it is the best and only snapshot of its kind, and the picture we see has some major red flags”
“The differences between islands are interesting and potentially important – they might tell us something about the best environmental conditions for tortoise nests” said Hemmings. “We must be cautious with our interpretations though, as we only have limited data. It really highlights the need for more intensive long-term monitoring.”
To protect the species over the long term, the team argues that conservation groups need to look past adult counts, allocate funding towards identifying and monitoring active nests and their outcomes, and actively expand research into the potential factors affecting the quality of tortoise nests.