High early embryo mortality and low hatching success observed in Aldabra giant tortoise populations

Turtles and tortoises are highly threatened yet understudied, particularly regarding reproductive traits and early life stages. In long-lived species, reliance on adult census data can mask reproductive failure for decades before population declines become apparent. This knowledge gap is evident in the Aldabra giant tortoises (Aldabrachelys gigantea), despite its widespread use in conservation translocations and ecosystem restoration. We present preliminary data on fertilisation success, early embryo survival and hatching success, across one natural and five translocated populations in the Seychelles. Of 317 eggs from 24 clutches, only 16% successfully hatched. Most failed eggs (97%) were undeveloped. Using recently developed microscopic methods to assess egg fertility, we provide the first population-level comparisons of fertilisation and hatching outcomes for this species. Although sample sizes are limited, our results indicate low and variable hatching success, driven primarily by embryo mortality. Complete clutch failure was common (67%), and embryo mortality was prevalent even among clutches that had some degree of hatching success. Hatching rates were markedly reduced in translocated populations (0%–26%) compared to a natural Aldabra Atoll population (46%) and all fell below historical estimates from ~50 years ago (60%–80%). We also found a negative association between hatching success and a Human Modification Index, although interpretation of this result should be cautious considering the small sample size. These findings provide the first reproductive success data for translocated Aldabra giant tortoise populations and the first for the species in the last five decades. We argue that incorporating productivity metrics into monitoring frameworks is essential to improve vulnerability predictions and inform effective conservation management.