Changes in breeding periodicity of a long-lived vertebrate over 30 years

Abstract Changes in breeding periodicity in vertebrates can highlight important trends in population demographics that have conservation implications. Following widespread conservation measures, some previously endangered species have shown population recoveries across their range, with green turtles (Chelonia mydas) being an exemplar of global species recovery. At sites in the Indo-Pacific, green turtle nesting numbers have increased fourfold in 30 years. As sea turtles are generally non-annual breeders, we used a total of 1399 measurements of the breeding interval for marked individuals to reveal that the relative proportion of individuals breeding with a 4-year versus a 3-year interval has increased over time: 28% in 1997–2012 versus 37% in 2013–2024. Furthermore, this percentage of 4- versus 3-year nesting intervals was linked to annual nest numbers (R2 = 0.23, F1,23 = 6.92, p = 0.01). This lengthening of the breeding (remigration) interval may reflect density dependence, restricting foraging success so that individuals need longer to build body condition for migration and breeding. These results provide the first evidence for long-term lengthening of breeding intervals in this taxon and might have important global conservation implications in terms of reduced per capita reproductive output, a finding that echoes recent observations with cetaceans.