Changes in plasma oestradiol, testosterone an progesterone concentrations during an annual reproductive cycle in wild Aldabra giant tortoises (Aldabrachelys gigantea)

Abstract

E human colonisation of the Indian Ocean Islands led to the extinction of one of two lineages of endemic giant tortoises, Cylindraspsis, and the almost complete elimination of the other lineage, Aldabrachelys, by the end of the 19th century due to excessive harvesting and translocation (Gerlach et al., 2013; Hansen et al., 2010). Today, only one species, the Aldabra giant tortoise (Aldabrachelys gigantea), survives. In the 1970s, conservation efforts including population monitoring and research programs were initiated, resulting in a significant increase of the single remaining population on the Aldabra Atoll and introductions of populations outside the historic range (Bourn et al., 1999). Recent introductions of captive-bred A. gigantea to new island habitats, such as Ile aux Aigrettes and Round Island in Mauritius, have not only served the conservation of the species, but also the restoration of ecosystem function, especially in regard to plant seed dispersal (Falcón & Hansen, 2018; Falcon et al., 2018; Hansen et al., 2010). However, natural density-related population regulatory mechanisms, late onset of reproductive age, and the extremely restricted range render A. gigantea population dynamics vulnerable to external impacts, be they human or natural in origin (Bourn, 1977; Gerlach et al., 2013; Haverkamp et al., 2017). The species is currently listed as Vulnerable on the IUCN Red List of Threatened Species (IUCN, 2018), but an overall population decline of 40-65 % over the next 100 years is projected due to severe negative impacts of sea-level rise, which will render the species Endangered by IUCN criteria (Gerlach et al., 2013). Captive propagation is an important tool for inand ex-situ species conservation. However, breeding success outside the Aldabra tortoises’ native island habitats has been very limited. Of the 159 zoological institutions registered as holding Aldabra tortoises, only one institution (Tulsa Zoo, USA) has been producing and changes in plasma oestradiol, testosterone and progesterone concentrations during an annual reproductive cycle in wild Aldabra giant tortoises (Aldabrachelys gigantea)