Journal Article

Persistence of distinctive morphotypes in the native range of the CITES‐listed Aldabra giant tortoise

Hansen, Dennis M., Richards, Heather, Pistorius, Pierre, Crommenacker, Janske, Turnbull, Lindsay A., Accouche, Wilna, Doak, Naomi, Baxter, Rich, Bunbury, Nancy, Fleischer‐Dogley, Frauke, Ozgul, Arpat, ChongSeng, Lindsay, Currie, Jock C., Brandis, Rainer

2015

Overview

Understanding the extent of morphological variation in the wild population of Aldabra giant tortoises is important for conservation, as morphological variation in captive populations has been interpreted as evidence for lingering genes from extinct tortoise lineages. If true, this could impact reintroduction programmes in the region. The population of giant tortoises on Aldabra Atoll is subdivided and distributed around several islands. Although pronounced morphological variation was recorded in the late 1960s, it was thought to be a temporary phenomenon. Early researchers also raised concerns over the future of the population, which was perceived to have exceeded its carrying capacity. We analyzed monthly monitoring data from 12 transects spanning a recent 15‐year period (1998–2012) during which animals from four subpopulations were counted, measured, and sexed. In addition, we analyzed survival data from individuals first tagged during the early 1970s. The population is stable with no sign of significant decline. Subpopulations differ in density, but these differences are mostly due to differences in the prevailing vegetation type. However, subpopulations differ greatly in both the size of animals and the degree of sexual dimorphism. Comparisons with historical data reveal that phenotypic differences among the subpopulations of tortoises on Aldabra have been apparent for the last 50 years with no sign of diminishing. We conclude that the giant tortoise population on Aldabra is subject to varying ecological selection pressures, giving rise to stable morphotypes in discrete subpopulations. We suggest therefore that (1) the presence of morphological differences among captive Aldabra tortoises does not alone provide convincing evidence of genes from other extinct species; and (2) Aldabra serves as an important example of how conservation and management in situ can add to the scientific value of populations and perhaps enable them to better adapt to future ecological pressures. In his quest to undermine the immutability of species Darwin famously cited the morphological variation displayed by the Galapagos giant tortoise, where distinctive morphotypes are found on different islands. Here we test whether the only other extant species of giant tortoise – the Aldabra giant tortoise – displays similar morphological variation. We find that it does and these differences have been apparent for around 50 years, despite the better‐connected nature of the subpopulations on Aldabra.

• Publisher: John Wiley & Sons, Inc,John Wiley and Sons Inc
• Subject: Aldabra giant tortoise / allopatric variation / Animals / Atolls / Carrying capacity / Censuses / Conservation / Data analysis / Endangered & extinct species / Extinct species / Females / Genes / Islands / morphological variation / Morphology / Original Research / Population / population dynamics / Populations / Reintroduction / Reptiles & amphibians / Sexual dimorphism / spatial subdivision / Subpopulations / Testudines / Tortoises / Variation / Vegetation type