Abstract
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.
Highlights
Morphological variation in the Galapagos giant tortoise is said to have inspired Darwin’s theory of evolution by natural selection
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
That distinction belongs to the Aldabra giant tortoise (Aldabrachelys gigantea (Schweigger 1812); Fig. 1), the sole survivor of a radiation of Indian Ocean giant tortoises that once occupied many islands in the region (Palkovacs et al 2002; Austin et al 2003)
Summary
Morphological variation in the Galapagos giant tortoise is said to have inspired Darwin’s theory of evolution by natural selection. Darwin was told that the people of Galapagos could assign island of origin to any tortoise because of differences in the size and shape of the shell. Darwin attributed these differences to adaptive changes resulting from the varied ecological conditions that prevailed among islands. The giant tortoises of Galapagos, despite their fame, are not the most abundant free-living species of giant tortoise alive today. That distinction belongs to the Aldabra giant tortoise (Aldabrachelys gigantea (Schweigger 1812); Fig. 1), the sole survivor of a radiation of Indian Ocean giant tortoises that once occupied many islands in the region (Palkovacs et al 2002; Austin et al 2003). The species is thought to number somewhere in the region of 100,000 animals (Bourn et al 1999), in comparison with an estimated 19,000 free-living tortoises on Galapagos (Marquez et al 2004)
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