The Komodo dragon (Varanus komodoensis) is infamous for being the world's largest lizard, reaching a maximum length of 3 m and a maximum body mass of up to 87 kg (Fig. 1). As adults, this monitor lizard is capable of killing and consuming mammals, including water buffalo (Bubalus bubalis), Timor deer (Cervus timorensis), and wild pigs (Sus scrofa), which coexist on five rugged islands in eastern Indonesia. Monitor lizards are monophyletic and comprise three clades; the Komodo dragon is assigned to the Indo-Australian lineage. Its closest sister species, as inferred from a mitochondrial gene tree, is the Eastern Australian lace monitor (Varanus varius). The Komodo dragon is estimated to have differentiated from Australasian ancestors ~4 million years ago, and historically, its range extended further east across to the island of Timor.
Ungulate prey constitutes the majority of the diet of large Komodo dragons (i.e., >15 kg in body mass), whereas smaller dragons consume small reptiles, birds, and rodents. Within Komodo National Park, where this species is best studied, Komodo dragons persist on four islands that vary in area from 10 km2 up to 350 km2. Across these islands, the availability and distribution of ungulate prey varies considerably, with deer being found on all islands but buffalo and pigs only on the two larger, and human-habited, islands. Differences in large-prey density and availability appear to be an important factor underpinning ecological and evolutionary processes influencing Komodo dragons.
Major differences in the morphology of the Komodo dragon appear to be associated with conspicuous island differences in large-prey availability. For instance, maximal body size among the four islands varies nearly fourfold, indicating that the Komodo dragon exhibits both dwarf and giant populations. Maximal body size is positively correlated with insular prey density, independent of genetic relatedness among lizard populations. Hence, even genetically similar island populations can exhibit large differences in body size associated with local prey density, suggesting considerable phenotypic plasticity in body size. Other ecological hypotheses that are implicated in morphological divergence among island populations, including character displacement due to competition or predation, are intuitively unlikely given that this lizard is the sole large predator persisting on these islands.
Differences in island prey density appear to be associated with conspicuous differences in the population demography of Komodo dragons in Komodo National Park. For example, low densities of dwarf Komodo dragons persist on the two small islands, in contrast to high densities of large-bodied lizards occurring on the two big islands. Despite individuals exhibiting a much smaller body size, and in turn decreased energetic requirements, the two small-island dragon populations appear to be uncompensated by increased population density. Thus an inverse relationship between body size and population density, as predicted by the energetic equivalence rule, is not observed for Komodo dragons.
On emergence from their subterranean nests, hatchling Komodo dragons (~100 g) directly climb trees and exhibit an arboreal life-history stage. Limited natal dispersal is apparent, with juveniles moving slowly in a mostly linear direction, away from their nests. Arboreal living by juvenile Komodo dragons presumably reduces predation from larger terrestrial dragons, while enabling access to smaller arboreal prey. Once reaching several kilograms, juvenile lizards transition to predominantly terrestrial activity and develop home ranges of ~o.25 km2. Spatial requirements increase considerably with body size, with the largest adult lizards occupying home ranges of ~7 km2.
Long-distance dispersal of Komodo dragons within and among islands appears to be an ecologically rare event. Even within the large islands, the exchange of individuals (based on mark-recapture estimates and telemetry) between closely adjacent valleys is infrequent. Komodo dragons thus exhibit strong philopatry to their resident valleys (and to islands), a feature also generally supported by molecular data indicating significant genetic structure within, and among, island populations.
Neutral genetic estimates suggest varying population differentiation among extant populations. The populations of Rinca, Nusa Kode, and Western Flores are the most closely related, reflecting their proximity to one another and their higher rates of gene flow. In contrast, the more isolated populations are more differentiated. The Komodo island population exhibits the highest level of genetic divergence and allelic distinctiveness. In contrast, the small island of Gili Motang exhibits a low level of hetrozygosity, consistent with its small population size, suggesting that this island is most at risk from stochastic processes.
Demographic evidence from Komodo National Park suggests the two small island populations are declining in abundance, while the two large island populations appear relatively stable. The factors underpinning the declines on the two small islands are not clearly identified. Nevertheless, given the inherent vulnerability of island populations to extinction, and that Komodo dragons appear to exhibit limited dispersal among islands, ongoing and robust population monitoring is advocated to ensure that extant populations of Komodo dragons, both within and outside Komodo National Park, are conserved.
Dispersal / Dwarfism / Gigantism / Indonesia, Biology / Island Rule / Lizard Radiations
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