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An Introduction to Gollum's Rule With an Assessment of Convergent Evolution among Cave-dwelling Malaysian Bent-toed Geckos
Convergent evolution is a widely studied evolutionary phenomenon that has been documented in taxonomically diverse groups representing numerous ecosystems and specialized habitats. Yet one traditionally understudied system is the cave environment. Cave life is often associated with novel adaptations for coping with the unique challenges of a subterranean existence. Accordingly, cave-adapted species are expected to converge on a common suite of adaptive differences-introduced here as Gollum's rule-relative to their surface-dwelling relatives. Here I tested for differences in morphology, performance, and physiology of cave-dwelling Malaysian bent-toed geckos (Cyrtodactylus pulchellus complex) relative to their close relatives living on granite boulders in the surrounding forests. I hypothesized that the measured traits would be adaptive as each lineage independently transitioned into its respective cave environment. I found strong evidence for convergence among cave-dwelling Cyrtodactylus. Conventional statistics and comparative analyses accounting for the phylogenetic relationships among the taxa generally agreed that, relative to non-cave-dwelling species (n = 4), twilight zone-dwelling cave species (n = 3) had shorter digits and small palmar and plantar surface areas, higher cling scores to cave substrata, and lower rates of resting metabolism. These data provide evidence that geckos occupying the twilight zone undergo a common suite of changes in traits that are likely adaptive for life in caves thus supporting Gollum's rule.