Evolution in Body Size in Monitor Lizards

The Monitor lizards of the genus Varanus have radiated into a variety of habitats that range from extreme desert to very wet tropical forests, as well as grasslands and mangroves, and the genus contains species that are dedicated carnivores as well as omnivores, and fruit-eaters. In fact, even a quick survey of the 73 species of monitors and it obvious that their common ancestor was remarkably successful with descendants now distributed from Africa and Asia to Australasia. Body size in these lizards is also diverse, ranging from the largest living lizard, the Ora or Komodo Dragon, Varanus komodoensis, which can weigh more than 100 kg and reach 3 m total length, to the quite small pygmy monitors (V. brevicauda and V. primordius) with weights of about 10 g and lengths of about 20 cm total length. David C. Collar and colleagues have now examined the factors involved in producing the huge range of body sizes seen in monitor lizards. They found adult body size spans four orders of magnitude in adult body mass and more than an order of magnitude in total length. Undoubtedly monitor lizards have the largest size range found in any genus of living vertebrates. Therefore Varanus provides an opportunity to investigation the causes and consequences of body size evolution. Their research shows that the diverse body sizes that occur in this clade is a consequence of different selective demands imposed by three major habitat use patterns—arboreal, terrestrial, and rock-dwelling. The authors reconstruct phylogenetic relationships and ancestral habitat use and applied model selection to determine that the best-fitting evolutionary models for species’ adult size are those that infer oppositely directed adaptive evolution associated with terrestrial life styles and rock-dwelling, with terrestrial lineages evolving extremely large size and rock-dwellers becoming very small. Additionally, they found habitat use affects the evolution of several ecologically important morphological traits independently of body size divergence. They suggest that habitat use exerts a strong, multidimensional influence on the evolution of size and shape in monitor lizards.

The small, rock-dwelling Varanus baritji,
the large, semi-aquatic Varanus salvator, and the
medium -sized  arboreal, Varanus prasinus.
Citation
Collar, D. C., Schulte II, J. A. and Losos, J. B. (2011), EVOLUTION OF EXTREME BODY SIZE DISPARITY IN MONITOR LIZARDS (VARANUS). Evolution. doi: 10.1111/j.1558-5646.2011.01335.x

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