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Question on herbivorous lizard theory


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Question on herbivorous lizard theory

Postby Brian - LCRC » Sat Dec 02, 2006 8:01 pm

Leeway Corucia Research Center (LCRC)

I wish to question a theory proposed by Keith A. Metzger and Anthony Herrell on herbivorous lizard evolution.

Here is the abstract form the paper:

Correlations between lizard cranial shape and diet : a quantitative, phylogenetically informed analysis

METZGER Keith A. ; HERREL Anthony

Biological Journal of the Linnean Society (2005)

Résumé / Abstract

Although the relationship between dietary and phenotypic specialization has been well documented for many vertebrate groups, it has been stated that few such general trends can be established for lizards. This is often thought to be due to the lack of dietary specialization in many lizards. For example, many species that are reported to be insectivorous may also consume a variety of plant materials, and the reverse is often true as well. In this study, we investigate whether a correlation exists between general cranial form and dietary niche in lizards. Additionally, we test previously proposed hypotheses suggesting that herbivorous lizards should be larger bodied than lizards with other diets. Our data indicate that lizards specializing in food items imposing different mechanical demands on the feeding system show clear patterns of morphological specialization in their cranial morphology. True herbivores (diet of fibrous and tough foliage) are clearly distinguished from omnivorous and carnivorous lizards by having taller skulls and shorter snouts, likely related to the need for high bite forces. This allows herbivores to mechanically reduce relatively less digestible foliage. Carnivores have relatively longer snouts and retroarticular processes, which may result in more efficient capture and processing of elusive prey. When analysed in an explicit phylogenetic context, only snout length and skull mass remained significantly different between dietary groups. The small number of differences in the phylogenetic analyses is likely the result of shared evolutionary history and the relative paucity of independent origins of herbivory and omnivory in our sample. Analyses of the relationship between diet and body size show that on average herbivores have a larger body size than carnivores, with omnivores intermediate between the two other dietary groups.

Here is a pronounced exception.

The Gray's Monitor (Varanus olivaceus) or Butaan has a slender neck, body, short height, long narrow pointed head. Indeed, it looks identical to it's other carnivorous tree monitor relatives. It does not possess the shortened snout with associated strong jaw apparatus as indicated in the above theory. The Butaan consists as an adult, only on a diet of oily fruit from a native Luzon palm tree. This fruit is so tough that pliers are needed to extract it. But the Butaan removes it with ease. It can be argued too if one were to say that this is a case of inital evolution towards a non -carnivorous diet, this would not be so as this species has an enlarged Caecum as well as an enlarged intestine which would have taken time to evolve.

Brian - LCRC
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