Lepidosaur herbivory describes herbivorous lepidosaurs. Living non-avian reptiles form a paraphyletic group that consists of over 9,000 species of crocodiles, turtles, and lepidosaurs. The most diverse group, Lepidosauria, is first known from the Middle Triassic (240 million years ago) fossils, but likely originated in the Permian (approximately 300-250 million years ago).Jones, M. E. H., Anderson, C. L., Hipsley, C. A., Müller, J., Evans, S. E., and R. R. Schoch. 2013. Integration of molecules and new fossils supports a Triassic origin for Lepidosauria (lizards, snakes, and tuatara). BMC Evolutionary Biology 13:208–229. Living lepidosaurs, which include snakes, lizards, and rhynchocephalians, occupy a wide range of environments and niches. The lepidosaurs have many similar anatomical morphology like transverse cloaca, distal tongue, superficial teeth attachment, fused pelvic bones etc. Though widely viewed as obligate carnivores, a small number of lepidosaurs are known to consume plant material. For example, there are roughly 3,300 species of living lizards and approximately 3% of them eat at least some plants. Though the exact definition of herbivory varies significantly between scientists, most define herbivorous lepidosaurs as those that consume plants for approximately 70-90% of its diet.
Evolution of herbivory in lepidosaurs
Herbivory in fossil lepidosaurs
The fossil record of herbivorous lepidosaurs extends into the Late Triassic period, nearly 227 million years ago. A recently described sphenodontian reptile is one of the oldest known herbivorous lepidosaurs. This animal possesses a number of features that demonstrate an herbivorous lifestyle, such as a specialized jaw movement (propalinal) and closely packed teeth that appear to be useful in the shearing of plant material.Martínez, R. N., C. Apaldetti, C. E. Colombi, A. Praderio, E. Fernández, P. Santi Malnis, G. A. Correa, D. Abelin, and O. Alcober. 2013. A new sphenodontian (Lepidosauria: Rhynchocephalia) from the Late Triassic of Argentina and the early origin of the herbivore opisthodontians. Proceedings of the Royal Society B 280:20132057. Phylogenetic analysis shows that herbivory likely evolved in the sphenodontian clade once, specifically within a group called the Opisthodontia. This group of herbivorous sphenodonts possessed wide, shredding teeth and became extinct during the Late Cretaceous (approximately 65 million years ago). These ancient herbivorous animals clearly demonstrate that the ability to consume plant material evolved very early in the history of lepidosaurs.
The evolution of herbivory in squamates (the group that includes lizards and snakes) is less well known, though herbivorous species are found in the fossil record. A large bodied lizard, Polyglyphanodon sternbergi, from the Late Cretaceous of Utah is likely an herbivore. The diet is reconstructed based on the tooth shape; a thin transverse blade-like tooth, with fine serrations, similar to living iguanas.Nydam, R. L. and R. L. Cifelli. 2005. New data on the dentition of the scincomorphan lizard Polyglyphanodon sternbergi. Acta Palaeontologica Polonica 50: 73–78. Others, such as Barbaturex morrisoni, illuminate important ecological niches that living squamates do not fill in modern environments.Head, J. J., Gunnell, G. F., Holroyd, P. F., Hutchison, J. F., and R. L. Ciochon. 2013. Giant lizards occupied herbivorous mammalian ecospace during the Paleogene greenhouse in Southeast Asia. Proceedings of the Royal Society B 280:20130665. B. morrisoni is an herbivorous acrodontan lizard found in the middle Eocene of Myanmar and is notable for its large body size (approximately 1 meter). The large body size demonstrates that competition with herbivorous mammals did not limit the mass of this genus. B. morrisoni also provides evidence that large lizards can occupy ecological niches that are commonly thought to belong solely to mammals, such as a large bodied terrestrial herbivore.
Modern herbivory
In their 2002 study, William Cooper Jr. and Laurie Vitt investigated the dietary patterns of over a hundred lizards and found that many groups have representatives that consume plant material. Omnivory has evolved independently several times within lepidosaurs, whereas the evolution of obligate herbivory has proved to be more rare. For example, in the Iguania clade, omnivory and herbivory have independently evolved at least nine times. When other lepidosaur clades are included, Cooper and Vitt (2002) demonstrate that omnivory evolved at least 32 separate times. Obligate herbivory, on the other hand, has only evolved 10 times in living, non-liolaemid lizards (nearly 7,700 species). Though, one clade seems to be an exception to this pattern of infrequently developed herbivory. A 2004 study investigated a unique group of South American lizards called the Liolaemidae. The authors of this study found that this clade evolved herbivory approximately 65 times faster than other squamates. Specifically, herbivory has evolved 18 times in liolaemids (approximately 170 species).Espinosa, R. E., Wiens, J. J. and C. R. Tracy. 2004. Recurrent evolution of herbivory in small, cold-climate lizards: Breaking the ecophysiological rules of reptilian herbivory. Proceedings of the National Academy of Sciences of the United States of America 101:16819–16824. It has been clearly demonstrated that these herbivorous animals have evolved from omnivorous ancestors and not directly from carnivores. Unfortunately, many biological studies have not investigated the possible timing that key herbivorous characteristics evolved; leaving the amount of time these organisms have possessed these traits unknown. The distinctions in dietary ecology that are observed today are likely a result of early ecological differences between clades. The separation of the Iguania and Scleroglossa (a group including geckos, snakes, skinks, and varanids) clades in the late Triassic best demonstrates this. It is hypothesized that the early Scleroglossa possessed traits that were better suited for hunting, such as the ability to distinguish prey by means of chemical processes, whereas the Iguania did not possess these traits.Vitt, L. J., and E. R. Pianka. 2005. Deep history impacts present-day ecology and biodiversity. Proceedings of the National Academy of Sciences of the United States of America 102:7877–7881. Therefore, the current dietary diversity within the Iguania (including a large number of herbivores) is due to certain traits held by early representatives of this clade.
Living herbivorous lepidosaurs
There are more than 7,800 species of squamate reptiles living today and approximately 2% of them are herbivorous, though the number is continually increasingStruck, U., Altenbach, A. V., Gaulke, M., Glaw, F. 2002. Tracing the diet of the monitor lizard Varanus mabitang by stable isotope analyses (δ15N, δ13C). Naturwissenschaften 89:470–473.). Herbivorous squamates are known from all major groups, including varanids. Less than two decades ago it was believed that there were no herbivorous Anguinomorpha (the clade that includes monitor lizards and possibly snakes;King, G. M. 1996. Reptiles and Herbivory. Chapman & Hall, London, UK, 160 pp.), but recent researched has demonstrated that the species Varanus mabitang was a strict herbivore and its sister species, Varanus olivaceus, may be an omnivore.Gaulke, M. 2004. Varanus mabitang; pp. 208–211 in E. R. Pianka and D. R. King with R. A. King (eds.), Varanoid Lizards of the World. Indiana University Press, Bloomington, Indiana.Pianka, E. R. 2004. Varanus olivaceus; pp. 220–224 in E. R. Pianka and D. R. King with R. A. King (eds.), Varanoid Lizards of the World. Indiana University Press, Bloomington, Indiana. Additionally, a new giant (>2 m in length) varanid (Varanus bitatawa) may also solely consume fruit, though a thorough examination of its dietary ecology has not been conducted.Welton, L. J., Siler, C. D., Bennett, D., Diesmos, A., Duya, M. R., Dugay, R., Rico, E. L. B., Van Weerd, M., and R. M. Brown. 2010. A spectacular new Philippine monitor lizard reveals a hidden biogeographic boundary and a novel flagship species for conservation. Biology Letters 6:654–658. Though present in all major squamate clades, herbivory is most prevalent within the Iguania.Cooper, W. E., Jr., and L. J. Vitt. 2002. Distribution, extent, and evolution of plant consumption by lizards. Journal of Zoology 257:487–517. In this group alone, herbivory and omnivory developed at least nine times. Herbivory is well developed in the Iguanidae, a clade within the Iguania. The majority of the animals that have been studied utilize plant material for nearly 100% of their diet, and therefore, the common ancestor of this group was likely an herbivore. Within the liolaemids, herbivory has evolved at least eight times (though it is estimated to have evolved upwards of 18 times). Additionally, the Agamidae evolved herbivory twice, in the genera Hydrosaurus and Uromastyx.
Herbivory is rare in species outside the Iguania clade, but is present in isolated squamate groups. Within the Scincomorpha (a clade that includes skinks), herbivory independently evolves at least seven times. Specifically, herbivory independently develops at least once in the Lacertidae, Gerrhosauridae, Teiidae clades, and four times in the Scincidae (in the Corucia, Egernia, Macroscincus, and Tiliqua genera).
Diet
There is a diverse range in diets among herbivorous lepidosaurs. The marine iguana, Amblyrhynchus cristatus, feeds primarily on algae. The spiny-tail lizard (Uromastyx aegyptia), which is found throughout the deserts of the Middle East, feeds mainly on desert grasses. This lizard likely also eats flowering plants, but due to no rain falling during the study period, the animals were not observed consuming these plants.Cunningham, P. L. 2001. Notes on the diet, survival rate, and burrow specifics of Uromastyx aegyptius microlepis from the United Arab Emirates. Asiatic Herpetological Research 9:30–33. Other herbivores, such as Ctenosaura pectinata (the spiny-tail iguana), have a mixed diet, with 52% of its diet composed of leaves, 35% flowers, and 2% of fruits. This diet stands in stark contrast with an iguana from the same genus, Ctenosaura similis. This animal's diet is composed of primarily fruit (73%) and leaves (25%), with no consumption of flowers.
The green iguana (Iguana iguana) has a similar diet to the spiny-tail iguana, with its diet composed of 52.1% leaves, 7.7% fruit, and 35.2% flowers. Research has demonstrated that green iguanas are selective in their plant choice, specifically in leaves. Green iguanas, regardless of age, preferentially select immature leaves to eat. Additionally, younger individuals (hatchlings and juveniles) consumed a higher proportion of immature leaves than adults. This selection allows them to access leaves high in protein.Troyer, K. 1984. Structure and function of the digestive tract of a herbivorous lizard Iguana iguana. Physiological Zoology 57:1-8. The diet of the herbivorous varanid, Varanus mabitang, is composed of fruits from palms augmented by the seeds of screw palms and figs. Many herbivorous lizards will eat insects in captivity.Pianka, E. R. and L. J. Vitt (eds). 2003. Lizards: Windows to the Evolution of Diversity. University of California Press, Berkeley, California, 333 pp. For example, the desert iguana (Dipsosaurus dorsalis) consistently eats mealworms in zoos, whereas in the wild its diet consists of solely plants. The desert iguana also provides an excellent example of seasonal diet changes in herbivorous reptiles. Flowers from the creosote bush make up its diet in the springtime, whereas after the month of May the diet is made up of a more diverse range of plant material.Pianka, E. R. 1971. Ecology of Two Lizards. Copeia 1:129–138.
Ontogenetic effect on diet