SOME REMARKS ON HERBIVOROUS LIZARDS

It is a striking fact that although many of the dinosaurs were herbivorous, in recent times the only reptilian group displaying a large number of plant-eating species is the Order Chelonia. An overwhelming majority of the dominant reptiles of today, namely the Squamata, feed on other animals. It cannot, however, be assumed that an adaptation to a plant diet is impossible for lacertilians. A few herbivorous species belong to the families Iguanidae, Agamidae, Scincidae, and Teiidae. It is therefore highly probable that during the past history of the Squamata other herbivorous species also occurred. Plant food is more abundant and much easier to collect than animal; it is therefore astonishing that no radiation ever took place among plant-eating lizards. The writer is not aware of any attempt to explain this fact. Some suggestions are therefore put forward below. Plant food is digested with greater difficulty than is animal food. Vertebrates able to thrive exclusively on plants (e.g., birds and mammals) possess highly efficient grinding devices and maintain a constant high body temperature. Many also make use of symbiotic bacteria, fungi, and protozoa. Among recent reptiles, any marked development of such adaptations is absent. It is therefore probable that herbivorous reptiles use only a small part of the calories potentially present in their food. In consequence they may have a low metabolic rate and their females may produce fewer eggs than females of related carnivorous forms. As a result, plant-consuming lizards would be more susceptible to predation, and here lies the factor which may always have hampered their radiation. Some facts can be listed in favor of this suggestion. The extreme slowness and the poor efficiency of digestion of plant material was recently described by Rick and Bowman for tortoises (1961). Two herbivorous iguanids, the seaweedeating Amblyrhynchus cristatus and Conolophus subcristatus, live on the Galapagos Islands which are notorious for the lack of predators. The plant-eating skink, Corucia zebrata, occurs in the Solomon Archipelago. Agamids belonging to the genus Uromastix, which feed on plants, have a spiny tail, used as a defence against predators entering rock crevices in which they live. Mexican lizards of the genus Enyaliosaurus, also herbivorous in the adult state, have a similar tail, used for blocking holes in trees in which they seek refuge. The herbivorous green iguana, Iguana iguana, is comparatively very large; it feeds in trees; when frightened it often enters water and swims well; finally it uses its powerful tail as a weapon with great skill-all factors affording some form of protection from enemies. Most herbivorous lizards are larger than their carnivorous relatives and consequently are sought by fewer kinds of predators. Probably the bestknown plant-consuming lizard is the American desert iguana, Dipsosaurus dorsalis. It is not strictly herbivorous, as it takes important quantities of insects and feeds also on carrion. But among causes of its biological success, a great part is probably played by its adaptation to an environment extreme in its low humidity and high temperature (Norris, 1952), where the competition from other animals is greatly diminished. Finally, the members of the only large group of herbivorous reptiles, turtles and tortoises, possess a unique device of passive resistance in their shells. Other herbivorous lizards include the genera Ctenosaura and Sauromalus; Macroscincus coctaei, Egernia, Tiliqua, and Dicrodon guttulatum. The present writer is not familiar enough with the biology of these forms to discuss it here. Further arguments for or against the author's hypothesis can be found, perhaps, in an examination of the digestion, metabolism, and fecundity of herbivorous lizards. This can be done only by scientists who have available a rich supply of these animals.