Iguana

The great majority of lizards are predators of insects, other invertebrates, or small vertebrates, because animal prey is easy to digest and rich in protein, while leaves are tough, low in nutrients, and full of cellulose that a vertebrate cannot break down on its own. Iguanas are among the few lizards that have crossed to a mostly leaf diet. They do it by keeping a large population of fermenting microbes in an enlarged, chambered hind gut, which digest the plant fibre and release usable nutrients, much as a cow's rumen does. This fermentation works best at a fairly high, steady temperature, which is why iguanas spend so much time basking, and why hatchlings deliberately eat some adult droppings to seed their own gut with the right microbes.

They are widely sold cheaply as small hatchlings, and many people are not prepared for what they become. A green iguana can reach 1.5 to nearly 2 metres including the tail, live 15 to 20 years, and needs a large, tall, warm, humid enclosure, strong ultraviolet lighting, and a carefully balanced plant diet to avoid metabolic bone disease. Males in breeding condition can become aggressive and deliver a serious bite and tail lash. As a result, huge numbers of pet iguanas are abandoned, released, or surrendered to rescues, and in warm regions released iguanas have established damaging feral populations. An iguana is a demanding, long-term commitment best suited to an experienced keeper.

The marine iguana of the Galapagos is a green iguana relative that adapted to a shore with little land vegetation and abundant marine algae. It grazes algae from rocks, either in the intertidal zone at low tide or by diving, with large males going down several metres and staying under for many minutes, slowing the heart to conserve oxygen. Because seawater and salty food load the body with salt, the marine iguana has enlarged nasal salt glands that concentrate the excess and expel it in a fine spray, so a basking colony is punctuated by loud sneezes and the lizards often have white salt crusts on their heads. Between feeds they bask in tight groups to rewarm, since the cold water chills them quickly.

It comes down to range and adaptability. The green iguana is a widespread, generalist, fast-breeding species that thrives in disturbed and human-altered habitat and tolerates a range of conditions, so where it has been introduced, from escaped and released pets, it multiplies and damages gardens, crops, and infrastructure and competes with native wildlife. The threatened iguanas are the opposite, mostly island specialists such as the Fijian crested iguana, the Grand Cayman blue iguana, and the Anegada and other West Indian rock iguanas, each confined to a small area, slow to breed, and highly vulnerable to the clearing of their forest and to cats, dogs, rats, mongooses, and pigs that eat their eggs and young. Several of these are the subject of intensive captive-breeding and island-restoration programmes.