It is an interesting fact of the Cenozoic Era that whereas mammals dominate much of the continental fauna on Earth, islands are often home to unique families of reptiles, particularly lizards. Once the essential difference between reptilian and mammalian anatomy is understood, the reasons become obvious. Modern reptiles, with their poikilothermic body metabolism, require something like ten per cent of the food intake of a similar-sized mammal, and can therefore successful form a reproductive and self-sustaining population in a much smaller area. Conversely, the arrival of domestic or other mammals on islands has often spelt disaster as creatures such as goats have stripped the limited flora available and thus jeopardised the survival of other animals, a step that has only recently - and drastically - been reversed by the removal or even elimination of these animals from the area.
Birds, of course, are also endothermic (warm-blooded), but their power of flight allows them access to areas that most reptiles cannot reach, namely the air (for flying insects) or sea (for marine life), and thus the two classes are not usually in competition. Indeed, in some ways reptiles and birds may indirectly benefit each other, since birds may prey on young lizards (eg the Galapagos hawks on young marine iguanas) and adult lizards may take bird eggs, as some skinks do on islands in the Indian Ocean. (The skinks on Cousins Island in the Seychelles, Mabuya seychellensis and M. wrightii, achieve large numbers because they occupy a unique ecological niche, living off the detritus of seabird colonies: bird faeces, fish dropped by the birds and broken eggs. Mattison notes that these pickings are so rich that even though they are seasonal, the lizards can live off them indirectly for the rest of the year due to the food reserves accumulated).
What follows is a rough guide to some of the islands of the world and their endemic lizard species.
Madagascar, one of the largest islands in the world, contains not only many endemic lizard species but also an anomaly, namely the presence of iguanid lizards (the Ophlorus species) and boa snakes, both of which groups are otherwise only found in the New World.
Chief among Madagascar's unique wildlife are the zonosaurs, also known as Malasy Plated Lizards, members of the Gerrhosauridae family. There are at least twelve species of these lizards, which look rather like the better-known plated lizard Gerrhosaurus major of southern Africa but which are slightly more flattened and angular in appearance, particularly around the snout, and often quite interestingly coloured, many having longitudinal stripes. The two Tracheloptychus lizard species are members of the same family and somewhat similar, if bulkier, in appearance.
The day geckos, Phelsuma species, should need no introduction. Although they are not unique to Madagascar - some species occur elsewhere in the Indian Ocean and more rarely in Africa - most of the species of this brightly coloured genus occur on the island and in many varied habitats. Overcollecting in the past has threatened the populations, and indeed at least one was rendered extinct in the nineteenth century.
The Oplurus species are the two iguanid lizards found on Madagascar, and are occasionally seen in captivity as well. Finally, no mention of Madagascar would be complete without the many chameleons which are found on the island.
Today the biggest threat to all the wildlife of Madagascar is deforestation and slash-and-burn agriculture. Fortunately many of the lizards mentioned here, especially the day geckos, are now being bred in captivity.
See also Day Geckos: Phelsuma spp., Zonosaurs and Tracheloptychus..
The Galapagos are a group of about seventeen volcanic islands in the Pacific ocean some seventy miles off Ecuador on the equatorial line. After a spotty history of human settlement which threatened to cause great damage to the environment they are now virtually unpopulated apart from scientists and closely supervised tourists. It was Darwin's observations of the differing subspecies of birds and reptiles on these islands that contributed towards his work on evolution.
The islands are deservedly famous for the giant tortoises, Geochelone nigra, but equally so for the marine iguana, Amblyrynchus cristatus and to a lesser degree the land iguana Conolophus. The marine iguana is unique in that it is the only lizard in the world to be "marine", although this is stretching the meaning of the word somewhat: the iguanas do not live in the sea but take their food from below its surface, the kelp that grows on the rocks on the seabed around Galapagos. That they can do this is nevertheless a tribute to their powers, since the water itself is cold despite its equatorial position. To survive the daily quest for food, the lizards first bask on the bare rocks until sufficiently warm, and then dive in during the warmest part of the day. While underwater they can control their physiology to the extent of reducing the blood supply to the extremities and slowing their metabolism down. Even so they do not spend overly long underwater, up to an hour but often only about ten minutes. When back on land they must bask again to raise their body temperature. They have special glands that allow them to remove excess salt from their body, often by sneezing. Incidentally, despite the sociable and harmless nature of these creatures, they seem to have made a profoundly negative psychological impact on the first human encounters. An American sea captain landing on the islands feared that he and his men would be killed by the iguanas (in fact the sailors subsequently slaughtered hundreds) while Darwin, actually a fairly conservative man, referred to the lizards as "imps of darkness".
The land iguanas, genus Conolophus, comprise two species, C. subcristatus and C. pallidus. These are somewhat similar in appearance to the Horned or Rhinoceros Iguanas, genus Cyclura spp., and it is speculated that they may indeed be descendants of these lizards which may have arrived on the islands by rafting millions of years ago. The land iguanas feed normally on plant matter like their cousins in other parts of the Americas, in this case usually the fruit and leaves of the prickly pear cactus Opuntia. Their real ordeal comes when reproducing, for the female then descends into the crater of the huge volcano on their home island to lay her eggs. This is arduous enough, but then upon hatching the baby iguanas must run the gauntlet of hawks and other predators as they ascend the caldera again. Although casualties are inevitably high, those which survive into adulthood have no natural predators on the islands.
Smaller lizards are also found on the island, of which there are three endemic species of gecko (all genus Phyllodactylus) and six of so-called 'lava lizards', iguanids of the genus Tropidurus. None grow larger than a foot and the geckos are smaller than this.
The very mention of the word Komodo probably brings to mind one of the few lizards in the world that needs no introduction, namely the Komodo dragon, Varanus komodensis. In some ways it may seem odd that such a huge and fearsome predator should be confined to such a relatively small space (about three middling-sized islands) in the Indonesian archipelago. But again, upon reflection the reasons become more apparent. Despite their hunting capabilities, the dragons still operate on the relatively low overheads of the lizard metabolism and do not need to eat every day, or even every week. Therefore these islands, which possess a fairly stable prey population of wild pigs, deer and other animals, can easily sustain the known population of about five thousand dragons. (In times of trouble the dragons can also swim from one island to another). On the other hand, were the dragons to seriously attempt to reestablish themselves on islands outside this "home area", they would be in competition with mammals, a contest that in fact they seem to have been steadily retreating from over the past several thousand years. In evolutionary terms even compared with other monitor lizards, V. komodoensis is considered somewhat primitive. If many islands are compared to laboratories of evolution, then Komodo and its neighbouring islands are more like a zoo, or a museum, where time has somehow stood still in the days when reptiles still dominated the earth and were at the top of the food chain.
Few holidaymakers would associate these near-equatorial islands with rare reptiles, but in fact the Canaries are home to an endemic genus, the Gallotia members of the Lacertidae family, as well as a couple of native species of skink. Although the Canaries have probably been inhabited for nearly two thousand years by man, other mammals probably only arrived aboard European ships, and even today there are no endemic mammals and only a few that have become native, as it were. In fact there is not a great deal of fauna on these islands in general. This is probably because the Canaries are volcanic and in places very bare of vegetation indeed, and also due to their isolation from the continents of Europe and Africa. The six species of Gallotia lizard are now protected, especially the giant member of the genus, which was only rediscovered a few years ago.
See also Gallotia lizards of the Canary Islands
The Caribbean islands can be considered the locus of two related genera: the myriad of different species of Anolis, many of which are bigger and brighter than their better known relatives in the USA, and the large rock iguanas, Cyclura. There are eight species of these giant lizards which are further divided into subspecies that often occupy small ranges on the islands of Cuba, Hispaniola, Jamaica, the Caymans and the Bahamas. Most if not all are severely threatened with extinction, while captive breeding is difficult due to their great need for space and also the small numbers involved, which may give rise to inbreeding problems (Mattison). Fortunately some action is being taken, but the outcome still appears to be in the balance.
The Balearic islands in the Mediterranean, with their current reputation as the holiday destination for package holiday Europeans and hedonistic clubbers, may sound an unlikely place for rare wildlife. But in fact they are home to several Podarcis species, many of which have colonised small islets around the islands and become subspecies owing to the phenomenon known as reproductive isolation (ie the population is isolated from others of the same species and therefore its gene pool receives no fresh input from outside). A great number of subspecies, each with its own range, has thus sprung up around the four islands.
The distribution of many geckos around the various islands of the Pacific illustrates another aspect of the distribution of lizards, namely that of their spread by rafting from one island to another. This is not so much a deliberate process (it is hard after all to imagine a group of iguanas building a boat) but rather a sort of accident of nature that does take place fairly frequently over the passage of time. Geckos in particular are often small enough to cling to a tree that has been uprooted and cast into the sea, and if it should reach another island with surviving male and female geckos then a potentially new colony and possible subspecies has arisen. In fact it seems that this process is even more assured with the transport of gecko eggs, since most are calcareous (hard-shelled) and can resist the effects of at least a limited amount of seawater (Mattison).
While this theory was readily accepted for small lizards such as geckos, there was some scepticism as to whether much larger species such as iguanas could raft between islands, especially over long distances. However recently a group of Caribbean iguanas was discovered to have rafted between two islands following a storm, so it now seems possible that this could have been the method whereby some islands in the Pacific could have been populated.
Apart from the Galapagos Islands, which have been already covered (see above), the obvious curiosity was that of the Fijian Iguana, genus Brachylophus, one of the most beautiful of the large iguanid lizards. Apart from the fact that Fiji and Tonga and their neighbouring islands are considered part of the Old World, whereas iguanids are almost entirely a New World family, there is the mystery of how the ancestors of these iguanas could have reached the islands. Rafting, albeit incredible given the distance from the Americas to Fiji and Tonga, seems to provide the only answer for the moment.
New Zealand might be considered under the previous section, but it is somewhat different in its geographical position and geological history, possessing a cooler climate and being isolated from other sources of potential colonisers. The country is famous for its endemic birdlife and the unique tuatara, but less well known is its somewhat surprising number of endemic lizard species. All of these are either geckos of the Diplodactyline subfamily or skinks. The geckos are represented by the genera Hoplodactylus and Naultinus (at least one authority also includes a third genus, Woodworthia) and the skinks by the genera Cyclodina and Oligosoma.
Apart from Madagascar, there are other islands of unique lizard interest in this area. Connected with Madagascar, of course, is the radiation of some Phelsuma day geckos to other islands in the area. Most of the day gecko species are still found on Madagascar, but some have colonised the Seychelles, the Mascarene Islands (Mauritius and Reunion), the Comoros and Aldabra Islands, and even as far as the Andaman Islands and Pemba Island in the Bay of Bengal. One or more species, incidentally, can also be found on the mainland of Africa, demonstrating the ease with which geckos seem to travel the world, either via the traditional method (rafting) or through inadvertent transport by man.
The other notable example is Sri Lanka (formerly Ceylon), a large island to the east of India that is home to a large number of unique agamid and skink genera. These constitute a large part of the fauna on the island, in addition to a number of gecko species. That there is still much herpetological discovery to be made on Sri Lanka is shown by a recent amphibian survey, which turned up a large number of as yet unknown frog species and placed the country ahead of the former champion Costa Rica in terms of frog density.
For a look at Sri Lanka's lizards and its biodiversity in general, go to The Wildlife Heritage Trust of Sri Lanka.
Lizards of the World, Chris Mattison, Blandford 1989. Excellent work on not only on the classification of individual families and species but also covering general themes such as distribution of lizards worldwide.
Odyssey Illustrated Guide to the Galapagos Islands, Pierre Constant, The Guidebook Company Ltd, Hong Kong, 1995. Very comprehensive guide to all aspects of the islands, covering not only fauna but also flora, geology and history. Constant has apparently studied the islands for years, including two years spent living there.
"Ground Iguanas of the Carribean, Cyclura" Reptilia 2, M Aresté, is a useful article on these lizards and their environmental plight. See also the article "Horned Iguanas of the Caribbean", Reptile & Amphibian Hobbyist 5:12, Robert Powell Ph.D.
For articles on the iguanas of the Galapagos Islands, see "Galapagos Island Iguanas: Breeding Cycles" Reptile & Amphibian 11-12/90 and "Galapagos Marine Iguanas: Natural History" Reptile & Amphibian 5-6/94.
A Photographic Guide to Reptiles & Amphibians of New Zealand, Tony Jewell, New Holland 2008. Slim but very factual volume.
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