Reptiles


 

Land Iguana 

Darwin was not much impressed with the land iguana:

"...they are ugly animals, of a yellowish orange beneath, and of a brownish-red colour above: from their low facial angle they have a singularly stupid appearance."

The land iguanas are typically divided into two species, Conolophus subcristatus and Conolophus pallidus, although there are some data to suggest that the later is merely a local variant, a subspecies, of the former. C. subcristatus is more widespread throughout the archipelago while C. pallidus is restrected Santa Fe. C. subcristatus is pretty much as Darwin described it, but since he had not visited Santa Fe he never saw C. pallidus , which is much more uniform and paler in color, and possesses a more pronounced ridge of spines.

C. subcristatus once once had a much broader distribution and higher population numbers than it does today. In 1835 Darwin was impressed with their numbers, remarking that:

"...when we were left at James [Santiago], we could not for some time find a spot free from their burrows on which to pitch our single tent."

Today, there is not a single land iguana to be seen on Santiago. They have become extinct at the hands of man, or man's introduced animals. C. subcristatus can be found on Santa Cruz, Isabela, Fernandina and, most dramatically, on Plaza. Plaza is a very tiny islet that surprisingly supports a large population of iguanas. Visitors are often disappointed by the size before disembarking but are always surprised by the richness of the islet, especially the iguanas. There is also an introduced population on N. Seymour. In the early 1930's William Randolph Hearst visited the Galapagos and was surprised to find no land iguanas on N. Seymour while just a few hundred yards acoss a small channel, Baltra supported a large population. Since he could see no difference between the two islands, he tried the experiment of transplanting (gasp!) some Baltra iguanas to N. Seymour. The disappearance of the Baltra population after World War II is generally blamed on US airforce and navy personnel who were stationed there to protect the Panama Canal. According to the standard story, the sailors were desperately bored, and would shoot at iguanas for ammusement. Reports from the Darwin Station, however, suggests that the Baltra iguanas were already dying out for unknown reasons. The Baltra iguanas still thrive on N. Seymour, where they can still occasionally be seen if you're lucky. Recently, conservationists at the Darwin Station have reintroduced iguanas back to Baltra.

Land iguanas, like all iguanas, are vegetarians, subsisting mostly on the fruit and pads of Opuntia cactus. It is not unusual to see them sitting under a cactus, waiting for pieces to fall. They normally use their front feet to scrape the larger thorns from the pads, but they don't seem to mind the smaller thorns. Usually they will gulp down a cactus fruit in just a few swallows. I once watched an iguana dislodge a thorn from its tongue by sticking the tongue out and dragging it back across its teeth several times. In addition to cactus, they also eat a variety of other plants including Alternanthera, Tiquilia, and Portulaca..Portulaca. is a low-growing plant that is found in abundance on the western part of Plaza. After a wet period, they blanket the island in yellow flowers. The flowers, however, last only a few days because they are eaten by the iguanas. Occasionally land iguans will dine on carrion or arthropods that crawl in their burrow. Again like other iguanas, the juveniles feed primarily on insects.

 

 

Land iguanas are territorial and signal their aggression with head-nodding. During mating a male literally catches the female and copulates with her. Following copulation, the female flees and finds a spot to dig a nest in which to bury a clutch of up to 25 eggs. On Plaza land iguanas occasionally hybridize with marine iguanas. My Galapagos guide, Walter Campoverde, has reported that male marine iguanas, in a mating frenzy, have occasionally been seen attacking and raping female land iguanas. Molecular studies on the hybrids would seem to bear this out. While offspring get half of their nuclear DNA from each parent, their mitochondrial DNA, found in the cytoplasm, comes only from the mother. The mitochondrial DNA in the hybrids matches the land iguana. In general form, the hybrids, such as the one below, resemble the land iguana and seem to live the land iguana lifestyle. Their coloration, however, is very dark, like the marine iguana, and they exhibit dorsal striping which is characteristic of juvenile marine iguanas, but not juvenile land iguanas. There are only a few hybrids and nobody knows if they are fertile.

The fact that there is crossbreeding between the two genera of iguanas, Conolophus and Amblyrhynchus, the marine iguana, raises interesting questions about the evolutionary relationship between the two. Standard definitions of the term "species" include the presence of a fertility barrier. Some closely related species can hybridize to form viable, fertile offspring, as is the situation among the Darwin Finches. Less closely related species can interbreed, such as horses and donkeys, but their offspring are infertile. It appears that Ctenosaura, the black or spiny-tailed iguana of Central America, is ancestral to both Galapagos iguanas. It has always been assumed that the marine iguana

evolved from the land iguana, and there are some data to support that assumption. On the other hand, other data sets suggest that the land and marine iguanas are no more closely related to each other than to any of the other iguana genera. Molecular clock studies of the marine iguana suggest that these iguanas diverged from other iguanas around 8 million years ago. The oldest Galapagos island, however, is only around 4 million years ago. Either the marine iguana originally evolved on the mainland and colonized the Galapagos in their present form, or there have been older Galapagos Islands that have now sunk beneath the waves. Indeed, there does seem to be some evidence for this later possibility.

 



    for more info, contact Dr. Robert Rothman: rhrsbi@rit.edu