…They are as black as the porous rocks over which they crawl & seek their prey as from the Sea. — Somebody calls them “imps of darkness”. — They assuredly well become the land they inhabit. — When on shore I proceeded to botanize & obtained 10 different flowers; but such insignificant, ugly little flowers, as would better become an Arctic, than a Tropical country. — The birds are Strangers to Man & think us him as innocent as their countrymen the huge Tortoises. Little birds within 3 & four feet, quietly hopped about the Bushes & were not frightened by stones being thrown at them.” [Darwin’s Beagle Diary (1831-1836)].
And thus we get a hint of Darwin’s impressions of the Galapagos, and in particular, that Island’s marine iguanas.
The Iguana family is Iguanidae, but most Iguana’s you’ve cuddled in the pet store are members of the genus Iguana (and most likely species Iguana iguana.) The Galapagos Islands have two or three species of iguana: The Land Iguana is Conolophus subcristatus and Conolophus pallidus, or perhaps is actually the subspecies Conolophus subcristatus pallidus. The marine iguana is Amblyrhynchus cristatus.
The two genera of iguana on the Galapagos seem able to interbreed, though they otherwise also seem to make good, distinctive species. (No, it is not really true that inability to inbreed is “THE biological definition of species….” it is more complex than that. A topic for another time, perhaps.) The phylogenetic relationship among the Galapagos iguanas and continental iguanas is similar to that among the finches and other Galapagos animals… complex and more complex because of the apparent fact that while the oldest of the Galapagos islands is about four million years old, earlier islands, perhaps going back twice that age, formerly existed but are now eroded down below sea level. One wonders what will happen next ice age (or what happened last ice age) when a 120 -150 meter drop in sea level exposes some of these islands! The point is that these volcanic islands have a complex history, and it is likely that the islands themselves have a complex relationship to the distant continent. Again, the topic of another post perhaps.
The following passages from Darwin (1839) exemplify the relationship between gathered information (from knowledgeable sources), experiment and in situ observation, and the process of synthesizing a reasonable scientific description of a species or natural phenomenon. Darwin read many primary sources and met several individuals in the Galapagos (and elsewhere) who gave him valuable information but he seems to have as often as possible verified and often rejected this information. Also in these passages is a hint of playfulness that tends to emerge in the field…
[The marine iguana] is extremely common on all the islands throughout the Archipelago. It lives exclusively on the rocky sea-beaches, and is never found, at least I never saw one, even ten yards inshore. It is a hideous-looking creature, of a dirty black colour, stupid and sluggish in its movements. The usual length of a full-grown one is about a yard, but there are some even four feet long: …
These lizards were occasionally seen some hundred yards from the shore swimming about; and Captain Collnett, in his Voyage, says, “they go out to sea in shoals to fish.” … I believe he is mistaken; but the fact stated on such good authority cannot be doubted. When in the water the animal swims with perfect ease and quickness, by a serpentine movement of its body and flattened tail,—the legs, during this time, being motionless and closely collapsed on its sides. A seaman on board sank one, with a heavy weight attached to it, thinking thus to kill it directly; but when an hour afterwards he drew up the line, the lizard was quite active. Their limbs and strong claws are admirably adapted for crawling over the rugged and fissured masses of lava, which every where form the coast. In such situations, a group of six or seven of these hideous reptiles may oftentimes be seen on the black rocks, a few feet above the surf, basking in the sun with outstretched legs.
I opened the stomach of several, and in each case found it largely distended with minced sea-weed, of that kind which grows in thin foliaceous expansions of a bright green or dull red colour. I do not recollect having observed this sea-weed in any quantity on the tidal rocks; and I have reason to believe it grows at the bottom of the sea, at some little distance from the coast. If such is the case, the object of these animals occasionally going out to sea is explained. The stomach contained nothing but the seaweed. Mr. Bynoe, however, found a piece of a crab in one; but this might have got in accidentally, in the same manner as I have seen a caterpillar, in the midst of some lichen, in the paunch of a tortoise. The intestines were large, as in other herbivorous animals.
The nature of this lizard’s food, as well as the structure of its tail, and the certain fact of its having been seen voluntarily swimming out at sea, absolutely prove its aquatic habits; yet there is in this respect one strange anomaly; namely, that when frightened it will not enter the water. From this cause, it is easy to drive these lizards down to any little point overhanging the sea, where they will sooner allow a person to catch hold of their tail than jump into the water. They do not seem to have any notion of biting; but when much frightened they squirt a drop of fluid from each nostril.
One day I carried one to a deep pool left by the retiring tide, and threw it in several times as far as I was able. It invariably returned in a direct line to the spot where I stood. It swam near the bottom, with a very graceful and rapid movement, and occasionally aided itself over the uneven ground with its feet. As soon as it arrived near the margin, but still being under water, it either tried to conceal itself in the tufts of sea-weed, or it entered some crevice. As soon as it thought the danger was past, it crawled out on the dry rocks, and shuffled away as quickly as it could. I several times caught this same lizard, by driving it down to a point, and though possessed of such perfect powers of diving http://and swimming, nothing would induce it to enter the water; and as often as I threw it in, it returned in the manner above described. Perhaps this singular piece of apparent stupidity may be accounted for by the circumstance, that this reptile has no enemy whatever on shore, whereas at sea it must often fall a prey to the numerous sharks. Hence, probably urged by a fixed and hereditary instinct that the shore is its place of safety, whatever the emergency may be, it there takes refuge.
During our visit (in October) I saw extremely few small individuals of this species, and none I should think under a year old. From this circumstance it seems probable that the breeding season had not commenced. I asked several of the inhabitants if they knew where it laid its eggs: they said, that although well acquainted with the eggs of the other kind, they had not the least knowledge of the manner in which this species is propagated;—a fact, considering how common an animal this lizard is, not a little extraordinary.
Darwin, C. R. 1839. Narrative of the surveying voyages of His Majesty’s Ships Adventure and Beagle between the years 1826 and 1836, describing their examination of the southern shores of South America, and the Beagle’s circumnavigation of the globe. Journal and remarks. 1832-1836. London: Henry Colburn.
In this text, we see Darwin’s collection of data, observation, and reasoning, nail down most of the important things one ever wants to know about a species; Body size, special physical adaptations, diet, response to predators, life history patterns. Not bad for a geologist.
Here is a list of adaptations found in the Marine Iguana related to it’s aquatic habits:
- Tails flattened vertically (so they swim like a fish rather than like a whale)
- Long and sharp claws, better to hold on to the slimy intertidal rocks and perhaps to facillitate foraging among aquatic plants
- A shortened facehttp:// and snout to facilitate grazing on algae
- Teeth adapted to algae grazing
- Very active nasal salt gland for removing salt from ingested water.
- Ability to affect metabolism and oxygen use in a way that allows them to spend more time under water