september 2, 2012 4 reacties
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Charles Darwin brengt slechts vijf weken op de Galapagosarchipel door, nadat hij er tijdens de vijf jaar durende reis met de Beagle op 15 september 1835 is aangemeerd.
Toch zet zijn bezoek de eilanden definitief op de kaart, als blijkt dat ze een unieke dieren- en plantenwereld hebben, met soorten die zelfs per eiland verschillen.
Nadat Darwin zijn evolutietheorie heeft bekendgemaakt, krijgt de Galapagos archipel zelfs een hoofdrol in de feitelijke (eerste) bewijsvoeringen van Darwin’s gelijk.
Twaalf kleine en twaalf grote eilanden telt de Galapagosarchipel, en onder die laatste zijn Isabela, Santa Cruz, San Cristobal en Fernandina de allergrootste. Vijf zijn er bewoond; in totaal wonen er zo’n 30.000 mensen.
De eilanden betreden is lopen op een veelkoppige vulkaan, die zich duizenden meters vanaf de oceaanbodem omhoog heeft gespoten.
De bodem bestaat er dan ook louter uit vulkanisch gesteente, het landschap oogt dor en droog.
Darwin verbaasde zich al over dat gebrek aan schuwheid van de toen aanwezige dieren
. „Een geweer is hier bijna overbodig, want met de loop ervan heb ik eens een roofvogel van een boomtak geduwd.”
Ook hij besefte de grote kwetsbaarheid als gevolg van dit gedrag, vooral omdat het (volgens hem )overgeërfd gedrag is dat zich niet zomaar laat corrigeren zodra er vijandige wezens op ze afkomen.
„Wat een verwoesting kan de introductie van een nieuw roofdier aanrichten voordat de instincten van de inheemse bewoners zich hebben aangepast aan de kracht van de vreemdeling.”
Hoe zijn al die beesten hier eigenlijk gekomen?
Da’s geen eenvoudige vraag, want ze zijn bijna 1000 kilometer van het vasteland verwijderd, en om de eilanden heen ligt een oceaan van water.
De enige verklaring voor hun aanwezigheid en van die van veel planten is dat ze in een ver verleden vanaf het vasteland zijn komen aanwaaien -wat planten en landvogels betreft- of zijn komen aandrijven – en dan hebben we het over niet gevleugelde landdieren of zee(zoog)dieren . Wellicht waren het boomstammen of scheepswrakken die ze een lift hebben gegeven.
Maar welke dieren houden zo’n zeereis vol?
In de brandende zon en zonder veel voedsel of water?
In ieder geval geen zoogdieren; reptielen zijn een stuk taaier, en daarom is het geen toeval dat op de Galapagos van nature alleen deze dieren te vinden zijn.
Overigens is het juist dat isolement dat de archipel voor de evolutietheorie zo interessant maakte. Daardoor vormden de eilanden als geheel, maar ook afzonderlijk een soort laboratorium( ook vandaag nog ) voor evolutiebiologen ……
My travels to this amazing land of fire! This is where it all started… I only wish to be back among the Marine Iguana’s
(Jerry Coyne ) ” ….The courtship of the blue-footed booby (Sula nebouxii) is a droll dance, involving with slapping of turqoise feet on the ground, that never fails to charm visitors to the Galápagos. This bird is not endemic to the archipelago but is one of its iconic animals. The male birds are highly sexed, and I saw many of them performing courtship dances when no female was in sight; these involve foot-slapping, brandishing of pebbles and twigs, and the climactic raising of head, tail and wings (they also utter a strange whistling cry)… ”
The blue-footed booby is an extraordinary-looking bird. It has fairly dull plumage but strikingly coloured blue legs and feet. What could be the evolutionary benefit of such a conspicuous feature? Both sexes have blue feet so they don’t seem to be for impressing potential mates
The evolutionary relationship between the various booby species is shown below in a figure take from Friesen and Anderson (1997).
The Peruvian and blue-footed boobies apparently diverged from a common ancestor about 200,000 years ago. The Blue-footed booby’s range is northern Peru and the Peruvian booby lives in southern Peru. They do not form hybrids where their ranges overlap.
Friesen, V.L. and Abderson, D.J. (1997) Phylogeny and Evolution of the Sulidae (Aves: Pelecaniformes): A Test of Alternative Modes of Speciation. Molec. Phylo. Evol. 7:252-260 [doi:10.1006/mpev.1996.0397]
Creatures of the Galapagos A living monument
Six hundred miles off the coast of Ecuador, on the small islands of the Galapagos, Darwin conducted much of the research for his theory of evolution. It was there that concepts like natural selection, adaptation, and variation took shape.
And ever since On the Origin of Species was published 150 years ago this year, the modern field of evolutionary biology has grown from such seemingly disparate disciplines as genetics, ecology, and even mathematics. For these reasons, the islands remain a cherished place of our scientific past, where the theory that changed the way we understand the natural world first got its spark.
And that’s how we tend to view the Galapagos: a monument to a great paradigm shift. But like Darwin’s theory, the islands are still very much alive and evolving, continuing to provide researchers with new opportunities to dig deeper into the origins of life. With 95% of their prehuman biodiversity still intact today, they give us a rare opportunity to observe iguanas, brown pelicans, blue-footed boobies, and other animals living much as they did centuries ago.
The following stunning images of these Galapagos creatures offer a glimpse of the diverse fauna that inspired Darwin many years ago — and continue to inspire new generations of scientists today.
Photographs courtesy of Peter Norvig.
The islands of the Galápagos are supposedly named after their most famous endemic animal, the giant tortoise (“Galapagos” is Spanish for “tortoise”, and may derive from the Spanish word for saddle, derived from the shape of their shells.) While some zoologists recognize as many as 15 species, the current consensus is that there is only one –Geochelone nigra– with several subspecies. These differ among islands in several traits, most notably shell shape. Several of the subspecies are extinct, and one is represented by a single surviving animal, the famous “Lonesome George” (see below). All the subspecies (or species) descend from a single colonization of the archipelago by one ancestor a few million years ago.
A big male tortoise can weigh more than 500 pounds, and the animals clearly live a long time: some estimates are 150 years or more, giving rise to the possibility that some living tortoises were alive when Darwin wrote The Origin. In fact, one tortoise in an Australian zoo, Harriet, was originally, and probably incorrectly, thought to have been collected by Darwin himself. Harriet died in 2006 at the presumed age of 175 years.
DNA-based dating from Jeffrey Powell’s lab at Yale puts the age of the original colonization between 3 and 2 million years ago. That lab also determined that the closest living relative of the Galápagos animals is the much smaller (8″) chaco tortoise of South America (Geochelone chilensis), with the ancestor making its way to the islands across 600 km of water.
Islands with lush vegetation harbor animals with shallow domed shells (see Santa Cruz tortoise below), but the drier islands have animals with “saddle-backed” shells (see Lonesome George below), which enables them to stretch out their necks and browse on taller plants, notably the cacti such as Opuntia that attain tree-like heights on the islands. The tortoises are completely herbivorous.
Tortoise in the wild (well, on a farm in Santa Cruz island where they roam freely); this beast shows the dome-shaped shell characteristic of wetter islands with lush ground vegetation:
Fig. 1. Santa Cruz (Indefatigable) Island tortoise, G. n. porteri (Rothschild, 1903)
One tortoise showed a bizarre behavior that was explained to us by our naturalist, and is also described in Wikipedia:
Tortoises have a classic example of a mutualistic symbiotic relationship with some species of Galápagos finch. The finch hops in front of the tortoise to show that it is ready and the tortoise then raises itself up high on its legs and stretches out its neck so that the bird can pick off ticks that are hidden in the folds of the skin (especially on the rear legs, cloacal opening, neck, and skin between plastron and carapace), thus freeing the tortoise from harmful parasites and providing the finch with an easy meal. Other birds, including Galápagos Hawk and flycatchers, use tortoises as observation posts from which to sight their prey.
: Fig. 2. Clean me!
We were told that we might be able to elicit this behavior by moving an index finger up and down in front of the tortoise, but that didn’t work. Whether this behavior is evolved or learned is an interesting question.
Rescue of endangered subspecies is carried out by breeding at the Charles Darwin Research Station on Santa Cruz island. Here the adults are mated to produce eggs, and young animals are brought up (separated by subspecies) in enclosures, safe from predation by dogs and other feral animals. The adults have no known predators, either natural or introduced. Let me revise that: their most serious predator is Homo sapiens. Humans have driven at least four subspecies to extinction. It’s appalling to read the chronicles of early sailors, who simply took dozens of these animals aboard as a living food source. In Voyage of the Beagle, Darwin claims that some ships left the islands with as many as 700 animals!
As is well known, Darwin botched his collections of these animals, taking only a few shells and a handful of living animals. He didn’t follow up suggestions by others that each island had its own morphologically distinguishable population of tortoises.
Fig 3. Lunchtime at the Charles Darwin research station. This individual (whose subspecies I didn’t record) has the flaring, saddle-shaped shell characteristic of tortoises from dry islands.
At the Charles Darwin Research station on Santa Cruz lives the most famous last-individual-of-the-group animal in the world, Lonesome George. Collected in the early 1970s, George is the last remaining individual of the Pinta Island subspecies, G. nigra abingdoni. He’s currently confined (see below) with two females of the Isabel island subspecies in a desperate attempt to bring back his kind. The females have produced several clutches of eggs, but none have hatched (I was told that another batch is incubating now.)
Fig. 4. Lonesome George (left) and a would-be bride.
Fig. 5. Nice perch if you can get it. The endemic Galápagos lava lizard Tropidurus albemarlensis atop a tortoise.
Fig. 6. Channelling the inner reptile (h/t: Otter).
Note that there’s one other archipelago-bound giant tortoise, the Aldabra Giant tortoise that lives on three Indian Ocean islands in the Seychelles. (Its population, around 150,000 animals, is far more numerous than that of the Galápagos tortoise, whose total population is around 10,000.) Although the Aldabra species is placed in the same genus as the Galápagos tortoise, G. gigantea, it’s not a close relative, and is often given the name Aldabrachelys gigantea, which appears to be taxonomically invalid.
It’s hard not to feel affection for this gentle, lumbering giant who doesn’t harm another creature but is so beleaguered itself. Kudos to the scientists and conservationists of the Galápagos who have brought the giant tortoise back from extinction.
For more information:
Caccone, A. G. Gentile, J.P. Gibbs, T.H. Fritts, H.L. Snell, J. Betts and J.R. Powell. 2002. Phylogeography and history of giant Galápagos tortoises. Evolution 56:2052–2066. Full Text via CrossRef
Caccone, A. G., J. P. Gibbs, V. Ketmaier, E. Suatoni, and J. R. Powell. 1999. Origin and evolutionary relationships of giant Galapagos tortoises. Proc. Nat. Acad. Sci. USA. 96:13223-13228 (link here).
Powell, J. and A. Caccone. 2006. Quick guide: Giant tortoises. Current Biology 16:R144-R145
Sulloway, F. 2009. Tantalizing tortoises and the Darwin-Galápagos legend. Journal of the History of Biology 42:3-31 (link here).
Darwin on tortoises (from the Beagle journal):
The inhabitants believe that these animals are absolutely deaf; certainly they do not overhear a person walking close behind them. I was always amused when overtaking one of these great monsters, as it was quietly pacing along, to see how suddenly, the instant I passed, it would draw in its head and legs, and uttering a deep hiss fall to the ground with a heavy sound, as if struck dead. I frequently got on their backs, and then giving a few raps on the hinder part of their shells, they would rise up and walk away; — but I found it very difficult to keep my balance. The flesh of this animal is largely employed, both fresh and salted; and a beautifully clear oil is prepared from the fat. When a tortoise is caught, the man makes a slit in the skin near its tail, so as to see inside its body, whether the fat under the dorsal plate is thick. If it is not, the animal is liberated and it is said to recover soon from this strange operation. In order to secure the tortoise, it is not sufficient to turn them like turtle, for they are often able to get on their legs again.
The chaco tortoise, closest living relative of the Galápagos giants: