Tortoises and Finches

Galapagos Islands Travel Blog

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Our Frequent Flier had time zone confusions, so we were up an hour early for our flight to the Galapagos. We flew from Quito to the island of Baltra, where we drove to the ferry to cross onto Santa Cruz. Just during the ferry transfer we saw Brown Pelicans, Galapagos Sea Lions, Sally Lightfeet and Red Billed Tropic Birds. We drove across Santa Cruz to board our boat, watching marine iguanas and Sally Lightfeet bask on the rocks, Darwin’s Finches flitter around chasing flies, Brown Pelicans watching Blue Footed Boobies dive deep into the water for fish and a Great Blue Heron idly picking at the seaweed.

 

We were in the amazing Galapagos Islands, made famous by the 1835 voyage of the HMS Beagle. Charles Darwin was invited on the Beagle as gentleman company for Captain Robert FitzRoy, and was not the official naturalist. When he first saw the islands he wrote "nothing could be less inviting than the first appearance". The islands are most arid and barren, volcanic outposts. They are also actually extremely poor in biodiversity, as only a few species have been able to reach and thrive on the islands. In fact, over the past 3.5 million years since the oldest island formed only 400 colonisation events have occurred, one every 12 000 years. The rate may be even slower than this, genetic testing shows that marine and land iguanas are 15 million years apart, therefore they either represent separate arrivals or they arrived 15 millions years ago when none of the current Galapagos Islands had formed and the sea mounts reached the surface.

 

Instead of being rich in biodiversity, the Galapagos are rich in endemics as the long isolation allows each arrival species to evolve to their new environment. We saw the most famous endemics that afternoon, when we drove across Santa Cruz to the moist highlands to see the Galapagos Tortoises. The tortoises were large and still. Enormous rocks with a prehistoric face hiding under the shadows or reaching out on a veined and wrinkled neck. They reminded me of the skeksis from The Dark Crystal, with a cruel vulture beak and dead grey skin. While they sat like boulders in the mud, when they started to move they looked like implacable tanks.

 

There are two types of Galapagos Tortoise, the Dome-shelled tortoises of the cool moist highlands, and the Saddlebacks of the low arid lands.

Each has evolved to its environment. The Dome-shelled is larger (with shells over a metre long the males reach 300kg) and they have short limbs, to be able to keep their heat better. The Saddlebacks are smaller (around 50kg on average) and have long limbs and a saddle on their shell so their neck can reach up higher. This allows them to be able to reach up high and eat the Opuntia, a type of cactus that has evolved into tree-like proportions, and it also gives rise to the name “Galapagos”, Spanish for “saddle”. While the two types of tortoise live separate lives most of the year, in order to lay eggs the female domed tortoises need to descend to the lowlands to lay their eggs. Galapagos tortise eggs become male if they are incubated under 28.5 C and female if they are incubated above 28.5 C, so if the females stayed in the cool highlands all the eggs would hatch into males.

 

As well as being shaped by the islands, the Galapagos Tortoise has altered the islands in return.

The Opuntia has grown so tall in order to escape the Saddleback, reaching far greater heights on islands that have the tortoise. The Galapagos tomato seeds now need to spend time in a tortoise’s digestion system before they can germinate.

 

There were once 14 different subspecies of tortoise. Over 100 000 tortoises were harvested by sailors because they were easy to collect and they could be kept alive on deck without food or water for months, giving them fresh meat throughout the voyage. This harvesting drove three subspecies extinct. A fourth might be extinct; there is only one Pinta tortoise – Lonesome George. He is only 90 years old and he could easily live for another hundred years, so they are searching for another Pinta tortoise on other islands, but genetic testing indicates that he may actually be an Espanola tortoise, making the Pinta extinct.

The other 10 subspecies are now healthy. The Espanola tortoise dropped down to fourteen individuals, but captive breeding and release has built the numbers up to over 1000.


As well are Galapagos tortoises we watched Golden Warblers bathe, and saw some of Darwin’s Finches, egrets and the introduced Ani. There are fifty one species of land birds in the Galapagos Islands, with a very high proportion (43%) being endemic – twenty two endemic species and three endemic subspecies. The proportion of endemics is even higher when the thirteen vagrant species (observed once, but not colonised) are counted out, 66%. The observed vagrant species show that while colonisation events are improbable, they do occur.

Yet the 22 endemic species represent only seven different colonisation events – a rail, a hawk, a dove, a martin, a flycatcher, a mockingbird and a finch. Two endemic owl subspecies, the Barn Owl and Shorteared Owl, came from two separate colonisation events. The expansion of a well suited immigrant is reflected by the success of goats on the islands. In 1959 two females and one male were released on Pinta. By 1970 the population had expanded to 100 000 and it took until 1986 to eradicate them.

 

Not all vagrants are so lucky though, it was only those vagrants that reached the islands in large enough numbers at the ideal time that were able to flourish. The ones that did diverged from the mainland populations, as the islands are far enough away that additional individuals arriving to keep up gene pool flow is extremely unlikely.

Because the small islands are close enough that most birds will travel in between them, almost all endemic successes have stayed as a single species (with flightless reptiles the story is very different, with seven separate endemic species of lava lizard, six endemic species of gecko and three endemic snakes on different islands). The mockingbirds (separated into four species) and the finches (separated into thirteen species) are the exceptions to this rule, for different reasons. The mockingbirds are very reluctant to fly across water, so the four different species and two subspecies are all found on separate islands, with travel rare enough to allow geographical divergence.

 

The finches were different. Unlike the other colonisation species they have highly specialised diets, so to take advantage of an array of unexploited food sources they had to diverge by adaptive radiation.

So the finches show different species living together on the same islands, but with beaks adapted to unique food supplies. On every island with more than one finch species, the finches have at least a 15% difference in beak size, with small and large forms to specialise in different food supplies, while the same finch species on islands with only a single finch tend to have medium-sized beaks to inefficiently take advantage of a variety of foods. The larger islands that support multiple Finch species show a very high level of divergence in eating behaviour, with seed eaters, blood drinkers (the Vampire Finch pecks Boobies to draw blood to drink, and kicks eggs against rocks to break them), insect eaters and tool users (the Woodpecker Finch breaks off spines from cacti to delve under the wood to find beetles). Interestingly there is actually a fourteenth species of Galapagos Finch, the Cocos Island Finch, which was seeded onto the Cocos Islands from the Galapagos and not from the mainland.

 

While these finches are one of the most striking examples of adaptive evolution, Darwin didn’t actually include them in “The Origin of Species”. The finches all look pretty much the same (we certainly couldn’t spot the difference) and he thought they were just variants of the one species. It was only when he brought his specimens back that a specialist told him they were separate species, and he saw how only evolution could adequately explain their existence. Unfortunately his finch collection was poorly labelled and he wasn’t sure which were from which islands, so he played it safe and left them out of the book, concentrating on the mockingbirds.

 

After watching the tortoises and finches we walked down a hollow lava tube, the long winding tunnel caused by the surface of a lava flow solidifying while the centre flowed on, and then had a cocktail in town before heading back for our first night on the boat.

 

Sunflower300 says:
Great blog. Your photos are wonderful.
Posted on: Dec 01, 2007
mellemel8 says:
GOSH I CAN'T WAIT TO SEE THIS. YOU HAVE AWESOME PICS AS ALWAYS :)
Posted on: Nov 28, 2007
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Galapagos Islands
photo by: Melboorn