Tuesday, February 8, 2011

Galapagos Tortoises and Evolution

Prior to the European presence in the islands, there were an estimated 400,000 plus tortoises.  Today approximately 20,000 remain.  The naturalists tell us that there were 14 distinct species of tortoises, and today there are eleven species remaining (I see the Wikipedia write up would not exactly agree with these numbers).  Interestingly, the identity of the island a species has evolved on can be ascertained by the shape of the tortoise shell.  Tortoises became so plentiful because they had few natural predators and so large because the larger ones could dominate the smaller tortoises and increased size helps in water and fat retention.

Whaling expeditions and pirates were two of the culprits for decimating the population.  The tortoises were used as food for the seafarers, who would stack them upside down alive in their ships, and kill/eat them as necessary. The tortoises were reputed to have made excellent soup.  Charles Darwin ate tortoise.  

Beyond the sailors and whalers, the fragility of the island’s ecosystem can be highlighted by two items.
 1.- The Galapagos Tomato seed will not germinate unless passed through the digestive system of a tortoise. How specialized is that?
2. - Female tortoises must migrate to the arid and warmer lowlands to lay their eggs. The temperature of the eggs is crucial in their development. Below 28 degrees Celsius (82.5 degrees Fahrenheit), only males will be hatched: If the temperature is above 29.5 degrees Celsius (85 degrees Fahrenheit) only females are hatched.

Tortoises are thought to have forced the development of the Giant Galapagos Cacti.  As the tortoises evolved so they could reach higher to eat the cactus pads (with the tortoises having longer legs, necks and shells that were not so restrictive), the cacti evolved to grow higher and higher, attempting to thwart the nibbling.  An evolutionary arms race.

Helping the tortoises survive is being done with the aid of tortoise farms (see pics).  Additionally, park rangers are removing rats, feral cats, goats and introduced animals that were eating the tortoise eggs and young and destroying the habitat necessary for the tortoises. And of course, today tortoises are not being eaten by mankind!
Private tortoise farm that caters to tourists on Santa Cruz Island. Tortoises have free range.

We are not supposed to get too close.

Well, I am closer , but he cannot see me.

Charles Darwin Research Station - Santa Cruz Island.  These small tortoises are two years old.
National Park tortoise farm - San Cristobal Island. These tortoises have limited range and are fed three times a week. Our timing was good.

I'm hungry and don't want the others to get everything!

Would you quit bothering me!
Love the salad!

How am I doing?

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