Oct 17, 2018 | Updated: 04:34 PM EDT

The Energy for a New Crawl—Galápagos Tortoises Feast on Invasive Plants for a New Foraging Technique

Apr 07, 2015 03:35 PM EDT

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When it comes to invasive flora, most conservation ecologists know that the ramifications that come with these primary producers often appear much higher in the food chain. Some animals are unequipped to utilize the plants for food, while others simply find the energy spent foraging for it is far too much for the energy gained. And it's a dynamic that often leads some species to coevolve. But looking into one of the first Darwinian subjects, Galápagos Giant Tortoises, some researchers have found that the unique species may be getting far more out of the invasive flora than they once thought-enough to even change their foraging behavior altogether.

Weighing in excess of 500 pounds, the Galápagos' Giant Tortoises are peculiar creatures that, to date, have revealed that not only are they not active foragers, but that they can even survive for up to a year without food or drink. It's an impressive feat, and an adaptation that has allowed them to survive on the archipelago for quite a long time. But when researching a pair of the tortoises on the island of Santa Cruz, researchers were stunned to find that the tortoises there would trek up and down an extinct volcano now populated with humans in search of vegetation. And the question that arose was: why? If the species could live without food for such a long time, then why would they be seeking out food year-round.

"Why would a 500-pound animal that can fast for a year and that carries a heavy shell, haul itself up and down a volcano in search of food?" lead author of the study published this month in the journal Biotropica, Stephen Blake says. "Couldn't it just wait out the dry season until better times came with the rains?"

Together with Blake, a researcher with Washington University, the pair of tortoises were followed and studied by Fredy Cabrera of the Charles Darwin Foundation in the Galápagos. The researchers outfitted the giant tortoises with GPS tracking devices to track their movements throughout the year, and after four years in search of an answer, they finally came to the conclusion that the slow-moving tortoises spent more time foraging non-native plant species which forced them to change their travel plans quite a bit. With their diets now being comprised of more than half non-native flora, the tortoises are making an advantageous choice in seeking out fruits and greens that are higher in sugar content in that they will give the tortoises a richer diet, packed with more energy.

"Consider it from a tortoise's point of view" Blake says. "The native guava, for example, produces small fruits containing large seeds and a small amount of relatively bitter pulp in a thick skin. The introduced guava is large and contains abundant sweet pulp in a thin, pliable skin."

So the tortoises are searching out the invasive alternatives instead. But it's not all sugary sweet treats for the tortoises. The foraging process also takes its toll, and requires a lot of energy as well. But Blake and Cabrera are hopeful that the species is adapting well, in spite of failed conservation efforts that have allowed 750 species of invasive plants onto the island. After consulting with a veterinarian, it appears that the giant tortoises' new foraging behavior may have a new positive effect for the animals, and may even help them stay even healthier during the dry months.

"Fortunately, tortoise conservation seems to be compatible with the presence of some introduced species." 

Lucky them, they like guava too!

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