Giant Galapagos Tortoises Get a Taste for Invasive Plants

Galapagos Tortoise

Story at-a-glance -

  • The Galapagos Islands are facing a problem with more than 750 species of invasive plants now growing there
  • Eradicating the plants is virtually impossible, but the giant tortoises living on the island have started to eat them
  • About half of the tortoises’ diets are made up of invasive species, which may be providing them with an extra nutritional boost

By Dr. Becker

Giant tortoises once lived on most continents around the world, but today they’re found only on the Galapagos Islands and one other locale, Aldabra Atoll, in the Indian Ocean. There they eat grass, leaves, fruit, and cactus plants at their leisure, basking in the sun and sleeping for up to 16 hours a day.

Giant tortoises are not only the world’s largest tortoises (growing up to five feet long and weighing up to 550 pounds); they’re also the longest-lived vertebrate. They average a lifespan of 100 years plus, although the oldest giant tortoise on record lived to be 152.1

Part of their longevity is certainly due to their healthy diet, which researchers recently discovered is remarkably adaptable – and for good reason. Many giant tortoises have taken a liking to invasive species that have populated the island, and it turns out the plants may be giving the tortoises an extra nutritional boost.

Giant Tortoises Feast on Invasive Plants

The Galapagos Islands are facing a problem with more than 750 species of invasive plants now growing there. Eradicating the plants is virtually impossible, but it turns out at least one species has learned to live in harmony with the plants.

Researchers outfitted tortoises with GPS devices to monitor their foraging behavior. The animals migrated between lowlands, which have lush vegetation only during the wet season, and highland meadows, which have lush vegetation all year long.

Considering giant tortoises can go up to one year without food or water, it seemed strange that they would make such a trek rather than simply waiting for a more steady food supply to come to them. Stephen Blake, PhD, an honorary research scientist at Washington University in St. Louis, and study author put it this way:2

"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? Couldn't it just wait out the dry season until better times came with the rains?"

It certainly could, but after tracking the tortoises for four years, it was revealed that they spent more time foraging for non-native plants than they did those native to the island. About half of their overall diets were made up of invasive species.3 Blake explained:4

"Consider it from a tortoise's point of view… 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."

Fortunately in this case, the invasive species appear to be having either a neutral or an overall positive effect on the animals’ diets, perhaps even adding a nutritional boost.

How Did Giant Tortoises Become Endangered?

The tortoises once flourished but soon whalers and fur sealers realized the animals would survive for long periods with no food or water. So they began storing them on their ships during long voyages as a source of fresh meat. The tortoises were also used for their oil to light lamps, and after about 200 years an estimated 100,000 to 200,000 tortoises were lost.

Today only 20,000 to 25,000 wild giant tortoises live on the Galapagos Islands. As noted by the Galapagos Conservancy:5

In addition to their direct exploitation by humans for both food and oil, giant tortoises faced other challenges due to the introduction of exotic species by human visitors.

They suffered — and continue to suffer on some islands — predation on tortoise eggs and hatchlings by rats, pigs, and voracious ants, and competition for food and habitat with goats and other large mammals.

With the establishment of the Galapagos National Park and the Charles Darwin Foundation in 1959, a systematic review of the status of the tortoise populations began.

Only 11 of the 14 originally named populations remained and most of these were endangered if not already on the brink of extinction. The only thing saving several of the populations was the longevity of tortoises, keeping some old adults alive until conservation efforts could save their species.”

Did You Know Some Tortoises Make Noise?

Tortoises aren’t exactly known for their vocalizations, and females of the species actually make none. The males, however, make a bellowing noise similar to a cow mooing while they’re mating. Tortoises also communicate dominance by facing each other with their mouths open and necks outstretched. Whoever has the longest neck “wins” while the loser retreats.6

Also interesting, some tortoise species are incredibly intelligent. Red-footed tortoises, which are native to Central and South America, were taught how to do certain tasks using touch-screen devices – and they learned the tasks faster than dogs (it didn’t hurt that they received fresh strawberries as motivation).

If you have a tortoise as a pet, this is important to keep in mind, as these animals require ample cognitive stimulation when kept in captivity. Giant tortoises, of course, are definitely best off left in the wild, and are beneficial for biodiversity.

When a group of Aldabra giant tortoises were introduced to an island called Ile aux Aigrettes in the Indian Ocean, they ate the fruit of native ebony trees, a species that had been devastated on the island.

The tortoises then helped spread the seeds throughout the island, and the seeds germinated better once they passed through the tortoises’ guts. Even better, like on the Galapagos, the tortoises also feasted on a lot of the non-native plants on the island, representing a clear example of how wild animals can help to restore entire ecosystems.7