Black Rats: Deadly for Galapagos tortoises

The giant Galapagos tortoise, a symbol of the islands Photos by Sophia Caride

They go for the young And they're hard to catch

SANTA CRUZ, Galapagos Islands, Ecuador – The National Park rangers in khaki shorts and shirts were looking for nests of the endangered giant Galapagos tortoise in a patch of sunburned scrub brush and cacti.

Their job was to gather up tortoise eggs and take them back to the Charles Darwin Research Station here for protection and hatching.  

They heard faint scratching and nibbling sounds that day and they knew they were too late for at least one nest. A black Galapagos rat was ahead of them, clawing at a nest of half a dozen or so eggs.

The rat was gone when the rangers got there. One egg was broken, but mainly intact. The rangers took the damaged egg to the Darwin Station and two months later a baby tortoise hatched.

Giant tortoise eggs and baby tortoises are a favorite meal of the ferocious black rats of the Galapagos Islands. The black, furry rats began arriving hundreds of years ago with pirates and explorers. The rats, often a foot long with a thin whip-like tail, are a constant threat to tortoise eggs and newborn tortoises and to the eggs and young of marine iguanas, sea turtles, blue-footed boobies and other birds.

“Rats are a problem,” said Roslyn Cameron, a development officer at the Galapagos Conservancy, a non-profit organization focused on the islands. “They will always be a problem.”

The giant tortoise is one of the Galapagos’ best-known creatures. The tortoises can weigh up to 200 pounds and live for more than 100 years. They are featured in advertising for the islands and they are high on tourists’ lists of things to see.

But the number of tortoises has declined sharply over the centuries. Pirates, explorers, whalers and people who settled the forbidding volcanic islands hunted the tortoises. One buccaneer wrote that they were “excellent good food.”   Researchers say the rats have had a dramatic impact on the decline of the tortoises. They are harmless to adult tortoises, but devastating to young tortoises and tortoise eggs. The relentless attacks on tortoise eggs and newborn are one reason, scientists say, that restoration efforts have not been moving faster.

By the 1960s scientists estimate that there were no more than 20,000 giant tortoises on the islands, down from perhaps 500,000 when Charles Darwin arrived on Sept. 17,1835 and made discoveries that heavily contributed to his theories of evolution. Once there were 15 species of giant tortoises. Now there are 11. Four have been declared extinct. The tortoises are quiet animals. Their most distinctive sounds are their throaty breathing and their crunching of grass and fruit, their favorite food.   

As a result of restoration efforts by the Darwin Station and the National Park, the tortoise population has been rising gradually. Scientists estimate that there are perhaps 50,000 tortoises in the Galapagos now. They are listed as vulnerable, or one step below endangered, by the International Union for Conservation of Nature, a global environmental organization in Gland, a suburb of Geneva.

One of the objectives of the National Park is to increase the population of tortoises and get them off the vulnerable list.

The rats are like phantoms, rarely seen. But wildlife experts say there are lots of them lurking around, usually in ponds and in scrub brush throughout the islands.

“They are hard to find, catch and eradicate,” said Javier Castillo, who has been a National Park Ranger for 15 years.

Castillo and the other rangers use small rectangular wire cages to trap the rats. They hunt them with rifles and handguns and blowtorches. They put poison pellets in the traps and they scatter the pellets and spray the poison in places where the rats hang out.

But the rats are wily. Christian Sevilla, a coordinator for the National Park Service, said it can take weeks to trap one. The rangers check their traps regularly. But mostly, they say, the rats get away.

In 2012, the park began using a small, blue poison cube made by Bell Laboratories, in Holmdel, N.J. The poison gives off an aroma that the black rats love, but that is repulsive to other animals.
The poison was used most heavily on Pinzon Island, an outcropping of about seven square miles of volcanic rock, 22.9 miles west of Santa Cruz. As a precaution, the park took off the island 34 Galapagos Hawks and 40 marine iguanas while the poison was in use. Authorities in the National Park say the poison killed nearly 180 million black rats on Pinzon. The park has used the blue poison on four more of the dozen or so main islands that make up the Galapagos archipelago – all uninhabited by people. 

The pale white tortoise eggs, about the size of chicken’s eggs, are easy to find, the rangers say. Female tortoises dig shallow, dirt pits, lay half a dozen eggs, cover them with dirt and leave. Black rats look for the disturbed earth and go after the eggs. Sometimes the rats arrive as the eggs are hatching and attack the wobbly-legged baby tortoises as they take their first steps.

The baby tortoises are no larger than a man’s hand. They have fragile skin. For the first four years of their lives they are at the mercy of the rats.

The Charles Darwin Research Station opened in 1959 with the restoration of the giant Galapagos tortoise as its main goal. Nine years later, the National Park took control of the breeding center and now oversees it.

Tourists flock to the center. They see cages with baby tortoises with their ages painted on their shells. They see weathered wooden and rough stone corrals with bigger tortoises and land iguanas. Rangers talk with visitors about the life-cycle of tortoises and the work of restoring the tortoise population.

Rangers and scientists collect tortoise eggs throughout the islands during the nesting season from July through November. Then scientists put the eggs into incubators for hatching. Just as in nature, a few degrees difference in temperature determines gender. A temperature of 29.5 Celsius or 85.1 Fahrenheit produces a female.  Males are born at 28 degrees Celsius or 82.4 degrees Fahrenheit.  Seventy percent of the tortoises produced at the center are female, 30 percent are male.

When tortoises at the center are four years old and able to defend themselves, the rangers tag them and set them free on their native islands.

The giant Galapagos tortoises live on six islands:  Santa Cruz, Santiago, San Cristóbal, Pinzón, Española and Isabela. Santa Cruz has the most tortoises at an estimated 2,500. Pinzón has the fewest with about 600.

The authorities operate two other tortoise breeding centers, one on San Cristobal Island, the other on Isabela Island.

“They have the captive breeding program because they can’t breed in the wild,” said Cameron, the Galapagos Conservancy representative.

The giant tortoises are a treasure for mankind.  As tourism has grown in the islands to more than 200,000 visitors annually, the survival of the tortoise has become a pocket-book issue of sorts for the roughly 35,000 people who live in the Galapagos. Seventy percent of island earnings come from tourism.

“Everything economic in the Galapagos depends on tourism,” said Sevilla, the National Park Service coordinator. “If we don’t take care of our native animals, there will be no tourism and no means of economy.”

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