The Galápagos tortoise (Chelonoidis nigra) is the largest living species of tortoise and tenth heaviest living reptile, reaching weights of over eight hundred and eighty pounds and lengths of over five feet nine inches. With life spans in the wild of over one hundred years, it is one of the longest-lived vertebrates. A captive individual has lived at least one hundred and seventy years.

The tortoise is native to seven of the Galápagos Islands, named them after the Spanish galápago, meaning tortoise.

Shell size and shape vary between populations. On islands with humid highlands, the tortoises are larger, with domed shells and short necks - on islands with dry lowlands, the tortoises are smaller, with "saddleback" shells and long necks. Charles Darwin's observations of these differences on the second voyage of the Beagle in 1835, contributed to the development of his theory of evolution.

Tortoise numbers declined from over a quarter of a million in the 16th century to a low of around three thousand in the 1970’s. This decline was caused by exploitation of the species for meat and oil, habitat clearance for agriculture and introduction of non-native animals to the islands, such as rats, goats and pigs. Ten subspecies of the original fifteen survive in the wild; an eleventh subspecies (Chelonoids nigra abingdoni) had only a single known living individual, kept in captivity and nicknamed Lonesome George until his death in June 2012. Conservation efforts, beginning in the twentieth century, have resulted in thousands of captive-bred juveniles being released onto their ancestral home islands, and it is estimated that the total number of the species exceeded nineteen thousand at the start of the twenty-first century. Despite this rebound, the species as a whole is classified as "vulnerable" by the International Union for Conservation of Nature (IUCN).

The Galápagos giant tortoise is now strictly protected and is listed on Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora.

   

 

Egg-laying: Females journey up to several kilometres in July to November to reach nesting areas of dry, sandy coast. Nest digging is a tiring and elaborate task which may take the female several hours a day over many days to complete. It is carried out blindly using only the hind legs to dig a foot deep cylindrical hole, in which the tortoise then lays up to sixteen spherical, hard-shelled eggs ranging from nearly three to five and a half ounces in mass, and the size of a billiard ball. Some observations suggest that the average clutch size for domed populations (nine point six per clutch for porteri on Santa Cruz) is larger than that of saddlebacks (four point six per clutch for duncanensis on Pinzón). The female makes a muddy plug for the nest hole out of soil mixed with urine, seals the nest by pressing down firmly with her plastron, and leaves them to be incubated by the sun. Females may lay one to four clutches per season. Temperature plays a role in the sex of the hatchlings, with lower temperature nests producing more males and higher temperature nests producing more females. This is related closely to incubation time, since clutches laid early will incubate during the cool season and have longer incubation periods (producing more males), while eggs that are laid later incubate for a shorter period in the hot season (producing more females).