THE Book of the Week: The Last Tortoise: A Tale of Extinction in our Lifetime
Eleanor Sterling discusses a species' rapid decline at our hands ... and what we can do about it
In The Tortoise and the Hare, a tale attributed to Aesop, the unhurried tortoise prevails in a race with the speedy hare, the moral of the story being that slow and steady wins the race. In real life, tortoises not only move slowly, but also possess a suite of ecological strategies geared towards efficiency and longevity. Individuals of some species can live for 150 years, far longer than most species with backbones; even the smallest species has individuals who can live into their fifties. Many cultures revere tortoises for this very trait.
Tortoises begin reproducing late in their lives (sometimes not until their thirties, for instance), but then can steadily lay eggs for decades thereafter, defying the normal trend in other vertebrate species towards senescence and infertility late in life. Remarkably, in some species, ancient female tortoises are as fertile as - and may even produce more offspring than - young tortoises.
Why and how they are able to stave off reproductive decline is a mystery gerontologists would love to solve. However, we may never know, as the same strategies that have helped tortoises to survive for more than 200 million years may lead to their demise at the hands of the ultimate predator - humans.
In The Last Tortoise, biological anthropologist Craig Stanford passionately lays out the reasons why tortoises may face extinction in the near future and why we should care. After pointing out the differences between tortoises and their turtle cousins, Stanford notes that we know far more about the latter than the former. This book goes a long way towards filling the information gap, tracing the history of tortoises from deep time to the present.
The ancient lineage encompassing tortoise and turtle species developed a unique protection against predators of all sizes and capabilities by relocating the internal skeleton to fuse with a bony exterior, encasing its soft body parts in an impervious shield.
Variations on this blueprint range from tiny tortoises that fit easily in the palm of a human hand - what the author describes as a "tennis ball with legs" - to gargantuan island-dwelling species that can weigh up to 225kg. Stanford notes frequently that more than 40 tortoise species inhabit the Earth today, although there seems to be some uncertainty about the number of subspecies, reflected by the varying numbers reported in the book. However, they all share a laudable ability to adapt.
Tortoises can thrive in some of the most inhospitable climates on Earth - from rocky landscapes to dry grassland, or desert climes where annual rainfall is measured by droplets - because their metabolisms allow them to go for months without a constant food source. This feature caught the attention of buccaneers and whalers, who gathered giant tortoises by the tonne to store in their ships' holds and thus provide fresh meat for months with little or no maintenance.
Stanford describes in meticulous detail this and other threats that humans pose to the future of tortoises. Humans cherish tortoises as food, toys and pets to the detriment of existing tortoise populations. The statistics are sobering. More than 10 million tortoises and turtles change hands in Asia annually; in China alone, this amounts to $700 million (£464 million) each year. The black market in some species of rare tortoise rivals that of illegal narcotics. A single individual of the gorgeous radiated tortoise from Madagascar can sell for $5,000.
Tortoise populations are further hit by changes in their habitat, including development that bulldozes their burrows and roads that isolate populations so that individuals cannot find mates and contribute directly to losses through road kills. In island systems such as the Galapagos Archipelago, tortoises face further blows from introduced species that out-compete with them for food, such as goats, or that directly eat their young, such as rats.
Tortoises, with their late arrival at sexual maturity and their low annual reproductive output, are not able to overcome these increasing losses to their populations; hence Stanford's concern that they may be the next vertebrate group to become extinct.
Stanford writes in an engaging, storytelling style that speaks of his passion for the topic and his personal experiences both as a young naturalist and a seasoned biologist. He details the importance of tortoises to the various ecosystems they inhabit and builds a case for our need to be concerned about their declining population sizes, both from the standpoint of tortoise species and whole ecosystems.
Across the globe, from the Caribbean to the Indian Ocean, we have clear evidence for the extinction of tortoise species, particularly giant tortoises in island systems, subsequent to human colonisation of those areas. Hunting and wildlife trade have driven tortoise populations across Asia to the bare minimum, if not the brink of extinction. It is clear that if we do not undertake comprehensive management strategies for these species, we may well lose the majority of them in the near future.
From a moral standpoint, it is concerning that humans may be driving other species to extinction, but there are greater concerns, too, based on the ecological role that tortoises play. They can modify their environment through grazing and browsing and may serve as important predators as well as seed-dispersal agents. In fact, tortoises, particularly the giant tortoises currently found only in the Galapagos and Aldabra in the Seychelles, may serve as engineers of whole ecosystems, modifying the reproductive strategies, outputs and physical structure of their food sources and thus affecting the habitat and food for other, often endangered, species.
While the situation tortoise species face is precarious, Stanford presents some good-news stories from various sites around the world. Captive breeding efforts for different species of giant Galapagos tortoise, for instance, result in the reintroduction of species to islands from which they were extirpated. The involvement of local communities in conservation efforts and the careful development of ecotourism strategies may help in some cases to address the problems leading to population decline.
One strategy that Stanford champions - introducing giant tortoises into less remote areas where other giant tortoise species have become extinct in order to promote ecotourism - could dilute the current ecotourism programmes. Furthermore, it is not certain that these efforts would help the less gargantuan tortoise species. Finally, given that the systems have changed in the absence of these giant grazers, their introduction could affect whole ecosystems in unpredictable ways.
Even in the Galapagos, there is concern that the reintroduction of giant tortoises to islands where they only recently became extinct will affect endangered cactus populations that have flourished without these grazers.
Nevertheless, Stanford's main point is that we are the major cause of tortoise population decline and are just as capable of reversing these trends and contributing to their conservation.
Early in the book, he notes that the discovery of an etched shell in western China may be the first documented evidence of writing by humans. Given the importance of tortoises to our cultural history, as well as to healthy ecosystems across the globe, we must be better stewards of the species and their habitats in order to avoid being the writers of their epitaph. This book helps to lay the groundwork.
A terse note from chimpanzee expert Jane Goodall launched Craig Stanford's career. After studying for a BA in anthropology and zoology at Drew University, an MA in anthropology at Rutgers University and a PhD in anthropology at the University of California, Berkeley, Stanford wrote to many people looking for a job. Only Goodall replied, telling him that if he wanted to be a guinea pig at her sanctuary in Gombe, that would be up to him. He took Goodall up on her offer and since then has conducted extensive field research on wild great apes, monkeys and other animals. His work has often focused on the ecological relationships among the primate species sharing a tropical forest ecosystem. He is currently professor of anthropology and biological sciences at the University of Southern California, where he has taught since 1992.
The Last Tortoise: A Tale of Extinction in our Lifetime
By Craig B. Stanford
Harvard University Press
Published 26 May 2010
Eleanor Sterling is director of the Center for Biodiversity and Conservation, the American Museum of Natural History.