One of the most fascinating and repeatable patterns in evolution is the dramatic change in body size that accompanies island colonization. Large mammals, such as elephants, tend to become smaller on islands, whereas small mammals, such as mice, tend to become bigger. Examples of this phenomenon also come from human evolution, including an extinct hominid species with a height of 1m that lived on an Indonesian island. As one of only a few empirical generalities that characterize the evolution of natural populations, this “island rule” has inspired intensive study and debate among biologists for more than 100 years. Understanding the evolutionary causes of this provocative trend requires the identification of genetic changes that underlie these size differences. Remarkably, virtually nothing is known about the genetic basis of changes in the body sizes of island mammals.
We have found a system that holds remarkable promise for understanding the evolution of body size on islands. Gough Island, located in the South Atlantic about 3000 km southwest of South Africa, is home to the largest wild house mice in the world. These animals are about twice the mass of wild mice in the United Kingdom. Body size evolution has been rapid: mice were recently introduced to the island (in the 19th century) and a major size increase was observed over just a 40-year period. The drastic size change of Gough Island mice has been attributed to low temperature, extended longevity, reduced predation, and carnivorous eating habits. In a highly unusual behavior, these mice cooperatively feed on live and dead chicks of severely endangered albatrosses. In addition to contributing to increased body size, this dietary shift links evolutionary understanding of the Gough Island mice to crucial avian conservation concerns and policy decisions.
Giant Gough Island mouse feeding on the carcass of a Tristan albatross. Photo taken by Ross Wanless, available here.
The Gough Island mice are members of the same species as the classical inbred mouse strains used as model systems for biomedical research. As a result, a wealth of genetic resources is available, including a complete genome sequence, thousands of molecular markers, mutant lines with exceptional body sizes, and transgenic technologies. In collaboration with Dr. Peter Ryan (University of Cape Town), we are using these tools to dissect the evolution of large body size on Gough Island. We are currently reconstructing the colonization history of Gough Island mice.
In addition to dissecting the genetics of island gigantism for the first time in any animal, this research has important implications for human health. Genes known to influence body size in mice are strong candidates for size-related diseases, including obesity, in humans. The rapid evolution of body size on islands provides immense power to find genes of major effect using natural variation.