Abstract
Background Islands offer a classic topic for evolutionary studies. Few other subjects have historically raised as much fascination as the island large mammals that having evolved into dwarfs. Consensus has been gained that multivariate ecological causes are behind those changes, but what remains largely unexplored are the processes involved. Recent studies focused on associated change of body size and life history (age of reproduction, growth rate, longevity) – a key to understand the process of insular dwarfism. The Japanese Archipelago offers worldwide a unique natural experiment, as in its numerous islands of different sizes the same group of organisms, deer, have evolved into different sizes. We investigated eight deer populations exhibiting body size variation, both extant and fossil, to clarify the effect of insularity on life history traits.
Results We applied several methods to both extant and extinct populations for resolving life history change among deer populations. Skeletochronology using lines of arrested growth successfully reconstructed body growth curves and revealed a gradual change in the growth trajectories reflecting degree of insularity: slower growth with prolonged growth periods in more isolated deer populations. An extensive examination of bone microstructure further corroborated it, and clearly indicated much slower growth and later somatic maturity in fossil insular deer, which had been isolated for more than 1.5 Myr. Finally, mortality patterns revealed by demographic analysis varied among deer populations and life history of insular populations shifted toward “slow-life” of K-strategists.
Conclusion We clarified the evolutionary and ecological process behind insular dwarfism, which occurred in conjunction with life history shifts. The process initiated with phenotypic plasticity responding to the resource-limitation on predator-free islands after the settlements of islanders. Under the insular environments, natural selection favored K-selected animals and the life history traits were genetically fixed. Extreme K-strategy found in fossil dwarfs on islands would make them vulnerable to anthropogenic changes, that would explain termination of insular dwarfs after human arrival on islands.
Competing Interest Statement
The authors have declared no competing interest.
