Abstract

The island rule describes a graded trend in insular populations of vertebrates from gigantism in small species to dwarfism in large species. The dwarfing of large mammals on islands has been observed both in the present fauna and in the fossil record. Elephants, hippopotami, deer, and other species became dwarfed on islands scattered all over the world, from the Mediterranean Sea to Indonesia, from the Eastern to Western Pacific Ocean, from the Caribbean to Canary Islands. The most rapid and well documented cases of island dwarfing known thus far took place over thousands of years. Here, we describe a rapid example of dwarfing of a large mammal - the feral cattle of Amsterdam Island, southern Indian Ocean, which dwarfed to about three quarters of its body size in slightly more than one century. This population provides us with a rare opportunity to assess the rapidity of demographic, life history, and morphological responses of large mammals to a very isolated and ecologically simple, insular environment.

Highlights

  • These sometimes remarkable shifts in morphological, physiological, behavioral, and ecological characteristics of insular populations of mammals may be the products of the ecological simplicity of the islands they inhabit[3, 4]

  • We describe the case of the feral cattle (Bos primigenius taurus) of Amsterdam Island (South Indian Ocean), which provides us with a rare opportunity to assess the rapidity of demographic, life history, and evolutionary responses of large mammals to the ecological simplicity of a very isolated, insular environment

  • Results of one-sample t-tests confirm a significant body size reduction of Amsterdam Island males and females with respect to average reference values obtained for the ancestral population

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Summary

Introduction

These sometimes remarkable shifts in morphological, physiological, behavioral, and ecological characteristics of insular populations of mammals may be the products of the ecological simplicity of the islands they inhabit[3, 4]. The descendants of a founding population may undergo rapid increases in their populations, quickly followed by expansions into habitats, diets, and other ecological strategies considered atypical for the species on the mainland (invading niches left vacant in the absence of their mammalian competitors and predators)[8] Following these early phases of the island syndrome, which may occur within just a few generations, descendants of these initial populations may undergo phyletic evolution which allows them to progressively adapt to their new niches and insular environments[8, 10]. While the great wealth of evidence on body size evolution in large mammals is derived from palaeontological studies, anthropogenic introductions of mammals to islands in recent history can provide key insights into the rate and nature of the island rule and, more generally, the island syndrome Such introductions can serve as valuable manipulative experiments on the ecology and evolution of isolated biotas. At the time of introduction of the cattle, its maximum elevation was 881 m above sea level and the island habitat ranged from Phylica nitida trees and grasses in the lowlands to dwarf shrubs (Acaena magellanica), sphagum bogs and mosses along the higher elevations[12]

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