Le nanisme insulaire au Quaternaire

Abstract

Insular dwarfism and Quaternary Mammals. Insular dwarfism appears to be a general evolution rule for large mammals, as the giantism is for the small ones, but the phenomenon concerns some kind of islands only, and only a small number of Mammalian Families. Necessary conditions for one island to develop such an insular fauna rely on : 1. a not too long way off nor a too reduced one from the nearest continent, peopling occurring through temporary landbrige, by «sweepstake » and/or by natural rafts ; coastal as well as oceanic islands are hence excluded. 2. never a too small nor a too large surface ; very large geographically continental masses such as South America, Australia, New Guinea, Malagasy and other succeeded in developping equilibrated faunas when isolated. 3. a biological imbalance testimonied by the scarcity or the absence of predatorious mammals and by a poor biodiversity ; individuals may be very numerous but the number of mammalian species is always very low, usually well inferior to ten. So the dwarfing (and giantism) process is known for the Quaternary Era only in Mediterranean (Balearic Archipelago, Corsica, Sardinia, Maltese Archipelago, Crete, Cyprus, some Aegean Islands), Arctic Ocean (Wrangel Island), Northern Pacific (Northern Chanel Islands in California), Indonesia (Timor, Flores, Sulawesi), Japan and West Indies. Seven Mammalian families only are involved in dwarfing processes, pertaining to three Orders : Proboscidea (Elephantidae, Stegodontidae) , Artiodactyla (Suidae, Hippopotamidae, Cervidae, Bovidae) and Xenarthra (Megalonychidae) . Among these families, four (Elephantidae, Hippopotamidae, Cervidae and Megalonychidae) are more affected than the other, giving a stature reduction up to one third or one fourth of that of the continental ancestor. Families presenting an insular giantism (some among Rodentia and Insectivora) are limited in number in the same way. Other (all these included in Primates, Carnivora, Lagomorpha) are totally indifferent to any insular environment, and do not show any tendency to reduce nor increase their stature. Lastly, some families (like all the Perissodactyla ones) were never encountered in true insular conditions. The cause of insular dwarfism phenomenon is not well known : the most classical explanation rests in the supposed advantage of smaller individuals requiring less feeding facing a limited quantity of available food together with a lack of predators. Such an explanation is clearly insufficient in many cases, and further research is necessary for a better understanding of the actually observed facts.