Size matters.
Well, it matters when you are a mammal. And if you’re a mammal which has has found itself on an island. Let’s make that clear before we continue.
Islands are different from the continent, aka the mainland. They are smaller in land area, and separated from surrounding landmasses, be it the mainland or adjacent islands, by a certain amount of water. The degree of isolation can take on extreme forms for oceanic islands; in contrast to islands on the continental shelf, which may have been connected to the mainland by dry land at times of low sea level, oceanic islands arose from the sea floor and have always been surrounded by water. These features have a profound effect on island ecosystems and their inhabitants, and the field of island biogeography has inspired naturalists since the days of Charles Darwin and Alfred Russel Wallace.
One of the key tenets of island biogeography is that the number of species on an island is mainly determined by the immigration and extinction of species (although speciation on islands does occur). As both immigration and extinction are strongly affected by island size (larger islands have more habitats so that more species can survive) and degree of isolation (very isolated islands receive less immigrants), islands tend to have fewer species than similarly-sized patches of land on the mainland. Moreover, the subset of species that does manage to successfully colonise an island is not a balanced representation of the mainland ecosystem.
Unless there is a land bridge, you can only reach an island by swimming, flying or floating. Animals like elephants, deer and rats are quite good at swimming, and for many birds and bats, a water barrier does not pose a significant problem. In contrast, bovids, giraffes and many carnivores such as hyenas and big cats are less eager to cross water. Concomitantly, island ecosystems often are unbalanced, meaning that major groups of species, such as mammalian carnivores, can be entirely absent from remote oceanic islands. For example, the only native mammals on New Zealand and the Hawaiian islands are bats.
A newly colonising species therefore finds itself in an environment with few, if any, predators and competitors. For such islands, the evolutionary selection forces that are at play can be very different than those on the mainland, and evolutionary strategies that were necessary on the mainland to avoid being eaten and to find enough food to reproduce may then become obsolete. Island species respond to this ecological release by losing adaptations that were necessary on the mainland, and evolve new ones by adaption to their new island home, which can result in a cascade of changes in behaviour, reproduction, population dynamics and anatomy.
In mammals, one of the most dramatic responses to an insular environment is a change in body size. First noted in 1964, this pattern of body size changes in insular mammals became known as the “Island Rule”. Although it is not a true rule, it describes a trend in which small mammals increase in size (insular gigantism), whereas large mammals will become smaller (insular dwarfism).
Although there are a number of exceptions to this pattern, the trend generally holds true for all living insular mammals, and even seems more pronounced for fossil species. Giant rats can still be found on many southeast Asian islands today, and the Mediterranean islands were once home to the giant “terror shrew” (Deinogalerix), very large pikas (Gymnesicolagus gelaberti) and giant rabbits (Nuralagus rex). Dwarf hippopotami are known from the Mediterranean (for instance, Phanourios minor from Cyprus) as well as Madagascar (Hippopotamus lemerlei), and dwarf proboscideans are an almost ubiquitous element of fossil insular faunas. Remains of dwarf mammoths are known from Wrangel and Santa Rosa Island in the US, whereas dwarf stegodonts, cousins of today’s living elephants, have been found on a number of islands across Asia. Many islands in the Mediterranean were once home to a species of dwarf elephant, with the smallest off them all, Palaeoloxodon falconeri from Sicily, clocking in at a weight of only 2% (yes, you read that right) of its ancestor’s body size. What a time to have been alive!
The main reason behind these changes in body mass was often thought to be the limited resources available on an island; a smaller landmass means fewer available things to eat. Studies based on large data sets of insular mammals found that island area is indeed a large factor in the gigantism of small mammals, but for everything larger than a rat, the absence of predators and decreased competition with competitors are the main drivers behind these changes in body size.
Other factors like climate and isolation do affect body size to a lesser degree, and may account for much of the variation in body size seen between different groups of insular mammals. A new study on fossil insular proboscideans shows that differences in each island’s physical attributes and ecological structure affect the degree of dwarfing in this group. For instance, for species on the mainland and on relatively balanced and species-rich islands, competition with other species resulted in a relatively less dwarfed body size. Proboscideans become even smaller on islands where competitors are lacking.
Furthermore, time seems to play a role, as the more pronounced changes in body size in the fossil record result from a longer time span, as species simply had more time to evolve.
Lomolino, M.V., Sax, D.F., Palombo, M.R. and van der Geer, A.A. 2012. Of mice and mammoths: evaluations of causal explanations for body size evolution in insular mammals. Journal of Biogeography, 39, 842–854.
Van der Geer, A.A.E., van den Bergh, G.D., Lyras, G.A., Prasetyo, U.W., Awe Due, R., Setiyabudi, E., and Drinia, H. 2016. The effect of area and isolation on insular dwarf proboscideans. Journal of Biogeography 10.1111/jbi.12743
