Evolution of Island Mammals: Adaption and Extinction of Placental Mammals on Islands

I was spending time with family this weekend, and as the environment was somewhat noisier than usual I had difficulties reading the Handbook. So I decided to engage in some lighter reading, which is where this Wiley-Blackwell publication enters the picture. Along the way I realized it wasn’t actually much lighter reading, but I enjoyed it and so decided to keep going…

I was very uncertain if I should give the book three stars or four on goodreads, and when I finished it yesterday I gave it three stars. I have now changed that rating to four stars. It is a fascinating book, but it is a bit dry at times. It probably deserves two posts, but I couldn’t be bothered to write more than one. I watched this Gresham College lecture a while back, and that lecture provided part of the motivation for reading the book. Wilson touched upon some of the themes covered in the book as well – his name does come up, naturally. Still, it should be emphasized that there was a lot of stuff covered in this book which I’d never really thought about and about which I knew nothing, and reading books which deal with such things is always nice.

Say you have an island formed in the middle of the ocean. There’s water all around it, perhaps lots of water, and it’s always been far away from continents and other islands. Which kinds of animals turn up there, how many different kinds, and how do they get there? Does the size of the island matter, and how does it matter? What usually happens after a new visitor has become established on an island?

Animals don’t just pop up and start hanging around – we are used to there being so many species around us that it would be easy to forget that it’s not necessarily the natural state of affairs that there are hundreds of species of animals all around you. On an isolated island which literally rose from the ocean animals have to somehow establish themselves before they can start a life there, and until they get there and find mates and enough food to survive, the island will be rather boring from the biased perspective of living organisms such as ourselves. Some animals are better colonizers than others, and that goes for some species/genera/families of mammals as well. Not all islands incidentally rise from the ocean; some get separated from the continent from which they originated, perhaps due to a combination of plate tectonics and/or eustatic sea level rise. In biology islands in a more general sense, understood as habitats where some types of organisms are isolated from their conspecifics, play an important role in speciation processes, so there are many reasons why one might be interested to learn more about such things – though it should be pointed out here that this book only deals with ‘proper’ islands, i.e. the kinds with water around them and so on.

Some islands display a great deal of species diversity, whereas a lot of others have a very unbalanced and impoverished fauna. By impoverished, I mean really impoverished – it’s amazing how few types of mammals sometimes made it to a specific island and got themselves established before humans started messing around with stuff. Or perhaps the amazing thing is rather that any of them did at all? I don’t know. To take an example of an impoverished island fauna, Cyprus is a good illustrative example. The only mammals on Cyprus (that we know about) during the Pleistocene were pygmy hippopotami, dwarf elephants, and perhaps bats. No cats, no dogs, no mice or rats, no goats, no pigs, no bovids, no deer, no nothing. When I set out reading this book I had in my mind some not-particularly-well-thought-out ideas about which sorts of animals normally hang around in the environment, and sometimes reading books can really mess with such ‘ideas you were not even aware that you had’; the fact of the matter is that in the case of some specific islands every single mammal species you’ll encounter on that isolated island (which incidentally today may not actually seem particularly isolated on account of technology, human transport methods etc.) will have some unique and probably quite fascinating story, explaining how it got there – and members of species which don’t have such a story to tell simply aren’t/weren’t around at all. Of course many of the island species mentioned can’t tell their story either anymore because they’ve gone extinct – again Cyprus provides an example. We talked about Pleistocene – well, at the onset of the Holocene a few more species were added (a genet, a mouse, and some fruitbats). They did okay and nothing much changed. But then later on the island stasis was interrupted, presumably because of human agency, and all the mammals that had established themselves before that time went extinct. A wonderful story that makes you proud to be a human.

As mentioned rather than being isolated species-poor places, some island groups have large species diversity; the West Indies is an example of that type. The book deals with a lot of different islands, and although there are some common patterns and trends there is also a lot of variation – and the variation seems to be reasonably well understood in most cases, although lack of evidence sometimes make things a bit harder to figure out than one would like them to be. A thing I feel compelled to note and emphasize is that an impoverished fauna does not an uninteresting fauna make: Some awesome animals have inhabited various islands around the world, and I’ve occasionally been saddened to read a chapter because of the realization of what has been lost (after having been first greatly fascinated by what was actually there). I’ve probably have had in the past a tendency to think of mammals as the bad guys in the story of islands because of how anthropogenically introduced invasive species such as rats and rabbits have wreaked havoc around the world, but birds aren’t the only types of animals that colonize islands and lots of mammal species around the world have been natural elements of island ecosystems for millions of years. To get back to the awesome animals and our example of Cyprus from above, the pygmy hippopotamus which used to live there is estimated to have been about 125 cm long, with a shoulder height of about 70 cm. The dwarf elephant that lived there was a bit larger; it’s estimated to have had a shoulder height of 1.40 metres. These animals were actually much larger than the pygmy elephants which inhabited nearby Sicily roughly 450.000 years ago, which had female shoulder heights of about 0.9 metres and male shoulder heights of about 1,3 metres, and an estimated body weight of around 100 kg. Yep. You’re probably used to think of mammoths as huge creatures (I was), and in that case you may be interested to know that an estimate of the size of the Cretan pygmy mammoth (…yes, there were mammoths living on Crete! I know! I had no idea either!) is 1.5 meters. Proboscideans were incidentally quite successful island colonizers, showing up all over the world:

“Endemic proboscideans have been reported from islands all over the world. Apparently, proboscideans were the most successful lineage of large-sized island colonizers, ranging from stegodons and mastodons to mammoths and elephants. Everywhere they developed a smaller size, eventually reaching dwarf or even pygmy proportions compared with their mainland ancestors. It is hard to think of an island with a rich fossil record lacking any proboscidean remains. Exceptions are the Balearics, Gargano, the central Ryukyu Islands, and the West Indies.”

Again, I had no clue! Mammoths used to live as far apart as Crete and Japan… Lions and hyenas used to live on Sicily, and tigers used to inhabit Japan. Incidentally proboscideans got smaller on islands, but many small mammals have tended to grow in size instead once they established themselves on isolated islands. On Minorca there used to live giant hares weighing 14 kg, and giant rats still inhabit the island of Flores; these guys are up to 45 centimeters long, with tails up to 70 centimeters. Body size is not the only thing that tends to change on islands; a stockier build (shorter limbs, stiffer joints), and changes in dentition also often happens, but body size changes are certainly noteworthy. Note that size changes don’t just mean that animals will get smaller or larger; greater size variation on islands is common due to adaptive radiation. You’ll have both small and large animals of a similar species, at least to start with – if the process is allowed to continue for millions of years you may easily end up with multiple different species derived from the same ancestor. Lemurs are a good example of where you may end up. Life is different on islands. Generally intraspecific competition tend to increase and interspecific competition tend to decrease (as a general rule carnivores are much less likely to become established on islands than are herbivores), but interspecific competition is still very important; the existence or absence of competing taxa can have major effects on the course of evolution. For example the Sicilian pygmy elephants developed during a time period where there were no deer present on the island; during the time where there were deer on Sicily, proboscideans reached dwarf proportions but never went smaller than that (in the book they apply the pygmy label to species half the ancestral size or less, and the term dwarf for forms which are 80–60% of the ancestral body size).

The book has three parts. The first part is called ‘Beyond the mainland’, and aside from a few introductory remarks it briefly talks about the history of island studies and then moves on to talk about ‘how islands are different’ from the mainland and how this affects the fauna. Part two is the main part of the book, and it deals with how mammal species have lived, died, evolved etc. on many different islands around the world; each island is to some extent unique, so each chapter deals with one island. The islands included in the coverage are: Cyprus, Crete, Gargano, Sicily, Malta, Sardinia and Corsica, The Balearic Islands, Madagascar, Java, Flores, Sulawesi, The Philippines, Japan, the Southern and Central Ryukyu Islands (the northern islands are included in the coverage of Japan), the Californian Channel Islands, and the West Indies. So the book does give a reasonably global view of island mammal life, although there are many chapters dealing with the Mediterranean. The book deals with evolutionary biology, but in order to tackle this topic you also need to deal with other areas such as geology. For example the Mediterranean looked very different 5-6 million years ago, and such changes have had huge consequences for how species adapted and evolved to their local environments. Some of the most fascinating stuff in this book in my mind related to how different the world used to look like, even if you don’t go back all that far. Japan used to be part of the mainland, Crete was a submerged mountain chain at the bottom of the ocean a while back, and the island of Flores emerged above the sea during the Early Miocene. Roughly 2 million years ago much of the current island of Java was at the bottom of the ocean, but on the other hand 800.000 years ago the island, as well as Sumatra and Borneo, may well have been connected with the mainland due to changes in global sea levels. Madagascar and India stuck together when they left Africa a long time ago, but they picked different routes and ended up different places, with different fauna. The chapters in this part of the book also provides some history of how we came to know what we know, who were some of the key people involved in figuring these things out, and so on, which is often rather interesting. In a way you sort of have to deal with such matters to some extent e.g. because the taxonomy is sometimes a bit messy when analyzing island data. Lumpers and splitters will disagree violently about the number of species, and understanding how the people who first looked at the bones arrived at the conclusions they did is often interesting.

Part three of the book deals with ‘Species and Processes’. After dealing with all the various islands an attempt is made here to sum up what we’ve learned from the coverage; first by taking a closer look at the species we’ve encountered along the way, then towards the end of the book by adding some general remarks and observations. The species covered in this section are naturally the species which have engaged in most island colonization activities. I was a little frustrated along the way while reading the book that few general principles were formulated, and that the coverage focused a great deal on ‘the specifics’ of the island in question, but this part sort of partly makes up for it and some people would surely argue that the method applied is perfectly justified. The species covered in the chapters in this part of the book are: ‘Elephants, Mammoths, Stegodons and Mastodons’, ‘Rabbits, Hares and Pikas’, ‘Rats, Dormice, Hamsters, Caviomorphs and other Rodents’, ‘Insectivores and Bats’, ‘Cervids and Bovids’, ‘Hippopotamuses and Pigs’, and ‘Carnivores’. The last two chapters of the book deal with some general ‘Patterns and Trends’ as well as ‘Evolutionary Processes in Island Environments’; I’ve covered some of that stuff above already.

Overall I really liked the book. Aside from the small problems such as ‘too specific, a few too many remarks about Cuvier‘, the only problem I had with the book was that there was very little focus on mammalian interactions with non-mammals on the islands. But given the focus of the book this was perhaps only to be expected. It’s a nice book and I enjoyed reading it.