Artwork by John Megahan / Wikimedia Commons, CC BY-ND
Isolated and Adrift from the Rest of the World, Evolution on Islands Often Takes Strange Turns
Sloth Lemurs and Fake Cats
From about 335 mya (million years ago) until 175 mya all the land on Earth was squashed together, forming the super-continent Pangea. This sucked for the planet’s lifeforms for several reasons. Not only was the continent’s interior an enormous, arid, waste, due to rain clouds being unable to penetrate so far inland, the lack of mid-depth coastal regions, thanks to all the land being in one place, also reduced the amount of habitable reefs; meaning the ocean was similarly barren. However, there was another, more subtle, way Pangaea’s over-sized booty screwed with Earth’s biodiversity: there were no islands.
Why are islands important to life on Earth? Well, in addition to providing those nice coastal regions where reefs can flourish, they have a habit of preserving genetic lineages and accelerating evolution. Meaning many island species can be found nowhere else. When animals can easily move into each other’s habitats, as they could in Pangea, eventually one body-plan (the best adapted) will come to dominate in each niche. This is why there is a big cat sitting at the top of many ecosystems, on almost every continent on Earth…and it’s probably the reason we’re the only humans left as well. Cats and sapiens may not live in every environment on the planet but they’ve come to dominate the ones they do.
Cats, nature’s favourite murder weapon…after humans of course.
Islands can provide sanctuaries where animals can evolve away from fear of predation or competition, allowing them to adapt and diversify into different niches; maybe even taking up the apex predator mantle for themselves.
In the modern world, Madagascar is a great example of this. The island has been isolated from mainland Africa for a really long time (like 20 million years before the dinosaurs went extinct long) and, as such, many of the most successful mammal forms never set foot there. Monkeys would eventually out-compete almost all of their primate cousins elsewhere on the planet, to the point of extinction. However, the primates in Madagascar, known as lemurs, never had to deal with this threat and evolved to fill many niches similar to their monkey relatives (and beyond).
Ring-tailed lemurs spend a lot of time on the ground and are highly social, moving around in gangs, much like macaques or baboons. Red-ruffed lemurs live in the trees, grooming each other and making a hell of a racket, similar to howler monkeys. Archaeoindris was a large, ground-dwelling, lemur without a tail, that walked on its knuckles, akin to a gorilla. There were even ‘sloth lemurs‘ like Babakotia who had long, curled, fingers and spent their lives hanging upside-down underneath branches.
In the absence of true cats, another animal, the fossa, took up their mantle on Madagascar and did a damn good tribute act! So good, in fact, that several scientists suspected it was a cat, or at least closely related, after examining its characteristics. They were wrong though. Genetic studies have since shown it’s more closely related to civets and mongooses (mongeese?) but so distinct, it should be in a group of its own, with other Malagasay carnivores. People still insist on calling it a cat sometimes…because? I don’t know, people are annoying. The fossa, along with Archaeoindris and Babakotia, are excellent examples of convergent evolution, where unrelated animals adapt in similar ways to similar environments.
Flightless Parrots and Foster’s Rule
As well as rushing in to fill unclaimed ecological niches and doing stellar impressions of their mainland rivals, there are other ways life on islands gets seriously weird. One well documented effect of the island lifestyle for birds is a trend towards flightlessness. A process that has happened over 1000 times independently. The isolation of islands can often mean there are few, if any, ground dwelling mammals lurking about, hunting or scavenging for eggs. Making them quasi-paradises for any avian that lands there.
The most stunning example of this is effect happened on New Zealand, where the only mammals to reach the island before humans were seals, bats, and the primitive St. Bathans mammal, which has been extinct for a very long time. As such, many birds from many different lineages gave up flying altogether. As well as the famous kiwi and extinct moa, who despite both being Ratites (related to ostriches and emus) evolved flightlessness independently, New Zealand is home to the kakapo, the world’s only flightless parrot; the takahe, a flightless moorhen; and the weka, a flightless rail; as well as 2 species of flightless teal and several other living and extinct birds that gave up on the air.
New Zealand: a twitcher’s wet dream…
Another way islanders start getting their evolutionary freak on is through ‘Foster’s Rule’, which states that animals will get smaller or larger depending on the resources, threats and competition in the environment. With smaller species trending towards gigantism, when there is a lack of predation or competition from larger lifeforms; and larger species trending towards dwarfism, due to their being fewer resources to go around or big predators to hassle them.
Living displays of island gigantism are possibly less spectacular than they were in the past because humans have a habit of fucking things up for megafauna, in particular. They can be found, though they are usually big examples of typically smaller creatures. Such as the New Caledonian giant gecko, which can grow well over a foot long, and giant rat species found across the islands of south-east Asia. People often cite the Komodo dragon and Galapagos tortoises as modern giants (including myself, in this very article, until making edits). However, these are dubious at best, due to fossils of similar sized relatives being found on mainland Australia and South America, respectively.
However, if we look further back in time, we will find some truly enormous creatures! Like the Malagasay elephant bird, the largest bird species in history, 3m tall and weighing in at 860kg, that only went extinct about 1000 years ago due to *sigh* human activity (which also brought down another island giant: an enormous, flightless, pigeon known as the dodo). The largest eagle that has ever existed was only found in New Zealand. Haast’s eagle had a wingspan of up to 3 metres and preyed upon the enormous moa, mentioned earlier. This is the scene depicted in the main picture of this article.
The opposite effect, insular dwarfism, can be seen in tamaraws and anoas, Filipino buffalo species that are 2 or 3 times smaller than their mainland counterparts. The same was true for Honshu wolves and Bali tigers, before they fell foul of *sigh* human activity as well. However, perhaps my favourite example of insular dwarfism is the Channel Islands mammoth. A descendant of the largest of all proboscidians, which became one of the smallest, after swimming over to the Channel Islands off the coast of California, about 60,000 years ago. It only stood as tall as your average man and weighed 760kg, about 1/12th the weight of its ancestor, the Columbian mammoth.
Massive bastards: dodo, elephant bird and the New Caledonian giant gecko.
Tiny little pricks: Channel Islands mammoth, Honshu wolf and anoa.
We have now identified 5 reasons why life on islands can sometimes be a little strange:
1. Primitive lineages can cling on to life (lemurs).
2. Convergent evolution rushes to fill empty niches (fossa).
3. Birds become flightless (kiwi).
4. Gigantism in typically smaller species (New Caledonian gecko).
5. Dwarfism in typically larger species (Channel Islands mammoth).
Over the coming weeks, I am going to examine some island habitats, from both prehistory and the modern day, to explore more of the weird and wonderful turns evolution takes in these places. Starting in the late Cretaceous, on the island of Hateg, home of dwarf dinosaurs and a monster that preyed on them…
Read more of our series on island evolution:
The Horror of Hateg – Learn about Hateg Island, where giant pterosaurs preyed on tiny sauropods.
The Hobbits of Flores – The story of our strangest relative, Homo Floresiensis, also known as ‘the hobbit’.
If you have enjoyed this article and found it valuable, please consider supporting us by liking the post, following us on social media, or sharing it yourself. What’s your favourite island dwelling creature? Let us know in the comments below!
NB: Some corrections were made to this article, to improve its accuracy. These were:
– Removing reference to the Komodo dragon as an example of island gigantism
– Adding in information about the St. Banthams mammal having lived on New Zealand
Other information was added about the New Caledonian giant gecko, giant rats, Galapagos tortoise and Haast’s eagle, purely for education and stylistic reasons.
Do Your Own Damn Research!
Read more about The Theory of Island Biogeography in this book by Robert H. MacArthur and Edward O. Wilson
Watch this brilliant video on the Biology on Islands by Trey the Explainer:
Ologies podcast on island ecology with Alie Ward and Andy Kraemer.
Want to dig deeper but don’t want to buy a book? Watch this lecture by Harvard professor Philip Lehner: