Smaller emus on smaller islands

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Background photo for meme © 2012 Rex Boggs, Flickr | CC-BY-ND | via Wylio

Have you ever wondered how animals evolve when they are on a permanent diet? Over long time frames? Well, looking at what happens on islands is a perfect way to find this out.

In our recent study (‘Genetic diversity and drivers of dwarfism in extinct island emu populations’ published in Biology Letters), we found that the average size of each island’s emus seems to be related to the size of the island they were stranded on. It has long been known that large animals tend to become smaller on islands, known as island dwarfism. This is what appears to have happened to groups of emus isolated on Kangaroo and King Islands, as well as Tasmania, when sea levels rose after the last glacial period and cut them off from the mainland.

When early Europeans first discovered these small emus on the islands they thought they were completely different species and quickly hunted them to extinction, as they provided an easy food source to hunt. We know this from the early written records from the time, but we don’t know much else.

When we look at the DNA of the island emus, we notice that it looks pretty similar to that of the mainland emu. This suggests to us that they are the same species, just slightly smaller in size (approximately 30% smaller on King Island, 20% smaller on Kangaroo Island, and 10% smaller on Tasmania). The reduced amount of food available on the islands and the lack of need to walk long distances to find food, likely resulted in incrementally smaller emus over each generation. What we don’t know for certain is whether they each reached an optimal size for their island, or if we hadn’t driven them extinct, would they still be getting progressively smaller?

We think it is the former: with the smaller the island, the smaller the emus rather than the longer the isolation, the smaller the emus.

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