Giant dormice the size of cats used to live on Sicily

Despite their name, dormice are not actually mice but belong to a separate family of rodents which contains around 30 different species.

Some time between around 6 and 5.3 million years ago, one of these species managed to get across to different islands of the Mediterranean before being cut off and isolated. Over the next few million years, on each island the rodents grew to extraordinary sizes. 

The dormice on Formentera are still about 50% bigger than the mainland ones. Those on Majorca used to be slightly bigger still, and those on Sicily about the size of a cat.

Jesse Hennekam is a PhD student at the University of York who has been looking into how and why these dormice got so big.

'At one point, dormice got on to Sicily and the Balearic Islands and they appear to get bigger and bigger,' explains Jesse. 'But we realised that whereas some species got big, others got gigantic.

'We wanted to know why there was such size variation happening between the different island settings.'

By looking at each fossil species and comparing them to living dormice, Jesse and his team were able to show that even though the giant rodents all likely started off at a similar size, the subtle environmental differences on each island meant that they went down slightly different evolutionary routes.

This resulted in some of the dormice becoming gigantic herbivores and others becoming giant carnivores.

The work has been published in the journal Proceedings of the Royal Society B.

An archipelago of oddities

The Mediterranean Sea is littered with islands that have been cut off from each other and the mainland for millions of years. But it hasn't always been that way.

Around 6 million years ago, the movements of the Earth's crust caused the Strait of Gibraltar to close, sealing off the Mediterranean. Over a long period of time the sea eventually evaporated leaving behind a huge salt plain in an event now known as the Messinian Salinity Crisis.

This also meant that the many islands that used to be cut off by water were now suddenly connected to the mainland, allowing lots of animals from both Europe and Africa to make the crossing.

When the Straits of Gibraltar opened up again around 5.3 million years ago, the Mediterranean basin was once again flooded, and the islands and their new inhabitants isolated. These included a whole retinue of animals, from goats and hares to elephants, hippopotamuses and the dormouse.

Dr Victoria Herridge is a researcher at the Museum whose focus is on miniature island elephants, and has been involved with this research on the giant dormice.

'One of the most common evolutionary responses of mammals to becoming isolated on islands is to change size, with many large mammals evolving to become smaller and many small mammals evolving to become bigger,' explains Victoria. 'This is known as the Island Rule or Island Syndrome, and in some cases, this can be really extreme.'

For example, on Mallorca there was a species of goat that underwent miniaturisation. The Balearian mouse-goat only reached 50cm at the shoulder, and due to the lack of predation it even moved its eyes from the side of its head to forward facing. It was joined by a giant shrew and a species of rabbit that got to a size larger than the goat itself.

On Sicily, straight-tusked elephants had colonised the island and underwent insular miniaturisation until they were only about a metre tall at the shoulder, while the later swans that lived there grew to sizes greater than that of the elephants.

It was also on Sicily that the dormice achieved their greatest sizes, around that of a domestic cat. It is thought that this could be related to another giant that lived there: the giant barn owl. One suggestion is that as the rodents got bigger, so did the owls meaning that the rodents got bigger again, in a process known as an evolutionary arms race.

'If you look at an island ecosystem, both now and in the past, the one thing that they have in common is that they are isolated and that not a lot of new things get in,' explains Jesse. 'But at the same time, they are very different from each other when it comes to the other species present, the climate and the geography.'

It is these differences in the environments that have likely led to the rodents on different islands getting bigger to alternative degrees.

Different ways to get big

By looking at the dormice that live on the mainland and comparing them to the giants from Majorca, Formentera and Sicily, Jesse was able to see if the rodents were increasing their sizes in the same way.

The giant dormice on Formentera are unique. They are still with us, and they have a particularly carnivorous diet. By comparing the skeletons of all the giants, Jesse could test whether the other, much bigger dormice followed a similar trajectory towards carnivory. 

'If you compare the Formentera dormice with the giant fossil ones, they are clearly more different from each other than either are from the mainland species,' explains Jesse. 'The fossil ones from Majorca also seem to be different from the fossil ones from Sicily.'

'We propose that the biggest dormice did not develop the carnivorous diet that we see in the Formentera ones, but instead had a more herbivorous, plant-based diet, and that by getting larger they were able to eat this tougher vegetation.'

This is possibly because when the dormice arrived on Formentera there were no predators, and so the rodents effectively filled the role of rats. But when the dormice arrived on Sicily, the role that was available to them was one of a large herbivore and so they took that path instead.

Despite the general rule that small animals get bigger on islands, different species grow for different reasons.

'This has important ramifications, as it suggests that one size doesn't fit all and that body size alone might not determine the ecological role of an insular species,' says Victoria.

'In some ways, however, this complexity highlights just how remarkable it is that the Island Rule is such a general phenomenon.'