Island castaways

Islands account for one sixth of the Earth’s land area, and are created in one of two main ways. Oceanic islands, such as the Galapagos, rise from the sea as volcanic eruptions build up layer upon layer, slowly surging towards the ocean’s surface. Madagascar, on the other hand, is a little different. It is classed as a continental island, separating from India some 88 million years ago as a standalone landmass.

Islands become integrated ecosystems where species can develop their own unique physical characteristics and ways of life.

On continental islands, species become separated from their mainland parent populations as their newly-formed island home goes its separate way. From the moment of separation, the island population is treading a unique evolutionary pathway. For oceanic islands, new land created from below the waves, any inhabitant species must swim, fly or float ashore: castaways washing up in a strange new world.

Whichever path they follow, islands become integrated ecosystems where species can develop their own unique physical characteristics and ways of life. In short, islands breed new species, and this specialisation means that our planet’s islands have higher concentrations of endemic species - those that are found nowhere else - than its continents.

Madagascar’s most famous residents are its lemurs. Closely related to monkeys, lemurs live exclusively on this island and have evolved into many different species adapted to exploit the extreme habitats they live in. Verreaux's Sifakas (Propithecus verreauxi), or ‘dancing lemurs’, have made their home in the island’s spiny forests. Their bodies are perfectly adapted to arboreal life. Sifakas powerful hind legs can propel them in leaps of over 30 feet through the air, whilst on the ground they skip along using a peculiar two-legged hop. With hips that can’t support a walking posture, sifaka life revolves around the trees, they are even comfortable sleeping high aloft in the forest canopy.

Miles from the Verreaux's Sifakas’ spiny kingdom, their relative the golden bamboo lemur scurries around in search of its favourite food, bamboo. This species has developed a taste for the soft bamboo stalks and growing tips which are ignored by other lemurs in their habitat. This is hardly surprising, given that they contain high levels of cyanide, a poison to deadly to most animals. But the highly specialised golden bamboo lemur has evolved a digestive system to cope. Most days they happily munch around 500g of bamboo, containing over 12 times the lethal dose of cyanide for most mammals.

Over in the Pacific, incidentally the ocean where Tom Hanks filmed Castaway, lie the oceanic islands of the Galapagos. Although the islands’ birth may have been very different, their endemic species have evolved along similar extraordinary lines to those cast away on Madagascar.

One such species is the Galapagos penguin (Spheniscus mendiculus), the only species of penguin whose range crosses the equator and even strays into the northern hemisphere! Living proof that animals can adapt to the most unlikely new habitats, these little birds have learned how to avoid not ice-rash, but sun-burn. They have a strange, hunched posture, that shades their feet. With a rich blood supply, their feet act like a pair of radiators, getting rid of excess warmth and stopping the penguins over-heating. Most of their day is spent in the shallow waters around the islands, keeping cool and collecting food while avoiding sharks. Once the sun goes down and the temperature drops, the colonies huddle together to keep warm.

Another fish out of water story relates to a group of spotted pigs living the high life on Big Major Cay, an uninhabited island in the Bahamas. More commonly found in muddy fields, these farmyard friends are thought to have been left here by a group of sailors who planned to come back to eat them, but never returned. Left to their own devices the pigs have worked out that the passing yachts habitually dumped excess food in the surrounding waters, and there was only one way to get to it: go swimming. After these well-earned aquatic feasts they piggy-paddle back to the island, to lounge in the shade of their tropical paradise home.

For plant and animal castaways, islands are crucibles of innovation. Sometimes it’s as simple as tweaking their behaviour, other times these island inhabitants will go out on an evolutionary limb, and a new species is forged.

And as for Wilson, well who knows where he has ended up. There are reports that those Bahaman pigs play with flotsam and jetsam washed up on their beach. Perhaps this includes a happy volleyball, quietly adapting to his new castaway life.

Featured image by Emma Brennand © BBC NHU 2016