This is the second in a series of articles on the Andaman Islands launched by Nature inFocus, in an attempt to showcase the natural history of these islands from the unadulterated perspective of its unique biodiversity. Each article from the series is titled with a line from J.R.R. Tolkien's Lord of the Rings poem - “All that is gold does not glitter."
The Andaman Islands, shaped like a stretched rubber band, has a long coastline with several beaches. Jahazi beach in Rutland island is by far my favourite, spectacularly beautiful and relatively free of sandflies. Here, we have often camped in some of the last remaining littoral forests in the world, a fragile army of tall trees fighting a belated and failing battle against “development”. At the feet of these refugees, white sand slopes away into blue sea, the only location deep enough for ships to dock, giving the beach its name.
One orange moonlit night, we walked this coast to our usual phone call spot a few kilometres away, making mildly bioluminescent footprints and sending Ghost Crabs scuttling. Suddenly, there was excited chatter and a chain reaction of finger pointing; I woke up from my trance to see tracks in the sand all the way to a raised sandbank. A turtle had climbed to nest and lay eggs! We scrambled to the top, but she was long gone and she hadn’t laid any eggs. “Leatherback,” said Sabian, who has been assisting the turtle monitoring project for a few years now – tagging turtles and measuring beaches to see when and where they nest.
Sea turtles are known for their marathon migrations, but the Leatherback sea turtle holds the record. On hatching, baby Leatherback turtles hitch rides with flotsam into the open ocean, where they feed; in adulthood, they travel tens of thousands of kilometres, riding currents, apparently wandering, but reaching their breeding grounds with perplexing accuracy. Scientists are still piecing together bits of this puzzle.
Animals move anything from a few millimetres to a few thousand kilometres, over seconds or years, using anything from flagellae to wings to any number of pairs of legs. But, they always move for the same few reasons – to eat, to breed, to escape predators or bad weather. Most life is adapted to function optimally in a narrow range of conditions, but the environment is changing constantly. As conditions or seasons change, a common strategy for animals is to move away, just far enough to feel comfortable again.
We often think of this migratory behaviour as hardwired into the brain of the species. An inherited behaviour flips a switch every year to prompt every Leatherback to just keep swimming or every Bar-headed Goose to cross the tallest mountain range in the world, from Tibet to India. However, life is not so black and white for most other species.In Asian Glossy Starlings, loud (glossy) blackbirds that flock in the Andaman islands, not all individuals in a population migrate during the dry season. Even after a wave of migration out of the islands, dense forests have remnant groups that end up surviving the year! All of these different phenomena, together called partial migration, are thought to have preceded full migrations in the history of evolution. It is surprisingly common in the natural world, widespread among birds, mammals, insects and fish. Why is this so?
Do or (maybe not) die
A species of animal or bird may be distributed across a large range of latitudes. While the northern limits of the range might get too cold to survive in the winter, temperatures in the southern limits may not be so. In the lean season – the cold or the dry season, there is often some food around, but in poor quality or not enough for everyone. There are fewer insects flying, fewer fruits on trees or the good ones have all been eaten already. The choice now is to stay and eat less or risk a flight, maybe across thousands of kilometres of sea or inhospitable land, for the promise of food. But not everyone has to leave – remember, there is still some food left. If part of the population leaves, the remainders can survive on the scarce food. Then what is the advantage for the migrants, you might ask, for taking these risky flights?
Stone, paper, scissors!
To a behavioural ecologist, the two choices – to stay or to leave – that a starling has to make are “strategies” in a “game”. To each individual, a strategy comes with a benefit and cost depending on many factors like age, body condition and available food. It would also help, like in a game of stone-paper-scissors, to anticipate the move of your opponent. This is because your chance of survival or “winning” depends not only on your move but on your opponents’ decisions. If most of your flock wants to fly out, you will do well by staying in. On the other hand, if too few fly out, you should fly to escape famine.
Wolves of Wall Street make such predictions with cloud computing but these birds have to do it with their tiny brains and without even a primary education, and yet it works! Besides a couple of well-studied species, ecologists don’t know whether these decisions are driven by individual genetic variation or social learning. Although this seems abstract, such strategies and outcomes have been observed in bird species that show partial migration. But these birds have it easy when you look at freshwater fish.
In small islands like the Andamans, most streams are rain-fed and ephemeral. Come summer and rippling waters transform into disconnected pools. For freshwater fish and invertebrates like water fleas and shrimp, this means careful strategies and perfect movement timing to survive the dry season. As the climate becomes more unpredictable, the ability to assess conditions accurately and change behaviour becomes key in ensuring survival. This “plasticity” not just results in the survival of the population, but also in maintaining levels of nutrients and gases in the water, and the proper functioning of the ecosystem. Maybe shapeshifters like partial migrants hold the key to adaptation in these uncertain times.
We once spotted a Chital, a deer species introduced from the open grasslands of mainland India – with genomes that have long forgotten the smell of salt – swimming across a large stretch of sea. Headed straight to a tiny island in the distance, he was defying reason and evolution. To me, that day was a testament to the fact that nothing in nature is really binary, black-or-white, yes-or-no. It’s leaky and sticky and that’s charming.
To read the next story in the Andamans series click here.