© Jurgen Freund / Nature Picture Library / Rex FeaturesDriven by blood sugar levels.
Claws aloft they flow in a vast carpet of red, across busy roads, through shops and over the island's golf course. The annual march of the Christmas Island red crabs has been described as one of the most spectacular animal migrations on Earth.

Now researchers have identified some of the metabolic changes that occur to transform tens of millions of red crabs from inactive, antisocial creatures that hide in their burrows into an unstoppable sea of individuals capable of walking for several days on end in order to fulfil their desire to reproduce.

"This [migration] requires a major physiological change," says Steve Morris of the University of Bristol, UK, who led the study. "If they are made to walk for 5 minutes in the dry season they build up high levels of [lactic acid], yet when the rains come they'll walk a kilometre a day quite easily."

The red land crab, Gecarcoidea natalis, is unique to Christmas Island, which is located in the Indian Ocean, south of Indonesia. Their migration usually occurs at the start of the wet season, and is timed so that females can release their eggs into the sea at the turn of high tide during the last quarter of the moon.

Previous studies have shown that injecting crustacean hyperglycaemic hormone (CHH) into crabs boosts blood sugar levels, suggesting that it is roughly analogous to insulin in humans, triggering the breakdown of glycogen into glucose to be used as fuel when required. To find out whether this hormone played a role in the migration of red crabs, Morris and colleagues measured concentrations of CHH in the crabs' eyestalk glands and blood during the wet and dry seasons.

They found that there was an increase in CHH in the crabs immediately before dawn during the wet season but not during the dry season. "We've shown the level of this hormone goes up and is important in encouraging the release of glucose for muscle activity during the migration," says Morris, who presented the findings at a Society for Experimental Biology conference in Glasgow, UK, earlier this month. However, it is unclear whether this is the direct trigger for the migration, or if another mechanism is at work too.

Morris warns that climatic changes could threaten the viability of the species. The monsoons have been arriving late in recent years, causing the crabs to die of dehydration and exhaustion when they eventually migrate in the absence of heavy rain. Meanwhile, conditions have also boosted numbers of the yellow crazy ant, which spray the crabs with poison before eating them.

"No rain means that the chance of a recruitment event is greatly diminished," says Stuart Linton of Deakin University in Melbourne, Australia. "Combine this with the invasive crazy ant problem and the population may crash in the near future."