How mass migration evolved

A new study of migration's origins has suggested that even small changes in the social behaviours of solitary animals may set in motion an evolutionary cascade ending in massive, globe-spanning migrations.

Such migrations - caribou across the Arctic and wildebeest across the Serengeti, birds and butterflies over oceans - are among nature's most beautiful and mystifying phenomena, reports Wired.com

Many models suggest how migration works now, in terms of individual actions producing collective behaviour; but how it could have started in the first place is far harder to explain.

"Despite the ubiquity of collective migration, and the key function it plays in the ecology of many species, it is still unclear what role social interactions play in the evolution of migratory strategies," said Lain Couzin and Vishwesha Guttal of Princeton University.

In their evolutionary model, Couzin and Guttal assumed two fundamental traits.

First, the digital animals needed the ability to respond to a direction-linked environmental cue, of the sort provided in reality by temperature, geomagnetism, wind and chemical gradients.

The second required trait was sociability, or an ability to be attracted toward moving neighbours and physically align with them.

Each adaptation came with a cost, reflecting the energy required to follow a cue and the dangers of disease associated with group exposure.

They ran the model again and again, for a wide range of population densities and migration costs and benefits.

Over and over, the same pattern emerged. Evolution tossed up two distinct types of individuals: "leaders," who followed environmental cues and ignored everyone else, and "sociable" individuals, who were attracted to others but themselves oblivious to the cues.

The findings also raised some interesting hypotheses. In migrating populations, a few individuals often and inexplicably fail to migrate; maybe they're just not getting the message from their leaders. In the models, it was also possible for migration to evolve when individuals were widely scattered - which fits with the existence of migration in such insects as dragonflies and Monarch butterflies, which live independently.

The findings were published in the Proceedings of the National Academy of Sciences.