Just a few small changes in the social behaviors of even solitary animals may set in motion an evolutionary cascade ending in massive, globe-spanning migrations, suggests a study of migration's origins.
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. Many models suggest how migration works now, in terms of individual actions producing collective behavior; 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," wrote Princeton University evolutionary biologists Iain Couzin and Vishwesha Guttal in a study published Sept. 14 in the Proceedings of the National Academy of Sciences.
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 neighbors 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. The evolutionary benefit was scored according to how far organisms migrated. 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.
Extrapolation from computer models to real-world behavior is always tricky, but the findings do correlate with observed patterns in migrations; a swarm of bees, for example, can follow a few scouts to a new hive. The findings also raise 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.
"Guttal and Couzin add evolutionary dynamics to the mix and set the scene for a new generation of experimental tests and applications," wrote University of Sydney biologists Stephen Simpson and Gregory Sword in a commentary on the experiment.
Some of the the study's implications are, however, troubling. Over time, the simulated populations tended to settle into a ratio with far more followers than leaders. If leader-producing mutations are rare in the wild, then lost leaders could be very difficult to replace, and migration easily compromised. This lesson is reflected in the traditions of Inuit hunters, who once allowed lead animals to pass, hunting only from the middle of the pack.
In the models, migrations lost to habitat fragmentation were also difficult to rekindle. Even after restoring the habitat, "a population's migratory ability does not recover at the same habitat recovery at which it declined," wrote Guttal and Couzin. Migration could disappear in a few generations, and take many more to come back, if at all. Indeed, bison in North America no longer seem able to migrate, a fate that may soon be shared by wildebeest in the Serengeti.
Migration may vanish at a scale measured in human years, and recover at time scales measured in planetary cycles.
Image: Caribou./Flickr, Sami Keinanen.
*Citation: "Social interactions, information use, and the evolution of collective migration." By Vishwesha Guttal and Iain D. Couzin, Proceedings of the National Academy of Sciences, Vol 107. No. 37, September 14, 2010. *
*"Evolving migration." By Stephen J. Simpson1 and Gregory A. Sword. Proceedings of the National Academy of Sciences, Vol 107. No. 37, September 14, 2010. *
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