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Role of Social Interactions in The Evolution of Migration

August 22, 2010

Migration of birds, animals, microbial organisms is a very interesting phenomena, much of which is still not clearly understood. In few of my earlier posts, I have mentioned about recent studies focusing on collective migration of birds and how they follow magneto-reception cues to guide them during migration. Other cues which organisms use while migrating can be odor, temperature gradient, concentration gradient etc. In another study, it was shown that the birds follow a pecking order, a kind of hierarchy with a leader and followers while flying long and short distances. Organisms migrate as it gives them certain benefits such as better resource availability, better breeding locations etc but it also involves some cost and organisms try to balance the cost and benefits and these studies try to explain how and why the migration pattern evolves while balancing the costs and benefits. One of the key factors which has not been extensively studied is the role of social interaction between different individuals and how it results in collective migration pattern. Well, that was until now! A recent study by researchers at Princeton University, which is going to be published in the next edition of PNAS, focuses on this key factor. Vishwesha Guttal and Iain Couzin at Princeton have developed a mathematical model which includes social interaction parameter and provides a generic temporal and spatial solution to various collective migration scenarios ranging from zero interaction groups to highly interactive groups. The model is not just limited to birds but can be applied to various organisms from bacteria to wildebeest, that’s the beauty of the model.

So what benefit do organisms get by social interactions? If an organism is error prone in sensing directional cues, then by having a larger group and by interacting with individual members, these errors can be minimized. This is based on principle called ” many wrongs principle” proposed by Bergmann and Donner in 1964 for explaining benefits of group size in animal navigation. By utilizing directional cues and being in group one can take advantage as proposed by many wrongs principle, but utilizing directional cues also incurs some cost. Organism can still enjoy the benefits of being in group even after switching off directional sensing abilities (no cost plus benefits). Ultimately all the members of the group will switch off their directional sensing ability, as there is no incentive to do so, resulting in the whole group losing migration capability. So how does migratory groups evolve a strategy wherein they can use utilize benefits of large group as suggested by “many wrongs principle” and still maintain their direction sensing capabilities. This model by Guttal and Couzin provides answer to such questions in the evolution of migration.

They define two main parameters, ωsi and ωgi, where ωgi is defined as gradient detection ability parameter while ωsi is defined as social trait paramater. ωgi=0 implies random walk by the organism while high ωgi gives high directional accuracy and provides the organism higher velocity in migration, but with high ωgi, comes higher energy related costs. Same with ωsi, higher social interactions comes with higher costs. Fitness of the individuals in the group is obtained by subtracting costs from benefits and thereby an optimum fitness of an individual can be determined. By having various combination of different values of these two parameters, various migration pattern emerges, this study shows. They found that in groups where individuals interact with each other, two modes exist in the optimum solution. One mode consists of individuals which have very high ωgi but very low ωsi, implying they don’t interact much with others but have good directional attributes- these are the leaders. The second mode consists of individuals high on social traits and low on directional traits- these are followers. The study also shows that larger groups require lower proportion of leaders. The paper discusses several other scenarios including effects of habitat destruction on migration pattern and I will recommend you to read the complete paper.

I liked the results of this study alot, not because its my friend’s work, but mostly because I like the simplicity of the model and how nicely it explains various scenarios of migratory pattern and strategies in various organisms. Good work Vishu!!

Here is the abstract of the paper:

Social interactions, information use, and the evolution of collective migration

Vishwesha Guttal and Iain D. Couzin

Abstract

Migration of organisms (or cells) is typically an adaptive response to spatiotemporal variation in resources that requires individuals to detect and respond to long-range and noisy environmental gradients. Many organisms, from wildebeest to bacteria, migrate en masse in a process that can involve a vast number of individuals. 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. Here, we explore the evolution of migratory behavior using an individual-based spatially explicit model that incorporates the costs and benefits of obtaining directional cues from the environment and evolvable social interactions among migrating individuals. We demonstrate that collective migratory strategies evolve under a wide range of ecological scenarios, even when social encounters are rare. Although collective migration appears to be a shared navigational process, populations typically consist of small proportions of individuals actively acquiring directional information from their environment, whereas the majorities use a socially facilitated movement behavior. Because many migratory species face severe threat through anthropogenic influences, we also explore the microevolutionary response of migratory strategies to environmental pressures. We predict a gradual decline of migration due to increasing habitat destruction and argue that much greater restoration is required to recover lost behaviors (i.e., a strong hysteresis effect). Our results provide insights into both the proximate and ultimate factors that underlie evolved migratory behavior in nature. [doi: 10.1073/pnas.1006874107]


Picture credit: Flickr user
rwjensen

Plot credit: Article doi: 10.1073/pnas.1006874107 Video credit: PNAS and authors of the article

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