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The Tapeworm Made Me Do It

Modeling study suggests the opportunity to shed parasites could help drive the evolution of migratory behavior.

Allison Shaw

Traveling takes time and energy, but we travel anyway for the benefits it provides: a chance to do business, visit friends or family, get a reprieve from winter or see new sights. In the same way, because animals that migrate from one habitat to another expend energy and expose themselves to new risks, there has to be a payoff as well. Commonly cited candidates include improved food supply, reduced competition for breeding habitat and more tolerable climate. But are these the only evolutionarily sensible reasons to get up and go?

Putting their heads together, CBS faculty member Allison Shaw, an expert in mathematical modeling of animal behavior, and colleague Sandra Binning of the University of Neuch√Ętel in Switzerland came up with an intriguing idea: Might migration help animals get rid of unfriendly worms, fleas and other parasites by exposing them to changes in temperature, salinity or other aspects of their physical environment?

To find out, the researchers created a mathematical model that allowed them to adjust and assess the costs and benefits of various combinations of parasite pressure, migration patterns, lifespan, reproduction rate and more. As reported in the a recent issue of The American Naturalist, the model predicted that some combinations produce sufficient parasite-shaking benefit to make either migration or partial migration — in which only a portion of a population migrates — worthwhile from an evolutionary standpoint.

Along with the more clear-cut results, the model also yielded a mystery: For some life-cycle assumptions, it predicted that higher parasite cost would actually favor staying put. It wasn’t until Shaw presented the puzzle to an informal gathering of modeling experts at the U that an explanation emerged: If the benefits accruing from migration takes longer to outweigh the cost of migration than the animal’s lifespan, it makes more sense from an evolutionary standpoint to stay put.

Shaw and Binning identified a number of species, including ducks and their worms, manatees and their barnacles, and mourning doves and their lice, in which the cost of infection — and the vulnerability of parasites to changing conditions — might be high enough to drive the evolution of migration. Their hope is that researchers familiar with those species might take a look to see if the model stands up to experimental testing.

“Models are really helpful in studying evolution of behavior, but one of the caveats is that a model will tell you what is possible, but it doesn’t really tell you what’s going on in the real world,” Shaw says. “We’re hoping that people who know those species will [take a look].” – Mary Hoff


April, 2016