Every autumn, monarch butterflies embark on a quite unbelievable 2000 mile migration from Canada and the USA to the toasty surroundings of central Mexico. It is well known that the butterflies navigate this journey by using both the time of day and the sun’s location in the sky to point them in the right direction.
However, researchers were curious to understand exactly how the butterfly’s brain receives and processes this information. To gain an insight into the mechanisms responsible for this, Eli Shilzerman and his colleagues at the University of Washington have developed a model to explore how the internal compass of the monarch butterfly is organised within its brain.
“We wanted to understand how the monarch is processing these different types of information to yield this constant behaviour – flying southwest each fall,” said Shlizerman.
Monarch butterflies are equipped with large, complex eyes, which give them the ability to be able to monitor the position of the sun in the sky. They couple this knowledge with an internal clock, which many animals possess, that controls their daily pattern of behaviour. The internal clock of the monarch butterfly is contained within the antennae.
“We created a model that incorporated this information – how the antennae and eyes send this information to the brain,” said Shlizerman. “Our goal was to model what type of control mechanism would be at work within the brain, and then asked whether our model could guarantee sustained navigation in the southwest direction.”
Their proposed model was quite simple. Two mechanisms controlled signals passing from the antennae to the brain, one which would stimulate behaviour and one which would inhibit behaviour. The signals from the eyes to determine the sun’s position were also under similar control. It is the balance between these signals that allows the monarch butterfly to calculate their direction of migration.
Implementing this model, the researchers could then run mathematical flight simulations and compare them to actual monarch migration flights.
The flights produced by the model were similar to those that had been recorded from butterfly migrations, with details such as course corrections due to wind being faithfully recreated in the simulations.
The simplicity of this model also provides a good explanation as to how the butterflies are able to navigate their return journey as well. With just 2 input signals from the eye and two from the antennae, the transmission of these signals would simply have to reverse for them to head back north.
“And when that happens, their compass points northeast instead of southwest,” said Shlizerman. “It’s a simple, robust system to explain how these butterflies – generation after generation – make this remarkable migration.”
Amid growing reports that the monarch butterfly population is plummeting due to the loss of their larvae’s sole food source, this research serves as a timely reminder of the incredible inner workings of this species and the importance of working to reverse their decline.
Shlizerman, E., Phillips-Portillo, J., Forger, D.B., Reppert, S.M. (2016). Neural integration underlying a time-compensated sun compass in the migratory monarch butterfly. Cell Reports, DOI:10.1016/j.celrep.2016.03.057
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