Animals whose magnetic compass leads them astray might still make it if they stay with a group, one model suggests. — Daily Science

How do migratory animals travel so far and reach their destination with such precision? For many animals, the answer is magnetoreception. New computer modeling research shows how sticking together helps migratory animals get where they need to go, even when their magnetic compass throws them off course. The researchers also found that the strategy breaks down when species decline in numbers.

Some of us live and die by the GPS on our phone. But if we can’t get a signal or we run out of battery power, we get lost on our way to the grocery store.

However, animals can find their way across great distances with amazing precision.

Take monarch butterflies, for example. Millions of them fly up to 2,500 miles across the eastern half of North America to the same wintering grounds each year, using Earth’s magnetic field to help them reach a small region in central Mexico that is about the size of Disney World size.

Or sockeye salmon: Starting in the open ocean, they return home each year to spawn. Using geomagnetic signals, they manage to identify their source stream out of thousands of possibilities, often returning to within a few feet of their birthplace.

Now, new research offers clues about how migratory animals get where they need to go, even when they lose their signal or their internal compass leads them astray. The key, said Duke Ph.D. student Jesse Granger: “You can get there faster and more efficiently if you travel with a friend.”

Many animals can sense the Earth’s magnetic field and use it as a compass. What has puzzled scientists, Granger said, is that magnetic sense is not fail-safe. These signals from the planet’s molten core are subtle on the surface. Phenomena such as solar storms and man-made electromagnetic noise can disturb or drown them out.

It’s as if your internal compass ‘needle’ would sometimes fly off or point in random directions, making it difficult to get a reliable reading. How do some animals manage to chart a course with such a noisy sensory system and still do well?

“This is the question that keeps me up at night,” said Granger, who did the work with her adviser, Duke biology professor Sönke Johnsen.

Multiple hypotheses have been proposed to explain how they do this. Perhaps, some scientists say, migratory animals average multiple measurements taken over time to get more accurate information.

Or perhaps they switch from consulting their magnetic compass to using other ways of navigating as they near the end of their journey, such as scent or waypoints, to get closer to their goal.

In an article published on November 16 in the magazine Proceedings of the Royal Society BThe Duke team wanted to test these ideas against a third possibility: that some animals manage to find their way, even when compass readings are unreliable, simply by sticking together.

To test the idea, they created a computer model to simulate virtual groups of migrating animals and analyzed how different navigation tactics affected their performance.

The animals in the model begin their journey spread out over a wide area, meeting others along the way. The direction an animal takes each step of the way is a balance between two competing drives: to group and stay with the group, or to head towards a specific destination, but with some degree of orientation error.

The scientists found that even as the simulated animals began to make more errors when reading their magnetic map, those that stayed with their neighbors made it to their destination, while those that didn’t care to stay together didn’t.

“We show that the animals are better at navigating in groups than at navigating alone,” Granger said.

Even when their magnetic compass led them off course, more than 70% of the animals in the model made it home simply by joining others and following suit. Other ways to compensate weren’t up to the task, or they would need to guide them perfectly for most of the journey to accomplish the same feat.

But the strategy breaks down when species decline in number, the researchers found. The team showed that animals that need friends to find their way are more likely to get lost when their population drops below a certain density.

“If the population density starts to drop, it will take them longer and longer along their migration route before they find someone else,” Granger said.

Previous studies have made similar predictions, but the Duke team’s model could help future researchers quantify the effect for different species. In some model runs, for example, they found that if a hypothetical population was reduced by 50%, similar to what monarchs experienced in the last decade and some salmon in the last century, 37% fewer of the remaining individuals would make it. to his destiny.

“This may be an underappreciated area of ​​concern when studying population loss,” Granger said.

This research was supported in part by the Air Force Office of Scientific Research (FA9550-20-1-0399) and by a National Defense Science and Engineering Graduate Fellowship to Jesse Granger.

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