It's incredible to watch: each year thousands of animals, including zebras, wildebeests and gazelles, migrate in turn with the seasons between foraging grounds. The animals migrate long distances in their search for sufficient and highly nutritious forage. While science has explained certain aspects of this migration, it is not fully understood how the animals know where to go.
Chloe Bracis, a researcher at the German Senckenberg Biodiversity and Climate Research Centre and the Goethe University Frankfurt, has found that memory is the key to directing zebra migration: "Zebras appear to migrate to the location where foraging conditions were best in the past. They seem to navigate to their destination based on memory, and importantly, forecast conditions several months after arrival."
As part of the study, Bracis and her colleague Thomas Mueller modelled migration routes of zebras using computer simulations. Zebras migrate around 250 kilometers from the Okavango Delta, Botswana to the Makgadikgadi grasslands in November. "We tested two mechanisms which can influence the direction. Simulated zebras could use perception and sense, for example, the vegetation green up in their current surroundings. Alternatively, zebras could use memory, i.e. information from previous migrations, to forecast where to go ", explains Mueller.
The researchers compared the simulated tracks with real-life tracks from GPS-tagged zebras which were collected by other researchers. Memory using past average conditions was able to predict the migration destination of the modelled zebras up to four times closer than those modelled using perception to find their way. "Memory even beats perception at the largest - that is omniscient - perceptual ranges", says Bracis and adds "However perception is still important. Other studies have shown the importance of perception of current local conditions on the timing and speed of the zebra migration, but these may be less important for zebras in terms of direction."
Migration routes of zebras are threatened by climate change and land use change in southern Africa. The zebra migration examined in this study, for instance, was blocked by a fence from the late 60's until 2004. The researchers therefore see the study as contributing to the conservation of large migratory terrestrial mammals such as zebras. "One can only protect migration routes efficiently if one knows how the animals migrate. If memory of past conditions informs the direction migrating animals take, this suggests migration routes could be relatively inflexible", concludes Mueller.
Bracis, C., Mueller, Th. (2017): Memory, not just perception plays an important role in terrestrial mammal migration. Proceedings of the Royal Society B, doi: 10.1098/rspb.2017.0449
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