horse migration

When wild horses move toward water or food sources, are they making decisions based on what they see, smell, and hear? Or what they (or their ancestors) remember?

Recent study results suggest that zebras and other equids tend to rely strongly on memory when migrating. That means that, less than “smelling that the food’s probably over there” or “following visible changes in landscape toward a possible lake or river,” they’re remembering what worked for them or their group in the past, said Chloe Bracis, PhD, of the Goethe University Frankfurt Department of Biological Sciences Senckenberg Biodiversity and Climate Research Centre, in Germany. In fact, she said, they might even be relying on memory that’s been passed down culturally or genetically from previous generations.

Using complex simulation models, Bracis and colleague Thomas Mueller, PhD, investigated the migration movement of a group of plains zebra in Botswana. They used computer simulation modeling to compare how the zebras would reach their migration destination if they were using only memory or only perceptual cues. Perceptual cues are what the animal can sense in its surroundings; perceptual cues for migration could include rainfall, tree cover, plant quality, and vegetation conditions. They found that, with the memory-based model, zebras were two to four times more accurate in finding the actual migration destination, Bracis said.

This knowledge could give critical information for herd management, especially regarding how building fences and other structures could affect migration, Bracis said.

“Protecting migration routes is crucial for conserving migratory species, and care must be taken if migration routes shift,” the researchers stated.

Scientific proof of the use of memory in migration is important (as a basis for administrative arguments, for example)—but very hard to come by.

“That’s the thing with memory,” said Bracis. “On one hand it seems obvious. We ‘know’ these animals have memory capabilities, so why wouldn’t they be using them? But it’s surprisingly hard to demonstrate, given we can’t directly measure memory, or in the case of individual learning, know their past experiences for what they may have learned. In fact, most previous models of ungulate (hooved animal) migration have focused on perceptual cues rather than memory.”

In their study, Bracis and Mueller used data from seven female zebras fitted with GPS collars as they migrated with their herd across Botswana. Interestingly, the team said, their migratory path had recently been restored because authorities took down a fence set up nearly 40 years earlier as ordered by veterinarians hoping to stop disease spread at the time.

The researchers input the collected data into their complex models to compare the ways the zebras would have moved based on perceptual information alone or on memory alone. Their results showed that the memory model was most consistent with the way the zebras actually moved, according to the GPS data from their collars.

“With the data set we used in particular, there had been a fence blocking the migration route for 36 years (about three zebra lifespans), so it’s less clear what the role of memory is,” Bracis said.

Although they didn’t specify how the memory was encoded in that group of zebras, they suggested it could have been learned from earlier generations or possibly inherited.

That doesn’t mean they don’t use perceptual clues, however, Bracis added. In fact, they likely do, in addition to using memory. Their scientific models looked at the extreme situations: relying only on memory or only on perceptual cues, for the sake of clear research results. But that wouldn’t imply that the animals didn’t benefit from both during migration. And previous research has shown that perceptual cues are likely important in determining migration timing.

“If any groups of mustangs, brumbies, or other free-roaming horses have patterns of migration or seasonal movements to exploit different areas, the findings of our research could apply in their cases,” Bracis said.

The study, “Memory, not just perception, plays an important role in terrestrial mammalian migration,” was published in Proceedings: Biological Sciences.