An agouti can bury hundreds or thousands of seeds in the forest. This scatter-hoarding behavior is interesting to many scientists because it raises the question of how these animals can remember the locations of so many seeds. Agoutis may not be the Einstein’s of the animal world, but the ability to remember the locations of all these seeds is really impressive. The problem is, nobody really knows if these agoutis actually remember the location of their seed caches! Our new paper published in the journal Animal Behaviour sheds some light on this interesting question.
As previously discussed in this blog, we placed camera traps next Astrocaryum seeds that had been buried by agoutis to determine if they were eventually dug up by cache owners (i,e, the animal who stored it there in the first place), or cache thieves. However, these cameras also recorded a lot of other animal action in the area. When looking through the photos, I noticed that cache owners often visited their hidden seeds. It looked like the agouti cache owners were purposely re-visiting their buried seeds every few days. But why would they do that?
Could the agoutis be monitoring their seeds to determine if/when they have been stolen, or is there another reason why the agoutis would want to visit their seeds?
To investigate this question I talked to Tim Roth (now a professor at Franklin and Marshall) who is an expert on cognition and seed caching birds. Tim recently wrote a paper on long-term memory and seed caching in black-capped chickadees which gave me some great ideas. Previous studies of chickadees indicated that these small birds were able remember the locations of buried seeds for about a month, but not much longer. In those experiments, birds were allowed to cache a seed in an enclosure, and then were allowed to return after a given amount of time. After a month, the chickadees were not able to recover seeds at better than random levels. Just like in humans, the spatial memory of the chickadees degraded over time. But in the wild, these birds (as well as agoutis) often need to recover seeds that have been stored longer than a month. So, if memory degrades over time, how can scatter-hoarders remember where their seeds are over long time periods? Well, it turns out that Tim discovered a difference between those experiments and wild animals – free living birds may be able to revisit their cache locations to reinforce their spatial memory. This is important because it provides a potential mechanism for a species with limited brain power to still be able to remember the location of their seeds for longer time spans. When Tim tested this hypothesis, he found that chickadees allowed to revisit cache sites could indeed remember the location of seeds after 6 months!
Agoutis need to save their seeds until the season of lowest food availability, and so they should want to store their Astrocaryum seeds in the ground for at least 3-4 months. It would make sense if agoutis also acted like the chickadees in Tim’s experiment. If agoutis repeatedly revisit their caches to remind themselves of the locations of their caches this should lead to increased long-term memory.
To test this idea using our videos, we counted how many times agoutis visited their caches and then compared this to control cameras that were placed nearby (where no caches were known to be buried). We found that cache owners were almost four times more likely to pass in front of cache cameras than control cameras, and that cache owners visited their caches about once every five days. We also noticed that cache owners acted differently when they passed the cache location, often walking directly above the seed location and sniffing the ground where the seed is. In these cases, we believe that these agoutis are reaffirming that their cached seed is indeed still buried at that location. Sometimes non-cache owners would also investigate the cache locations before stealing the seed. In contrast, cache owners purposely left their caches in the ground to save for later consumption. The behaviors we’ve seen are consistent with the idea that agouti cache owners are revisiting their seeds to reinforce their memory (and acting as a census of caches for stolen seeds). If this is indeed the case, these agoutis may be using this behavioral strategy to lengthen their memory. This is a really cool result, because if true, scatter-hoarding animals that want to increase their long-term memory abilities don’t have to evolve bigger hippocampuses (the part of the brain responsible for memory), they can simply behave differently. We expect that similar behavioral patterns may be commonly used by other scatter-hoarding animals, but nobody has tested it in the wild yet!
By: Ben Hirsch