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Saturday, April 19, 2014

Nature’s Farmers Are Pretty Smart!

Posted by jlwile on March 1, 2013

The Central American agouti collects seeds and buries them for later use. (Click for credit)

When I was in Costa Rica last year, I saw several Central American agoutis, such as the one pictured above. I didn’t know anything about them, so when I got back home, I looked up some information. They can be omnivorous, but they prefer to eat seeds and fruit. One of their interesting behaviors is to follow troops of monkeys. They “hang out” underneath the trees that the monkeys climb, and they eat the fruit that the monkeys drop or inadvertently shake off the trees.1

Another really interesting thing about the Central American agouti is that it’s a scatter hoarder.2 This means it collects seeds and buries them in multiple locations. It remembers these locations and returns to them when food is scarce. However, it doesn’t just bury them once and leave them there. It often revisits its stores of seeds, digs them up, and reburies them somewhere else.

While this behavior is beneficial to the agouti (it provides storehouses of food for when food is scarce), it is also beneficial to the trees that drop the seeds. That’s because the agouti rarely uses all of its stored seeds. As a result, some of the buried seeds grow and develop into new trees. This means that the Central American agouti is, in fact, a “farmer” for the trees. It moves seeds away form the tree that drops them and plants the seeds so they can grow into new trees.

Why is this beneficial to the trees? If a seedling grows too near the tree that dropped the seed, it ends up competing with its parent tree. That’s not good for the parent or the seedling. By carrying the seed far from the tree and planting it, the agouti allows the seedling the chance to grow without competing with its parent. Pretty nifty, huh? Well, recent research shows its even niftier than that. It turns out that these “tree farmers” are smarter than we originally thought.

While it’s great that a scatter hoarder like the agouti takes a seed to a place where it will not compete with its parent, what about other trees of the same species? If a scatter hoarder moves a seed away from its parent tree but near to another tree of the same species, the seedling still has to compete for the same resources. That’s not much of a benefit to the seedling or the species as a whole. The question is, “How often does this happen?”

Ben T. Hirsch and his colleagues decided they would answer this question. They radio tagged 589 black palm seeds so they could follow the seeds as they were picked up by agouti and moved from place to place. It turns out that 224 of them were actually picked up and buried so they could be followed. Hirsch and his colleagues followed these seeds for a total of 1 year.

In their analysis, they compared how the agoutis moved the seeds to a model that assumed the movements were random. What they found was that the movements were not random at all. Instead, the agoutis moved the seeds so that they were more likely to be in areas with few black palm trees! In other words, the agoutis were moving the seeds to places where they would experience less competition with other trees of the same species!3

Why would they do such a thing? The authors do not know. However, they speculate that perhaps the agoutis understand that black palm trees are where other agoutis will look for seeds, and so they hide the seeds they have found far from such places, out of fear that other agoutis will find them. They also suggest that perhaps the agoutis bury their seeds in places where they don’t see as many other agoutis. Once again, since the agoutis hang around black palm trees, this results in the seeds being buried in areas with few black palm trees. Regardless of the reason, the authors make this point:

In conclusion, we found that the behaviour of agoutis results in directed dispersal towards areas with low conspecific tree densities. As the same ecological pressures that lead to directed seed dispersal by agoutis are likely present for other scatter-hoarding animals, we expect that the patterns observed here are a widespread phenomenon in habitats in which food trees are unevenly distributed. Scatter-hoarding animals can generate seed dispersal patterns that are fundamentally different from patterns produced by other dispersal types (e.g. frugivores, wind or water), and may be far more effective seed dispersers than previously assumed.

If you don’t recognize the term, “conspecific” just means “of the same species.”

I certainly agree that the phenomenon they found in their research is probably widespread in nature. After all, that’s exactly what I would expect from a cleverly-design ecosystem!

REFERENCES

1. Mark Wainwright and Oscar Arias, The mammals of Costa Rica: a natural history and field guide, Comnstock Publishing 2007, p. 220
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2. Laurie Agopian and Illene Miller, Rain Forest, Teacher Created Resources 2004, p. 37
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3. Ben T. Hirsch, Roland Kays, Veronica E. Pereira1, and Patrick A. Jansen, Directed seed dispersal towards areas with low conspecific tree density by a scatter-hoarding rodent, Ecology Letters 15:1423–1429, 2012
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Comments

4 Responses to “Nature’s Farmers Are Pretty Smart!”
  1. Jonathan B. says:

    Agoutis are cool. I’ve also read somewhere that they are the only species in their ecosystem with jaws capable of cracking Brazil-nut seed pods, and that without the Agouti, the Brazil-nut trees would be unable to reproduce. But I could be wrong.

  2. jlwile says:

    Thanks for your comment, Jonathan. I had not heard that about agoutis, but I looked into it a bit. While a lot of sources state that agouti are the only animals in the forest that can crack open the seeds, those references tend to be “soft” ones. What I would consider more reliable sources, such as Forests and Grasslands by Britannica Educational Publishing, say that agoutis are the main dispersers of Brazil-nut trees, but there are other large rodents that can do it as well, such as the paca. In any event, they are cool!

  3. Eric H. says:

    I found several things that were highly interesting about the Agouti and really do appear that they would be rather smart. First of all, they pretty much capitalize on the work of others!! Free food with a minimum amount of work is always a good option and not really taking to much from the monkeys, since going all the way down to the ground to pick up a bruised fruit would not be worth it for the monkeys.

    I find it fascinating that they have a good enough memory to bury these seeds in multiple locations and yet remember most of them when times get tough, and I presume they remember ones they’ve reburied as well. Since I wasn’t able to see the entire transcript, because I’m not subscribed to the journal, I wonder, what exactly did the burying pattern look like? If it was very evenly spaced, with plenty of space even between individual seeds, than I would find it VERY unlikely that the trait doesn’t show hallmarks of design and that it is just a happy side effect of their behavior. I am not sure how much time lapses between when they bury these seeds and dig them up, but if it was long stretches, that would make their memory quite impressive.

    The burial mechanism would serve a two way function for the Agoutis. It would spread out the seeds so that the tree benefits by not competing with its parent tree, but it would also supply a generous future supply of seeds to future generations of Agoutis as well, and the cycle would continue indefinitely as long as the Agoutis don’t go extinct from predators or some other cause. Would evolutionists think that these trees and the Agouti co-evolved? Because it doesn’t seem likely the spreading and or even survival of these trees would be very efficient before the Agouti came along.

    “However, they speculate that perhaps the agoutis understand that black palm trees are where other agoutis will look for seeds, and so they hide the seeds they have found far from such places, out of fear that other agoutis will find them.”
    My first question would be, are the seeds in that short of a supply when the trees are producing seeds that there would be intense competition for seeds? It seems like alot of seeds are never even eaten after being buried. I would think this would be considered an evolutionarily wasteful trait, burying way more than they would ever need, spending all kinds of energy, putting themselves at more risk with potential predators by going in open areas. If an evolutionist says not, it would sound eerily like design, because it would give you the impression that nature was actually planning ahead with the goal in mind that there might be a famine. We know that the very tenent of nature is that it is blind and doesn’t plan for anything, only the here and now means something. I’m not sure that competition for seeds is that great during growing seasons. That is definitely an idea that scientists could test though. Do squirrels perform similar behaviors? It seems like around here, there is always a multitude of acorns that no squirrels ever touch around trees, giving them all plenty for winter.

    If we could isolate the Agouti for several generations and give it palm nuts and it returned, doing the exact same burial methods as before, it would show that it’s a programmed trait. I would be interested to see if it is a learned trait or programmed. I’d make a bet that it is programmed.

    How do you think they could go about testing these ideas to see if they hold any merit? I fear that they will take these ideas for the reason why the Agouti does what it does, and hold onto them blindly without any scientific verification to see if they are true. Don’t get me wrong, the reasons for what the agouti does that they give may be true, but they most certainly may not be either. What do you think?

  4. jlwile says:

    Thanks for your comment, Eric. The authors do have a graph in the paper showing the burial pattern, and it is not evenly spaced. Of course, that’s not surprising, because the black palms aren’t evenly spaced, either. In order to go to the areas of low black palm density, the pattern cannot be even.

    Based on the observations of many species of scatter-hoarders, there is a lot of competition for the seeds. That’s because not only are they eating the seeds, they are also burying them. So…think about an entire local population eating what they need and storing up food for the entire year. That does produce a lot of competition.

    I am not sure how your ideas could be tested. The problem is that as soon as you isolate an animal or set of animals, they don’t behave the same as they do when they are not isolated. In addition, I would not expect the pattern to be the same every year, because new black palms grow and old ones die. As a result, the places where black palm density is low keeps changing.

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