Squirrel-Nut Economics And Other Agility Tricks
In many parts of the country, the lead-up to winter is a busy time for squirrels, furiously collecting and hiding acorns and nuts for the cold months ahead. But how can squirrels recall where it has stashed all its stores? And what can studying squirrels tell researchers about memory, learning, and economic decision-making in other species?
Ira talks with Lucia Jacobs, a professor in the department of psychology and the Institute of Neuroscience at UC Berkeley, about her studies of the campus squirrels—from learning about their cognition, learning, and memory to recording the acrobatic movements of a squirrel on the ground and in the treetops. Jacobs co-leads a ‘squirrel school,’ observing rescued and orphaned juvenile squirrels as they learn normal squirrel behavior, and is contributing to a project seeking to develop robots using agility tricks learned from the rodents.
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Lucia Jacobs is a professor in the department of psychology and the Institute of Neuroscience at UC Berkeley.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. In many parts of the country we’re just past peak squirrel season when the bushy tailed rodents are in overdrive hiding nuts for the winter ahead. And if you’ve ever spent time like I have, watching a squirrel hide a nut only to zoom away, run along a cable, or devise some acrobatic assault on your bird feeder, which I have watched too many times, you’ve probably got squirrel questions.
Joining me now is Lucia Jacobs. She’s a professor in the Department of Psychology and the Institute of Neuroscience at UC Berkeley, and she co-leads something called squirrel school. Welcome to Science Friday, Dr. Jacobs. Is school in session now?
LUCIA JACOBS: School’s in session. Thanks for having me.
IRA FLATOW: You’re welcome because I have such mixed feelings about squirrels. Like I said, they attack my bird feeder. They dig up my bulbs. Yet I am in admiration of them for their athletic and acrobatic abilities. They’re unbelievable.
LUCIA JACOBS: Well, I think squirrels are very polarizing. And there are many people who would state what you just said much more strongly, that they actually hate, detest squirrels.
IRA FLATOW: We are on the radio, so I couldn’t really tell you.
LUCIA JACOBS: OK. No, it’s fascinating. It seems some people are fascinated and walk to the park with bags of nuts, and other people can’t stand them. And you know my response is squirrels are like humans. I mean, they’re very similar.
The eastern gray, we’re talking about the eastern gray squirrel– they’re extremely smart. They’re extremely destructive. They’re generalists. They can kind of eat most anything. At the same time, they’re specialists because they have this fascinating co-evolved relationship with oaks and hickories, so that because of them we have forests. So people have a lot of mixed feelings, I think.
IRA FLATOW: Yeah, so tell me about your feelings. How did you get interested in squirrels in this whole universe of animals to study cognition in?
LUCIA JACOBS: Well, it was actually– it did come down to a food story because I was fascinated that there are different squirrel species on the east coast that store food differently. And the red squirrel puts all its eggs in one basket, defends the basket, larder hoarding. And the eastern gray is obligate scatter hoarder, where every large seed is individually buried and without defense.
And so I was fascinated by the kind of economics of these two species that often overlap. How did they work that out? So I was really interested in their kind of economic systems. And since then I found that was a good place to start because the eastern gray squirrel does have, in fact, a fascinating and complicated economic system.
IRA FLATOW: Well, when you say economic system does that mean, making choices about burying the acorns, or eating them, or exactly what are you talking about? I don’t think of squirrel economics, but I will now.
LUCIA JACOBS: No, I think if you think about humans, you get some income and you have to decide. Do I spend it now? Do I save it for the future? And that’s what squirrels do also. That’s why squirrels and humans are so similar. All these nuts come in. Do I eat the nut? Do I cache it for the future? If I cache it, where do I cache it? How do I protect it? How do I defend it? When do I get it back again? So squirrels have months and months of foraging decisions about one nut and they have to do that about 3,000 times every year.
IRA FLATOW: Do they obsess over this? I mean, you know, get crazy because they have to make all these decisions?
LUCIA JACOBS: What’s fascinating is when we looked at these decisions in the summer. For squirrels, the summer is a terrible time because they’ve eaten all their stores from the year before. And the new crop hasn’t showed up. And so the summer is when they’re actually very thin, very starving. That’s when they’re really going to attack your bird feeders.
And what we found is when you give squirrels nuts in the summer, they make that decision much more slowly, much more carefully. We actually quantify a squirrel picks up a nut, and they roll it in their paws. And then they put it in their mouth, and they shake their head from side to side. It’s very quick. You have to look very carefully to see it.
And what we showed is they’re actually weighing the nut. And depending on how much the nut weighs, they will carry that nut a little further away from the place where they found it. Because the place where they found it is generally where all the other squirrels are. And so they want to get it far away as possible.
So it’s this really careful economic decision, and we can actually see them doing it because we can quantify these paw movements. So you can actually see an animal thinking, what is this worth? And then you can see their response. And there’s all these different kind of ecological factors that go into this one little decision. But in the fall– the fall is when they turn into caching machines. That’s when you’ve got all this availability and the decisions are very fast.
IRA FLATOW: Do we have any idea, speaking of burying those nuts, of how they remember all those hiding places?
LUCIA JACOBS: I mean, we’ve shown with captive squirrels that they remember and that’s not a surprise. This is what we have been able to show in the lab is that they’re very accurate. And when they’re very hungry, they go first to their own caches, and then they go to other squirrels caches because squirrels have a very good sense of smell. And they, in fact, if you plant 80 walnuts in your backyard a squirrel will come and find 79 of them within a month.
But the way we’ve interpreted this is you can see, when they’re very hungry, they go first to the sure thing. I mean, you would do the same thing. If you’re starving, you’re not going to cook an elaborate meal. You’re going to grab something right on the counter. Right? And that’s what the squirrels do.
But if you think about squirrels, what the natural conditions– they’re living in areas with snow. And when you’ve got deep snow, or if you’ve got an ice storm, you’ve got a frozen layer, using your sense of smell is not going to help you. You’re going to freeze to death before you can find someone else’s caches. And so when things are really cold and really frozen, squirrels tend to stay in their nests.
But then when things warm up, they’ll come out, and they’ll beeline to their own caches because they know where they are. And you’ll see in snow, you’ll see a squirrel running across the snow, and then going straight down, coming up with a nut, and going off to a safe place to eat it.
IRA FLATOW: Wow, so do they have any memory tricks? I know if I buried something, I’d say, well, it’s five feet from the oak tree, four feet from this tree. It must be right here. Do they do that kind of calculation or have any tricks they use?
LUCIA JACOBS: That is a fascinating question that we don’t have the complete answer on. What we have shown in the campus squirrels is that they do seem to organize their caches by nut species. And this is a mnemonic trick. It’s called chunking.
So if I give you a list of 10 items to remember, and you can categorize those items into a kind of fruit, and a kind of dog, for example, then you’re going to remember and recall that much more accurately. And we know this from lab rats. They do this.
And so what we found with the squirrels is we gave them a random series of five different kinds of nuts. They would actually cache the same species of nuts, statistically, in a clumped distribution. So you would see a clump of almonds, and a clump of peanuts, and a clump of walnuts. And we interpret that as the mnemonic.
What they’re doing is spatial chunking to improve their retrieval. What’s interesting is they not only– when they’re doing that, imagine you’re a squirrel, and you’re getting this rapid fire presentation of different nuts. And the nuts are different species, and sizes. And what the squirrel is not only thinking, Oh, OK, this is an almond. Where did I cache the other almonds?
But they’re also measuring the weight of that almond. And we’ve shown that within almonds they carry a slightly heavier almond significantly farther. And they’re also caching the almonds at a specific density. So the more valuable the nut, which is basically its fat content, the lower the density.
Because they don’t want to– this is a problem. They’re constantly stealing from each other. So if you have these great nuts close together, someone’s going to come in, say aha, here’s an almond. And they’re going to look around quickly and find the others. So the better the nut, the more widely dispersed they are.
IRA FLATOW: They’re all thieves, then.
LUCIA JACOBS: Oh, absolutely. I mean they’re very human-like. That’s what I keep saying. They’re– humans are the only primate that stores food. And I think we can learn a lot from squirrels for that reason.
IRA FLATOW: I want to talk about what you started our conversation– or what, actually, I started it with, which is something that you run, called the squirrel school. What do you teach in that school? Or do they teach you, or what exactly is that?
LUCIA JACOBS: So this is an idea I have that squirrels are actually very commonly orphaned in the United States and other countries, where a tree nest will be blown down, and the baby squirrels are brought to a wildlife rescue organization, who raise them for two months and then release them again, back where they were found.
And so I was very curious to know what do squirrels– what can they learn during those two months? And so we’re working with rescue organizations to study the development of cognition in these orphaned squirrels, and with the goal that we’ll then be able to follow them after release, and see what influences early rearing have on their survival. That’s the blue sky goal.
IRA FLATOW: Interesting. If you have a bird feeder, which I have a few, you might not be feeling so warmly towards squirrels because they have a knack, they have an incredible– they can break into almost any bird feeder that we come up with. How do they have that skill? I mean, what– and they’ll keep trying over and over again. What is there about their personalities, if I might call it, that allows them to do that?
LUCIA JACOBS: Well, they’re generalists. They’re facing a year where for the fall they know exactly what to do. But then come March through August they have to eat anything they can find. And that’s tree buds, and leaves, and insects, and mushrooms, and all kinds of things. So they really have to be creative to survive.
IRA FLATOW: But they also seem equipped to break into stuff with, like, their little hands, and claws, and stuff like that.
LUCIA JACOBS: Well, I mean, right. So think about a squirrel as a burglar with who has built into her head a little chainsaw that can cut through metal. And they’ve got incredibly– they’ve got front paws that can manipulate and hold things. They can run on the ground. They can jump. They can climb up a tree.
They can go up a tree as fast as they can go down a tree. Their wrists actually completely can rotate 180 degrees. So they can hang easily from their back feet. With their bushy tails they can leap over great gaps. They really own the physical environment. You putting up a bird feeder is just a challenge to them.
IRA FLATOW: Yeah, I noticed that because I know of a lot of people who– it’s like Spy vs. Spy, trying to outwit the squirrel. I often see these squirrels around my house all the time. Am I seeing the same ones all the time or are there a lot of different ones roving through my neighborhood, scavenging?
LUCIA JACOBS: They’re very likely the same. In fact, I think the squirrel in your backyard knows you better than you know them.
IRA FLATOW: I’m sure he does.
LUCIA JACOBS: They can live to be 20 years in the lab but even in the field 12 years. We’ve got squirrels at Berkeley that we followed for seven years. Yeah, they’re very long-lived. You know, they’re– I mean, that’s what’s so fascinating, actually, about their cognitive development is that it takes them weeks and weeks to learn how to be a squirrel. Which compared to a lab mouse, who’s born and out by three weeks– very different situation.
IRA FLATOW: This is Science Friday from WNYC Studios. In case you’re just joining us, we’re talking squirrels with squirrel aficionado and expert Lucia Jacobs of UC Berkeley.
Do they know when predators are around? Because there are days where there’ll be no squirrels at all, and then there’ll be a dozen of them. I mean, it seems like they all know to stay away. And I asked the neighbor once, who has a lot of bird feeders or whatever, and she said, you know, there must be a falcon flying around here. Would that be true?
LUCIA JACOBS: No, there’s actually a new study showing that they eavesdrop on birds. And they use birdsong to know when predators are around. And they’re also, of course, listening to other squirrels. But yeah, they’re actually listening to the birds, yeah. So absolutely.
IRA FLATOW: Would you say then, if they’re talking to birds, they know what’s going on? Would you say they’re smart, or just very skilled, or how would you put that?
LUCIA JACOBS: Well, I’d say that– I would say are humans smart? They’ve got all this skill and motor learning, plus they’ve got all these spatial cognition challenges, which is they’re both moving in three dimensions, the tree, and they’re foraging in three dimensions. And then they’re caching in two dimensions.
And they have this months and months of caching, and retrieving, and pilfering from each other. And, of course, they’re very sensitive to who’s stealing. We’ve also got some preliminary evidence that squirrels are actually caching in areas near where other squirrels they’re related to are caching.
So what you’re seeing in your backyard is squirrels who know each other very well. There’s probably kind of extended kin groups. We know that, particularly, eastern gray squirrels have these winter nesting organizations that we don’t really understand what’s going on, but it’s clearly something interesting and social. So, yeah, I think they’re– you’ve got a posse out there who’s keeping an
IRA FLATOW: It certainly seems that way. And some people who are not squirrel fans, and certainly here in the New York City area, have called them rats with bushy tails. I mean, how accurate is that?
LUCIA JACOBS: Well, like I said, I could insult humans in various ways, too. They’re not– they’re obviously– they’re rodents but rodents also include species like beavers, which are very similar to squirrels in that they have this massive effect on the landscape. Beavers change the landscape. Squirrels change the landscape. They actually create their own forests.
And, of course, squirrels are not not in the rat family. They’re a very ancient lineage. They’re about 36 million years old. And the first squirrels, from actually quite remarkably well-preserved skeletons, look like squirrels. They look like– the first squirrel might have been something very similar to a tree squirrel.
They’re also very unusual because they’re diurnal. So they have better color vision. They have their visual cortices are more similar to primate. They look more like a monkey than a rat. So there’s lots of ways that, I think, we should think of squirrels as these very smart, innovative monkeys, really, that live in our cities.
IRA FLATOW: Well, you have given me a different way to think about squirrels, and I think I’m a better person now for that.
LUCIA JACOBS: I have done my job.
IRA FLATOW: Thank you very much for taking the time to talk with us today, Lucia.
LUCIA JACOBS: Thank you. Thank you it was a pleasure.
IRA FLATOW: You’re welcome. Dr. Lucia Jacobs is a professor in the Department of Psychology and the Institute of Neuroscience at UC Berkeley.