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Why Rats Can’t Go Grocery Shopping

ALTHOUGH ROBIN DUNBAR’S social-brain hypothesis has, for the past several decades, held primacy among scientists as the leading explanation of brain expansion in primates, there is an alternative explanation: what has been called the ecological-brain hypothesis.

As we have seen, early primates were not only uniquely social but also had a unique diet: they were frugivores. Fruit-based diets come with several surprising cognitive challenges. There is only a small window of time when fruit is ripe and has not yet fallen to the forest floor. In fact, for many of the fruits these primates ate, this window is less than seventy-two hours. Some trees offer ripe fruit for less than three weeks of the year. Some fruit has few animal competitors (such as bananas in their hard-to-open skin), while other fruit has many animal competitors (such as figs, which are easy for any animal to eat). These popular fruits are likely to disappear quickly, as many different animals feed on them once they ripen. Altogether this meant that primates needed to keep track of all the fruit in a large area of forest and on any given day know which fruit was likely to be ripe; and of the fruit that was ripe, which was likely to be most popular and hence disappear first.

Studies have shown that chimpanzees plan their nighttime nesting locations in preparation for foraging on the subsequent day. For fruits that are more popular, such as figs, they will go out of their way to plan where they sleep to be en route to these fruits. They do not do the same for less competitive but equally enjoyable fruits. Further, chimpanzees will leave earlier in the morning when pursuing a competitive fruit than when traveling to forage for a less competitive fruit. It has been shown that baboons also plan their foraging journey in advance and will leave earlier when fruit is less abundant and likely to be depleted more quickly.

Animals who feed on non-fruit plants do not have to cope with this same challenge; leaves, nectar, seeds, grass, and wood all last for long periods and are not localized in sparse patches. Even carnivores don’t have as cognitively challenging a task—prey must be hunted and outsmarted, but there are rarely only short time windows in which hunting is possible.

Part of what makes this frugivore strategy so challenging is that it requires not only simulating differing navigational paths but also simulating your own future needs. Both a carnivore and a non-fruit-eating herbivore can survive by hunting or grazing only when they are hungry. But a frugivore must plan its trips in advance before it is hungry. Setting up camp en route to a nearby popular fruit patch the night before requires anticipating the fact that you will be hungry tomorrow if you don’t take preemptive steps tonight to get to the food early.

Other mammals, such as mice, clearly stock up on food as winter months approach, storing vast reserves of nuts in their burrows to survive the long stretch when trees produce little to no food. But such seasonal hoarding is not nearly as cognitively challenging as the daily need to change your plans based on how hungry you will be tomorrow. Further, it isn’t even clear that mice hoard food because they understand that they will be hungry in the future. Indeed, lab mice—although they have never suffered from a cold winter without food—automatically start hoarding food if you simply lower the temperature of their environment, an effect seen only in northern species of mice who have had to evolve to survive winters. Therefore, this doesn’t seem to be an activity that they have learned from past winters and cleverly responded to; it seems that such hoarding is an evolutionarily hard-coded response to the changing seasons.

The ecological-brain hypothesis argues that it was the frugivore diet of early primates that drove the rapid expansion of their brains. In 2017, Alex DeCasien from NYU published a study examining the diets and social lives of over 140 species of primates. Some primates are primarily frugivores; others are now primarily folivores (feeding on leaves). Some primates live in very small social groups; others live in large ones. He surprisingly found that being a frugivore seemed to explain the variation in relative brain size perhaps even better than the size of a primate’s social group.

The Bischof-Kohler Hypothesis

In the 1970s, two comparative psychologists by the name of Doris Bischof-Kohler and her husband, Norbert Bischof, proposed a novel hypothesis about what was unique about planning in humans: They hypothesized that while other animals can make plans based on current needs (like how to get to food when they are hungry), only humans can make plans based on future needs (like how to get food for your trip next week, even though you are not hungry right now). The evolutionary psychologist Thomas Suddendorf would later call this the “Bischof-Kohler hypothesis.”

Humans anticipate future needs all the time. We go grocery shopping even when we are not hungry; we bring warm clothes on trips even when we are not cold. Given the evidence available at the time of Bischof-Kohler, it was a reasonable hypothesis that only humans could do this. But recent evidence has called this into question. There are now anecdotal stories of chimpanzees bringing straw from inside a warm cage to make a nest outside when they know it is cold outside but before they were cold. There have been findings of bonobos and orangutans selecting tools for future tasks up to fourteen hours in advance of that task. Chimpanzees will carry stones from faraway locations to open nuts in areas that have no suitable stones and will manufacture tools in one location for use in another location. Indeed, if frugivorism requires planning ahead before you are hungry, then we should expect primates to be able to anticipate future needs.

In 2006, Miriam Naqshbandi and William Roberts of the University of Western Ontario measured a squirrel monkey’s and a rat’s ability to anticipate their own future thirst and change their behavior accordingly. Squirrel monkeys and rats were both given two options by being presented with two cups. Cup 1 was a “small treat option,” which contained a tiny morsel of food, and cup 2 was a “high treat option,” which contained lots of food. For squirrel monkeys the treats were dates; for rats they were raisins. Under normal conditions, both animals would choose the high treat option; they love dates and raisins.

But Naqshbandi and Roberts then tested these animals in a different condition. Dates and raisins induce large amounts of thirst in these animals, often requiring them to consume over twice as much water to rehydrate themselves. So what happens if these animals are forced to make a trade-off, incorporating their future state of thirst? Naqshbani and Roberts modified the test such that if animals select the high treat option (the cup with many dates or raisins), they will only get access to water hours later; however, if animals select the low treat option (the cup with few dates or raisins) they get access to water between 15 and 30 minutes later. What happens?

Fascinatingly, squirrel monkeys learn to select the low treat option, while rats continue to select the high treat option. Squirrel monkeys are capable of resisting the temptation to have treats now, in anticipation of something—water—that they don’t even want yet. In other words, monkeys can make a decision in anticipation of a future need. In contrast, rats were entirely unable to do this—they stuck with the flawed logic of “why give up extra raisins for water, I’m not even thirsty!”*

This suggests that perhaps Suddendorf’s Bischof-Kohler hypothesis was correct that anticipating a future need is a more difficult form of planning and was correct that some animals should be able to plan but unable to anticipate future needs (such as rats). But it may not be the case that only humans were endowed with this ability. It may instead be the province of many primates.

How Primates Anticipate Future Needs

The mechanics of making a choice based on an anticipated need, one you are not currently experiencing, presents a predicament to the older mammalian brain structures. We have speculated that the mechanism by which the neocortex controls behavior is by simulating decisions vicariously, the outcomes of which are then evaluated by the older vertebrate structures (basal ganglia, amygdala, and hypothalamus). This mechanism allows an animal to choose only simulated paths and behaviors that excite positive valence neurons right now, like imagining food when hungry or water when thirsty.

In contrast, to buy groceries for the week, I need to anticipate a pizza is going to make a great addition to Thursday’s movie night, even though I don’t currently want pizza. When I imagine eating pizza while I’m not hungry, my basal ganglia doesn’t get excited; it doesn’t accumulate votes for any decisions to pursue pizza. Thus, to want pizza, I need to realize that in this imagined future state of hunger, the smell and sight of food will excite positive valence neurons, even though imagining it right now does not. How, then, can a brain choose an imagined path in the absence of any vicarious positive-valence activation? How can your neocortex want something that your amygdala and hypothalamus do not?

There is another situation we have already seen where brains need to infer an intent—a “want”—of which it does not currently share: when they’re trying to infer the wants of other people. Might brains be able to use the same mechanism of theory of mind to anticipate a future need? Put another way: Is imagining the mind of someone else really any different from imagining the mind of your future self?

Perhaps the mechanism by which we anticipate future needs is the same mechanism by which we engage in theory of mind: We can infer the intent of a mind—whether our own or someone else’s—in some different situation from our current one. Just as we can correctly infer the cravings of someone deprived of food (“How hungry would James be if he didn’t eat for twenty-four hours?”) even though we ourselves might not be hungry, perhaps too we can infer the intent of ourselves in a future situation (“How hungry would I be if I didn’t eat for twenty-four hours?”) even though we are currently not hungry.

In his paper discussing the Bischof-Kohler hypothesis, Thomas Suddendorf brilliantly foreshadowed exactly this idea:

Figure 18.1: The similarity between theory of mind and anticipating future needs

Original art by Rebecca Gelernter

Future need anticipation . . . might be only a special case of animals’ general problem with simultaneously representing conflicting mental states. Like 3-year-old children, they may be unable to imagine an earlier belief (or state of knowledge, or drive, etc.) that is different from a present one or to understand that another individual holds a belief different from their own. This may apply to future states as well as to past ones. That is, a satiated animal may be unable to understand that it may later be hungry, and therefore may be unable to take steps to ensure that this future hunger will be satisfied.

Although Naqshbandi and Roberts’s experiment with squirrel monkeys and rats suggests that Suddendorf may have been wrong that only humans can anticipate future needs, Suddendorf may have been prescient in proposing that the general ability to model a dissociated mental state from your own can be repurposed for both theory of mind and anticipating future needs.

There are two observations that support this idea. First, it seems that both theory of mind and anticipating future needs are present, even in a primitive form, in primates, but not in many other mammals, suggesting both abilities emerged around the same time in early primates. Second, people make similar types of mistakes in tasks of theory of mind and of anticipating future needs.

For example, we saw in chapter 16 that thirsty people become incorrectly biased to predict that other people must also be thirsty. Well, it is also the case that hungry people seem to incorrectly predict how much food they will need in the future. Take two groups of people and bring them to the grocery store to shop for themselves for the week, and those that are hungry will end up buying more food than those that were well fed, even though they were both shopping for food to feed themselves for the same window of time, namely, a single week. When hungry, you overestimate your own future hunger.

The ability to anticipate future needs would have offered numerous benefits to our ancestral frugivores. It would have enabled our ancestors to plan their foraging routes long in advance, thereby ensuring they were the first to get newly ripened fruits. Our ability to make decisions today for faraway, abstract, and not-yet-existent goals was inherited from tree faring primates. A trick that, perhaps, was first used for getting the first pick of fruits, but today, in humans, is used for far greater purposes. It laid the foundation for our ability to make long term plans over vast stretches of time.