A few weeks ago NPR’s All Things Considered did a story on recent research supporting Einstein’s Theory of Relativity (see Full Story). Einstein posited that time is affected by your position relative to a gravitational field and by how fast you are moving through space. Dr. Chin-Wen Chou and colleagues demonstrated the validity of Einstein’s theory using atomic clocks placed at different heights. The clock placed a mere 33 centimeters above the other ticked at a faster pace than the one below. Of course, the time difference was infinitesimally small, but different nonetheless. This research got me wondering that since time is relative, not only literally but perceptually as well (hadn’t you noticed), how do animals perceive time? More interesting perhaps, do they perceive time at all?
It seems that my random thoughts led me to stumble across a contentious issue, one almost as controversial as language. A member of the Brambell Committee, Bill Thorpe, raised the question about whether or not animals live solely in the present moment in the 1960’s. He argued that few animals remember the past, even fewer can fear the future, but that animal welfare regulations should account for the capacity of some animals to suffer in their own mind. In general, this ability to mentally time travel has been, like language, argued to be an expressly human trait. To further this argument, researchers have suggested that language itself allows for mental time travel. This makes a nice circular line of reasoning that no animal can penetrate. If animals do not have language, then they cannot time travel. If they only feel pain in the moment and are not traumatized by their memory of that pain or the anticipation of future pain, this provides a neat excuse for human behavior towards animals. However, science often moves forward through dissent and many researchers have challenged the conclusion that animals are “stuck in time”. The result is decades of research that is beginning to provide new insights into the capacity of animals to remember and anticipate the future.
The primary type of memory dealing with time is episodic memory and there are several definitions. A classic definition incorporates: what happened, where did it happen, and when did it happen. Keep in mind that the focus is on unique events or episodes. Because we can ask people about their episodic memory using a common language, it is clear that humans have episodic memory. When dealing with non-verbal animals this becomes a bit tricky and behavioral criteria replace verbal responses. In the examples below episodic memory in animals is frequently referred to semantically as “episodic-like” due to the inability to verbally confirm the temporal nature of the memory, but functionally it is the same.
Scrub jays have led the way on revealing the potential for animals to remember and have a working concept of time. Scrub jays are members of the Corvid family. Smaller than the closely related blue jay, scrub jays are frequently found in open habitats dominated by oak woodlands, chaparral, or pinyon-juniper woodlands. The Western scrub jay has long been a model species for studying food caching behavior, spatial memory, and cognitive behavior. Scrub jays store, or cache, their food in many different locations. Usually if you have to store your food it seems like a good idea to remember where you put it. Therefore you can predict that animals that store their food in different locations will, at the very least, have superb spatial memory. Unlike many humans who find that locating their car keys can present unique challenges. If you store your food and that food can spoil, it seems obvious that it would be beneficial to also remember what you stored when. Indeed, several clever experiments have revealed that these remarkable birds learn to avoid recovering food when a long time has gone by and the food has become inedible. Using two types of food sources and allowing the birds to recover stored food at different time intervals, the birds recovered their preferred food item (worms-yummy!) at the shortest time interval when the worms were still fresh and then switched to recovering the other food source (peanuts) at the longer time interval when the worms were decayed. Controls were used to eliminate the use of sight and smell for food recovery, demonstrating that the birds remembered what was stored where and, more importantly, when the items were stored.
The humble lab mouse also has episodic-like memory. Like the scrub jay, experiments have been used to test whether mice remember what item was stored, where it was stored, and in what order. Results show that mice recognize objects they have previously encountered, they remember where they came across these particular objects, and even discriminate the temporal order in which they were presented with different objects. So how exactly does one determine the time component here? Mice were presented with two different objects made of plastic and precautions were undertaken to ensure that odor cues could not be used to distinguish between the two objects and that the mice didn’t have a wacky preference for one or the other of the objects. The mice were placed in an open field with a set of four of one of the objects placed in each corner of the field in random order. After 50 minutes, four copies of the second object were placed in each of the corners. After another 50 minutes, the process was repeated but this time 2 copies of the object used the first time (100 minutes prior) and 2 copies of the “recent” object (50 minutes prior) were places in the corners. Spatial configurations of where the old and new objects were placed tested for the what/where component. Basically, the mice spent more time checking out the “old” objects. Similar experiments with rats have yielded the same result, indicating that there is a clear concept of what, where and when a novel item was encountered. By the way, the 50-minute interval in the mice experiments was chosen based on the time interval used for rats (65 minutes). Though not clear, the values of 100 minutes ago and 130 minutes ago, respectively, may represent the upper limit of time perception in these species. However, it may be more likely that we just have to get more creative in our experiments to assess how long ago they can remember.
Let’s hop back to birds for a moment and talk about the black-capped chickadee. I became fond of these little birds during my time on Long Island. There was little that I enjoyed about living there, but visiting a park where these little birds courageously landed on your hand to gently pluck a sunflower seed out of your palm always made me smile. Somehow I felt like singing a Snow White song and skipping merrily down the path. Like the scrub jays discussed above, black-capped chickadees store food. Therefore, we can already predict that it may be very important to their survival to remember what they store where and how long ago. Similar to the scrub jay experiments, birds visited sites at short intervals (3 hours) or long intervals (123 hours) selecting the food source at short intervals that would not yet be spoiled (mealworms) and visiting the sites that had sunflowers at the longer time interval. Since many birds store their food for access over the winter it is likely that their memory would extend beyond the 123 hours in the experiment. On side note, a second experiment of foraging in an aviary showed that chickadees remember when even if the task at hand does not involve storage and retrieval.
If birds, bees, humming birds, and rodents can do it, then surely apes can too, right? We’re still talking about remembering when…
A study on three species of great ape (bonobo, chimpanzee, orangutan) using food retrieval showed that all three integrated the what, where and how long ago components of the task. What was interesting though was that bonobos and chimpanzees individuals younger than seven and older than 18 were a bit slower on the time component. This is particularly striking because episodic memory in humans shows the same age-dependent pattern (though the specific ages vary). This may indicate similarities in the development of information encoding and storage processes. Not to be excluded, similar findings were reported for a male gorilla named King.
Thus far the discussion has only considered remembering the ‘past’. An intriguing question is can animals anticipate the future? The research on this aspect of mental time travel is under-explored, but once again, this is primarily due to the difficulties associated with identifying what indicates future planning in non-verbal species. However, some research has been done on scrub jays, chimpanzees, and orangutans that does support the existence of future thought by animals. There are several implications of this research, not the least of which directly involves the regulations surrounding animal welfare. Now that is food for future thought...
References:
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