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In thinking out our original experiment, we wanted to add a stage where the rats were in the tank together without the screen. There would be one pile of food and both their boxes would be in the tank. Unfortunately, we ran out of time to do a real test on this. We did however, do one trial just to see what would happen. The results were very interesting.
Penelope hoarded 28 pellets and stole 7 pellets from Louise’s tank.
Louise hoarded 25 pellets and stole 4 pellets from Penelope’s tank.
The rats were at some points rough with each other. It was interesting to see that it took no time for them to distinguish their own boxes. The rats seemed very competitive and protective over their box and food.
Here is a video that shows what happened during the trial.
We have finally completed 8 trials of each condition (alone vs. together) in which we have collected data on the number of pellets hoarded by each rat. The following graphs summarize the data for each rat for each of the 16 trials, as well as averages:
You can see that although we had a rocky start at the beginning of the semester when we were unable to make the rats hoard, we ended strongly with each rat continually hoarding throughout the course of the experiment. Honestly, the journey of trying to figure out why the rats weren’t hoarding was more strenuous and confusing than our actual experiment!
Although we have yet to assess the significance of our findings, you can see from the graphs that both rats hoarded more when in the presence of one another than when alone.
Louise’s averages: Alone = 30.75, Together = 37.63
Penelope’s averages: Alone = 28.88, Together = 45.00
We will continue to research possible explanations for this specific outcome, but for now, we are thinking the following:
According to Steven Van Der Wall’s book “Food Hoarding in Animals,” having another rat present would lead to one or more of the following:
*The other rat serves as a threat to the hoarder by retrieving food from their hoard after they have left (which would inhibit hoarding.)
*Each rat competes for the un-hoarded food that is available (which would increase hoarding.)
*Both rats work together to create a single larder (this would also mean an increase in hoarding.) We were unable to gain enough data on how the rats would react when put together without the screen due to time constraints. However, according to a study by Miller and Postman in 1946, rats do not work together to create a common larder, rather they carry pellets from the common food area back to their individual home cages.
Interestingly, most articles Van Der Wall references (including Stone and Baker 1989, Miller and Postman 1946, and Denenberg 1952) found that rodents hoarded less in the presence of other rodents than when alone in their home cage. However, Van Der Wall also mentions that when animals are competing for a limited resource, each animal may be stimulated to hoard more food in order to accumulate a larger proportion than the other animal present. Thus, it is possible that even though our rats were separated by the wire screen and did not share a common food pile, the mere presence of one another may have caused each rat to feel as if her food source was under threat and that she must hoard more in order to compete with the other.
We did find a few studies that supported our findings. For example, Sanchez and Reichman (1987) found that white-footed mice hoarded more when close to other mice that they could see and smell. Additionally, Hansson (1986) found that bank voles began to redistribute their hoards when in the presence of other animals, but they didn’t significantly increase the amount of food hoarded. However, this supports the idea that animals are affected by the competition caused by the presence of other animals.
We wish we had more time to observe what would happen when the rats were put together (without the screen) with a common food source. Goodwin (1956) suggests that when put together, animals will steal from one another’s food hoards. It would have been interesting to have a few more trials of this in order to see what would happen!
Over the last few days, we have been implementing phase 2 in which we put both rats in the tank together, separated by the wire mesh barrier. For this stage, each rat will remain on her particular side (which we marked with tape) and each rat’s tissue box and washcloth are placed next to each other on each side of the barrier. We run each trial the same way we did in phase 1, except that both rats are run at the same time! To refresh your memory, this is what the apparatus looks like:
And this is what it actually looks like (including both rats):
We are hoping to see either an increase or decrease in hoarding when they are together in comparison to the trials in which they were apart. We have reasoning to believe that a change could occur in either direction.
1) Explanation for why they may hoard more when apart than together:
-When together, they are distracted by the other rat’s presence and simply are too distracted to hoard.
-When apart they are more comfortable (less threatened by the possibility of another rat stealing their food) and therefore hoard more.
2) Explanation for why they may hoard more when together than apart:
-Social facilitation theory–Perhaps when in the presence of one another, the rats are aroused into performing better on the simple task of hoarding.
-Fear that if they do not hoard many pellets, the other rat may steal from their food source or even their hoard.
-Competition for resources
After three trials of running the rats together, Louise has hoarded an average of 29 pellets and eaten an average of 2.9 grams. Penelope has hoarded an average of 46.3 pellets and eaten an average of 3.9 grams.
We have noticed a number of interesting things:
-The rats seem to be hoarding faster than in “alone” trails but not necessarily significantly more pellets.
-When they see the other rat running toward her food pile, each rat (Penelope especially) tends to follow them to the “food end” of the tank, and then begin hoarding as well.
-The hoarding tends to occur in spurts, with distracted behaviors such as sniffing each other through the screen, jumping up on their boxes, and running around the cage in between.
-Louise likes to jump out of the tank! (This very suddenly started happening and we’re working on it!)
We have included a video that further explains what we have observed.
Here are some graphs that show what is going on with our experiment.
This graph shows the number of pellets hoarded by both Louise and Penelope during the first three trials together.
This graph shows Louise’s hoarding while alone compared to her hoarding with Penelope in the box.
This graph shows Penelope’s hoarding while alone compared to her hoarding with Louise in the box.
After making the previously described changes in our apparatus, we set out to gather data regarding number of pellets hoarded when each individual rat is alone on her particular side of the mesh divider. After 8 trials ( we would have liked to do more but couldn’t given our time constraint–it’s almost the end of the semester!) Louise has hoarded an average of 30.8 pellets and eaten an average of 6.3 grams per trial. Penelope has hoarded an average of 28.9 pellets and eaten an average of 6.2 grams per trial.
Thus, it is clear that the changes we made on our apparatus really made a difference. We’ve been doing some research and have found some possible explanations for the success of our changes.
1) Larger space: Many of the studies we have read use long “alleys” attached to the rats main cage. The food was placed at the end of these alleys, allowing the rats to travel a ways to retrieve the food, as they would in a more natural, wild situation.
2) Familiar environment: A study done by Miller and Viek (1944) suggested that rats may hoard more in their home cages because they see their cages as a place of maximum security. This study worked off the basis of a concept proposed by Bindra (1948b) called the security hypothesis. This hypothesis assumed that rats are relatively “anxious” and “insecure” when outside of their home cages, which would interfere with natural hoarding behavior. Bindra also suggested that the amount of anxiety experienced by the rat depended on its individual characteristics, as well as the characteristics of the situation. Thus, different rats could react differently in different situations. It is also interesting to note that the characteristics of the “home cage” that make it familiar to the rat are unknown. However, it seems as if we were successful in assuming that a familiar smell (their own urine) and object (the wash cloth) would make the box seem more familiar.
We have begun the second phase of the experiment which includes gathering data on the number of pellets hoarded while the rats are together in the tank, but separated by the wire mesh divider. We will attempt to get in at least 8 trials during this phase as well.
It looks like we’ve hit a home run! After a lot of struggling, problem solving, and changing different aspects of our experiment, we have finally managed to get the rats hoarding again. To review, we began our experiment hoping to discover whether rats hoard more pellets when alone or when they are together, but we ran into a problem when the rats were only averaging about 7 pellets per trial. Thus, we set off to answer the question of WHY the rats were no longer hoarding. Over the course of a couple of weeks we researched hoarding and changed the following things, hoping they would make a difference and increase the number of pellets each rat hoarded:
1) Placed the pellets in a container rather than in a pile on the ground
2) Delivered the pellets one by one by hand instead of in a pile
3) Varied the size of the pellets
4) Fed them before each trial to ensure they were no longer hungry (versus depriving them)
5)Took away hoarded food to motivate them to hoard more
6) Gave them free reign of the whole tank (more space)
Unfortunately, NONE of these manipulations worked. What DID work was the addition of a familiar washcloth and “cubby” (both concepts are described in previous posts) into the experimental tank, along with a slightly redesigned apparatus (which is also described in previous posts.) From research we learned that rats tend to hoard more in their own cage, and a larger space is more representative of a rat’s natural environment.
Thus, we have begun collecting data in hopes of pursuing our original proposal for the remainder of the semester. After 3 trials, Louise has hoarded an average of 34.33 pellets, and Penelope has hoarded an average of 29.0 pellets. You can see in the following graph that this is a vast difference from previous trials!
This graph shows the change in Penelope’s hoarding behavior between the old apparatus and the new apparatus.
This graph shows the change in Louise’s hoarding behavior between the old appartus and the new apparatus.
In the following video, you can see Penelope’s great hoarding technique! This consistent back and forth movement between the food pile and her box is a huge difference from previous trials.
We will continue with “alone” trials for the next week, and then switch to trials in which Penelope and Louise are in the tank together, separated by the wire mesh divider. We will stick with the following small changes:
1) We will stop counting the number of times each rat takes a bite of a pellet. Instead we will keep track of how many grams each rat eats.
2) Instead of ending each trial after 3 minutes of non-hoarding or up to 50 pellets hoarded, we will make each trial 25 minutes long or up to 50 pellets hoarded. We noticed that at times the rats would become distracted for a few minutes (standing on their hind legs, sniffing, jumping on their boxes, etc.) but then continue hoarding. Thus, we felt it was appropriate to wait and give the rats the opportunity to hoard even more pellets. Here is a video of Penelope NOT hoarding (due to distraction behaviors) in the beginning, and then getting “re-focused” and starting to hoard again:
Since our last update, we have made a couple of changes to our hoarding experiment:
1) In order to make the experimental tank more like the familiar home environment, we are incorporating a “cubby” (small tissue box) in which we place a small wash cloth that is covered in each rat’s sent. The cloths are kept in each rat’s cage 24/7 and are currently covered in urine and other rat smells! We place the tissue boxes in the cages for about an hour each day in order to make the rats familiar with them. Unfortunately, we cannot leave the boxes in the cages all the time because they take up so much of the already limited space, and Louise and Penelope have been chewing on them (Louise especially!)
Here is a picture of a tissue box and wash cloth inside Penelope’s cage.
We have found that when the “cubbys” are present and we deliver food to the rats, they bring the food into the cubbys nearly 100% of the time.
2) We redesigned the apparatus to give the rats a larger space, as well as to make the experimental situation more like a rat’s natural hoarding environment. We switched to a much larger tank, and used a newly designed wire mesh divider to split it in half longways. Thus, we are able to give them more space, while also maintaining the option of testing the rats together as well as alone as we had originally planned. We chose to give them long rectangular spaces rather than wider square spaces because this allows us to place their food further from the hoarding area, making them travel further for the food, and making the situation at least a little bit more natural. Additionally, each rat is kept on a specific side and the tank is deodorized using a natural cleaner between every trial.
Here is a picture of the new apparatus with Louise’s box (and Louise’s tail!) included.
To make things more clear, here is a representation of the above apparatus.
The following drawing represents the set up we will use when if/when we put both rats in the tank to assess how much they hoard when they are together.
Finally, we have included a video showing Penelope’s behavior when we added the tissue box to the experiment (this was not official, we simply wanted to see what she would do.)
Over the past few days, we have noticed that the rats have plateaued at an all time low number of hoarded pellets. Over the last 4 trials, Penelope has hoarded an average of 8.33 pellets and eaten an average of 7.67 times. Louise has hoarded an average of 8.67 pellets and eaten an average of 5.33 times. This wouldn’t be such a worry for us and we would continue with the current experiment plans; however, we know that 1) hoarding a large number of pellets is a known natural behavior for rats, and 2) we have observed the rats hoarding many more pellets under other circumstances (i.e. when Penelope hoarded 30+ pellets on the first day and when they hoard a large amount of pellets while in their home cages.) Thus, although we began our experimental process by investigating whether rats hoard more when alone or when together, we have molded our experiment to simply find out WHY both rats stopped hoarding.
Like we have said before, there are many things that affect hoarding behavior, and therefore many variables that we could manipulate. We will attempt to tackle a number of these variables to see what will and will not increase the number of pellets hoarded, from our low baseline. We know that our baseline includes a foreign environment, delivering the pellets in piles of five into a plastic container, and a relatively small space. We will keep these variables in mind when we change other aspects of the experiment, and we will attempt to control for as many things as possible.
We will be focusing on the following specific changes:
1) Although research suggests that hoarding is based on deprivation, we will try to make the rats as full as possible and see if they hoard more (or at least eat less!)
2) Changing how food is delivered to the rats — Instead of delivering it in piles, we will deliver each pellet one-by-one by hand.
3) Although pellet size has also been shown not to make a significant difference in number of pellets hoarded, we will use bigger pellets so that perhaps they are less likely to eat the pellets.
4) We have already attempted to take pellets away once they are hoarded to prevent the rats from eating and to see if this would motivate them to hoard more (or faster.) As a result, Louise began to “protect” her food by standing over it and watching for our hands. It also just made her eat the pellets faster!
5) We have read literature that suggests that rats will hoard more in a familiar environment that in a novel one (however, we also read that rats habituate to novel environments and hoarding will eventually even out.) We have also observed each rat hoarding food within their own cages, and when we continuously delivered individual pellets to the rats in this environment, they hoarded a large number of pellets! Thus, we would like to see how they would behave in a more controlled, yet familiar, environment. In order to keep things as controlled as possible, we will place a cloth in each rat’s cage, hoping that the cloths take on each rat’s scent. The hope is that when we place this in the experimental environment, the rats will feel comfortable, and hoarding will increase. We may also add a small “cubby” (probably made of a shoe box cut in half) inside which we will place the “familiar” cloth. This may also stand as a home environment, and may increase hoarding. Here is a picture of one of the rat’s cages with the cloth inside:
6) It is possible that our experimental environment is too small — it may be unnatural to find food so close to where they must hoard (maybe the environment is too ambiguous.) Thus, we could remove the wire mesh barrier and allow the rats free reign of the entire tank, delivering the food at one end of the tank. We were also brainstorming new apparatus ideas and came up with the following:
Using the same glass tank, we could section off two corners and part of the long side of the tank, creating a chute down the middle and an open section at one end. We would place the food at the end of the chute and hope that the rats would bring the food back to the open end. We could even add the cloth and cubby to this environment.
We were also thinking it could be as simple as just removing the wire mesh barrier, adding the “cubby,” and placing the food at the far end of the tank like this:
Finally, here is a video of the hoarding behavior we have observed in the home cage environment. We hope this will translate to the experimental condition given our new plans!