Archive for November 25, 2008
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!