More than a year ago, when we were working with some CD-1 mice on a separate project, our technicians brought a problem to our attention–they were having to feed some cages almost daily instead of weekly. The food was being ground up by the mice and the waste material, also known as orts (best word ever), was accumulating at the bottom of the cage. This resulted not only in increased feed usage, but increased cage cleaning, as the cages would fill up with this combination of ground food and bedding.
Not every mouse on the project was grinding food, however, suggesting that this was abnormal behavior and perhaps a welfare concern. Based on the dual concerns of husbandry and welfare, we decided to investigate the causes of the food grinding further.
Rodents are granivores—animals that preferentially eat plant seeds and grain. One thing that seeds have in common is an outer layer that must be stripped from the seed before the nourishing inner kernel is revealed. Rodents are also animals with constantly-growing teeth, which means they have a need to gnaw on hard objects to help wear those teeth down. Our question was: Does food grinding indicate a need to gnaw or a need to interact with seeds?
To try to determine possible motivations for the behavior, we placed our mice in one of three treatment groups. One group, the control, received no further enrichment. A second group received a nylon chewing device in their feeder. The third group received 50 grams of shell-on sunflower seeds in their feeder. We first collected baseline measurements, then followed the mice for four weeks after placing the enrichments. Ort production was calculated by taking the dirty bedding, orts and all, then drying and sifting it, and weighing the orts that were sifted out.
Both enrichment groups showed a significant decrease (p < 0.05) in ort production compared to baseline measurements, but only mice provided with sunflower seeds maintained the decreased rate of food grinding after the treatment was withdrawn. We also found that cages with greater average body weights had lower levels of ort production. This suggests that a simple need to gnaw doesn’t entirely explain food grinding, and that a nutritional motivation may also be involved.
The food we fed the mice in this study was the standard Charles River chow, which contains about 6% fat and 18% protein. Sunflower kernels are 51% fat and 21% protein1, which is quite a bit different! As other researchers have found—though we did not test—orts have less energy content per gram than food pellets2, which suggests that mice could be grinding food to get at nuggets high in fat or proteinthat are not completely mixed into the food pellet. The orts produced may be the equivalent of sunflower seed hulls—the mice are treating the food pellets as if they were giant seeds and chewing them in an attempt to get to the high-energy kernel that is supposed to be inside. This would also correlate with the work of yet more researchers who found that food grinding is increased at lower temperatures, when animals have greater energetic needs3.
We are currently working on a follow-up study, where we are comparing the response of mice to both sunflower seeds and a diet formulated to be a sunflower seed without the hull. We hope that we will be able to duplicate the decrease in ort production we found in the first study. We also hope that this study will help us figure out if the decrease in ort production was related to increased energy provided by the sunflower kernels or if it was related to meeting the frustrated need of a granivore to interact with seeds.
(This work has been published: Pritchett-Corning, K.R., et al., Can seeds help mice with the daily grind? Lab Anim, 2013. 47(4): p. 312-5.)
Composition of foods: raw, processed, prepared. USDA National Nutrient Database for Standard Reference, Release 25 [Internet] 2012. Available from: http://ndb.nal.usda.gov/
Cameron KM and Speakman JR. The extent and function of ‘food grinding’ in the laboratory mouse (Mus musculus). Lab Anim 2010; 44: 298–304.
Koteja P, Carter PA, Swallow JG and Garland T Jr. Food wasting by house mice: variation among individuals, families, and genetic lines. Physiol Behav 2003; 80: 375–383.