Work for Food – A Solution to Restricting Food Intake in Group Housed Rats?
Rodents spend a great proportion of their time searching for food. The foraging drive in rats is so strong that the animals readily work for food even when food is freely available. Commonly used ad libitum feeding is associated with a reduced life span, increased incidence of tumours and risk of liver and kidney diseases. It is also considered to be the most poorly controlled variable in rodent bioassays. The aim of this study was to assess whether rats will gnaw wood in order to obtain food hidden in wooden walls, whether this activity has a beneficial effect on controlling weight gain, and whether a typical diurnal activity rhythm is maintained. A total of 18 BN/RijHsd and 18 F344/NHsd male rats were housed in either open or individually ventilated cages (IVC), three rats in each cage. 10 of 36 were fitted with a telemetric transponder. Four groups were used: two groups (diet board and plain board) with a maze made of two crossed aspen boards, the third having a rectangular aspen tube. One maze was of plainboard, but the other included drilled holes snugly loaded with food pellets, the “diet board”, such that the rats had to gnaw wood to reach the food. The other two groups – and the controls – were fed ad libitum. The study used a crossover design and the added item was changed every two weeks. Rats, added items, and amount of food left at the end of the two week period were weighed. The statistical assessment showed that in terms of weight gain there was a significant interaction both in IVC- (p = 0.005) and in open cages (p < 0.001) between the strains and the group. In the F344 rats the diet board was more effective in controlling weight, but when combining the strains, all comparisons with diet board were significant (p < 0.05). Use of strain and added item as main effects, and age as covariate, showed that in the IVC-system there was a significant (p < 0.001) interaction between the strain and the group, this effect being rather clear in the F344 rats in terms of amount of food disappearing. In the open cage system, both strain and group were significant (p < 0.001) factors; all three comparisons with diet board were significant (p < 0.001) in the amount of food disappearing. In conclusion, the work-for-food approach appears to be a promising way of avoiding obesity without causing untoward effects on diurnal activity in rats. Hence, the approach may have considerable refinement and reduction potential.