Anyone who has ever owned a pet hamster can't fail to be impressed by the amount of food that these little creatures can stuff into their cheek pouches:at times, up to 20% of their body weight. When hamsters pouch food, they are behaving in a so-called `appetitive' way, meaning that they are driven by hunger to find and store food, rather than to immediately eat what they find. Because hamster cheek pouches are only involved in hoarding, but not eating,`they are a beautiful model system for studying appetitive behaviour', says Carolyn Buckley of Lehigh University, who investigated pouch filling behaviour in the Syrian hamster (Mesocricetus auratus) with her colleagues Jill Schneider and David Cundall(p. 3096).
The team wanted to know how hamsters could get so much food into their pouches, so they filmed hamsters at 60 frames per second as they pouched food pieces and measured their forepaw and jaw movements. They found that the movements differed slightly depending on whether the hamsters were pouching large 2.5 g pellets from their feed or smaller items such as sweetcorn kernels or sunflower seeds. The hamsters tended to use their forepaws to help manipulate large food pieces into their mouths, while they simply grabbed smaller food items with their tongues. Studying the jaw movements, the team found that the hamsters used repeated patterns of jaw movements to get food into their pouches, regardless of the size of the food item. These cyclical movements were very similar to chewing, except that they would pause regularly with their mouths open. `This is probably to allow the tongue to push food into the pouch', says Buckley.
Previously researchers had suggested that the retractor muscle, a large muscle which travels from the mid-spine to each side, over and under the pouch, merely helped support full rodent pouches. But the team's videos showed that the muscle was very active during pouch-filling, so to investigate further, the team operated on some hamsters and cut the nerve supplying the muscle to one pouch while leaving the nerve on the other side intact. If the muscle played a vital role in pouching, `we thought that they might not be able to pouch on the denervated side', Buckley explains. However, they found that hamsters could still pouch the same amount of food into denervated pouches, although they pouched slightly fewer larger items. They also noticed that hamsters filled their denervated pouches more to the side, not pushing food that far into the pouch.
This led the team to suspect that the retractor muscle might play a role in moving food down the pouch, so they stimulated the retractor muscle in one anaesthetised hamster while pushing a blunt probe into a pouch containing a single corn kernel to simulate a second piece of food entering the pouch. They found that retractor muscle contraction straightens folds in the pouch walls,allowing food to slide further into the pouch. Examining their videos once more, the team found that it took hamsters 40% longer to put a second food piece into their denervated pouch than into their unaffected pouch, showing that retractor activity helps move food out of the way as more is added. `The retractor helps make the pouch more efficient', says Buckley.