In various situations animals prefer to work for food even when free food is available. Several hypotheses have been put forward to explain this phenomenon, most of them either suggesting that a learned behaviour may be self-reinforcing or that the animal has to perform a certain species specific behaviour.

Mongolian gerbils (Meriones unguiculatus) were used in the present study. Half of them were given access to a cage with one bowl with sunflower seeds, and another bowl with sand and 30 seeds, 20 min/d for 5 d. The amount of seeds eaten from each bowl and the time spent digging in the bowls, on the floor, and in the corners was recorded. The animals were then put in the same cage with one bowl with seeds but the other bowl now contained only sand (20 min/d for 5 d). For the other group the treatment was reversed.

Almost all digging took place in the 30 seed bowl, and most seeds eaten were taken from this bowl. When this bowl contained only sand the amount of digging went down drastically for that bowl. Nor was there any compensatory digging on the floor or in the corners of the cage.

The present results indicate that the self-reinforcing hypothesis and the obligatory species specific response hypothesis are not enough to explain the contra-freeloading phenomenon. A possible alternative hypothesis, that the behaviour is a way of exploring and reducing uncertainty is discussed.