Blackshaw, J. K., Thomas, F. J., Lee, J.-A. 1997. The effect of a fixed or free toy on the growth rate and aggressive behaviour of weaned pigs and the influence of hierarchy on initial investigation of the toys. Applied Animal Behaviour Science 53, 203-212.

The toys were used as play objects ... and resulted in a lower level of aggressive behaviour. The animals displayed a preference for fixed toys and a tendency to decrease their level of play with the fixed and free toys [which they probably did not prefer because they quickly became soiled with excreta] over the 3 week observations period.