The observed frequencies of object-directed oral-nasal manipulation in the different treatment groups. At the time of observation, the pigs had had the objects for two and a half months continuously. All pens were equipped with a straw rack, a chain and daily provisions of wood shavings. The treatments were as follows: C: no additional objects (N = 17), B:branching chains (N = 15), P: a polythene pipe cross (N = 13), W: horizontal pieces of fresh wood (N = 14), WPB: all of the above (N = 14). The data points represent the mean numbers of manipulations per pig in each pen during the 20-minute observation period, and the bars show the means and standard deviations in each treatment. Letters above the bars indicate significant differences between treatments: frequencies with different letters differ at P < 0.05