String pulling tasks are commonly used to investigate recognition of means-end connections. Previous studies suggested that dogs base their choice on proximity rather than connectivity (Osthaus, Lea, & Slater, 2005), nonetheless, dogs performed successfully in the related support problem (Range, Hentrup, & Virányi, 2011). To reinvestigate dogs' means-end understanding, we tested 34 Border collies in string pulling tasks in which the proximity of the reward to the connected string's end was varied. First, subjects were presented with a four-string task (four parallel perpendicular strings, one baited, with the reward in line with the correct string's end). Dogs that performed above chance in this task were tested with a curved string task, involving one straight and one curved string. When the reward was attached to the curved string, it was equidistant from both strings' ends so that choosing by proximity was not possible. Although group level performance was significantly above chance, only three of 20 dogs met criterion individually, of which one dog subsequently solved a broken string task upon its first presentation. However, the dogs seemed to be unable to overcome their proximity bias in a parallel diagonal string task where proximity of the unconnected string's end to the reward was misleading. We conclude that although dogs may not demonstrate means-end understanding spontaneously, some can learn to pay attention to connectivity when proximity is not a confounding factor. This study supports the notion that animals may apply several alternative strategies to solve physical problems, which are influenced by the test-setup.
