Two strings to choose from: do ravens pull the easier one?

There are simple co-occurrences as well as functional relationships between events. One may assume that animals detect and use causation rather than mere co-variation. However, understanding causation often requires concepts of hidden forces. In string pulling, obstacles may hamper the access to food. Here, I studied whether ravens have an abstract concept of effort. First, in a competitive situation, ravens (Corvus corax) could choose one out of two strings. The strings differed in whether they were baited with meat and in how far away the meat was. Ravens pulled mainly the string containing meat and where the meat was nearer to the perch, respectively. Second, ravens could choose between two strings that had either a functional obstacle or a non-functional obstacle. Optimal performance required the integration of at least two cues: object and height. In 5 ravens, the model that best matched behaviour took into account only that meat was on a string, ignoring the obstacle. However, 2 ravens' performance was best explained by a model that took into account both an object's identity (meat or wood) and its height on the string. Third, one string out of two was loaded with a heavy meat piece. In this overloaded string condition, 5 out of 7 ravens did not try to pull the heavy meat piece but went straight for pulling the smaller piece. The pattern of results indicated that ravens can judge the effort required to pull a string.