The string-pulling paradigm in comparative psychology.

String pulling is one of the most widely used paradigms in comparative psychology. First documented 2 millennia ago, it has been a well-established scientific paradigm for a century. More than 160 bird and mammal species have been tested in over 200 studies with countless methodological variations. The paradigm can be used to address a wide variety of issues on animal cognition; for example, what animals understand about contact and connection as well as whether they rely on perceptual feedback, grasp the functionality of strings, generalize across conditions, apply their knowledge flexibly, and possess insight. Mammals are typically tested on a horizontal configuration, birds on a vertical one, making the studies difficult to compare; in particular, pulling a string vertically requires better coordination and attention. A species' performance on the paradigm is often influenced by its ecology, especially concerning whether limbs are used for foraging. Many other factors can be of importance and should be considered. The string-pulling paradigm is easy to administer, vary, and apply to investigate a wide array of cognitive abilities. Although it can be and has been used to compare species, divergent methods and unclear reporting have limited its comparative utility. With increasing research standards, the paradigm is expected to become an even more fundamental tool in comparative psychology.