The Piagetian invisible displacement task has been used extensively in the field of comparative cognition to measure representational capacity. In the task a desirable object is hidden beneath one of several hiding boxes via a displacement device, such as a small opaque container. Success on the task is deemed to require that the invisible trajectory of the object be mentally represented and its current location inferred. That is, the task is supposed to measure the ability to ”think” of something that is occurring outside of direct perception. However, simple associative strategies may also lead to success in the absence of stringent control conditions. Among mammals, only great apes and domestic dogs have consistently performed above chance on the invisible displacement task. There is much converging evidence from other tasks to suggest that great apes have a capacity for representational thought. However, dogs have shown few signs of possessing the representational abilities generally thought necessary to pass the task. Thus, in Chapter 2, four experiments investigated how dogs (n = 35) find an object that has been invisibly displaced behind one of three opaque boxes under four control conditions devised to separate associative search strategies from performance based on mental representation. Strategies involving experimenter cue-use, search at the last or first box visited by the displacement device, and search at boxes adjacent to the displacement device were systematically controlled for. Dogs passed invisible displacements, but only if the device used to displace the object was adjacent to the target box following displacements. These results suggest that the search behaviour of dogs was guided by simple associative rules rather than mental representation of the past trajectory of the object. In contrast, Experiment 5 found that, on the same task, 18- and 24-month-old children (n = 21) showed no disparity between trials in which the displacement device was adjacent or non-adjacent to the target box. In Chapter 3, two chimpanzees were tested on single invisible displacements under the same four control conditions that were administered to dogs. In contrast to dogs, chimpanzees showed no indications of utilizing these simple strategies, suggesting that their capacity to mentally represent single invisible displacements is comparable to that of 18- to 24-month-old children. Chapter 4 followed up reports of children and apes' difficulty with double invisible displacements in which an object is hidden at two non-adjacent boxes in a linear array. Experiment 1 eliminated the possibility that chimpanzees' previous poor performance was due to the hiding direction of the displacement device. Subjects failed double non-adjacent displacements, showing a tendency to select adjacent boxes. In Experiments 2 and 3, chimpanzees and 24-month-old children were tested on a new adaptation of the task involving four hiding boxes presented in a diamondshaped array on a vertical plane. Both species performed above chance on double invisible displacements using this format, suggesting that previous poor performance was due to a response bias or inhibition problem rather than a fundamental limitation in representational capacity. In Chapter 5, I conducted a pilot study examining the performance of siamangs and a spider monkey on single and double invisible displacements. Performance was mixed but provides some promising evidence that invisible displacements are within the capacity of siamangs. In contrast to siamangs and chimpanzees, but like dogs, the spider monkey showed a significant tendency to search at a box adjacent to the displacement device on single invisible displacements. However, the spider monkey performed above chance on an impromptu test of single invisible displacements presented in the vertical format. Further study is needed to eliminate alternative associative strategies in these two species. In Chapter 6, I discuss the findings of the studies with dogs, chimpanzees, 2- year-old children, siamangs, and a spider monkey on invisible displacement understanding. The results highlight the importance of associative strategies and inhibition problems. The thesis presents strong evidence for stage 6 invisible displacement understanding, and thus representational thought, in chimpanzees and 2- year-old children, but suggests that dogs are capable of only stage 5 object permanence understanding.

The Piagetian invisible displacement task has been used extensively in the field of comparative cognition to measure representational capacity. In the task a desirable object is hidden beneath one of several hiding boxes via a displacement device, such as a small opaque container. Success on the task is deemed to require that the invisible trajectory of the object be mentally represented and its current location inferred. That is, the task is supposed to measure the ability to ”think” of something that is occurring outside of direct perception. However, simple associative strategies may also lead to success in the absence of stringent control conditions. Among mammals, only great apes and domestic dogs have consistently performed above chance on the invisible displacement task. There is much converging evidence from other tasks to suggest that great apes have a capacity for representational thought. However, dogs have shown few signs of possessing the representational abilities generally thought necessary to pass the task. Thus, in Chapter 2, four experiments investigated how dogs (n = 35) find an object that has been invisibly displaced behind one of three opaque boxes under four control conditions devised to separate associative search strategies from performance based on mental representation. Strategies involving experimenter cue-use, search at the last or first box visited by the displacement device, and search at boxes adjacent to the displacement device were systematically controlled for. Dogs passed invisible displacements, but only if the device used to displace the object was adjacent to the target box following displacements. These results suggest that the search behaviour of dogs was guided by simple associative rules rather than mental representation of the past trajectory of the object. In contrast, Experiment 5 found that, on the same task, 18- and 24-month-old children (n = 21) showed no disparity between trials in which the displacement device was adjacent or non-adjacent to the target box. In Chapter 3, two chimpanzees were tested on single invisible displacements under the same four control conditions that were administered to dogs. In contrast to dogs, chimpanzees showed no indications of utilizing these simple strategies, suggesting that their capacity to mentally represent single invisible displacements is comparable to that of 18- to 24-month-old children. Chapter 4 followed up reports of children and apes' difficulty with double invisible displacements in which an object is hidden at two non-adjacent boxes in a linear array. Experiment 1 eliminated the possibility that chimpanzees' previous poor performance was due to the hiding direction of the displacement device. Subjects failed double non-adjacent displacements, showing a tendency to select adjacent boxes. In Experiments 2 and 3, chimpanzees and 24-month-old children were tested on a new adaptation of the task involving four hiding boxes presented in a diamondshaped array on a vertical plane. Both species performed above chance on double invisible displacements using this format, suggesting that previous poor performance was due to a response bias or inhibition problem rather than a fundamental limitation in representational capacity. In Chapter 5, I conducted a pilot study examining the performance of siamangs and a spider monkey on single and double invisible displacements. Performance was mixed but provides some promising evidence that invisible displacements are within the capacity of siamangs. In contrast to siamangs and chimpanzees, but like dogs, the spider monkey showed a significant tendency to search at a box adjacent to the displacement device on single invisible displacements. However, the spider monkey performed above chance on an impromptu test of single invisible displacements presented in the vertical format. Further study is needed to eliminate alternative associative strategies in these two species. In Chapter 6, I discuss the findings of the studies with dogs, chimpanzees, 2- year-old children, siamangs, and a spider monkey on invisible displacement understanding. The results highlight the importance of associative strategies and inhibition problems. The thesis presents strong evidence for stage 6 invisible displacement understanding, and thus representational thought, in chimpanzees and 2- year-old children, but suggests that dogs are capable of only stage 5 object permanence understanding.

The Piagetian invisible displacement task has been used extensively in the filed of comparative cognition to measure representational capacity. In the task a desirable object is hidden beneath one of several hiding boxes via a displacement device, such as a small opaque container. Success on the task is deemed to require that the invisible trajectory of the object be mentally represented and its current location inferred. That is, the task is supposed to measure the ability to “think” of something that is occurring outside of direct perception. However, simple associative strategies may also lead to success in the absence of stringent control conditions. Among mammals, only great apes and domestic dogs have consistently performed above chance on the invisible displacement task. There is much converging evidence from other tasks to suggest that great apes have a capacity for representational thought. However, dogs have shown few signs of possessing the representational abilities generally though necessary to pass the task. Thus, in Chapter 2, four experiments investigated how dogs (n=35) find an object that has been invisibly displaced behind one of three opaque boxes under four control conditions devised to separate associative search strategies from performance based on mental representation. Strategies involving experimenter cue-use, search at the last or first box visited by the displacement device, and search at boxes adjacent to the displacement device were systematically controlled for. Dogs passed invisible displacement’s but only if the device used to displace the object was adjacent to the target box following displacements. These results suggest that the search behaviour of dogs was guided by simple associative rules rather than mental representation of past trajectory of the object. In contrast, Experiment 5 found that, on the same task, 18- and 24-moth-old children (n=21) showed no disparity between trials in which the displacement device was adjacent or non-adjacent to the target box. In Chapter 3, two chimpanzees were tested on single invisible displacements under the same four control conditions that were administered to dogs. In contract to dogs, chimpanzees showed no indication of utilizing these simple strategies, suggesting that their capacity to mentally represent single invisible displacements is comparable to that of 18- to 24-month-old children. Chapter 4 followed up reports of children and apes’ difficulty with double invisible displacements in which an object is hidden at two non-adjacent boxes in a linear array. Experiment 1 eliminated the possibility that chimpanzees’ previous poor performance was due to the hiding direction of the displacement device. Subjects failed double non-adjacent displacements, showing a tendency to select adjacent boxes. In Experiments 2 and 3, chimpanzees and 24-month-old children were tested on a new adaptation of the task involving four hiding boxes presented in a diamond-shaped array on a vertical plane. Both species performed above change on double invisible displacements using this format, suggesting that previous poor performance was due to a response bias or inhibition problem rather than a fundamental limitation in representational capacity. In Chapter 5, I conducted a pilot study examining the performance of siamangs and a spider monkey on single and double invisible displacements. Performance was mixed but provides some promising evidence that invisible displacements are within the capacity of siamangs. In contrast to siamangs and chimpanzees, but like dogs, the spider monkey showed a significant tendency to search at a box adjacent to the displacement device on simple invisible displacements. However, the spider monkey performed above chance on an impromptu test of single invisible displacements presented in the vertical format. Further study is needed to eliminate alternative associative strategies in these two species. In Chapter 6, I discuss the findings of the study with dogs, chimpanzees, 2-year-old children, siamangs, and a spider monkey on invisible displacement understanding. The results highlight the importance of associative strategies and inhibition problems. The thesis presents strong evidence of stage 6 invisible displacement understanding, and thus representational thought, in chimpanzees and 2-year-old children, but suggests that dogs are capable of only stage 5 objective permanence understanding.