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Humans and nonhuman great apes share a sense for intuitive statistics, making intuitive probability judgments based on proportional information. This ability is of tremendous importance, in particular for predicting the outcome of events using prior information and for inferring general regularities from limited numbers of observations. Already in infancy, humans functionally integrate intuitive statistics with other cognitive domains, rendering this type of reasoning a powerful tool to make rational decisions in a variety of contexts. Recent research suggests that chimpanzees are capable of one type of such cross-domain integration: The integration of statistical and social information. Here, we investigated whether apes can also integrate physical information into their statistical inferences. We tested 14 sanctuary-living chimpanzees in a new task setup consisting of two “gumball machine”-apparatuses that were filled with different combinations of preferred and non-preferred food items. In four test conditions, subjects decided which of two apparatuses they wanted to operate to receive a random sample, while we varied both the proportional composition of the food items as well as their spatial configuration above and below a barrier. To receive the more favorable sample, apes needed to integrate proportional and spatial information. Chimpanzees succeeded in conditions in which we provided them either with proportional information or spatial information, but they failed to correctly integrate both types of information when they were in conflict. Whether these limitations in chimpanzees' performance reflect true limits of cognitive competence or merely performance limitations due to accessory task demands is still an open question.

Eurasian jays (Garrulus glandarius) do not habitually use tools, yet they can be trained to solve object-dropping tasks, i.e., to insert a tool into an apparatus to release a food reward. Previous research suggests that these jays can learn a preference toward functional tools – objects allowing them to obtain a food reward placed inside an apparatus – according to their density (Cheke et al., 2011). However, it is not yet known whether they can also select functional tools (tool selectivity) according to other physical properties such as size and shape and use different kinds of tools to solve a similar task. Here we conducted three object-dropping experiments aimed at exploring these abilities in Eurasian jays. In Experiment 1, jays tended to select large stones as tools irrespective of the diameter of the apparatus. However, jays progressively developed a preference for the small tool, which was functional with both the wide and the narrow apparatuses. In Experiment 2, only vertically oriented long stones could fit into the narrow apparatus, whereas both long and round stones were functional with the wide apparatus. Jays showed a preference for the long stone and, with the narrow apparatus, tended to achieve the correct manipulation after one or more unsuccessful attempts. In Experiment 3, jays were able to use sticks and adopt a novel technique on the same object-dropping apparatus, thus providing the first evidence that Eurasian jays can use sticks as tools. Taken together, these results do not support the notion that tool selectivity abilities can be found in Eurasian jays but nonetheless show that these corvids can use different kinds of tools to solve similar tasks.

Scientific disciplines face concerns about replicability and statistical inference, and these concerns are also relevant in animal cognition research. This paper presents a first attempt to assess how researchers make and publish claims about animal physical cognition, and the statistical inferences they use to support them. We surveyed 116 published experiments from 63 papers on physical cognition, covering 43 different species. The most common tasks in our sample were trap-tube tasks (14 papers), other tool use tasks (13 papers), means-end understanding and string-pulling tasks (11 papers), object choice and object permanence tasks (9 papers) and access tasks (5 papers). This sample is not representative of the full scope of physical cognition research; however, it does provide data on the types of statistical design and publication decisions researchers have adopted. Across the 116 experiments, the median sample size was 7. Depending on the definitions we used, we estimated that between 44% and 59% of our sample of papers made positive claims about animals’ physical cognitive abilities, between 24% and 46% made inconclusive claims, and between 10% and 17% made negative claims. Several failures of animals to pass physical cognition tasks were reported. Although our measures had low inter-observer reliability, these findings show that negative results can and have been published in the field. However, publication bias is still present, and consistent with this, we observed a drop in the frequency of p-values above .05. This suggests that some non-significant results have not been published. More promisingly, we found that researchers are likely making many correct statistical inferences at the individual-level. The strength of evidence of statistical effects at the group-level was weaker, and its p-value distribution was consistent with some effect sizes being overestimated. Studies such as ours can form part of a wider investigation into statistical reliability in comparative cognition. However, future work should focus on developing the validity and reliability of the measurements they use, and we offer some starting points.

When chimpanzees (and other animals) use tools to pound, crack open, retrieve, soak up, pry apart, probe into, and/or dig up other objects (just to name a few of the operations of which they are capable), are these actions modulated by higher-order, structural, role-based representations of , , , and so forth? We report a study that was designed to shed light on what chimpanzees understand about and between objects. We presented expert tool-using chimpanzees with very familiar objects (ropes) in a very familiar testing context (hooking and retrieving an object containing food). We investigated whether they could transfer the known operation (hooking and pulling) to two distinct setups involving ropes (a looped rope that could effectuate the transfer of force, and a draped length of rope that could not¾unless both ends were grasped at the same time). We show that first-order, perceptually-based relational reasoning is both necessary and sufficient to explain not only the impressive pattern of results we did obtain, but any possible pattern of results we could have obtained. If correct, any claim that higher-order reasoning is necessary to explain the results of such a test would, by definition, be false. More importantly, we show that this is not an idiosyncratic limitation of this experiment. Instead, we show that all experimental protocols claiming to assay higher-order reasoning in animals rest upon an extremely suspect, and ultimately unprincipled, titration. Specifically, researchers implicitly assume that the tasks are perceptually similar enough to what their subjects have previously encountered to allow them make sense of the problem, but perceptually different enough that the subjects must (somehow) rely upon higher-order reasoning to navigate their way through it. We show why such assumptions are false.

Humans form abstract representations about the physical properties of objects, with very young infants having ‘core knowledge’ about solidity and continuity. Whether nonhuman primates also form abstract representation of physical properties is debated. Despite studies showing that some nonhuman primates can discriminate between functional and non-functional tools, whether they achieve this by recognizing an object’s physical properties or via associative learning of perceptual cues remains contested. One method for exploring an individual’s physical cognition is the trap-tube (where subjects push a reward out of a pipe whilst avoiding a trap). The trap-tube investigates whether participants understand that rewards cannot pass through solid objects (solidity) and that unsupported objects will fall (gravity). Initial research reported that chimpanzees and capuchin monkeys solved the test using associative rules; however, modifying the task by removing the tool revealed an understanding of solidity in chimpanzees. In this series of studies, we tested 12 squirrel monkeys and 14 capuchin monkeys on a modified trap-box, where the monkeys could move the reward using their fingers rather than a tool. No individual of either species passed the task at above chance levels. In Experiment 2, the same trap blocked one of two cups: both species learned to avoid it. In Experiment 3, only capuchins generalized this solution to novel materials, performing better when cues had a functional versus a non-functional relationship to the outcome. Squirrel monkeys appeared to learn via association, while results from capuchins indicated a sensitivity to the concept of solidity.

Play is widespread among vertebrates. Some animal groups stand out in their play behaviors in levels of complexity, innovativeness, sociality, and volume. Despite the vast phylogenetic distance between corvids, parrots, great apes, and dolphins, all are usually identified as among the most playful. These groups also have several complex cognitive skills in common. There is growing agreement that play has evolved multiple times under different selective pressures in different lineages. As these groups appear similar in their complex play but are separated by considerable evolutionary distance, the similarity is unlikely to result from homology. Far more probable is that the similarity has arisen from convergent or parallel evolution. It is important to conduct comparative ontogenetic play studies on these groups to learn more about what basic processes underlie complex play and whether such play is, indeed, related to complex cognition. Toward that end, we explored the play behavior of raven nestlings over the last ten days before they fledged. We found high levels of play both in terms of instances initiated and duration. The play behaviors were at level with – or above – maintenance behaviors and flight training. Most of the play was object play, but social object play and apparent play contagion was also recorded. The importance of play in developing young ravens is clear. The reasons might be less clear, however play could underlie both object-related and social development.