Chimpanzees (Pan troglodytes) accurately compare poured liquid quantities

Trading up: chimpanzees (Pan troglodytes) show self-control through their exchange behavior

Self-control is defined as the ability or capacity to obtain an objectively more valuable outcome rather than an objectively less valuable outcome though tolerating a longer delay or a greater effort requirement (or both) in obtaining that more valuable outcome. A number of tests have been devised to assess self-control in non-human animals, including exchange tasks. In this study, three chimpanzees (Pan troglodytes) participated in a delay of gratification task that required food exchange as the behavioral response that reflected self-control. The chimpanzees were offered opportunities to inhibit eating and instead exchange a currently possessed food item for a different (and sometimes better) item, often needing to exchange several food items before obtaining the highest valued reward. We manipulated reward type, reward size, reward visibility, delay to exchange, and location of the highest valued reward in the sequence of exchange events to compare performance within the same individuals. The chimpanzees successfully traded until obtaining the best item in most cases, although there were individual differences among participants in some variations of the test. These results support the idea that self-control is robust in chimpanzees even in contexts in which they perhaps anticipate future rewards and sustain delay of gratification until they can obtain the ultimately most valuable item.

The long and the short of it: rule-based relative length discrimination in carrion crows, Corvus corone

Birds and other nonhuman animals can choose the larger of two discrete or continuous quantities. However, whether birds possess the conceptual grasp and cognitive control to flexibly switch between relative more-or-less-than judgments remains elusive. We therefore tested carrion crows in a rule-based line-length discrimination task to flexibly select lines presented on a touchscreen according to their relative length. In the first experiment, the crows needed to discriminate a shorter from a longer line, and vice versa. In the second experiment, the crows were required to choose a medium long line among three lines of different length (intermediate-size task). The crows switched effortlessly between "longer than/shorter than" rules, showing no signs of trial history affecting switching performance. They reliably chose the relatively longer and shorter line length, thus demonstrating a concept of greater than/less than with a continuous magnitude. However, both crows failed to discriminate a line of 'medium' length embedded in longer and shorter lines. These results indicate that relational discrimination exhibits different cognitive demands. While a greater than/less than concept requires only one relational comparison (with the respectively greater or smaller magnitude), the discrimination of a 'medium' magnitude demands to relate two or more comparisons, which might overburden crows and maybe animals in general.

A shared system of representation governing quantity discrimination in canids

One way to investigate the evolution of cognition is to compare the abilities of phylogenetically related species. The domestic dog (Canis lupus familiaris), for example, still shares cognitive abilities with the coyote (Canis latrans). Both of these canids possess the ability to make psychophysical "less/more" discriminations of food based on quantity. Like many other species including humans, this ability is mediated by Weber's Law: discrimination of continuous quantities is dependent on the ratio between the two quantities. As two simultaneously presented quantities of food become more similar, choice of the large or small option becomes random in both dogs and coyotes. It remains unknown, however, whether these closely related species within the same family - one domesticated, and one wild - make such quantitative comparisons with comparable accuracy. Has domestication honed or diminished this quantitative ability? Might different selective and ecological pressures facing coyotes drive them to be more or less able to accurately represent and discriminate food quantity than domesticated dogs? This study is an effort to elucidate this question concerning the evolution of non-verbal quantitative cognition. Here, we tested the quantitative discrimination ability of 16 domesticated dogs. Each animal was given nine trials in which two different quantities of food were simultaneously displayed to them. The domesticated dogs' performance on this task was then compared directly to the data from 16 coyotes' performance on this same task reported by Baker et al. (2011). The quantitative discrimination abilities between the two species were strikingly similar. Domesticated dogs demonstrated similar quantitative sensitivity as coyotes, suggesting that domestication may not have significantly altered the psychophysical discrimination abilities of canids. Instead, this study provides further evidence for similar non-verbal quantitative abilities across multiple species.

The role of numerical competence in a specialized predatory strategy of an araneophagic spider

Although a wide range of vertebrates have been considered in research on numerical competence, little is known about the role of number-related decisions in the predatory strategies of invertebrates. Here, we investigate how numerical competence is expressed in a highly specialized predatory strategy adopted by the small juveniles of Portia africana when practicing communal predation, with the prey being another spider, Oecobius amboseli. Two or more P. africana juveniles sometimes settle by the same oecobiid nest and then share the meal after one individual captures the oecobiid. Experiments were designed to clarify how these predators use number-related cues in conjunction with non-numerical cues when deciding whether to settle at a nest. We used lures (dead spiders positioned in lifelike posture) arranged in a series of 24 different scenes defined by the type, configuration and especially number of lures. On the whole, our findings suggest that P. africana juveniles base settling decisions on the specific number of already settled conspecific juveniles at the nest and express a preference for settling when the number is one instead of zero, two or three. By varying the size of the already settled juveniles and their positions around the nest, we show that factors related to continuous variables and stimulus configuration are unlikely explanations for our findings.

Tracking of food quantity by coyotes (Canis latrans)

Previous studies have demonstrated that Weber's Law mediates quantitative discrimination abilities across various species. Here, we tested coyotes' (Canis latrans) ability to discriminate between various quantities of food and investigated whether this ability conforms to predictions of Weber's Law. We demonstrate herein that coyotes are capable of reliably discriminating large versus small quantities of discrete food items. As predicted by Weber's Law, coyotes' quantitative discrimination abilities are mediated by the ratio between the large and small quantities of food and exhibit scalar variability. Furthermore, in this task coyotes were not discriminating large versus small quantities based on olfactory cues alone.

Quantity judgments of auditory and visual stimuli by chimpanzees (Pan troglodytes)

Many species can choose between two visual sets of stimuli on the basis of quantity. This is true when sets are both visible, or are presented one set at a time or even one item at a time. However, we know comparatively little about how well nonhuman animals can compare auditory quantities. Here, three chimpanzees (Pan troglodytes) chose between two sets of food items when they only heard each item fall into different containers rather than seeing those items. This method prevented the chimpanzees from summing the amount of visible food they saw because there were no visual cues. Chimpanzees performed well, and their performance matched that of previous experiments with regard to obeying Weber's law. They also performed well with comparisons between a sequentially presented auditory set and a fully visible set, demonstrating that duration of presentation was not being used as a cue. In addition, they accommodated empty sets into these judgments, although not perfectly. Thus, chimpanzees can judge auditory quantities in flexible ways that show many similarities to how they compare visual quantities.