Defining value through quantity and quality-Chimpanzees (Pan troglodytes) undervalue food quantities when items are broken

The less-is-better effect: a developmental perspective

The less-is-better effect emerges when an option of lesser quantitative value is preferred or overvalued relative to a quantitively greater alternative (e.g., 24-piece dinnerware set > 24-piece dinnerware set with 16 additional broken dishes; Hsee, 1998, Journal of Behavioral Decision Making, 11, 107-121). This decisional bias emerges when the option of lesser quantitative value is perceived as qualitatively better (e.g., smaller set of intact dishes > larger set of partially broken dishes). Interestingly, this effect emerges for adult humans when options are evaluated separately but dissipates when options are considered simultaneously. The less-is-better bias has been attributed to the evaluability hypothesis: individuals judge objects on the basis of easy-to-evaluate attributes when judged in isolation, such as the brokenness of items within a set, yet shift to quantitative information when evaluated jointly, such as the overall number of dishes. This bias emerges for adult humans and chimpanzees in a variety of experimental settings but has not yet been evaluated among children. In the current study, we presented a joint evaluation task (larger yet qualitatively inferior option vs. smaller yet qualitatively superior option) to children aged 3 to 9 years old to better understand the developmental trajectory of the less-is-better effect. Children demonstrated the bias across all choice trials, preferring an objectively smaller set relative to a larger yet qualitatively poorer alternative. These developmental findings suggest that young children rely upon salient features of a set to guide decision-making under joint evaluation versus more objective attributes such as quantity/value.

Bundling and segregation affect pheromone deposition, but not choice, in an ant

Behavioural economists have identified many psychological manipulations which affect perceived value. A prominent example of this is bundling, in which several small gains (or costs) are experienced as more valuable (or costly) than if the same total amount is presented together. While extensively demonstrated in humans, to our knowledge this effect has never been investigated in an animal, let alone an invertebrate. We trained individual Lasius niger workers to two of three conditions in which either costs (travel distance), gains (sucrose reward), or both were either bundled or segregated: (1) both costs and gains bundled, (2) both segregated, and (3) only gains segregated. We recorded pheromone deposition on the ants' return trips to the nest as measure of perceived value. After training, we offer the ants a binary choice between odours associated with the treatments. While bundling treatment did not affect binary choice, it strongly influenced pheromone deposition. Ants deposited c. 80% more pheromone when rewards were segregated but costs bundled as compared with both costs and rewards being bundled. This pattern is further complicated by the pairwise experience each animal made, and which of the treatments it experiences first during training. This demonstrates that even insects are influenced by bundling effects. We propose that the deviation between binary choice and pheromone deposition in this case may be due to a possible linearity in distance perception in ants, while almost all other sensory perception in animals is logarithmic.

The role of item size on choosing contrasted food quantities in angelfish (Pterophyllum scalare)

Comparative studies on quantity discrimination in animals are important for understanding potential evolutionary roots of numerical competence. A previous study with angelfish has shown that they discriminate numerically different sets of same-sized food items and prefer the larger set. However, variables that covary with number were not controlled and choice could have been influenced by variables such as size or density of the food items rather than numerical attributes. Here using a recently developed approach, we examined whether contour length of the food items affects choice in a spontaneous binary choice task. In Experiment 1, a contrast of 1 vs. 1 food item was presented, but the ratio between the size (diameter) of the food items was varied. In Experiment 2, numerically different food sets were equated in overall size by increasing the size (diameter) of the items in the numerically small sets. In both Experiments, subjects showed a preference for the larger sized food items with a discrimination limit. These results show that item size plays a prominent role in foraging decisions in angelfish. Experiment 3 placed numerical and size attributes of the sets in conflict by presenting one larger-sized food item in the numerically smaller set that also had smaller overall size (diameter) of food items. Angelfish showed no preference in any of the contrasts, suggesting that they could not make optimal foraging decisions when these attributes were in conflict. Maximization of energy return is central to optimal foraging. Accordingly, here item size was also found to be a key feature of the sets, although the numerical attributes of the sets also influenced the choice.

Playing it safe? Solitary vervet monkeys (Chlorocebus pygerythrus) choose high-quality foods more than those in competition

An important goal in foraging ecology is to determine how biotic and abiotic variables impact the foraging decisions of wild animals and how they move throughout their multidimensional landscape. However, the interaction of food quality and feeding competition on foraging decisions is largely unknown. Here we examine the importance of food quality in a patch on the foraging decisions of wild vervet monkeys (Chlorocebus pygerythrus) at Lake Nabugabo, Uganda using a multidestination platform array. The overall nutritional composition of the vervet diet was assessed and found to be low in sodium and lipids, thus we conducted a series of experimental manipulations in which the array was varied in salt and oil content. Although vervets prioritized platforms containing key nutrients (i.e., sodium and lipids) overall, we found that solitary vervets prioritized nutrient-dense platforms more strongly than competing vervets. This finding was opposite to those in a similar experiment that manipulated food site quantity, suggesting that large, salient rewards may be worth competing over but slight differences in nutritional density may be only chosen when there are no potentially negative social consequences (i.e., aggression received). We also found that vervets chose platforms baited with oil-only, and oil combined with salt, but not salt-only, suggesting that energy was an important factor in food choice. Our findings demonstrate that when wild vervets detect differences in feeding patches that reflect nutritional composition, they factor these differences into their navigational and foraging decisions. In addition, our findings suggest that these nutritional differences may be considered alongside social variables, ultimately leading to the complex strategies we observed in this study.

Impact of stimulus format and reward value on quantity discrimination in capuchin and squirrel monkeys

Quantity discrimination abilities are seen in a diverse range of species with similarities in performance patterns, suggesting common underlying cognitive mechanisms. However, methodological factors that impact performance make it difficult to draw broad phylogenetic comparisons of numerical cognition across studies. For example, some Old World monkeys selected a higher quantity stimulus more frequently when choosing between inedible (pebbles) than edible (food) stimuli. In Experiment 1 we presented brown capuchin (Cebus [Sapajus] paella) and squirrel monkeys (Saimiri sciureus) with the same two-choice quantity discrimination task in three different stimulus conditions: edible, inedible, and edible replaced (in which choice stimuli were food items that stood in for the same quantity of food items that were given as a reward). Unlike Old World monkeys, capuchins selected the higher quantity stimulus more in the edible condition and squirrel monkeys showed generally poor performance across all stimulus types. Performance patterns suggested that differences in subjective reward value might motivate differences in choice behavior between and within species. In Experiment 2 we manipulated the subjective reinforcement value of the reward by varying reward type and delay to reinforcement and found that delay to reinforcement had no impact on choice behavior, while increasing the value of the reward significantly improved performance by both species. The results of this study indicate that species presented with identical tasks may respond differently to methodological factors such as stimulus and reward types, resulting in significant differences in choice behavior that may lead to spurious suggestions of species differences in cognitive abilities.