Elsevier

Behavioural Processes

Volume 111, February 2015, Pages 118-126
Behavioural Processes

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

https://doi.org/10.1016/j.beproc.2014.11.004Get rights and content

Highlights

  • Both the quantity and quality of food sets impact food quantity judgments.

  • Quality was defined as the cohesiveness or wholeness of items in a set (i.e., their original state).

  • Chimpanzees chose the larger food set when both sets contained whole or broken items.

  • Chimpanzees chose the smaller set if it was comprised of broken or partially broken items.

  • This occurred even when they were first shown the items in their original whole state.

Abstract

Decision-making largely is influenced by the relative value of choice options, and the value of such options can be determined by a combination of different factors (e.g., the quantity, size, or quality of a stimulus). In this study, we examined the competing influences of quantity (i.e., the number of food items in a set) and quality (i.e., the original state of a food item) of choice items on chimpanzees’ food preferences in a two-option natural choice paradigm. In Experiment 1, chimpanzees chose between sets of food items that were either entirely whole or included items that were broken into pieces before being shown to the chimpanzees. Chimpanzees exhibited a bias for whole food items even when such choice options consisted of a smaller overall quantity of food than the sets containing broken items. In Experiment 2, chimpanzees chose between sets of entirely whole food items and sets of initially whole items that were subsequently broken in view of the chimpanzees just before choice time. Chimpanzees continued to exhibit a bias for sets of whole items. In Experiment 3, chimpanzees chose between sets of new food items that were initially discrete but were subsequently transformed into a larger cohesive unit. Here, chimpanzees were biased to choose the discrete sets that retained their original qualitative state rather than toward the cohesive or clumped sets. These results demonstrate that beyond a food set's quantity (i.e., the value dimension that accounts for maximization in terms of caloric intake), other seemingly non-relevant features (i.e., quality in terms of a set's original state) affect how chimpanzees assign value to their choice options.

Introduction

Imagine yourself in the cereal aisle of your local supermarket. As you navigate the countless alternatives (e.g., wheat vs. corn, sweetened vs. healthy, bargain vs. premium), you weigh multiple options’ relative value in an effort to generate a clear consensus for one choice over the others. Thus, value guides choice behavior, but what determines value? Many factors likely add to the determination of value. However, in some contexts, one can generate relatively simple and easy to understand value relations. For example, using a two-option choice task, a food's relative quantity, size, or quality can dictate value in a combinatorial manner when an individual is choosing among alternatives. Interestingly, because value may be comprised of multiple features, one dimension of an alternative's value (e.g., the nutritional quality of the cereal) may be at odds with a second dimension of that same option's value (e.g., the price of the cereal). And, of course, for humans a number of these factors relate to issues of observational and cultural learning (e.g., what time of day one is supposed to eat certain kinds of food, etc.).

The study of choice behavior and value has been a major topic of inquiry in comparative cognition because, in many ways, one can assume that nonhuman animals make choices largely devoid of the cultural biases or expectations that frequently impact human food choice behavior. Choice behavior by animals often is assessed through food-based discrimination tasks. Food-based discrimination tasks present highly motivating choice options and the stimulus properties are easily manipulated on multiple dimensions. The commonly used relative quantity judgment paradigm requires subjects to choose between two sets of items that differ in the amount or number of items in each set. This paradigm, also known as the natural choice paradigm when food items are used (Silberberg et al., 1998) has many advantages for testing value assignment among animals. Relative quantity judgments of food items require little to no training because they make use of motivating edible rewards. In addition, there is high spatial contiguity between the stimulus, the response loci, and the reward (i.e., the animal immediately receives the chosen set for consumption). Finally, the paradigm produces direct and reliable measurements of food size and quantity preferences (Menzel, 1961). Because food quantity is an extremely salient factor relevant to an animal's survival, it is no surprise that a wide range of species have proven to be successful in these quantity discrimination tasks. The species include but are not limited to salamanders (Krusche et al., 2010, Uller et al., 2003), robins (Garland et al., 2012), dogs (Ward and Smuts, 2007), elephants (Perdue et al., 2012), marine mammals (Abramson et al., 2011), and a variety of nonhuman primates (e.g., Addessi et al., 2008, Beran, 2001, Beran, 2004, Beran, 2012, Beran and Beran, 2004, Boysen and Berntson, 1995, Call, 2000, Hanus and Call, 2007). Thus, the ability to accurately select the larger of two food quantities appears to have a long evolutionary history in the animal kingdom. These results are unsurprising given the highly adaptive nature of differentiating food quantities toward the end of maximizing intake (Stephens and Krebs, 1986).

Chimpanzees (Pan troglodytes) have been of particular interest in research regarding quantitative capacities of nonhuman animals. They demonstrate high proficiency in a wide range of testing conditions, including discriminating between slight differences in food sizes to a degree that rivals and sometimes exceeds the abilities of human adults (e.g., Menzel, 1960, Menzel, 1961, Menzel and Davenport, 1962). Chimpanzees are successful in discriminating between food sets when the items are presented in different temporal styles (i.e., sequentially or simultaneously) and in different modalities (i.e., visual or auditory). Moreover, they are successful in quantity discriminations after long durations and when only one set is made visible throughout the choice phase (e.g., Beran, 2001, Beran, 2004, Beran, 2012, Beran and Beran, 2004, Boysen and Berntson, 1995, Dooley and Gill, 1977, Hanus and Call, 2007, Rumbaugh et al., 1987, Sayers and Menzel, 2012).

Despite their aptitude on relative quantity judgment tasks, however, a number of seemingly non-relevant factors sometimes influence chimpanzees’ quantity judgments. For example, Parrish and Beran (2014a) demonstrated that chimpanzees were proficient in selecting the larger of two foods when both were plated on the same-sized plates. However, when foods were presented on different-sized plates, chimpanzees erroneously preferred an equal-sized food or a smaller food plated on a small plate relative to a truly larger alternative on a large plate. These same chimpanzees were accurate in differentiating food quantities presented in same-sized containers. However, they often showed a bias toward an equal or smaller amount of food when that amount was presented in a smaller, yet seemingly fuller cup (Parrish and Beran, 2014b). Thus, perceptual illusions appear to affect food choice behavior in chimpanzees in many of the same ways as they do for humans (e.g., Van Ittersum and Wansink, 2012, Wansink, 2006, Wansink et al., 2006). Moreover, chimpanzee quantity discrimination sometimes is influenced by the properties of the individual elements that comprise a set. For example, two studies reported that chimpanzees were proficient at selecting the larger of two sets of food items but performance suffered if the smaller set contained the overall largest single item of food within it (Beran et al., 2008, Boysen et al., 2001). These studies demonstrated that chimpanzees relied on multiple features of food sets to discriminate quantity. They assigned value to alternative dimensions, including non-relevant stimulus properties or contextual variables. Thus, multiple features of food sets likely play a key role in quantitative decision-making with the end result sometimes being choice of less food over more food. Demonstrating discrimination errors and biases in a nonhuman species like the chimpanzee aids in elucidating the variables that shape our own perception of food value. Furthermore, investigating how value is constructed within the chimpanzee is informative as we can study this phenomenon in a nonhuman animal species that is highly food motivated and extremely adept at quantity discriminations.

In addition to understanding how individuals react to how much of something is available in a given choice option, there is also value in understanding what happens when the quantity of a stimulus is at odds with its quality. Hsee (1998) conflated an option's quantitative value with its qualitative value in a decision-making study. Human participants were given an option between two sets of dinnerware. One contained fewer overall dishes, and none of them were broken. The alternative contained more overall dishes, but some were broken. Notably, the latter set contained at least as many unbroken dishes as the first set. Despite an equal number of unbroken dishes in each set, participants overvalued the first set with fewer overall dishes relative to the latter set that also contained some broken pieces in a sequential evaluation. This finding was labeled the “less-is-better” effect; the preferred option's qualitative value (cohesiveness) outweighed its quantitative value (numerousness of the intact pieces).

We investigated this “less-is-better” phenomenon in chimpanzees by exploring how value was affected by the relative quality of food items in a set (i.e., the cohesiveness of each set), even if this was inconsistent with the set's relative quantity (i.e., the overall quantity of food in each set). Here, quality was defined as the cohesiveness of the set as this reflects the originally intact state of the food items presented. We manipulated the degree of set cohesiveness by breaking some or all of the items within one set in a relative quantity judgment task. We compared the chimpanzees’ performance in these test trials to control trials in which the chimpanzees chose between sets that both were made up of broken or non-broken items. These control trials allowed us to establish baseline performance rates and to ensure that the chimpanzees were capable of choosing the larger quantity when the potential bias toward cohesiveness could not occur.

When the chimpanzees sometimes preferred lesser quantities of whole items, we next assessed whether we could attenuate such biases by providing the chimpanzees with additional information about each set prior to the choice phase. To do this, we broke food items in front of the chimpanzees after they had seen all items in their unbroken state. The motivation for Experiment 2 was based in Piagetian conservation tasks in which quantities are spatially transformed within the subject's view (Piaget, 1965). The question in such conservation tasks is whether the subject can retain the initial choice of a larger quantity despite its newly altered presentation style. The new presentation style may present the quantity in a manner that makes it appear to be a different amount because of the size and shape of the new container that holds it. Several great ape species including chimpanzees are successful in these paradigms, accurately selecting the larger quantity even after its spatial transformation (e.g., Call and Rochat, 1996, Muncer, 1983, Suda and Call, 2004, Suda and Call, 2005, Woodruff et al., 1978). In Experiment 2, we adopted this general approach by first presenting the chimpanzees with sets that were completely intact in their original form. Following this stage, the experimenter then broke all items in the larger set in front of the chimpanzees, after which the chimpanzees made their choice between the alternatives. If the chimpanzees conserved the value of each set from its initial, non-broken presentation, they should accurately select the larger set despite its newly broken state. This would help to differentiate a quantitative error (i.e., a misperception of how much food there is when it is broken) from a qualitative bias (i.e., a devaluation of broken food items that were formerly whole, regardless of knowledge regarding quantity).

Finally, we explored the reverse scenario to determine whether the chimpanzees truly were biased against non-cohesive sets, or instead were biased toward food items that retained their original state. For Experiment 3, we presented a set of discrete food items (mini marshmallows) that could be transformed into a larger cohesive unit. If the chimpanzees were biased toward cohesive sets, they should have preferred the set with marshmallows clumped together. But, if the chimpanzees instead were biased toward food items that preserved their original quality or state, they should have preferred the set of discrete marshmallows over the clumped option.

Section snippets

Subjects

Four chimpanzees housed at the Language Research Center of Georgia State University participated in this study including two females (Lana – 43 years, Panzee – 28 years) and two males (Sherman – 40 years, Mercury – 27 years). All chimpanzees participated in Phase 1 and three of the four chimpanzees participated in Phase 2 (before completing the experiment, Panzee died from complications related to a long-term illness for which she was receiving regular veterinary care). All chimpanzees were

Experiment 2

In Experiment 2, we explored whether we could attenuate the choice biases observed in Experiment 1. Here, we first presented the chimpanzees with whole foods that then were broken directly in front of them. This differed from Experiment 1 in which the items were broken out of the chimpanzees’ view. In previous studies examining the conservation of quantities, great apes have successfully conserved a food set's quantity across manipulations of the set's presentation style, accurately selecting

Experiment 3

In Experiment 3, we further explored the nature of the choice bias toward cohesive sets. Were the chimpanzees biased toward whole foods, or were they instead biased toward the set that retained its original value or state? Thus far we cannot differentiate between the two hypotheses as the food types used were cohesive in their original form. However, in Experiment 3 we presented a discrete food item (mini marshmallows) that we then transformed into a larger cohesive unit. This transformation

General discussion

In the current study, we investigated how food value is constructed by chimpanzees using the natural choice paradigm by pitting one dimension of a food set's value (quality) against another dimension of that same set (quantity). To establish baseline discrimination performance, we presented sets that only differed in the quantitative dimension. The chimpanzees were highly proficient in selecting the larger of two food sets when presented with two of the same set-types (whole vs. whole or broken

Acknowledgments

Support for this research was provided by a grant from NICHD (HD-060563), a 2CI Primate Social Cognition, Evolution and Behavior Fellowship, and the Duane M. Rumbaugh Fellowship from Georgia State University. We thank the animal care and enrichment staff for maintaining the health and wellbeing of the primates and making this research possible. This study is dedicated to the memory of Panzee.

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