Language‐Trained Chimpanzees (Pan troglodytes) Delay Gratification by Choosing Token Exchange Over Immediate Reward Consumption

Abstract

Token exchange inherently introduces an element of delay between behavior and reward and so token studies may help us better understand delay of gratification and self-control. To examine this possibility, we presented three language-trained chimpanzees with repeated choices involving different foods that could be eaten immediately or lexigram (graphic symbol) tokens that represented (and could be traded for) foods later. When both options were foods, chimpanzees always chose more preferred foods over less preferred foods. When both options were lexigram tokens representing those same foods, performance remained the same as chimpanzees selected the higher value token and then traded it for food. Then, when faced with choosing a token that could be traded later or choosing a food item that could be eaten immediately, most chimpanzees learned to make whatever response led to the more preferred food. They did this even when that meant selecting a high value lexigram token that could be traded only two to three minutes later instead of a medium value, but immediately available, food item. Thus, chimpanzees flexibly selected tokens even though such selections necessarily delayed gratification and required forgoing immediately available food. This finding illustrates the utility of symbolic token exchange for assessing self-control in nonhuman animals.

Chimpanzees (Pan troglodytes) transfer tokens repeatedly with a partner to accumulate rewards in a self-control task

“…In an earlier series of studies, one chimpanzee (Sherman) had experience requesting and exchanging tools and food items with a conspecific using the lexigram system (see Savage-Rumbaugh et al 1978a, 1978b). All four chimpanzees also have been tested previously in tasks involving tokens representing different quantities of foods as well as one task involving the choice between immediately consumable rewards and lexigram tokens exchangeable later for better rewards (Beran and Evans 2012; Beran et al 2011; Evans et al 2010). …”

The comparative science of “self-control”: what are we talking about?

“…Capuchin monkeys made many such errors although chimpanzees showed better self-control by often choosing the LL option but also waiting for it to accumulate fully (Beran et al, 2014 ). Other tasks that meet the criteria for assessing self-control include delayed exchange tasks (e.g., Dufour et al, 2007 ; Pele et al, 2010 ), accumulation tasks (Beran, 2002 ; Beran and Evans, 2006 ), some token collection or exchange tasks (Jackson and Hackenberg, 1996 ; Hackenberg and Vaidya, 2003 ; Judge and Essler, 2013 ), and similar approaches (e.g., Evans and Westergaard, 2006 ; Evans, 2007 ; Bramlett et al, 2012 ).…”

“…For example, there are concerns about exactly what different tasks involving intertemporal choice measure not only for nonhuman animals (e.g., Addessi et al, 2013 ) but also for humans (Duckworth and Kern, 2011 ). And, as Stevens ( 2014 ) noted, we need more cross-species assessments using some of the other methods that are designed to measure self-control such as delay of gratification tasks (e.g., Grosch and Neuringer, 1981 ; Beran et al, 1999 ), delayed exchange tasks (e.g., Dufour et al, 2007 ; Pele et al, 2010 ; Judge and Essler, 2013 ), accumulation tasks (Beran, 2002 ; Evans and Beran, 2007 ; Vick et al, 2010 ; Parrish et al, 2014 ), and other variations on these methods. With those data, one could compare the resulting correlations between performance and allometric factors to see if they match what Stevens found for the intertemporal choice task.…”

A Longitudinal Assessment of Vocabulary Retention in Symbol-Competent Chimpanzees (Pan troglodytes)

“…For example, apes that use lexigrams can communicate about past events that they experienced or about future intended actions [ 54 – 55 ]. The use of lexigrams as relevant task stimuli allow for asking questions about the role of symbolic rewards in areas such as self-control and delay of gratification [ 56 – 57 ], prospective memory [ 58 – 59 ], metacognition [ 60 ], bartering and exchange behavior [ 61 ], and analogical reasoning [ 62 ]. In these areas, the symbol-trained chimpanzees have shown positive results of using symbols to facilitate self-control, to show prospective memory and metacognition, and they engage in limited forms of analogical reasoning and bartering.…”