Elsevier

Behavioural Processes

Volume 89, Issue 3, March 2012, Pages 197-202
Behavioural Processes

A fruit in hand is worth many more in the bush: Steep spatial discounting by free-ranging rhesus macaques (Macaca mulatta)

https://doi.org/10.1016/j.beproc.2011.09.010Get rights and content

Abstract

Decision making is one of the principal cognitive processes underlying goal-directed behaviour and thus there is justifiably strong interest in modeling it. However, many of these models have yet to be tested outside of the laboratory. At the same time, field work would benefit from the use of experimental methods developed in the laboratory to determine the causal relationships between environmental variables and behaviour. We therefore adapted a laboratory-derived experimental paradigm to test decision making in the wild. The experiment used an indifference-point procedure to determine the influence of both the amount and distance of food on choice behaviour. Free-ranging rhesus monkeys were given the choice between a smaller amount of food at a closer distance and a larger amount farther away. In four conditions, we held the closer amount constant across trials and varied the farther amount to determine the point at which the monkeys were indifferent to the choice alternatives. For example, in condition one, we used one piece of food at the closer location, and determined how many pieces would be equivalent in the farther location. Four different closer amounts were tested to obtain an indifference point curve, with the indifference amounts at the farther location plotted against the closer amounts. The slope of the obtained linear indifference curve was surprisingly high, suggesting that rhesus monkeys significantly discount food that is farther away. Possible reasons for this steep spatial discounting are discussed.

Highlights

► Models of decision making need to be tested experimentally under natural conditions. ► Wild rhesus monkeys chose between different food amounts at two distances. ► The monkeys significantly discounted food that was farther away. ► Context and species-typical conditions influence the decision-making process.

Introduction

Decision making is a key process governing goal-directed behaviour, and models of decision making attempt to characterize how cost and benefit parameters influence choice behaviour. For example, with respect to the amount of a positive reward such as food and the delay to receiving it, animals will discount the overall value of the reward if it is not immediately available (Fantino, 1969, Rachlin and Green, 1972, Ainslie, 1974, Mazur and Logue, 1978, Fantino and Davison, 1983, Mazur, 1987, Green and Myerson, 1996). Thus, when choosing between a smaller and larger amount of food, the smaller amount may be preferred if the larger would be received after a significant delay.

To quantify the effects of amount and delay on choice behaviour, evidence has been found for the following relationship:V=A1+KDwhere V is the subjective value of a given option, A is the reward amount, D is the delay to receiving the reward, and K is a free parameter that determines how steeply subjective value changes with delay (e.g. Mazur, 1987, Mazur, 2000, Mazur, 2007, Tobin and Logue, 1994). Thus, there is a positive linear relationship between the subjective value of a choice option and the reward amount, and a hyperbolic relationship between subjective value and delay to reward. The hyperbolic relationship captures the fact that the value of a choice option decreases as delay increases, with the discounting becoming less severe with longer delays.

For animals foraging in the wild, delay is often associated with travel distance—individuals normally must travel to obtain food, and traveling takes time and energy (Stephens and Krebs, 1986). Moreover, many choices are based on food sources at known locations and thus the evaluation of a given food source may depend explicitly on the travel distance to the source. One might therefore hypothesize that the relationship of distance to value may also be represented in Eq. (1), with distance replacing delay. That is, one would expect value to decrease with increasing distance, and the severity of this discounting might diminish with increasing distances. We will consider this formulation of the influence of distance on choice – where delay is replaced by distance in Eq. (1) – as Model 1 (see Bateson and Kacelnik, 1996, Janson, 2007).

Other studies have found evidence for the addition of a parameter to Eq. (1):V=A1+KDBwhere B is a scaling factor or represents other factors such as effort or risk (Mazur and Kralik, 1990, McKerchar et al., 2009, Rachlin, 2006). In this second model, we will again consider D to be distance to the food source.

We conducted the current study to determine whether the value of food at a distance is better represented by Model 1 or 2, in cases where animals are making discrete choices before traveling. The experiment was also conducted to determine if models of decision making developed in the laboratory apply to animal behaviour in more natural environments. This ecological validation is critical because laboratory research has uncovered important findings that have yet to be verified in the field. Furthermore, much of the research in the development of these models was conducted on pigeons and rats, and thus we also conducted the experiment to determine if such models also applied to an Old World monkey, the rhesus macaque (Macaca mulatta) (also see Hayden and Platt, 2007, Janson, 2007, Kim et al., 2008, Stevens et al., 2005a, Stevens et al., 2005b, Szalda-Petree et al., 2004, Tobin et al., 1996). Finally, the current experiment was conducted to test whether a specific experimental procedure used in the laboratory to quantify the relationship of economic variables to choice behaviour could also be used in the field: Mazur's (1987) indifference point procedure, described in Section 2.2. If successful, it would provide another experimental paradigm with quantitative rigor to study decision making in the wild (also see Janson, 2007).

Section snippets

Subjects

We tested adult and subadult male rhesus macaques on Cayo Santiago in Puerto Rico. Subadult males were males estimated to be between three and five years of age, whose body size was approximately as large as those of adult males, but whose testes had not completely descended.

Indifference point procedure

To test between the two models, we utilized an indifference point procedure (Mazur, 1987). The procedure is a popular means for testing decision-making models because it takes advantage of the fact that when one is

Results

The results are shown in Table 1. Overall, for every condition the monkeys’ behaviour reflected the titration procedure: with the smallest amounts of apple slices on the farther tray, the monkeys chose the closer location most of the time; as the amount on the farther tray increased, the monkeys chose the farther location more frequently. The results for all four conditions are plotted in Fig. 2. Each graph plots every trial of the condition, with the farther amount along the x-axis and each

Discussion

We found a linear indifference point curve with a slope (9) significantly higher than the ratio of the two distances (1.5). This degree of devaluation appeared steep, given that the monkeys were only required to travel an additional five feet to obtain the larger amount of food, and yet more than 32.7 apple slices on the farther tray was necessary for them to prefer the farther tray with 4 apple slices on the closer one. Model 2 was able to account for this result, Model 1 could not, which

Acknowledgements

The project was supported by Grant Number CM-5 P40 RR003640-20 from the National Center for Research Resources (NCRR), a component of the National Institutes of Health (NIH), and the contents are solely the responsibility of the authors and do not necessarily represent the official views of NCRR or NIH. The experiment complied with all current laws and regulations of Puerto Rico and USA.

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