Orangutans (Pongo abelii) make flexible decisions relative to reward quality and tool functionality in a multi-dimensional tool-use task

Abstract

Making economic decisions in a natural foraging situation that involves the use of tools may require an animal to consider more levels of relational complexity than merely deciding between an immediate and a delayed food option. We used the same method previously used with Goffin´s cockatoos to investigate the orangutans' flexibility for making the most profitable decisions when confronted with five different settings that included one or two different apparatuses, two different tools and two food items (one more preferred than the other). We found that orangutans made profitable decisions relative to reward quality, when the task required the subjects to select a tool over an immediately accessible food reward. Furthermore, most subjects were sensitive to work-effort when the immediate and the delayed option (directly accessible by using a tool) led to the same outcome. Most subjects continued to make profitable decisions that required taking into account the tool functionality. In a final multidimensional task design in which subjects had to simultaneously focus on two apparatuses, two reward qualities and two different tools, the orangutans chose the functional tool to access the high quality reward.

Fig 1

Left: A) Ball-apparatus 1) Insertion hole 2) vertical tube shaft 3) position of food reward on collapsible platform 4) reward opening. Right: B) Stick-apparatus 1) Insertion hole 2) moveable tube-cup (open at the bottom), food reward is located inside of it 3) opening to slanted panel 4) reward opening.

Fig 2. Overview of the conditions within each of the five tests.

High quality food is banana-pellet, lower quality food is apple. 1) TST (Tool selection test): both tools are present while the high quality food is inside the apparatus. 2a) QAT (Quality allocation test): the tool is functional in each of the conditions. Left: High quality food is inside the apparatus. Immediate food option is the lower quality food. Right: Lower quality food is inside the apparatus. The high quality food is the immediate food option. 2b) MT (Motivation test): tool is always functional. High quality food is inside and outside the apparatus. 2c) TFT (Tool functionality test): High quality food is inside the apparatus in each condition. Left: Functional tool is present. Right: Non-functional tool is present. 2d) TSQAT (Tool selection quality allocation test): both tools and both apparatuses are present. Left: High quality food is located in the stick-apparatus. Right: High quality food is located in the ball-apparatus.

Fig 3. Mean percentages of tool choices per condition within each test.

1) TST (Tool selection test), 2a) MT (Motivation test), 2b) QAT (Quality allocation test), 2c) TFT (Tool functionality test), 2d) TSQAT (Tool selection quality allocation test); (n = 6). * p<0.05.