Innovative problem solving in great apes: the role of visual feedback in the floating peanut task

Nonhuman great apes show remarkable behavioural flexibility. Some individuals are even able to use water as a tool: They spit water into a vertical tube to make a peanut float upwards until it comes into reach (floating peanut task; FPT). In the current study, we used the FPT to investigate how visual feedback, an end-state demonstration and a social demonstration affect task performance in nonhuman great apes in three experiments. Our results indicate that apes who had acquired the solution with a clear tube maintained it with an opaque one. However, apes starting with an opaque tube failed to solve the task. Additionally, facing the peanut floating on a water-filled tube (i.e., an end-state demonstration) promoted success independent on the availability of visual feedback. Moreover, experiencing how water was poured into the tube either by a human demonstrator or by a water tap that had been opened either by the ape or a human did not seem to be of further assistance. First, this study suggests that great apes require visual feedback for solving the FPT, which is no longer required after the initial acquisition. Second, some subjects benefit from encountering the end-state, a finding corroborating previous studies. Electronic supplementary material The online version of this article (10.1007/s10071-019-01275-0) contains supplementary material, which is available to authorized users.

1 Four spits occurred after ten minutes had past (in the end the tube was quarter-filled). Additional figure

Fig. S1
The activated water tap in the water tap by ape condition (A) and the water tap by human condition (B) is shown from outside the test room in Experiment 2

Long-term memory
Subjects acquired the solution in the FPT at different times. Toba was tested nine years ago while her six-month-old daughter Tao was present (Mendes, Hanus, & Call, 2007). Padana and Dokana were also tested nine years ago (Mendes et al., 2007) and re-tested five years age and two years ago respectively. Dokana additionally participated in several TV documentaries, but did so for the last time about two years prior to the current study. Two-year-old Raja was present when her mother Pini was tested in the original study nine years ago (Mendes et al., 2007). Subsequently, Pini was re-tested two years ago and Raja was tested for the first time two years ago as well. Pini was not tested in the current study because she could not be separated from her current offspring whom we wanted to give the task later as well. Frodo was trained to function as a demonstrator about seven years ago (Tennie, Call, & Tomasello, 2010). Lome and Tai were also tested about seven years ago (Hanus, Mendes, Tennie, & Call, 2011). Tai additionally participated at least in one TV documentary one and a half years ago. Alexandra, Kara, Kofi, Lobo and Sandra were tested one month prior to Experiment 3 (mean: 29 days, minimum: 24 days, maximum: 33 days), but had only solved the task once, whereas the others had solved it multiple times.
As Raja, Dokana and Padana had already been re-tested, they were not given a reminder with the clear tube, but started with the opaque tube straight away. For reasons of consistency we A B 6 should have treated the chimpanzee Tai in the same way because she had been re-tested for a TV documentary, but we were not aware of this fact at the time of conducting the study.

Comment on the testing of Toba, Tao and Padana
Toba and Tao had been transferred to a different holding facility since the last study was conducted (Mendes et al., 2007). Unfortunately, the clear tube did not fit well to the mesh of the sleeping room at the new facility so that a rectangular plate at the bottom of the tube protruded.
Toba bit it off in her first session accessing the peanut in this way. We repeated the session on the next day with a tube that was closed at the bottom but lacking a rectangular plate. The caregivers reported beforehand that the orang-utans should probably know about the water dispensers in their sleeping rooms, although they had rarely seen them drinking from these dispensers. Especially with Tao they were unsure if she was actually aware of this drinking option. Thus, in her first session, we provided Tao with a bowl of water which she spilled after a while so that we do not know if she had a chance to solve the task at all. Before Toba broke off the bottom of the tube in her first session, she spat saliva into the tube several times so that we were also not sure if she was aware of the water dispenser. To give them a fair chance, we provided both with a large bucket of water in the second session that we placed on the floor of the test room and then, both orang-utans solved the task.
For Toba and Tao, the mesh surrounding the opaque tube could not be covered completely due to the conditions at the holding facility. However, we covered the hose at the back of the tube that led to a basin to collect the water. Importantly, the setup looked exactly the same for the opaque condition, the "visual cause" and the "no visual cause" condition from the perspective of the ape. However, the hose was not yet cut on the surface to let the air out so that Toba experienced a gurgling sound in the no visual cause condition when the water flew out. Padana was tested with the same setup as Toba in the first two sessions of the opaque condition. She did not solve the task and after receiving reminder sessions with the clear tube, she received the opaque tube again, but this time the mesh surrounding the tube was completely covered as it was for all the other apes.