Informed Forager Methods Introduction

Menzel introduced the informed forager paradigm in a series of experiments

published in his chapter “A Group of Young Chimpanzees in a One-Acre Field” (1974). The general procedure of his experiment was to inform a chimpanzee ‘leader’ of the location of a hidden item in the group’s outdoor enclosure—either a desirable piece of food or toy, or a frightening toy snake. After the baiting took place, the leader was released into the enclosure with the rest of the social group to search for the hidden item(s). Menzel recorded the

interactions of the group members in relation to each other’s behaviour, the incentive structure of the hidden items, and the dispersion of the items.

In several other studies in the informed forager paradigm, numerous containers such as boxes or film canisters were placed in the subjects’ living area as hiding locations for the food (Coussi-Korbel, 1994; Held et al., 2000; Hirata & Matsuzawa, 2001; Bugnyar &

Kotrschal, 2004). However, Hirata and Matsuzawa (2001) report that the dominant individual in their study gained information from the subordinate regarding her direction of movement, and as she was familiar with the experimental boxes, would run ahead of the subordinate toward the box in her line of travel. It was a very clever strategy on the part of the dominant to gain information about the food’s location in this way, as it made it difficult for the subordinate to withhold information and even more difficult for her to approach the food without high risk of losing it to the dominant.

For the purpose of the current experiment, I did not want the experimental setting to be too easy for the dominant to assess where the food was without genuinely searching, as in Hirata and Matsuzawa (2001). The subordinate should have sufficient opportunity to win the reward often enough to stay motivated to participate. Therefore, food was hidden at fixed locations but in more natural conditions: boxes specific to the experiment were not set up in the enclosure, rather, food was hidden using the natural features of the enclosure, for example in a tyre or under a barrel.

Another change that was made to the procedure was that the dominant was released from her cage before the subordinate into the outdoor enclosure. In Hirata and Matsuzawa (2001), the chimpanzees were moved into the same cage before release, because Menzel (1974) had not found conclusive evidence to suggest that the knowledgeable individual communicated about the food to it’s partner(s) before their release. Therefore, whether the partners were together or each in their own cages at the time of their release did not influence

whether the ignorant partner found the bait. In some of Hare’s experiments (Hare et al., 2000, 2001), the subordinate was released for competition before the dominant to control for the subordinate simply reacting to the dominant’s direction of movement. However, in my experiments, the dominant was not informed of the location of the hidden food (except in one control condition) and thus the subordinate was not expected to read the dominant’s

behaviour before entering the enclosure. Therefore I decided it would be best to release the ignorant dominant first. Additionally, a few of the hiding places were relatively close to the release doors, and I feared due to the delay in opening the hydraulic doors, that if the subordinate was released first, she might be able to find the bait at these closer locations before the dominant had time to assess the situation.

II. Methods: Informed Forager Experiment 1: Competition

A. Subjects

Captive chimpanzees (Pan troglodytes) housed at the Living Links Center, Yerkes National Primate Research Center Field Station at Emory University in Lawrenceville, Georgia, participated in this study. At the Field Station there are two groups, FS1 and FS2, each of which comprises eleven individuals (Tables 3.1, 3.2): these chimpanzees have been housed together since the 1970s and 1993, respectively (Seres et al., 2001).

Table 3.1: Chimpanzees at Yerkes FS1, from most dominant to least dominant Name Group Sex Birthday Rearing History Kinship

Socko FS1 M 23 Jan 1987 Half-brother of Donna

Georgia FS1 F 27 Aug 1980 Mother/Group Daughter of Borie, sister of Rita, mother of Katie Borie FS1 F 1 Jan 1964 Mother/Group Mother of Georgia, Rita,

grandmother of Katie, Tara Donna FS1 F 3 Apr 1990 Mother/Group Half-sister of Socko

Katie FS1 F 10 May 1989 Mother/Group Daughter of Georgia Rita FS1 F 23 Sep 1987 Mother/Group Daughter of Borie, sister of

Georgia, mother of Tara Anja FS1 F 9 Jan 1980 Mother/Group No kin in group

Reinette FS1 F 17 Dec 1987 Mother/Group No kin in group Mai FS1 F 1 Jan 1964 Mother/Group Mother of Missy Tara FS1 F 5 Sep 1995 Mother/Group Daughter of Rita Missy FS1 F 8 Jul 1993 Mother/Group Daughter of Mai

Table 3.2: Chimpanzees at Yerkes FS2, from most dominant to least dominant Name Group Sex Birthday Rearing History Kinship

Steward FS2 M 8 Sep 1993 Mother/Group Son of Vivienne Chip FS2 M 30 Mar 1989 Nursery/Group No kin in group Ericka FS2 F 20 Oct 1973 Home/Nursery No kin in group Cynthia FS2 F 7 Jun 1980 Nursery/Group Mother of Virginia Vivienne FS2 F 27 Jul 1974 Nursery/Group Mother of Steward Barbie FS2 F 14 Jun 1976 Nursery/Group No kin in group Virginia FS2 F 18 Apr 1991 Mother/Group Daughter of Ericka Daisy FS2 F 1 Oct 1989 Mother/Group Daughter of Tai, sister of

Julianne

Tai FS2 F 1 Jan 1967 Mother/Group Mother of Daisy, Julianne Julianne FS2 F 15 May 1998 Mother/Group Daughter of Tai, sister of Daisy Waga FS2 F 19 Mar 1982 Mother/Group No kin in group

Before data collection for the Competition or Unequal Rewards began, chimpanzees in both groups were tested against a linear dominance hierarchy. Subordinates from each group were then tested for their memory of the placement of banana slices in differently coloured tubes, and then different buckets (See Appendix 3); both those experiments were unsuccessful for various reasons. It became clear that changes to the finalised protocol of the

Competition and Unequal Rewards experiments would reduce the subject pool: only FS1 was used in the final informed forager experiment for reasons related to their housing. At FS1, a small Lexan window was placed into two of the four metallic hydraulic doors—doors 3 and 4—leading out to the enclosure from the ‘Cognition room’; the Lexan replaced a steel plate that had covered the window hole previously (Figure 3.1). However, at FS2, the plastic doors did not have holes in them already; a decision was made not to reduce the integrity of the doors by cutting new holes, or to not replace them altogether with metallic doors. While this problem was potentially surmountable, the issue of space was not: FS2 does not have a ‘Cognition room’ and therefore it was impossible to lock nine chimpanzees into two cages, and use the other three cages for the experiment. Ultimately, FS2 was abandoned as an experimental site. Therefore, Tai, Barbie, and Julianne, who had been tested with tubes and buckets, did not participate in the Competition or Unequal Rewards experiments.

Pairs of chimpanzees from FS1 were chosen to compete: pairs consisted of captive- born unrelated individuals, taken from the same social group of 11 individuals. In FS1, Missy (age 18) and Reinette (age 23) were chosen as subordinates for the study based on their

positions determined during dominance testing. Hierarchy testing was completed prior to the experiment, and since the hierarchy was not precisely linear, each subordinate was partnered with a dominant competitor that had at least three positions in rank difference, to be sure that one was dominant over the other. Many of the members of the FS1 group are kin, and I avoided pairing related individuals, because mothers and daughters, having shared a close bond throughout child-rearing, might be more tolerant of food sharing (van Lawick-Goodall, 1975). Therefore, Missy was paired with Rita (age 23), and Reinette with Georgia (age 30).

Of the four FS1 subordinates tested during tube and bucket training (See Appendix 3), only Missy and Reinette were included in the final analysis. Of the two females in FS1 that were not included in the final analysis, Katie and Tara, Katie completed all trials for the

Competition and eight trials in the Unequal Rewards condition with her partner Socko. Socko was the only male in the FS1 group, and was very tolerant of food sharing. During group feeding situations, he often let females take food before he ate, and even let some females take food directly from his mouth. During the course of the Competition, he did not put enough competitive pressure on subordinate Katie, who won the food reward on every trial. There was no need for her to act deceptively, and on many trials Socko would leave the ‘Cognition room’ but not follow Katie towards the food. Socko was removed from the study after completing eight trials in the Unequal Rewards condition. Tara was paired with both Anja and Donna as dominants, but neither were motivated to exit the ‘Cognition room’ on experimental trials. I attempted to pair Katie, as a dominant, with Tara. However, early in the data collection for the Competition, the two chimpanzees got in a fight and afterwards would not be locked inside together, and both were thus removed from the study.

B. Setting

Subjects were housed in an outdoor enclosure (24 x 30 m) and had access to two indoor areas, six Bedroom cages (3 x 3 x 3 m), and five ‘Cognition Room’ cages (1.74 x 1.74 x 1.74 m) (See Figure 3.1). There were mesh panels in the ‘Cognition Room’ between cages 1 and 2, and 2 and 3, measuring 28.5 by 20.5 inches. Most cages in each building had hydraulic doors connecting them, as well as hydraulic doors leading out to the enclosure. In the ‘Cognition Room’, a small Lexan window was placed into two of the four doors leading out to the enclosure, doors 3 and 4. The outdoor enclosure contained a three-story climbing structure with ladders and ropes, as well as several tyres, kegs, barrels, and other enrichment objects. All chimpanzees were fed twice a day with chow, fruit and vegetables, and water

was available ad libitum. Chimpanzees were not deprived of food at any time and were not subject to any invasive procedure.

Testing took place in the ‘Cognition Room’ and in the outdoor enclosure at FS1. All of the chimpanzees in the group were locked inside so that a human experimenter (E1) could enter the enclosure safely to hide a banana on each trial. Non-participating chimpanzees were called into the Bedroom area and came in voluntarily, and were rewarded with a quarter slice of apple. They could not witness the baiting procedure from the Bedroom cages. For

maximum safety, chimpanzees in the Bedroom area were locked in Cage #6, which was also at a 90-degree angle to the enclosure, behind Cage #5. When necessary, hydraulic doors leading to the enclosure were pinned and locked from the inside of enclosure.

Each pair was tested separately. While baiting took place, the participating

chimpanzees were locked in the ‘Cognition Room’: the informed subordinate subject was always in the third cage with the Lexan window to the enclosure, the Knowledge Cage #1, and the uninformed dominant competitor was always in the second cage without a window, the Ignorance Cage. Each chimpanzee could see her competitor through the mesh panel between the two cages, and the dominant was able to see that the subordinate had a window; however, the dominant could not see out the window from the Ignorance Cage. The first cage was always left empty so that E1 could enter the enclosure through the hydraulic chimpanzee door. After baiting was complete, both partners were released into the outdoor enclosure to search for the hidden food on each trial.

FIGURE 3.1: Diagram of the Testing Area and Procedure C. Apparatus

At FS1, a small Lexan window was placed into two of the four metallic hydraulic doors, doors 3 and 4 (also known as Knowledge Cages #1 and #2), leading out to the enclosure from the ‘Cognition Room’; the Lexan replaced a steel plate that had covered the window hole previously.