At present, there are several wrinkles to this story (Hauser 2006; Miklosi 2006), thereby precluding a firm conclusion concerning the origins and selective pressures that shaped the capacity to infer communicative intention. First, though domesticated animals seem to outperform the wild-types, as well as chimpanzees and most primates, there are a few examples of non-domesticated animals succeeding on some versions of these tasks, albeit with extremely small sample sizes of individuals (Neiworth et al. 2002; Pack & Herman 2004); further, even apes succeed if you pool data across subjects, and on some tasks, some individuals show above chance performance (Call et al. 2000). Second, there is some evidence that enculturated primates (i.e. reared by humans) outperform naturally reared primates (Call & Tomasello 1994; Carpenter et al. 1995; Call et al. 2000); this suggests that something about the human environment may enable this ability, in the absence of domestication. Third, we know of no studies testing animals under free-ranging conditions and with a large sample of individuals; as a result of the small sample sizes used thus far, tests entail repeated trials with the same conditions, thereby precluding quantitative analyses of first-trial effects. Fourth, all studies using gestures use some version of the human pointing gesture, as opposed to the potentially communicative gestures of the target species.

In the food-absent communicative gesture condition, rhesus were more likely to approach the box targeted by the gesture (30/40 subjects, binomial probability: P=0.001); this result was identical to the food-present communicative gesture condition. In the food-absent pointing gesture condition, rhesus selectively approached the box targeted by the gesture (31/40 subjects, binomial probability: P=0.0003); this pattern was identical to the food-present pointing gesture condition. Thus, rhesus not only use the communicative gesture and the pointing gesture to locate hidden food, but also use these gestures to infer the existence of potential goals.

Though our initial aim was to avoid retesting the same subjects, this was not possible across conditions. To assess whether prior experience in these experiments might influence subsequent performance, we reanalysed each of the conditions to assess whether experimentally naive individuals performed differently from experimentally experienced subjects. For the food-present communicative gesture condition, experimentally naive individuals selectively approached the targeted box (21/28 subjects, binomial probability: P=0.006), and there was no difference in performance when contrasted with experimentally experienced subjects (Χ²(1, N=68)=0, P=1). In the food-absent communicative gesture condition, experimentally naive subjects selectively approached the targeted box (24/34 subjects, binomial probability: P=0.01) and there was no difference in performance when contrasted with experimentally experienced subjects (Χ²(1, N=74)=0.181, P=0.67). In the food-present pointing gesture condition, experimentally naive subjects selectively approached the targeted box (22/28 subjects, binomial probability: P=0.002) and there was no difference in performance when contrasted with experimentally experienced subjects (Χ²(1, N=68)=0.01, P=0.92). In the food-absent pointing gesture condition, experimentally naive subjects selectively approached the targeted box (22/31 subjects, binomial probability: P=0.01) and there was no difference in performance when contrasted with experimentally experienced subjects (Χ²(1, N=71)=0.39, P=0.53). Thus, for all conditions in which we obtained success with all subjects included, experimentally naive subjects succeeded as well; thus, prior experience on this kind of task is not necessary, nor does the experience appear to improve performance. No subject was tested twice in the basic gaze and communicative gesture from opposite box conditions.

In most of the previous work focusing on the ability of animals to correctly interpret a human gesture as indicating a target goal, individuals were presented with multiple trials within a session, and often, multiple sessions. Therefore, at some level, it is difficult to compare the present results with prior work as we used only a single trial per individual, per condition. To more closely approximate prior work, we attempted to test a small number of individuals with 10 consecutive trials of the communicative gesture using food. These tests are difficult to run, as subjects often move off, are distracted by others approaching, and so forth; thus, we counted as valid only the subjects that were tested while alone, remained in the same general area from start to end, and ran with inter-trial intervals of no more than a few minutes, which included set-up time. We used the same abort criteria as in our single-trial-per-subject conditions. The final dataset included 10 subjects that ran 10 consecutive trials. We tested 17 subjects that ran fewer than 10 trials and 13 subjects that failed to run through one trial. We did not use these 30 subjects in the final dataset; we note here that the abort rate is higher than in the one-shot experiments due to the difficulty of repeated testing of the same subject.

In the first experimental condition, a human experimenter presented a piece of food, concealed it behind an occluder covering two boxes and then used this communicative gesture to indicate the location of the hidden food. Consistently, subjects approached the indicated box. In all but one of the suite of follow-up conditions, designed to break down this gesture into a set of necessary and sufficient cues, we found evidence that rhesus can use the communicative gestures of a human agent to find both an explicitly presented goal as well as an inferred goal. In particular, we found that rhesus could follow a pointing gesture to the target location, and that the species-specific communicative gesture provided sufficient information even when the experimenter stood in front of the non-target box but indicated the alternative; thus, even though the experimenter was spatially biased towards one box, thus providing potentially salient associational cues, rhesus used the experimenter's communicative gesture to find the target location. Perhaps, most surprisingly, rhesus monkeys appeared to infer the presence of a goal from these gestures. That is, in the absence of presenting a piece of food prior to the gesture, rhesus nonetheless showed a selective approach to the indicated box. This shows that rhesus use a communicative gesture to infer the existence of an object or goal in the absence of any explicit reference to the object or goal, paralleling prior studies of chimpanzees and human-raised ravens (Corvus corax) following eye gaze around a barrier (Tomasello et al. 1999; Bugnyar et al. 2004). The only condition that was insufficient to trigger approach to the target box was a simple orienting response or basic gaze. These results raise several significant theoretical and methodological issues, which we turn to next.

Why might this population of free-ranging rhesus monkeys succeed in situations involving the communicative gestures of a human agent in an object choice task where chimpanzees and other primates (including captive rhesus) generally fail? One possibility is methodological. As mentioned in §1, previous work on this problem has focused on relatively small numbers of captive individuals, using a repeated trial design in which the same subject is presented with multiple opportunities, both within and across sessions, to use a communicative gesture or physical cue to find a target goal. Although some of these studies have explored first-trial effects, this is often difficult and has typically led to ambiguous patterns of response. The advantage, of course, of looking at repeated trials is that it provides the animal with the opportunity to learn from such experience. Even in these cases, however, chimpanzees generally fail to pick up on the pattern of associations between the experimenter's actions and the location of the target goal (Call et al. 2000). At some level, this fairly consistent failure to uncover the target goal in the face of powerful associative cues (e.g. noises, lifting up or pointing at the target box) suggests a higher-order problem; that is, given the ease with which even the simplest organism can learn from association, it is surprising, but telling, when an animal fails to use this mechanism. For the rhesus monkeys observed in these experiments, we tested most subjects on only a single trial. Success was determined by the number of subjects selecting the target box. Therefore, for these subjects prior experience played no role in the pattern of successes. In fact, even for subjects tested on a previous condition, we found no evidence that such experience played a significant role in their capacity to use the experimenter's gesture as a relevant cue, and in our repeated testing condition, found no learning effects as well.

Another possible interpretation of the present data is that in contrast to the chimpanzees tested thus far, rhesus on Cayo Santiago are more like domesticated dogs (Canis familiaris) and silver foxes (Urocyon cinereoargentus): they are more attentive to the actions of a human experimenter. For over 15 years now, a number of researchers have been conducting experiments with rhesus on Cayo Santiago, and some of these experiments involve overlapping design features with the present studies. In particular, in a wide number of experiments (Hauser et al. 2000; Santos et al. 2001; Flombaum & Santos 2005), an experimenter presents one or two boxes or a stage, reveals some amount of food, lowers the food behind an occluder and then either allows the subject to search or reveals the outcome and films looking time. Although these researchers have certainly not tested every single animal on the island, they have exposed many of these individuals, either directly or indirectly, to such tasks. As a result, rhesus monkeys may show heightened attention to human action owing to prior experience in tasks that often provide access to novel food. Although this is certainly an accurate description of rhesus monkeys on Cayo, we do not believe that it provides a compelling explanation of the pattern of results because most, if not all, of the chimpanzees tested in the communicative gesture tasks have also been exposed to experiments involving food retrieval of some sort, and often with a human experimenter (Carpenter et al. 1995; Povinelli & Eddy 1996; Povinelli 2000; Call et al. 2004; Hare et al. in press).

A final possible explanation is that there are fundamental species differences which enable rhesus to succeed where chimpanzees fail. Although this is possible, the current literature on comparative anatomy and behaviour provides few insights. Anatomically, there are certain differences in brain volume and suggestive evidence of novel cell types in the apes (Allman et al. 2002), but all of these differences would point to an advantage for chimpanzees, not rhesus. Similarly, though early work on the relationship between the perceptual experience of seeing and the mental state of knowing pointed to a possible advantage for chimpanzees over monkeys such as capuchins (Cebus apella Hare et al. 2000, 2001, 2003), more recent work has entered rhesus into the picture (Flombaum & Santos 2005; Santos et al. 2006), with comparable abilities emerging. Therefore, at this point, though we have ruled out that either domestication, training or membership in our own species are necessary preconditions for reading the communicative gestures of a human experimenter vis-à-vis explicit or inferred goals, it is not at all clear why this population of rhesus monkeys succeeds under conditions that chimpanzees and other captive primates generally fail.