Source: Chapter
7 (pp. 227-261) of Comparative Vertebrate
Cognition: Are Primates Superior to Non-Primates? Edited by Lesley J.
Rogers and Gisela Kaplan (a featured volume in Developments in Primatology: Progress and
Prospects). New York: Kluwer Academic/Plenum Publishers, January 2004.
Moti
Nissani, Department of Interdisciplinary Studies, 5700 Cass Ave., Wayne State
University, Detroit, MI 48202 (USA)
E-mail: aa1674@wayne.edu
Note: For a more recent
summary of our elephant research, please click here.
Clickable Table of
Contents:
Do Elephants and
Chimpanzees Know that People See?
EXPERIMENT 1: DO ELEPHANTS KNOW
THAT PEOPLE SEE?
EXPERIMENT 2: DO CHIMPANZEES KNOW
THAT PEOPLE SEE?
RETRACTABLE CORD-PULLING IN
ELEPHANTS
DO ELEPHANTS KNOW WHEN TO SUCK OR
BLOW?
This chapter attempts to provide an accessible review of a fundamental scientific and philosophical question: Are animals conscious? Instead of trying to address the issue of consciousness as a whole, this partial review will for the most part touch upon just two facets of consciousness— theory of mind and insight.
This review takes it for granted that we need to place a question mark on the understandable but as yet unfounded assumption that our evolutionary next of kin—the great apes—are also our closest cognitive relatives. At the moment, we cannot rule out the view that “a classification of the animal kingdom based on intelligence would probably cut right across the classifications based on structure” (Hobhouse, 1915). As a matter of fact, at the moment we cannot even refute the counterintuitive claim that there are no differences in intelligence between one nonhuman vertebrate and another (Macphail, 1982; Thomas, 1986).
On first sight, the case for consciousness in
animals seems overwhelming. Given the
evolutionary notion of continuity, and given moreover the remarkable
physiological and genetic similarity between apes and humans, it seems scarcely
credible to argue that apes are devoid of any trace of consciousness. Furthermore, it seems reasonable to suppose
that consciousness confers an enormous evolutionary advantage, for it allows
animals to try out possible actions in their head without actually performing
them through costly trial and error (Griffin, 2001). Common intuition seems to point in the same
direction, as the following passage suggests. ”When I looked into Washoe’s eyes she caught my gaze and regarded me
thoughtfully, just like my own son did.
There was a person inside that ape ‘costume.’ And in those moments of steady eye contact I
knew that Washoe was a child” (Fouts, 1997).
Regretfully, the question of animal consciousness cannot be resolved by either intuition or theory. Although intuition often serves as valuable breeding ground for research ideas, it is notoriously fallible. And as far as theory is concerned, it could be just as well supposed that consciousness exerts its own evolutionary price (e.g., hesitation when swift action is called for), or that it is not readily achieved, even when favored by natural selection.
Much of the original work reported in this chapter deals with
elephants, so it may be worthwhile to briefly review their cognition. Because we know so little about the minds of
elephants, and because the three known species of elephants are similar, this
review will focus on the cognition of elephants in general terms, often leaving
aside the terra incognita of possible cognitive and behavioral differences
among the three species.
Although elephants have been
in close association with humankind for thousands of years, and although
anecdotes about their wisdom or witlessness are many, their mentality has only
been subject to a mere handful of controlled studies. This curious gap in the research literature
provides fertile grounds for speculations and controversies. Indeed, views on the subject range from the
assertion that “elephants are exceedingly intelligent; that they have a form of
intelligence which manifests itself in many ways that are very like our own;
and that, in these respects, they stand as far apart as we do from all other
living things—the great apes not excluded” (I. T. Sanderson, 1962), to the
assertion that the “elephant is a stupid animal; and I can assert with
confidence that all the stories I have heard of it, except those relating to
feats of strength or docility performed under its keeper’s direction, are
beyond its intellectual power, and are mere pleasant fictions” (G. P.
Sanderson, 1912; see also Carrington, 1958).
At 6 kg, the elephant brain
is the largest brain of all land mammals (Rensch,
1957). Sheer size however means little,
for a good part of the brain must support the bulk of an animal and some of its
special, non-cognitive, functions. Seen
this way, the elephant is a comparative lightweight: an elephant weighing 75
times as much as a human has a brain weighing only 2-3 times as large (Spinage, 1994).
Likewise, the elephant’s cerebral hemispheres do not cover the
cerebellum as they do in humans and apes (Spinage,
1994). But, in common with the brain of
dolphins, great apes, and humans, the brain of elephants is highly convoluted—a
possible indicator of high intelligence and of an advanced capacity for
learning (Alexander, 2000).
Elephants also have a large neopallium (the area associated with memory). Above all, most mammals are born with brains
that are around 90 percent of their weight as adults, but humans are born with
about 27 percent, elephants with 35 percent, and chimpanzees with about 54
percent (Eltringham, 1982; Poole, 1997, p. 38). There is thus considerable post-natal brain
growth in these three species, a fact which is believed to be linked to the
exceptional learning ability of their young.
The only complete study of elephant cognition was conducted on a 5-year-old
Asian female at the Munster Zoo. She
faced two small wooden boxes whose lids were painted with two different
patterns, for example., circle and cross, and had to remove the correct lid to
get a food reward. It took her about 330
trials, over a period of several days, to consistently choose the reinforced
pattern, e.g., the cross (Rensch, 1956, 1957). By the fourth single-pair discrimination
task, she reached criterion on the 10th trial.
After learning to discriminate between 20 symbol pairs, she performed
superbly on a test that combined all 20.
The test lasted several hours, yet her performance actually improved
toward the end. A year later, her scores
ranged from 63 to 100 percent—a scientific demonstration, Rensch
says, of the adage that elephants never forget.
In a partial study, Leslie Squier (reported in Stevens, 1978) confirmed the elephant’s ability to learn a simple discrimination task and to remember it 8 years later, “a remarkable performance and one that many Homo sapiens might have difficulty emulating” (Markowitz, 1982). In another study, Povinelli (1989) reported that elephants do not recognize themselves in the mirror (an observation that we have confirmed with our two Asian elephants—Nissani and Hoefler-Nissani, 2002, unpublished observations). Thus, the few controlled experiments on record seem to lend credence to Carrington’s conclusion that “elephants are not sufficiently intelligent to grasp an idea easily or quickly in the early stages, but once it has penetrated to their somewhat slow brains it is virtually ineradicable.”
In contrast to the picture
which emerges from these zoo studies, field observers of elephants in the wild
are often more impressed by their minds (e.g., “the more we learn about
elephants, the fainter the line we have drawn between man and other animals
will become”—Poole, 1997). In one field
study, McComb et al. (2000) found that an average
female can recognize the contact calls of about 100 other females. Equally remarkable, the “legend” of coming to
the aid of a wounded comrade is true. In
one confirmed instance, when a bull was shot, two younger elephants came to his
rescue and tried to lead him away (Denis, 1963; cf. Carrington, 1958). Romanes’ (1882)
seemingly tall tale that, during an operation, “elephants behave like human
beings, as if conscious that the operation was for their good, and the pain
unavoidable,” is likewise probably true (Blashford-Snell
and Lenska, 1997; Groning
and Saller, 1999; Shand,
1995). Aelian’s
fantastic 3rd century recount of an Ethiopian tale that “if one elephant sees
another lying dead, it will not pass by without drawing up some earth with its
trunk and casting it upon the corpse, as though it were performing some sacred
and mysterious rite on behalf of their common nature” has been likewise
confirmed by contemporary researchers.
During drought, the African elephant is the only animal known to dig for
water, and there is moreover an account of an elephant chewing bark to fashion
a plug, using it to plug a water hole, and hiding the hole from other animals
by covering it with sand (Gordon, 1966).
In Burma (=Myanmar), according to one observer (Williams, 1950), raiding
young elephants often stuff their bell with mud in order to avoid
detection.
Elephants are proficient
tool users (Chevalier-Skolnikoff and Liska, 1993; Hart and Hart, 1994). They often scratch themselves with sticks and
break fences by piling logs—or even young elephants—over them. There are likewise many reliable accounts of
elephants hurling objects (Chevalier-Skolnikoff and Liska, 1993; Wickler and Seibt, 1997). A
27-year-old bull, for instance, routinely concealed a rock in his trunk and
hurled it at his keeper (Linden, 1999).
Once the training of elephants in playing cricket or soccer is over,
they “play the game with the enthusiasm of boys having a knock-up on the
village green” (Carrington, 1958; I. T. Sanderson, 1962).
I have tried to confine my
very partial account of the elephant’s mind to the more trustworthy, or to
independently-corroborated, observations, omitting the more anecdotal, and even
more fantastic, tales. Even so, the
picture that emerges is of an animal whose mentality deserves closer scientific
attention than it has so far received.
Do Elephants and Chimpanzees Know that People See?
An organism can be said to possess a theory of mind when it is capable of attributing mental states to others—when it understands that others see, feel, and know. An organism’s theory of mind might be inextricably linked to its capacity for consciousness, insight, concept of self, and such emotions as pride, shame, empathy, and compassion. The knowing component of this complex concept can perhaps be illustrated by imagining the following sequence (following Zentall, 2,000). In the first scene, little Susan observes Wong Tsu hiding a candy under a pillow and then going outside to play. As the next scene unfolds, Susan sees an adult entering the room, removing the candy without disturbing the pillow, and hiding the candy in a desk drawer. Next, Susan sees Wong Tsu coming back. If Susan expects him to look for the candy under the pillow and not in the drawer (i.e., if she implicitly theorizes that Wong Tsu has a mind and that in his mind the candy is still under the pillow), we can say that Susan possesses the knowing component of a theory of mind.
As we shall see, attempts to prove the existence of a theory of mind are often exceedingly complex when we divert our gaze from young children to animals. According to Lorenz (cited in Thorpe, 1956), if a raven is observed hiding food, it often retrieves the food, scolds the observer, and hides the food out of human reach. On first sight, this behavior appears to involve the same conceptual understanding as Susan’s, but here the cause is slippery—it could be traceable to understanding the perspective of another, or to genetic programming, or to prior trial and error learning.
Probably owing to such conceptual roadblocks, the question of whether any animal besides ourselves possesses a theory of mind has never been scientifically resolved. Hunter-gatherers, ancient civilizations, and Buddhism seem to have taken it for granted that animals are conscious. But with the advent of Christianity and, later, Cartesian philosophy, most educated westerners came to view animals—including the ones closest to us in appearance and behavior—as mindless, unfeeling, automatons. A couple of centuries later came yet another paradigmatic shift, for evolutionary theory seemed to require a continuity of physical and mental characteristics among humans and their closest relatives.
Darwin’s views prevailed until the mid-1990s or so. On the experimental side, numerous studies and field observations seemed to independently converge on the belief that some animals, at least, possess a theory of mind. Chimpanzees, for example, were said to be capable of insightful problem solving, acquiring a rudimentary form of language, self-referential behavior in front of a mirror, and deception (cf. Russon et al., 1996). Moreover, chimpanzees split from the human line only a few million years ago, share close to 99 percent of the human genome, and show remarkable behavioral similarities to human beings. All these observations seemed to imply that chimpanzees, and perhaps other species, possess a theory of mind.
Many researchers, to be sure, rejected this seemingly irresistible avalanche, but theirs appeared to be the kind of rearguard action that is often seen in science. In the 1990s, however, the pendulum began swinging again in a neo-Cartesian direction, and here one set of elegant experiments stands out, for it appeared to provide the clearest experimental evidence to date that chimpanzees—and perhaps all other non-human animals—possess “clever brains but blank minds” (N. Humphrey, cited in Gallup, 1998).
In these experiments, Daniel J. Povinelli and his colleagues capitalized on captive chimpanzees’ tendency to beg from their keepers to ask: Do chimpanzees understand the seemingly elementary fact that people see? (Povinelli and Eddy,1996; Povinelli et al., 2000; Reaux et al., 1999). Six or seven young chimpanzees took part in a complex series of longitudinal studies. In a typical pre-training phase, a youngster entered a room where it faced, across a clear Plexiglas barrier, a familiar experimenter. The experimenter sat on either the right or left side of the barrier, in each case directly across a hole in the Plexiglas. In an average of 48 consecutive sessions consisting of 10 trials each, the chimpanzee learned to insert a hand through the hole which faced the experimenter, as opposed to the hole which did not. Once this skill was acquired, the chimpanzee had to choose between one experimenter who had appealing food in her hand and one who held a neutral block of wood. In this case, the chimpanzee received a food reward only when it inserted its hand through the hole facing the food-carrying experimenter.
In the experiment itself, the chimpanzees faced variations along a single theme: in all variations they had to spontaneously decide whether to insert their hand toward an experimenter who could see them or toward an experimenter who could not. Only a few of these variations need to be described here (cf. Povinelli and Eddy, 1996). In all these variations, one of the experimenters faced the right hole of the barrier while the other experimenter faced the left hole (cf. Figure 1). Thus, in a typical blindfold trial, both experimenters stood motionlessly in front of the right or left hole, one with a blindfold covering her mouth and the other with an identical blindfold covering her eyes. Thus, the former could see the chimpanzee, while the latter could not. In the buckets condition, the experimenters stood or sat in the same place, but one held a large bucket on her shoulder and could see the animal, while the other experimenter covered her head with a bucket and could not see the animal. In the similar screen condition, one experimenter held a cardboard screen on her shoulder while the other held it so that it completely obscured her face from the chimpanzee. In the Back/Front condition, one experimenter faced the Plexiglas partition and the chimpanzee, while the other had her back turned to both. In the looking-over-shoulder condition, both experimenters assumed the same position and both had their backs turned to the chimpanzee, but one experimenter had her face turned toward the animal, while the other experimenter turned her face away.
Startlingly, in the first few trials, in all but one variation, the chimpanzees consistently performed at chance level. That is, and in contrast to the performance of young children (3 years of age or older) in a similar task, Povinelli’s chimpanzees were just as likely to beg from a person who could see the begging gesture as from a person who could not. The one exception in the original set of experiments involved the back/front combination (Figure 2, frame B3), but further experiments suggested to the authors that the chimpanzees’ proficiency in this task was merely “a consequence of their reinforcement history in the training phase” (Povinelli and Eddy, 1996). Thus, chimpanzees “do not seem to appreciate that others ‘see’ things” (Povinelli and Giambrone, 2001).
|
Figure 1. Four probe conditions
in chimpanzee seeing experiments. Top left: blindfolds. Top right: buckets.
Bottom Left: sideways. Bottom right: screens. |
These results, along with similarly discouraging studies of pointing and gaze-following, led Povinelli to the neo-Cartesian view that only humans can represent explicitly their “own psychological states and those of others” (Povinelli, 1998). Later, Carruthers (1998) based his view that sentience is exclusively human in part on Povinelli’s results.
The question of animal consciousness in turn has profound implications for our very conception of the world around us, for evolutionary biology, for comparative psychology, and perhaps also for the treatment of human autism (Baron-Cohen et al., 2000; Carruthers and Smith, 1996). It has also practical and controversial implications, ranging from a call for putting into place “more rigorous standards for conducting and reporting empirical research” in comparative psychology (Povinelli and Giambone, 2001), to endorsing intrusive research on apes (Wynne, 1999).
Thus, if the neo-Cartesians are right, only one animal possesses a measure of understanding of its own and others’ actions and mental states—and that animal is us! It follows that the ongoing, extensive search for culture, imitation, deception, or genuine self-awareness in animals is misguided in principle, somewhat akin to the quest for the philosopher’s stone. Looking closely at any great ape, we cannot, indeed, help feeling that it thinks and feels, at least in some rudimentary fashion, as we do. But this, the neo-Cartesians insist, is an illusion.
Povinelli’s work is consistent with some well-known observations. For one thing, his work throws light on our failure to come up with a single unequivocal demonstration of a theory of mind in animals; we may have failed to come up with a clearcut proof, neo-Cartesians may say, for the very good reason that such proof does not exist. Computer models likewise raise the possibility that complex behaviors, of the kind that are often shown by chimpanzees, can emerge mechanically, through mere learning and the pressure of natural selection, from the interaction of simple behavioral building blocks (cf. van Hooff, 2000; see also Morgan, 1920).
Povinelli’s experiments, and the neo-Cartesian hypothesis they led to, have however been questioned (Byrne 2001; Byrne and Whiten, 1988; Call, cited in Tuma, 2000; Gallup, 1998; Hare et al., 2000, Heyes, 1998; Russon et al., 1996). Before describing collaborative research efforts with elephants and chimpanzees at the Detroit Zoological Institute, and in an effort to provide a rationale for their design features, I wish to highlight a few additional uncertainties in Povinelli’s experiments.
Povinelli tacitly assumes that, cognitively speaking, the chimpanzee is our closest relative. This assumption is not necessarily true, for reasons that emerge elsewhere in this chapter. Thus, the neo-Cartesian hypothesis might be wrong, even if it can be shown that chimpanzees lack a theory of mind.
Although inconclusive, there is a body of circumstantial and experimental evidence suggesting that apes do have a theory of mind (e.g., Povinelli et al., 1990; Premack, 1988; Uller and Nichols, 2000). There is, in particular, experimental evidence that “chimpanzees are capable of modeling the visual perspectives of others” (Povinelli et al., 1990) and that, at least in some situations, they know “what conspecifics do and do not see” (Hare et al., 2000; but cf. Povinelli and Giambrone, 2001 for a dismissal of this latter claim). The tentative confirmatory evidence extends to other species as well; for instance, marmosets are claimed to be capable of true imitation (Voelkl and Huber, 2000), bottlenose dolphins seem capable of mirror self-recognition (Reiss and Marino, 2001), and even the green bee-eaters of India seem capable of distinguishing a person who can see them from a person who cannot (Smitha et al., 1999, cited in Griffin 2001). Given these observations, more than one set of experiments, one basic design, and six or seven subjects of one species (Povinelli and Eddy, 1996) are needed to resolve the theory of mind controversy.
Chimpanzees are not only physiologically our closest kin (de Waal, 2001), but they often behave similarly to human beings, for example, like us, they too follow the gaze of another, use tools, and engage in tribal warfare. Occam’s Razor would seem to imply similarity of the cognitive processes which underlie such analogous behavioral patterns. But, if cognitive similarity is rejected, one must resort to an auxiliary evolutionary hypothesis. Thus Povinelli and Giambrone (2001) are forced to put forward a reinterpretation hypothesis, which counterintuitively suggests, for example, that what appears to us as deception in animals is in fact traceable to the evolution of “brain systems dedicated to processing information about the regularities of the behaviors of others.”
One unresolved issue of Povinelli’s work involves the age of his chimpanzees. Since the initial work was carried out with young chimpanzees roughly between the ages of 5 and 6 years, the experiments left open the possibility that theory of mind arises later in chimpanzee development, as Povinelli and Eddy (1996) noted. To meet this objection, Povinelli and colleagues re-tested the same chimpanzees twice, up to the age of 9 years. In the first retest, a year or so after the original work was completed, the chimpanzees forgot the little they had learned, and again seemed unable to tell the difference between a human being who could see them and a human being who could not. But in the second test, when the same chimpanzees approached 9 years of age, they performed extremely well from the start in the buckets, screens, and looking-over-shoulder tests (see Figure 1), and only did poorly in tests that involved a direct understanding of the role of human eyes in visual perception (Reaux et al., 1999). These striking improvements at close to age 9 could then be interpreted in two radically different ways. One could assume, with Povinelli et al., “that there was no development between 5 and 9 years of age in the animals’ understanding of visual perception as an internal state of attention,” and that throughout this longitudinal study, they merely learned stimulus-based rule structures. Alternatively, one could assume that the older chimpanzees of this third series of experiments have by now matured enough to understand the attentional aspects of seeing (or at least, have acquired on their own the broad rule of begging from the person whose face is visible), and, in turn, explain their random performance in subtle tests involving covering the eyes by assuming that chimpanzees, like some of Povinelli’s children, may understand that there is indeed someone behind those eyes—without understanding the working of eyes (Hare et al., 2000). That is, the first hypothesis is congruent with the radical assertion that chimpanzees lack a theory of mind; the second, with the trivial conclusion that chimpanzees do not understand how human eyes work. Needless to say, to decide which of these two hypotheses is more nearly correct, one must test adults who have not taken part in such tests before.
It is probable that the seeing experiments of Povinelli and colleagues lack ecological validity, a related set of problems that could only have been dealt with through a radical alteration of some features of their experiments.
To begin with, the experiments are claimed to have relied on the species-typical begging gesture. In point of fact, however, the chimpanzees had to learn the unnatural gesture of inserting a hand through one of two holes of a transparent plastic barrier. As might be expected, it took them an average of 479 trials, and dozens of sessions, to learn this allegedly natural response. Human observers are tempted to interpret such insertions as begging, but to the chimpanzees themselves they may have had no meaning whatsoever, thus accounting for their chance performance in the critical tests. In a comparable context, Gomez (1998) argues that the prolonged training of subjects in artificial tasks is a mistaken approach.
It is not clear either what the natural begging response of captive chimpanzees is. Our chimpanzees (but not our elephants) seemed to employ a variety of idiosyncratic begging postures, instead of just one. Thus, when begging services, toys, or food from their keepers, one of our captive chimpanzees often nodded her head while another whimpered, banged on the wire mesh, or clapped his hands. Indeed, often throughout our own experiments the chimpanzees employed one of these more natural responses first, and only then resorted to the required learned gesture of inserting their fingers through or under the wire mesh.
Moreover, Povinelli’s chimpanzees begged from people who often assumed unnatural, seemingly indifferent, postures. For instance, throughout any buckets trial, even the seeing experimenter remained motionless and made no eye contact with the animal.
Povinelli’s overworked subjects have been experimented with virtually all their lives. They were all born in captivity. Four were reared in a nursery from birth and two were reared by their mother for 1 year and then transferred to the nursery. This background raises serious questions about their natural curiosity and their emotional and cognitive development (Harlow and Harlow 1965; Thompson and Melzack, 1956). Needless to say, few if any of the children in Povinelli and Eddy’s initial experiment—which provided the backdrop, controls, and confirmation of the results—suffered the traumas of lifelong experimentation and maternal deprivation.
A related problem involves the over-reliance in most trials on the food versus block of wood configuration. This “baseline probe” was frequently used during extensive pre-training sessions, and it often outnumbered the “treatment probe” in experimental sessions by a ratio of 2:1. This might have led the chimpanzees to believe that their task in some of the critical trials involved guessing who had the food, not begging from the person who could see them. Indeed, Povinelli and Eddy’s data lend a measure of support to this interpretation. Thus, in their Experiment 7, when the majority of trials involved begging from a single person only, and only comparatively few trials involved food versus block of wood before and during critical sessions, the chimps performed significantly above chance in the screens test, leading the authors to suggest that some learning had occurred in this case. But, in their Experiment 8, when only food versus block of wood were used as spacer trials (and no singles), the chimpanzees’ performances in the screen test returned to chance level. In this context, Povinelli and Eddy are puzzled: “It is difficult to know why the subjects performed so poorly on the screen-over-face probe trials in this experiment as compared to Experiment 7.” Such unaccountable results can in turn be readily explained by the assumption that the chimpanzees were confused about the nature of the task they were to perform in these experiments. And again, when working with children, the equivalent of food versus wood was used in training, but only once during the trials themselves, thus raising doubts about the validity of the key comparison between the young chimpanzees and the younger children.
Another possible design flaw involves inadequate payoffs. In a typical experiment (for instance, their Experiment 1), 80 percent of the trials presented no choice at all, 8.9 percent presented the easy, immediate choice between a block of wood and a favorite food item, 4.4 percent the easy choice between an experimenter’s back and front, and only 6.7 percent presented the more difficult, experimental choices (e.g., looking over the shoulder). Thus, the subjects were assured of being rewarded in 168 of 180 trials (93.3 percent) by acting mechanically, and were likely to be rewarded in 174 of every 180 trials (96.7 percent). This high reward ratio seems to beg mechanical, behavioristic, action, rather than thought and contemplation—if you wish to encourage thinking, you must create situations where the payoffs for thinking before acting involve is a bit more than a 3.3 percent increase in the reward schedule. Thus, we may conclude that the choice between a seeing and non-seeing person was not as automatic as the choice between back and front, or food vs. wood, but not that it could not be made at all. This difficulty is compounded by the fact that the relevant fractions are entirely different for the human subjects, who had to make the more difficult choice in 30 percent of the trials. Acting mechanically, for the toddlers, would have involved a 15 percent loss of rewards vs. a 3.3 percent for the chimpanzees.
To the best of our knowledge, Povinelli’s ingenious experiments, despite their procedural simplicity, serious methodological and conceptual flaws, counterintuitive character, wide acclaim, and profound implications, have not been replicated with chimpanzees (although other researchers addressed similar questions by relying on different approaches, e.g., Hare et al., 2000), nor have they been extended to other species. It seemed worthwhile therefore to reproduce Povinelli’s essential procedure while safeguarding against some of its design flaws. Thus, to avoid reliance on just one species before rejecting Darwin’s cognitive continuity hypothesis, the experiments described here were carried out with seven chimpanzees