Byrne & Russon's proposal that stimulus enhancement, emulation, and response facilitation should be lumped together as priming effects conceals important questions about nonimitative social learning, fails to forge a useful link between the social learning and cognitive psychological literatures, and leaves unexplained the most interesting feature of phenomena ascribed to “response facilitation.”

We focus on Byrne & Russon's argument that program-level imitation is driven by hierarchically organized goals, and the related claim that to establish whether observed behavior is evidence of program-level imitation, empirical studies of imitation must use multi-stage actions as imitative tasks. We agree that goals play an indispensable role in the generation of action and imitative behavior but argue that multi-goal tasks, not only multi-stage tasks, reveal program-level imitation.

Byrne & Russon have argued that imitation is not an all-or-none phenomenon but may instead occur at different levels. Although I applaud their theoretical framework, their data provide little empirical support for the theory. Data from studies of human infants, however, are consistent with the view that imitation may occur at different levels. These data may provide better support for Byrne & Russon's hierarchical view of imitation than the nonhuman primate data that their theory was developed to explain.

It is suggested (1) that motivation must not be overlooked, (2) that most social imitation does not involve novel behaviors, and (3) that newborn babies do imitate.

Byrne & Russon use novelty as the primary requirement for providing evidence of true imitation in animals. There are three reasons to object to this. First, experiential learning cannot always be completely excluded as an alternative explanation of the observed behavior. Second, the imitator's manipulations performed during ontogeny cannot be known in full detail. Finally, there is at present only a weak understanding of how novel forms emerge. Data from our own recent experiments will be used to emphasize the need for a tighter methodology in imitation experiments.

Byrne & Russon's target article displays all the difficulties encountered when one fails to take a methodological behaviourist approach to imitation. Their conceptual apparatus is grounded in a mixture of introspection and folk psychology. Their distinction between action-level and program-level imitation falters on goal imputation for sequential acts. In an alternative gradient descent model, behaviour can be simulated as a frustration/satisfaction gradient descent in the animal's “potentiality space,” as defined by knowledge, inventiveness, and the surrounding environment.

Dolphins exhibit both action-level imitation (ALI) and program-level imitation (PLI). Dolphins may use ALI primarily for social cohesion, whereas PLI seems more likely to occur in goal-directed, problem-solving contexts. Both PLI and insightful problem solving require a recognition of the functional relations between actions and outcomes. Insightful problem solving, however, involves the creation of a program in the absence of a model, and therefore requires a higher order appreciation and application of the relations between actions and outcomes.

Byrne & Russon's account of program imitation in primates involves propositional attitudes (expectations and goals), which limits its falsifiability. Yet their account of priming shows exactly how imitation without attitudes would look. The challenge is to upgrade the notion of priming to give an account of low-level program imitation without invoking propositional attitudes.

We commend Byrne & Russon for their effort to expand and clarify the concept of imitation by addressing the various levels of behavior organization at which it could occur. We are concerned, however, first about the ambiguity with which these levels are defined and second about whether there is any particular need for comparative cognition to keep focusing on imitation as an important intellectual faculty. We recommend stricter definitions of hierarchical behavioral levels that will lend themselves to operational definitions and continued study of how animal subjects organize their goal-directed behavior as opposed to whether it is or is not imitation.

Byrne & Russon's notion of program-level imitation is based on the ability of apes to plan novel sequences of behavior and on how information gleaned by observation can aid the planning process. Byrne & Russon would have made a stronger case by focusing on social learning and planning and expending less effort interpreting their results as a new category of imitation.

I will argue that we cannot understand imitation unless we know more about its function. By comparing the two examples presented by Byrne & Russon I show how the imitative behaviour of orangutans can be interpreted as a homologue of human imitation during play. In contrast, the lack of data leave the role of imitation in gorillas doubtful.

Apes imitate the effects of others' actions, but the evidence for program-level imitation seems contradictory and the evidence against bodily imitation and trial and error in learning the organization of complex activities seems ambiguous. Action-level imitations are more flexible than described and may derive from imitation of the effects of others' actions on objects.

Byrne & Russon suggest that there are two kinds of imitation learning – action level and program level – and that the latter is critical for great apes' learning. I have interpreted this phenomenon from the standpoint of clinical neuropsychology and conjecture that action-level imitation might be related to parietal lobe function and program-level imitation might be related to frontal lobe function.

Byrne & Russon propose an account of imitation that mirrors levels of behavioral organization, but they perpetuate a tendency to dismiss imitation by members of most species as the result of more primitive processes, even though these alternative phenomena are often poorly understood. They argue that the prerequisites to program-level imitation are present in great apes, but the same prerequisites appear to be present in a broad range of species. The distribution of imitative capacity across species may be more limited by research methodology than by cognitive ability.

The target article by Byrne & Russon treats imitation as an achievement that originates from observation. In my commentary I propose extending the database to the role of listening. Referring to current studies on song learning in birds, I suggest that at least some features of this accomplishment also may be based on learning by imitation.

Byrne & Russon redefine the process of emulation learning as “goal emulation” and thereby distort its most distinctive characteristic: the criterion that the observer focuses on environmental rather than behavioral processes. The two empirical examples recounted – gorilla plant processing and orangutan manipulation of human artifacts – are hierarchically organized behaviors, but there is very little evidence that they involve imitative learning, program-level or otherwise.

In the context of the motor skill literature on observational learning and hierarchical skill structuring, Byrne & Russon's findings call into question their standpoint that great apes imitate the behaviour of role models at the programme level. The authors impose a hierarchical model on their observations without properly considering alternative explanations. One such possibility, which stems from a constraints perspective that they dismiss, is put forward.

We outline a view of imitative behaviour as largely internally driven and discuss, based on experimental research, the distinction between program versus action level imitation, the role of organismic constraints, observational learning as vicarious exploration, and imitation as selection in speeded response paradigms.

Byrne & Russon suggest that the production of action by primates is hierarchically organised. We assess the evidence for hierarchical structure in the comprehension of action by primates. Focusing on work with human children we evaluate several possible indices of program-level comprehension.

Byrne & Russon rightly draw attention to complex and neglected aspects of ape imitation. However, program-level imitation as a single, absolute category may mislead us in understanding abstractions involved in imitation. Designing the right experiments will offer clarity. One recent experiment has shown imitation of sequential structure: What is needed to test other components of what the authors propose?