We ask Heintz & Scott-Phillips (hereafter H&S-P) to clarify several issues related to “ostensive-inferential” communication in the animal kingdom, including the relationship between ostension and contextual interpretation, the role of metarepresentation in communication, and its relationship to enculturation. Because the unclarities we raise indicate problems with their view, we propose an alternative.

H&S-P suggest that a fruitful test of whether other species understand ostension qua ostension would be if animals respond differently to the same ostensively produced “informative intention” in different contexts. They predict that enculturated great apes may respond in contextually appropriate ways, because they have had more occasions for “interactions of mutual benefit” (target article, sect. 7, para. 4). We agree that enculturation improves great ape communication. Nonetheless, the context-sensitivity of interpretation is not a good test of ostensive communication. Great apes in the wild interpret gestures in context-specific ways (Hobaiter & Byrne, Reference Hobaiter and Byrne2014; Moore, Reference Moore2014) – suggesting that contextual interpretation is not specific to enculturation. Moreover, claims about whether communication is “ostensive-inferential” are independent of contextual variation in audience responses (Bar-On & Moore, Reference Bar-On, Moore, K. Andrews and J. Beck2017; Wheeler & Fischer, Reference Wheeler and Fischer2012). For this reason, we'd like H&S-P to clarify their view of the relationship between ostension and contextually variant interpretation.

Sometimes the authors claim that what distinguishes ostensive-inferential communication from other forms is that users can independently produce and attribute “informative” and “communicative” intentions (see also Scott-Phillips, Reference Scott-Phillips2014). They write that the “cognitive capacities for ostensive communication are foundational, because they unleash expression on a grand scale” (target article, sect. 9, “Conclusion,” para. 1). They also claim that object-choice tasks involving chimpanzees are a test of ostensive-inferential communication. Enculturated great apes do perform better in object-choice tasks (Lyn, Russell, & Hopkins, Reference Lyn, Russell and Hopkins2010). However, if enculturation makes chimpanzees better at pointing comprehension, it is unlikely that it is because it drives an ability to distinguish between informative and communicative intentions. Studies have shown that small methodological tweaks to these tasks (e.g., placing hiding locations further apart; Mulcahy & Call, Reference Mulcahy and Call2006) also improve great ape performance. These adjustments seemingly work not because they facilitate independent comprehension of “communicative” and “informative” goals, but because they support sustained attention and prevent apes from acting unreflectively (Berio & Moore, Reference Berio and MooreUnpublished manuscript). A more compelling explanation of the great ape gestural data is therefore that all chimpanzees are ostensive-inferential communicators, and understand informative and communicative intent (Berio & Moore, Reference Berio and MooreUnpublished manuscript; Moore, Reference Moore2016, Reference Moore2017a), but they perform poorly in object-choice tasks because they are inattentive or unmotivated.

If we understand H&S-P's argument correctly, enculturated great apes acquire expectations of mutual benefit, and so trust the information provided by pointers. However, it's unexplained whether or how this facilitates the development of the metarepresentational understanding of communication hypothesized to unleash expression. H&S-P suggest that an understanding of metarepresentations may be widespread in the animal kingdom (footnote 9), although perhaps “enriched” in humans. Is their idea, then, that unenculturated chimpanzees distinguish between “informative” and “communicative” intentions, but remain poor at pointing comprehension only because they lack the trust to interpret humans' messages pro-socially? If so, this seemingly retracts a central claim of Scott-Phillips's earlier work (Reference Scott-Phillips2014), in which the metarepresentational abilities needed for ostensive-inferential communication are uniquely human.

The extension of metarepresentational abilities to other species also raises further questions. If enculturated apes can acquire expectations of mutual benefit, and if their metarepresentational abilities are already in place, could their communicative abilities also be unleashed? This seems unlikely. Take Kanzi, perhaps communicatively the most sophisticated living nonhuman great ape. His productive communication is limited, although more expansive than in wild apes. While his comprehension is better than his production, it stops developing around the level of a child of 2.5 years (Savage-Rumbaugh, Taylor, & Shanker, Reference Savage-Rumbaugh, Shanker and Taylor1998). If Kanzi has the relevant metarepresentational abilities, the combination of metarepresentations and expectations of mutual benefit surely do not suffice for unleashing expression. Dogs present another puzzling case, because they excel in object-choice tasks – yet remain limited in what they can understand and express.

We propose that what unleashes expression in humans is not a difference in our metarepresentational abilities. Such abilities are needed for ostensive-inferential communication only in limited ways, and the relevant abilities are within the ken of all great ape species (Moore, Reference Moore2016, Reference Moore2017a). Moreover, the development of uniquely human forms of metarepresentation seems to be language-dependent (Berio, Reference Berio2021a, Reference Berio2021b; Grosse Wiesmann, Friederici, Singer, & Steinbeis, Reference Grosse Wiesmann, Friederici, Singer and Steinbeis2017; Low, Reference Low2010; Moore, Reference Moore2021) and acquired later than basic forms of beliefs (Newen & Starzak, Reference Newen and Starzak2020). Nor is what's missing in unenculturated great apes exclusively a matter of trust. Human expression was not unleashed by any radical new mechanism for communication in our species. Rather, what matters is that even enculturated great apes are domain-bound in the ways they use their skills for ostensive-inferential communication.

Wild chimpanzees communicate fluently with a small repertoire of signs (Hobaiter & Byrne, Reference Hobaiter and Byrne2014). Nonetheless, in captivity they fail to use this repertoire for solving coordination tasks, because they are poor at projecting existing skills into new contexts, to solve unfamiliar problems (Moore, Reference Moore2017c). They operate in what Susan Hurley called “islands of practical rationality” (Hurley, Reference Hurley2003), where they can solve specific social tasks without being able to transfer the relevant skills to more general domains. Hurley thinks it is language that bridges these islands in human reasoning. This may be, but something similar also happens in enculturation. Enculturation trains attention and builds trust, but it also expands great apes' hitherto minimal communicative ecology, leading them to discover the possibility of using their communicative abilities in new contexts. This insight arose in phylogeny only after the mechanisms of ostensive-inferential communication. Nonetheless, expression in enculturated great apes remains stalled by their limited working memory, weak analogical reasoning, lack of capacity for syntax, and poor inhibition control and social learning, among other factors.

In phylogeny, a communicative mechanism common to many species (Moore, Reference Moore2017b) expanded slowly. Many other abilities were relevant to this expansion. This makes expression one of many domains in which human and animal abilities are continuous (Andrews & Monsó, Reference Andrews, Monsó and Zalta2021; Laland & Seed, Reference Laland and Seed2021).