The authors discuss how the peculiar socio-ecological characteristics of our species would have allowed us to evolve a unique set of cognitive skills necessary to “unleash expression.” Although we applaud the authors' attempt to build a new theoretical framework, we are not sure how much it will advance the current state of the art from an evolutionary perspective. In particular, the authors suggest how, for unleashed expression to emerge, one would need cognitive skills to express and recognize communicative intentions, and a “partner choice social ecology” in which epistemic vigilance allows ostensive communication to become more reliable and “unleash expression.”
First, studying how individuals express and recognize communicative intentions implies investigating their attribution of mental states (by recipients to signalers), which is not an easy endeavor in non-linguistic subjects (Townsend et al., Reference Townsend, Koski, Byrne, Slocombe, Bickel, Boeckle and Manser2017). In the absence of language, even complex behaviors may appear simpler, as one cannot for instance ask receivers about their inferences of others' communicative intentions. The authors briefly address this problem by suggesting to test whether nonhuman species produce different behavioral responses when exposed to the same ostensive behavior in different scenarios in which only the individuals' common ground differs. However, while such an approach may be informative about whether individuals can attribute mental states to others (see Krupenye, Kano, Hirata, Call, & Tomasello, Reference Krupenye, Kano, Hirata, Call and Tomasello2016) and how they may flexibly combine this information to contextual cues and/or signals, it may still be hard to clarify what individuals understand about others' communicative intentions. In any case, it is at the moment not possible to exclude that species other than humans can reliably express and recognize communicative intentions.
Similarly, there are still important methodological issues to be addressed before claiming major differences in how humans and other species express and recognize communicative intentions. To date, the majority of vocal research has focused on how recipients react to vocalizations, while little is known about senders' communicative intentions (but see Crockford, Wittig, Mundry, & Zuberbühler, Reference Crockford, Wittig, Mundry and Zuberbühler2012). In the gestural modality, on the other hand, most studies focus on senders and their intentional gesture use, while there is little knowledge about how recipients interpret the senders' communicative intentions. Moreover, it is currently unknown if the interplay of different modalities might convey communicative and/or informative intentions. For example, a gesture produced in isolation may be perceived as the intent to inform others, whereas the combination of a gesture with gaze alternation between the recipient and an object might be perceived as having a communicative/ostensive function (note that these terms are used in slightly different ways across disciplines). Because most research about primate communication is unimodal (Liebal, Slocombe, & Waller, Reference Liebal, Slocombe and Waller2022), we might miss exactly those aspects of communicative interactions that would qualify them as ostensive communication. Addressing these methodological issues is therefore essential before drawing conclusions about interspecific differences.
Second, we do not think that epistemic vigilance is necessary for the evolution of open-ended communication. According to the authors, communication can only become open-ended when it is stable and mutually beneficial: Epistemic vigilance would be thus fundamental to assess the plausibility of what is communicated, reducing deception and allowing communication to thrive. However, as the authors incidentally recognize, there are several other ways in which mutual benefits in communication can be ensured, including genetic relatedness and direct reciprocity. In these systems, deception may be evolutionary disadvantageous and communication reliable even in the absence of epistemic vigilance. Moreover, communication can be seen as a form of cooperative behavior and therefore more general forms of social vigilance commonly used in cooperative interactions may also avoid the collapse of communicative systems when deception occurs. These mechanisms may be present in several species other than humans. Species engaging in complex forms of cooperation, like cooperative hunting and cooperative breeding, or some monogamous species, for instance, need to coordinate their actions and may especially rely on reliable trustful exchanges of communicatory signals. Therefore, it is not clear why a “partner choice social ecology” with epistemic vigilance would be a necessary prerequisite to evolve open-ended communication.
Third, the authors should better clarify whether humans, in their opinion, differ from other species in terms of cognitive skills or motivational aspects of communication. In their article, the authors suggest that, through experience with “altruistic human caregivers,” nonhuman primates may acquire the ability to recognize the expression of informative intentions. This would suggest that species other than humans do have the cognitive skills required for unleashed expression to emerge, but lack the predisposition to use them. However, in the “right” socio-ecological setting (e.g., a social environment of trust and mutual benefits), also other species may show communicative dispositions similar to human ones and engage in complex forms of communication.
Finally, we consider that the ability to combine meaningful elements into new combinations with novel meanings still better explains how open-ended communication emerges, especially if “stability in the face of incentives to deceive” is effectively achieved in other species as for other forms of cooperation (see above). In the last few years, studies in vocal and gestural communication systems of birds and nonhuman primates have provided evidence of compositionality – a property that had also long been considered one of the hallmarks of human communication. Campbell's monkeys (Cercopithecus campbelli), for example, can combine specific alarm calls into new vocalizations with a novel meaning (Arnold & Zuberbühler, Reference Arnold and Zuberbühler2006, Reference Arnold and Zuberbühler2008), whereas Japanese great tits (Parus minor) react differently to single notes and their combinations (Suzuki, Wheatcroft, & Griesser, Reference Suzuki, Wheatcroft and Griesser2016). Similarly, chimpanzees (Pan troglodytes) may compositionally recombine gestures with signals across different modalities (see Amici, Oña, & Liebal, Reference Amici, Oña and Liebal2022). Therefore, compositionality may be really a key factor for the emergence of open-ended communication.
Overall, we argue that accounting for combinatorial and multi-modal aspects of communication in other species is necessary to understand the complexity of their communication systems, before suggesting major differences with human means and modes of expression. Through mechanisms other than epistemic vigilance, individuals in other species can reliably communicate with each other, and some may compositionally recombine meaningful elements – a crucial property, after all, for the evolution of open-ended systems.
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The authors discuss how the peculiar socio-ecological characteristics of our species would have allowed us to evolve a unique set of cognitive skills necessary to “unleash expression.” Although we applaud the authors' attempt to build a new theoretical framework, we are not sure how much it will advance the current state of the art from an evolutionary perspective. In particular, the authors suggest how, for unleashed expression to emerge, one would need cognitive skills to express and recognize communicative intentions, and a “partner choice social ecology” in which epistemic vigilance allows ostensive communication to become more reliable and “unleash expression.”
First, studying how individuals express and recognize communicative intentions implies investigating their attribution of mental states (by recipients to signalers), which is not an easy endeavor in non-linguistic subjects (Townsend et al., Reference Townsend, Koski, Byrne, Slocombe, Bickel, Boeckle and Manser2017). In the absence of language, even complex behaviors may appear simpler, as one cannot for instance ask receivers about their inferences of others' communicative intentions. The authors briefly address this problem by suggesting to test whether nonhuman species produce different behavioral responses when exposed to the same ostensive behavior in different scenarios in which only the individuals' common ground differs. However, while such an approach may be informative about whether individuals can attribute mental states to others (see Krupenye, Kano, Hirata, Call, & Tomasello, Reference Krupenye, Kano, Hirata, Call and Tomasello2016) and how they may flexibly combine this information to contextual cues and/or signals, it may still be hard to clarify what individuals understand about others' communicative intentions. In any case, it is at the moment not possible to exclude that species other than humans can reliably express and recognize communicative intentions.
Similarly, there are still important methodological issues to be addressed before claiming major differences in how humans and other species express and recognize communicative intentions. To date, the majority of vocal research has focused on how recipients react to vocalizations, while little is known about senders' communicative intentions (but see Crockford, Wittig, Mundry, & Zuberbühler, Reference Crockford, Wittig, Mundry and Zuberbühler2012). In the gestural modality, on the other hand, most studies focus on senders and their intentional gesture use, while there is little knowledge about how recipients interpret the senders' communicative intentions. Moreover, it is currently unknown if the interplay of different modalities might convey communicative and/or informative intentions. For example, a gesture produced in isolation may be perceived as the intent to inform others, whereas the combination of a gesture with gaze alternation between the recipient and an object might be perceived as having a communicative/ostensive function (note that these terms are used in slightly different ways across disciplines). Because most research about primate communication is unimodal (Liebal, Slocombe, & Waller, Reference Liebal, Slocombe and Waller2022), we might miss exactly those aspects of communicative interactions that would qualify them as ostensive communication. Addressing these methodological issues is therefore essential before drawing conclusions about interspecific differences.
Second, we do not think that epistemic vigilance is necessary for the evolution of open-ended communication. According to the authors, communication can only become open-ended when it is stable and mutually beneficial: Epistemic vigilance would be thus fundamental to assess the plausibility of what is communicated, reducing deception and allowing communication to thrive. However, as the authors incidentally recognize, there are several other ways in which mutual benefits in communication can be ensured, including genetic relatedness and direct reciprocity. In these systems, deception may be evolutionary disadvantageous and communication reliable even in the absence of epistemic vigilance. Moreover, communication can be seen as a form of cooperative behavior and therefore more general forms of social vigilance commonly used in cooperative interactions may also avoid the collapse of communicative systems when deception occurs. These mechanisms may be present in several species other than humans. Species engaging in complex forms of cooperation, like cooperative hunting and cooperative breeding, or some monogamous species, for instance, need to coordinate their actions and may especially rely on reliable trustful exchanges of communicatory signals. Therefore, it is not clear why a “partner choice social ecology” with epistemic vigilance would be a necessary prerequisite to evolve open-ended communication.
Third, the authors should better clarify whether humans, in their opinion, differ from other species in terms of cognitive skills or motivational aspects of communication. In their article, the authors suggest that, through experience with “altruistic human caregivers,” nonhuman primates may acquire the ability to recognize the expression of informative intentions. This would suggest that species other than humans do have the cognitive skills required for unleashed expression to emerge, but lack the predisposition to use them. However, in the “right” socio-ecological setting (e.g., a social environment of trust and mutual benefits), also other species may show communicative dispositions similar to human ones and engage in complex forms of communication.
Finally, we consider that the ability to combine meaningful elements into new combinations with novel meanings still better explains how open-ended communication emerges, especially if “stability in the face of incentives to deceive” is effectively achieved in other species as for other forms of cooperation (see above). In the last few years, studies in vocal and gestural communication systems of birds and nonhuman primates have provided evidence of compositionality – a property that had also long been considered one of the hallmarks of human communication. Campbell's monkeys (Cercopithecus campbelli), for example, can combine specific alarm calls into new vocalizations with a novel meaning (Arnold & Zuberbühler, Reference Arnold and Zuberbühler2006, Reference Arnold and Zuberbühler2008), whereas Japanese great tits (Parus minor) react differently to single notes and their combinations (Suzuki, Wheatcroft, & Griesser, Reference Suzuki, Wheatcroft and Griesser2016). Similarly, chimpanzees (Pan troglodytes) may compositionally recombine gestures with signals across different modalities (see Amici, Oña, & Liebal, Reference Amici, Oña and Liebal2022). Therefore, compositionality may be really a key factor for the emergence of open-ended communication.
Overall, we argue that accounting for combinatorial and multi-modal aspects of communication in other species is necessary to understand the complexity of their communication systems, before suggesting major differences with human means and modes of expression. Through mechanisms other than epistemic vigilance, individuals in other species can reliably communicate with each other, and some may compositionally recombine meaningful elements – a crucial property, after all, for the evolution of open-ended systems.
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This research received no specific grant from any funding agency, commercial, or not-for-profit sectors.
Conflict of interest
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