We are sympathetic to Vaesen's view that no single cognitive trait differentiates human tool behavior from that of other animals, and we agree that comparative analysis has an important role in understanding the cognitive bases of human tool use. However, in our view, several vital issues are unaddressed. Have tool-using capacities driven human cognitive evolution, or is tool use the by-product of another ability? To what degree are the perceptual and cognitive traits underlying tool use and technological cultural evolution independent from each other and from morphological, societal, or ecological traits? What is the role of culture and development in shaping patterns of tool innovation and social learning? How much of cumulative cultural evolution rests on increases in causal understanding of tools, as Vaesen suggests, and how much on retention of “blind” variants (Simonton Reference Simonton, Reader and Laland2003)?

Here we focus on problems raised by the analysis of human tool behavior based on comparisons with one taxon, chimpanzees. Vaesen's aim is not to compare humans and chimpanzees, but to understand the cognitive bases of human tool use. As useful as comparisons with chimpanzees are, Vaesen's application of this tactic is critically flawed for at least four reasons. Although Vaesen admits his narrow focus on chimpanzees, the flaws are germane both to his conclusions and to other work in the field.

First, Vaesen's chimpanzee-human comparison assumes that shared ancestry explains similarities, whereas differences are explained by independent evolution of the trait in humans and not, for example, the loss of the trait in chimpanzees. However, the ancestral state must be established, which requires investigation of additional species (de Kort & Clayton Reference de Kort and Clayton2006).

Second, tool use is unlikely a unitary phenomenon. A variety of neurocognitive and genetic mechanisms can underlie a behavioral outcome such as tool use (Shumaker et al. Reference Shumaker, Walkup and Beck2011). Hence, it is not a given that similarities and differences between species in tool-related behavior or test performance equate to similarities and differences in underlying cognition, potentially compromising the explanatory power of species comparisons. Independent evolution may have produced similar behavioral specializations with different underlying mechanisms (de Kort & Clayton Reference de Kort and Clayton2006), or behavioral similarities may appear as a consequence of some third variable, such as enhanced social tolerance (van Schaik et al. Reference van Schaik, Deaner and Merrill1999). Furthermore, tool-using capacities may be present but unexpressed. For example, expression of true and proto-tool use (Shumaker et al. Reference Shumaker, Walkup and Beck2011) appears sensitive to variation in social and ecological conditions. Finches turn to tools in arid conditions, rarely using tools to extract prey where food is abundantly accessible (Tebbich et al. Reference Tebbich, Taborsky, Fessl and Dvorak2002); dolphins use sponges to locate prey that cannot be detected by other means (Patterson & Mann Reference Patterson and Mann2011); adult male capuchin monkeys are strong enough to bite open certain nuts, whereas females and juveniles require tools to open them (Fragaszy & Visalberghi Reference Fragaszy and Visalberghi1989); and grackles use water to soften hard food when the risks of kleptoparasitism are low (Morand-Ferron et al. Reference Morand-Ferron, Lefebvre, Reader, Sol and Elvin2004). These observations suggest tool use may frequently be a costly option employed flexibly, taken when other options fail or are unavailable. Similarly, innovation in tool use can be employed flexibly; for example, driven by the social milieu (Reader & Laland Reference Reader and Laland2003; Toelch et al. Reference Toelch, Bruce, Meeus and Reader2011). Hence, numerous variables could underlie species differences in tool-related behavior, and even apparent similarities may reflect different underlying mechanisms.

Third, chimpanzees may be well studied, and our close relatives, and provide much informative data (e.g., Hrubesch et al. Reference Hrubesch, Preuschoft and van Schaik2009; Marshall-Pescini & Whiten Reference Marshall-Pescini and Whiten2008), but other animals provide relevant data and counterpoints to Vaesen's proposals. For example, work on finches and crows demonstrates that social learning is not essential for the acquisition of tool use (Kenward et al. Reference Kenward, Weir, Rutz and Kacelnik2005; Tebbich et al. Reference Tebbich, Taborsky, Fessl and Blomqvist2001); meanwhile macaque observational data suggest that social transmission of nonfunctional object manipulation occurs outside humans (Leca et al. Reference Leca, Gunst and Huffman2007, who do not class nonfunctional behavior as tool use). Similarly, selective social learning may be rarely documented in apes but has been described in numerous other species, including monkeys, other mammals, fish, and birds (Laland Reference Laland2004; Lindeyer & Reader Reference Lindeyer and Reader2010; Seppänen et al. Reference Seppänen, Forsman, Mönkkönen, Krams and Salmi2011; van de Waal et al. Reference van de Waal, Renevey, Favre and Bshary2010). Selective social learning may be necessary for cumulative cultural evolution, but is clearly not sufficient, unless cumulative cultural evolution occurs unobserved in these animals. Researchers have demonstrated several other behaviors in non-primates that Vaesen identifies as distinctively human: ants, pied babblers, and meerkats teach; fish punish and image-score; birds use baits to trap prey, forgoing immediate rewards in a manner not unlike the human traps that Vaesen argues require foresight and inhibition (Bshary & Grutter Reference Bshary and Grutter2005; Reference Bshary and Grutter2006; Shumaker et al. Reference Shumaker, Walkup and Beck2011; Thornton & Raihani Reference Thornton and Raihani2011). We urge caution in interpreting even flexible and sophisticated tool use as necessarily the product of complex cognition.

Finally, any comparison based on an effective sample size of two is problematic. Humans and chimpanzees differ on numerous characteristics. In the absence of additional behavioral data on the role of underlying candidate mechanisms in tool use, any of these characteristics alone or in combination could account for differences in tool behavior. To robustly identify correlates of tool use with comparative data, repeated and independent co-evolution must be observed, using modern techniques to focus on independent evolutionary events and to account for multiple confounding variables (Nunn & Barton Reference Nunn and Barton2001). Confidence in such results is strengthened further if the same patterns are observed in multiple taxa. Such correlational comparative analyses, incorporating large numbers of species, reveal that avian and primate tool use has co-evolved with several cognitive traits and with brain volume measures, and (in primates) with manual dexterity (Byrne Reference Byrne, Whiten and Byrne1997; Deaner et al. Reference Deaner, van Schaik and Johnson2006; Lefebvre et al. Reference Lefebvre, Nicolakakis and Boire2002; Reference Lefebvre, Reader and Sol2004; Overington et al. Reference Overington, Morand-Ferron, Boogert and Lefebvre2009; Reader & Laland Reference Reader and Laland2002; Reader et al. Reference Reader, Hager and Laland2011; van Schaik et al. Reference van Schaik, Deaner and Merrill1999). These data, supported by discoveries of tool use capabilities in species previously not noted tool users (Reader et al. Reference Reader, Hager and Laland2011; Shumaker et al. Reference Shumaker, Walkup and Beck2011), are consistent with the idea that tool use can result from a generalized cognitive ability and that it forms part of a correlated suite of traits. However, such analyses would benefit from experimental data teasing apart the processes underlying tool behavior.

If human tool use really is unique, identification of its cognitive bases by comparison with any species will be problematic. We must unpack tool use, understand the underlying motivational and neurocognitive mechanisms in humans and other species, and study a range of species that both possess and lack these abilities in order to understand the consequences for tool behavior. Evolutionary approaches hence have an important role to play in investigations of cognition. Work with chimpanzees is but one part of solving this important issue.