Essays on Science and SocietyEcology and Environment

Of crows and tools

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Science  22 Nov 2019:
Vol. 366, Issue 6468, pp. 965
DOI: 10.1126/science.aaz7775

Hammers and chisels, pens and smartphones: Human life is built on tool use. Indeed, each of us likely uses tools every day. For a long time, crafting tools was thought to be uniquely human. And although we know that many species occasionally use tools, everyday tool use is extremely rare in the animal kingdom (1).

I study those rare animals that routinely use tools. My Ph.D. research took me to a small archipelago in the South Pacific called New Caledonia, the home of the New Caledonian (NC) crow, Corvus moneduloides (see the figure). Here, I explored the species's tool-oriented behavior using a series of non-invasive experiments with wild-caught birds.

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NC crows are the only nonhuman species known to craft hooks in the wild (1). The invention of the hook was one of the key technological advances in our own evolutionary history in the middle stone age, allowing us to develop productive fishing technologies and weapons with enhanced killing power (24).

For the NC crow, a hook—delicately crafted from a forked branch (5, 6)—is also used for “fishing.” NC crows feed on insects and grubs, which they extract from deadwood. Using a hook tool allows them to get the prey several times faster than using a simple stick (7).

Like humans, NC crows must find suitable materials for their hook manufacture. They show a strong site-specific preference for the material they use to make these tools (8), but how do they identify their preferred plant species in the densely vegetated forest, where there might be hundreds of suitable options?

To assess which features of a plant crows select for, I presented wild-caught birds with experimental leaf-stem combinations and found that they can identify their preferred species from its stems alone. Some birds can even identify it from only the leaves (9). Intriguingly, this suggests that NC crows may have the equivalent of a “search image” in mind when looking for good tool material that includes features of the tool they intend to make.

Keeping already made tools safe when you don't need them can allow you to avoid having to make new tools. I discovered that NC crows look after their tools in between uses, especially when the cost or likelihood of losing the tool is high (10). Strategies to prevent tool loss can increase foraging efficiency and help remove some of the constraints experienced by arboreal tool users.

Human tools show cultural variation: Different means of achieving the same end are preserved for generations through social learning. NC crows vary in their hook tool manufacture (11), which raises the question of whether this variation is perhaps “cultural,” as has been suggested for another NC crow tool type (12). But c ould the variation be explained by the properties of the available raw materials?

By giving crows the choice of material varying in thickness and rigidity, I showed that some of the observed variation in hook tool manufacture is indeed due to the material properties of the plants (11). My findings don't necessarily show that NC crows can't have cultural variation in their behavior, but they provide an alternative hypothesis for explaining regional differences observed in NC crows' tool making and, more generally, suggest another potential explanation for variation in animal tool behavior that appears to be culturally transmitted.

Despite the progress in understanding the NC crows' tool-oriented behavior, one mystery remained. Why do they, but no other crow species, use tools (6)?

A striking morphological feature of the NC crow is its unusually straight bill, which may reflect a specific adaptation for holding tools (6), as it allows the crow to position the tool tip in the center of binocular overlap (13). When checking the bill shapes of other living Corvus species, one with a relatively straight bill stood out: the Hawaiian crow or 'Alalā, Corvus hawaiiensis (6).

The 'Alalā was already extinct in the wild when we began our study—captive-reared birds have since been released—but San Diego Zoo Global has two breeding facilities in Hawai'i, where we tested all healthy individuals of the species (n = 104). We presented the birds with sticks and naturalistic extraction tasks and found that they are indeed very dexterous tool users: 93% of all adults and 47% of the younger birds spontaneously used sticks to probe for hidden food (14). They are not only very good tool users, they also exhibit an extensive repertoire of tool modification and manufacture skills (15). It is possible, then, that the marked population decline suffered by the 'Alalā in the late 20th century (16) might have been the result of lost opportunities to forage with tools as humancaused habitat loss spread across Hawai'i (14). Since 2016, cohorts of captive-reared birds are being released in habitats that contain raw materials the 'Alalā could use for tool building, and the future will show whether and how these charismatic birds will use tools in the wild.

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A wild New Caeldonian crow holds a hook tool


It is probably no coincidence that these two distantly related (17) tool-using crow species evolved on remote tropical islands. With no woodpeckers, there is little competition for embedded food sources, and in the absence of big predators, the crows can spend less time maintaining vigilance and use more time to develop tool use (6). Remote islands, therefore, combine rare ecological conditions that seem to aid the evolution of foraging tool use in birds (14). The evolution of human tool use might have been similarly facilitated by the reduction in predation risk that accompanied the transition from solitary life to living in groups.

Our own tool-making journey is unparalleled, having enabled us to walk on the Moon in what amounts to an evolutionary eye-blink after the first fishing hook was crafted. But two tool-using crow species on remote tropical islands can still offer insight into how we became the master tool users we are today.

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Barbara C. Klump

Barbara Klump received her master's degree from Heidelberg University, Germany, and a Ph.D. from the University of St Andrews, UK. After completing a postdoctoral fellowship at the University of St Andrews, she moved to the Max Planck Institute of Animal Behavior in Radolfzell, Germany, where she is currently a postdoc in the Cognitive and Cultural Ecology Lab. Her research explores how a species's ecology shapes its behavioral repertoire.

References and Notes

Acknowledgments: I am extremely grateful to my Ph.D. supervisor, C. Rutz, for inspiration, professional guidance, and scientific advice. This research would not have been possible without his enthusiasm, interest in my work, and high degree of confidence in me. I also thank K. Cross for advice, as well as the Rutz group and everyone who supported me throughout my Ph.D. I thank our local partners in New Caledonia and Hawai'i: Province Sud, DENV, SEM Mwe-Ara, and San Diego Zoo Global. These studies were funded through scholarships to B.C.K. from the BBSRC and from the School of Biology, University of St Andrews, and a BBSRC David Phillips Fellowship (BB/G023913/1 and /2) to C. Rutz.
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