Crows make tools by design, not by rote

The crows of New Caledonia have a reputation for being clever, but new research indicates they may be able to pass on tool design through observation and memory.

Individual birds may use or observe the tools made by others of their kind, form a mental template of the design, and then reproduce it – a process analogous to birdsong learning, in which juveniles hear the calls of others of their kind and adjust their own vocalisations until they match the memorised template.

The findings are contained in a study by scientists from the University of Auckland, in New Zealand, and published in the journal Scientific Reports, which says cultural traditions are common in the animal kingdom but cumulative cultural evolution is rare, in contrast to the human archaeological record, in which “clear indications of cumulative culture are present from at least 100,000 years ago”.

A 1996 paper by Gavin Hunt, from New Zealand’s Massey University, reported that New Caledonian crows (Corvus moneduloides) made and used two different types of hook tools to capture prey, and that these had “a high degree of standardisation, distinctly discrete tool types with definite imposition of form in tool shaping, and the use of hooks”.

Hunt said these features indicate that “crows have achieved a considerable technical capability in their tool manufacture and use”.

The new report, led by psychologist Sarah Jelbert, notes disagreement in the scientific community regarding the advancement of technologies and traditions. Some point to the uniquely human capacities for teaching, language and imitation.

However, the researchers write, “Social learning is not just underpinned by the copying of actions. It is defined as learning that is ‘influenced by observation of, or interaction with, another animal or its products’.”

Some researchers believe, they add, emulative mechanisms – learning from observing the end-products, rather than the actions which produced the products – can produce only low-fidelity copying, insufficient to support cumulative cultural change. “However,” they note, “it remains possible that copying end-products might offer an alternative route towards cumulative cultural evolution in some situations.”

To test their theory, Jelbert and colleagues trained eight crows to drop differently sized pieces of paper into a vending machine to receive rewards. The birds eventually learnt that only pieces of paper of a particular size were rewarded. Once the crows had been trained to recognise which sizes were rewarded, the researchers provided them with large pieces of paper but no physical templates of the previously rewarded paper sizes.

Despite being rewarded at random, and with no physical templates present, crows manufactured items that were more similar in size to what had previously been rewarded, than unrewarded.

Jelbert and her team confirmed that across the New Caledonian island of Grande Terre, crows manufacture basic tools, hooked stick tools and barbed tools torn from the leaves of pandanus plants. They routinely manufacture at least three distinct tool designs in the wild, including wide, narrow, and stepped models, where stepped tools taper from a wide base to a narrow working tip in a series of rips and cuts.

“The specific tool designs made in different areas do not have obvious ecological correlates, and have persisted for at least several decades, suggesting high-fidelity transmission,” the researchers conclude.

“The functionality of different pandanus tool designs is not yet well understood; however, their geographic distribution raises the possibility that stepped tools represent modifications made to the simpler wide design. Thus, New Caledonian crows may possess a material culture that has incorporated incremental changes over time.

“Our results provide the first evidence that this cognitive ability may underpin the transmission of New Caledonian crows’ natural tool designs.”