Using a taxonomic method called DNA barcoding, researchers have identified, from just a few recovered tool specimens, the plant species New Caledonian crows (Corvus moneduloides) use for crafting their elaborate hooked stick tools.
Identifying the raw material used by wild New Caledonian crows for manufacturing hooked stick tools: (A) New Caledonian crow holding a hooked stick tool manufactured from Desmanthus virgatus at site-2; (B) satellite photograph showing study sites on the west coast of Grande Terre, New Caledonia; (C) timeline of animal-centered approaches employed while attempting to identify the tool material used at site-3, with short explanations for their limited success; (D) timeline of the artifact-centered, DNA barcoding approach which ultimately led to successful material identification; (i) hooked stick tool recovered at site-3; (ii) Mimusops elengi trees, with close-up of a forked terminal branch, which would be suitable for tool manufacture; (iii) simplified maximum-likelihood ITS phylogenetic tree detailing clustering of tool samples and a subset of reference data; (iv) ‘refit’ of a hooked stick tool made from Mimusops elengi material by a temporarily captive crow from site-1: the tool is displayed along with stem and plant debris which were discarded by the crow during the manufacture process. Scale in mm. Image credit: Steele et al., doi: 10.1073/pnas.2020699118.
New Caledonian crows — a species of oscine passerine bird found on the South Pacific island of New Caledonia — are renowned for their ability to manufacture complex foraging tools.
When making a hooked stick tool, they select a forked plant stem, remove a suitable branch, trim off any leaves and twiglets, and often refine the tool by sculpting the remains of the nodal joint into a neat terminal hook, stripping bark near the functional end, and bending the tool shaft .
These processing steps substantially alter the appearance of the plant material. Importantly, properties of the raw material affect the morphology of the resulting tools, which in turn affects foraging efficiency.
“New Caledonian crows put a lot of effort into making their tools,” said Dr. Linda Neaves, a researcher in the Fenner School of Environment and Society at the Australian National University and the Royal Botanic Garden Edinburgh.
“They use specific plants with forked stems, which they remove and then process into hooked tools for foraging.”
“But they remove leaves and much of the bark, making it impossible to quickly identify the plant species.”
Dr. Neaves and colleagues recently discovered that three study populations of New Caledonian crows target different species despite living just a few km apart.
While they managed to identify raw materials at two sites, the scientists failed at the third, even after employing a wide range of well-established field methods aimed at observing tool manufacture directly.
“The plants used in one of the study sites is an introduced shrub, and in the other, they seem to use a variety of raw materials,” said Matthew Steele, a Ph.D. candidate in the Centre for Biological Diversity at the University of St Andrews.
“But no one had ever seen the crows make the tools at the third site, so we had no idea what species it was.”
In the new study, the authors extracted and analyzed DNA from seven hooked stick tools recovered at the third study site during 2016-2017.
They managed to match DNA from the tools to a large native tree, the Spanish cherry (Mimusops elengi).
“Because it’s native, this may be one of the original plants used by the crows for making hooked stick tools,” Steele said.
“The field team later confirmed that temporarily captive crows happily crafted hooked stick tools from this plant.”
“Clearly these crows have adapted their remarkable tool-making abilities over time to make use of introduced plant species, and it is exciting to find this population prefers a native tree,” Dr. Neaves added.
“It raises a lot of interesting questions about how and why crows select the plants they use and how this may be influenced by changes in their environment.”
“Our study demonstrates the broad usefulness of DNA barcoding for the essential task of separating and identifying plant species,” said Professor Peter Hollingsworth, a researcher at the Royal Botanic Garden Edinburgh.
The results appear in the Proceedings of the National Academy of Sciences.
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Matthew P. Steele et al. 2021. DNA barcoding identifies cryptic animal tool materials. PNAS 118 (29): e2020699118; doi: 10.1073/pnas.2020699118
