Mental template matching is a potential cultural transmission mechanism for New Caledonian crow tool manufacturing traditions

Abstract

Cumulative cultural evolution occurs when social traditions accumulate improvements over time. In humans cumulative cultural evolution is thought to depend on a unique suite of cognitive abilities, including teaching, language and imitation. Tool-making New Caledonian crows show some hallmarks of cumulative culture; but this claim is contentious, in part because these birds do not appear to imitate. One alternative hypothesis is that crows' tool designs could be culturally transmitted through a process of mental template matching. That is, individuals could use or observe conspecifics' tools, form a mental template of a particular tool design, and then reproduce this in their own manufacture - a process analogous to birdsong learning. Here, we provide the first evidence supporting this hypothesis, by demonstrating that New Caledonian crows have the cognitive capacity for mental template matching. Using a novel manufacture paradigm, crows were first trained to drop paper into a vending machine to retrieve rewards. They later learnt that only items of a particular size (large or small templates) were rewarded. At test, despite being rewarded at random, and with no physical templates present, crows manufactured items that were more similar in size to previously rewarded, than unrewarded, templates. Our results provide the first evidence that this cognitive ability may underpin the transmission of New Caledonian crows' natural tool designs.

Figure 1

The pieces manufactured by each bird in each condition. (A) Scanned images of all pieces ripped by each bird after learning that ‘small’ or ‘large’ templates were rewarded, in the order in which they were produced. Example templates are provided on the left. Condition order was counterbalanced. Bird names (from L-R) top row: Emma, Joe, Stella, Azzuro (all adults); bottom row: Anton, D3R, D4R, Blue (all juveniles less than 2 years old), order as in Panel (B) Panel B: The mean area of pieces ripped by each bird after learning that ‘small’ or ‘large’ templates were rewarded. Circles denote the median values. Horizontal lines indicate the area of the small and large templates. Mann-Whitney U-tests: ***p < 0.001, **p < 0.01, *p < 0.05.

Figure 2

Diagrams of the experimental set up. (A) Birds learnt that either small or large templates could be inserted into the vending machine to obtain rewards. (B) They then received two very large sheets of card (one presented loose and one fastened under wooden blocks), from which they could manufacture pieces to drop into the vending machine. Credit to Vivian Ward.