Insightful problem solving and creative tool modification by captive nontool-using rooks

Abstract

The ability to use tools has been suggested to indicate advanced physical cognition in animals. Here we show that rooks, a member of the corvid family that do not appear to use tools in the wild are capable of insightful problem solving related to sophisticated tool use, including spontaneously modifying and using a variety of tools, shaping hooks out of wire, and using a series of tools in a sequence to gain a reward. It is remarkable that a species that does not use tools in the wild appears to possess an understanding of tools rivaling habitual tool users such as New Caledonian crows and chimpanzees. Our findings suggest that the ability to represent tools may be a domain-general cognitive capacity rather than an adaptive specialization and questions the relationship between physical intelligence and wild tool use.

Fig. 1.

Choice of appropriate stone size. Mean stone sizes ± SEM: large (23.5 ± 2.0 g), medium (11.2 ± 0.4 g), small (3.8 ± 0.5 g). Graphs display percentage mean choices of 4 subjects across all trials. (A) Stone size choice when presented with large diameter tube. Subjects preferred to use the large stones (GLM, stone size: F_2,28 = 51.83, P = 0.00, [Tukey test: large–medium T = 6.49, P = 0.00, large–small T = 10.04, P = 0.00, medium–small T = 3.55, P = 0.004]), and this did not differ across blocks of trials (GLM, block: F_{2, 28} = 0.00) or between subjects (GLM, subject: F_3,28 = 0.00) (B) Stone size choice when presented with small diameter tube. Subjects switched their choice to the small size stones (GLM, stone size: F_2,28 = 258.88, P = 0.00, [Tukey test: large–medium T = 2.21, P = 0.086, small–large T = 20.72, P = 0.00, small–medium T = 18.51, P = 0.00], block: F_2,28 = 0.00, subject: F_3,28 = 0.00). ** P < 0.01, *** P < 0.001. Dashed line represents 33% chance level.

Fig. 2.

Actions when using sticks as a tool. Heavy sticks were 3.5 g (165 mm × 8 mm) and light sticks were 0.5 g (165 mm × 2 mm). Mann-Whitney U tests were used to compare stick type for 2 different actions (dropping and pushing down). (A) Stick dropped. Box plot (median, IQR, 95% CI) showing proportion of trials stick dropped (W = 25.5, n = 4, P = 0.043). (B) Stick pushed down. Box plot (median, IQR, 95% CI) showing proportion of trials stick pushed down (W = 10.0, n = 4, P = 0.03). *P < 0.05

Fig. 3.

Choice of tool type. Graphs display mean choice of 4 subjects across all trials. Subjects chose the functional tool (GLM, functional, or nonfunctional, F_1,40 = 771.56, P = 0.00), regardless of tool type (stick or stone, F_1,40 = 3.56, P = 0.07). There was no difference in choice between blocks of trials (GLM, block: F_2,40 = 0.00) or subject (GLM, subject: F_3,40 = 0.00). (A) Combination 1: functional stick versus non-functional stone. Subjects chose the stick nearly every time. (B) Combination 2: non-functional stick versus functional stone. Subjects chose the stone nearly every time. Dashed line represents 50% chance level.

Fig. 4.

Stick modification efficiency. Mean number (± SEM) of modifications made against number of modifications needed. Line shows linear regression (r² = 0.23, F_1,116 = 35.27, P = 0.00).

Fig. 5.

Metatool use task; trial by trial description of behavior. Initial choice indicates whether the tool provided was used to acquire the small stone (correct) or the large stone (incorrect). If the small stone was chosen, this stone was either used to acquire the worm (correct use, immediate success); used to acquire the large stone and subsequently used to acquire the worm (incorrect use, eventual success), although this never happened; or taken away (incorrect use, failure). If the large stone was chosen, this stone was either used to acquire the small stone, which in turn was used to acquire the worm (correct use, eventual success), redropped into the empty tube before being used to acquire the small stone which was in turn used to acquire the worm (incorrect use, eventual success) or taken away (incorrect use, failure).

Fig. 6.

Bending wire into hooks by rooks. (Left) Fry extracting the bucket containing a worm using a piece of wire she had just bent. This photo was taken after the experiment was completed but the hook and posture are typical of experimental trials. (Right) Photographs of the successful hooks used by all 4 subjects (excluding the 2 trials where the straight wire was used to stab the bucket), with the successfully used end facing right. Numbers indicate trial number.