Knower-guesser differentiation in ravens: others' viewpoints matter

Abstract

Differentiating between individuals with different knowledge states is an important step in child development and has been considered as a hallmark in human evolution. Recently, primates and corvids have been reported to pass knower-guesser tasks, raising the possibility of mental attribution skills in non-human animals. Yet, it has been difficult to distinguish 'mind-reading' from behaviour-reading alternatives, specifically the use of behavioural cues and/or the application of associatively learned rules. Here, I show that ravens (Corvus corax) observing an experimenter hiding food are capable of predicting the behaviour of bystanders that had been visible at both, none or just one of two caching events. Manipulating the competitors' visual field independently of the view of the test-subject resulted in an instant drop in performance, whereas controls for behavioural cues had no such effect. These findings indicate that ravens not only remember whom they have seen at caching but also take into account that the other's view was blocked. Notably, it does not suffice for the birds to associate specific competitors with specific caches. These results support the idea that certain socio-ecological conditions may select for similar cognitive abilities in distantly related species and that some birds have evolved analogous precursors to a human theory-of-mind.

Figure 1.

Sketch of the standard experimental set-up during caching and during testing. (a) E caches food in the presence of three ravens, two potential competitors on the left and the focal subject on the right. The opaque curtains on the doors of the back compartments are raised, allowing one competitor and the focal subject to witness the caching and to see one another (lines of sight are symbolized by arrows with dashed lines); the view of the other competitor towards the caching room remains blocked. (b) At testing, the focal subject heads in the direction of the caches (symbolized by arrow with solid line) while the competitor is still restrained in its compartment. The skewed positioning of the covers on the caches allows the competitor to see either of the ‘hidden’ food pieces (symbolized by arrows with dashed lines).

Figure 2.

Pilfering speed and -selectivity (mean ± s.e.m.) in the (a,b) stay-treatment and (c,d) switch-treatment. (a) Latency to pilfer caches with OB and NO, (b) Proportion of choosing cache 1 with OB and NO, (c) Latency to pilfer caches with O1 and O2, (d) Proportion of choosing cache 1 with O1 and O2. *p < 0.05, n.s., not significant; above bracket: comparison between conditions, within bar: comparison to chance (dashed lines).

Figure 3.

Focal subject's view of the competitor in the uninformed condition of experiment 2. Although the competitor's view towards caches made close to the door is blocked by the curtain, the competitor itself (sitting on the perch right behind the window of the pulled down curtain) is in full view of the focal subject during caching.

Figure 4.

Selectively in pilfering caches, depicted as mean (± s.e.m.) proportion of matching caches 1 and 2 to O1 and O2, respectively, in the competitor-informed and -uninformed condition (i.e. curtain's position at caching). Dashed line represents chance level. *p < 0.05.