Ravens, Corvus corax, differentiate between knowledgeable and ignorant competitors

Abstract

Human social behaviour is influenced by attributing mental states to others. It is debated whether and to what extent such skills might occur in non-human animals. We here test for the possibility of ravens attributing knowledge about the location of food to potential competitors. In our experiments, we capitalize on the mutually antagonistic interactions that occur in these birds between those individuals that store food versus those that try to pilfer these caches. Since ravens' pilfer success depends on memory of observed caches, we manipulated the view of birds at caching, thereby designing competitors who were either knowledgeable or ignorant of cache location and then tested the responses of both storers and pilferers to those competitors at recovery. We show that ravens modify their cache protection and pilfer tactics not simply in response to the immediate behaviour of competitors, but also in relation to whether or not they previously had the opportunity of observing caching. Our results suggest that the birds not only recall whom they had seen during caching, but also know that obstacles can obstruct the view of others and that this affects pilfering.

Figure 1

Sketch of aviary complex, indicating (a) the location of the test compartment (section B) and (b) the position of competitors at caching (section C). Observers (O) had visual access to the storer in section B (broken lines symbolize wire mesh), whereas the view of non-observers (NO) was totally obscured by an opaque wall (white bar) and closed curtains (solid bar). Subjects in the pathway (O below) could have visual access to both potential competitors (O above, NO), as the compartment's side wall was made of wire mesh.

Figure 2

Mean (±s.e.) percentage of caches recovered by storers (a) with previous observers (‘knowers’) and non-observers (‘ignorants’) and (b) when those competitors were moving towards the caches (black bars) or did not come close to the caches (white bars). Exact Wilcoxon signed-ranks test. Asterisk denotes p<0.05.

Figure 3

Mean (±s.e.) latency till pilfering of observers when together with knowledgeable and ignorant competitors and in private. White bars represent focal subjects as dominants, black bars as subordinates. Exact Wilcoxon signed-ranks test. Asterisk denotes p<0.05.