It has been suggested that the study of the domestic dog might help to explain the evolution of human communicative skills, because the dog has been selected for living in a human environment and engaging in communicative interactions with humans for more than 10,000 years [1
]. More specifically, it was assumed that the functional similarities in the socio-cognitive behaviour of dogs and humans emerged as a consequence of comparable environmental selection pressures. Earlier it was thought that selection might have acted directly on the cognitive capacity of dogs [3
]. However, subsequent studies have emphasized the role of auxiliary components of behaviour, changes in which may have facilitated the manifestation of the pre-existing cognitive abilities in an anthropogenic environment [see "emotional reactivity" hypothesis in [1
]]. Indeed, any performance in a cognitively challenging task does not depend only on mental machinery, but on other factors such as temperament. Recently, considerable attention has been paid to the so-called human-cued "two-way object choice" test, in which the subject can capitalize on the gestural cue of a human to choose between two containers [see [2
], for a review]. The relatively high performance of dogs in this task in comparison with apes [3
] and wolves [4
] was explained chiefly by reference to selective factors that acted directly on the cognitive ability of dogs to respond to human gestural cues. Recent findings on adult wolves' higher performance were attributed either to learning effects related directly to the task [6
] or to developmental changes in some behavioural traits [7
Importantly, however, there are at least two other situational factors that affect the performance of the subjects in this task. Firstly, Hare [8
] argued that the versions of the two-way choice task, where the subject has to select the social signal in preference to other cues (e.g. the location of the bait in the previous trial), are cooperative in nature. The poor performance of the chimpanzees in this test was attributed to their putative inability to appreciate the cooperative aspect of this task. Secondly, performance may be affected by enduring attention, i.e., that watching the cueing human more intensely or for longer could also facilitate performance because it increases the subjects' probability of noticing and correctly recognizing minute gestural signals. Differences in attention span appear critical in social learning [9
] where the quality of observation also affects subsequent performance. Previously we have shown that an increased tendency to look at humans may also play role in dog-wolves differences [4
Recent data on the performance of dogs and wolves in the comprehension of the distal pointing gestures suggest that claims about dogs' general superior ability in relying on subtle human communicative signals remain largely debatable [7
]. This is partly because individuals from some dog populations do not show high levels of performance [6
]. Recent reports and resultant discourse [see [3
]] have emphasized the possibility that the comparisons of dog and wolf groups have failed to exclude the influence of confounding environmental effects.
Without relying on species comparisons, in the current two studies we offer a novel approach to reveal the possible effects of artificial selection of dogs; selection that might affect their performance in communicative tasks. In the two-way object choice test, we applied the relatively demanding distal momentary pointing as a human cue, because it has been established as a benchmark by several previous studies [e.g. [5
]; 11; for review see [2
Our basic assumption is that, during domestication, dogs have been selected for 1) enhanced cooperative ability and 2) enduring attention, and these skills have been further differentiated during the process of breed formation.
Investigation of the working history of different breeds suggests that dogs have been required to cooperate in two fundamentally different contexts. Some work cooperatively, with continuous visual contact of their human partner, (e.g. herding dogs, gundogs), labelled the 'cooperative worker' breeds, whereas others work with no human visual contact (e.g. hounds, earth dogs, livestock guarding dogs and sled dogs), labelled the 'independent worker' breeds. Since the human cueing is necessary for the success in the two-way choice task, we firstly hypothesized that dogs selected for cooperation in visually guided tasks would show superior performance in comparison with the other working breeds.
Our second hypothesis emerged from a recent study that detected a significant difference in the distribution of retinal ganglion cells between brachycephalic ("short-nosed") and dolichocephalic ("long-nosed") dog breeds [12
]. It reported that ganglion cells in brachycephalic breeds occur more centrally in the retina. Since, in other species, such arrangement usually correlates with the retinal location of greatest visual acuity, McGreevy et al. [12
] suggested that brachycephalic breeds might respond most to stimuli in the central field (i.e., when looking forward) because they are less disturbed by visual stimuli from the peripheral field. Accordingly, we hypothesized that this morphological change could also influence performance in the two-way choice task and gave brachycephalic breeds an advantage over dolichocephalic breeds.