Abstract: Day-old chicks were exposed to point-light animation sequences depicting either a walking hen or a rotating cylinder. On a subsequent free-choice test (experiment 1) the chicks approached the novel stimulus, irrespective of this being the hen or the cylinder. In order to obtain equivalent local motion vectors, in experiments 2 and 3 newly hatched chicks were exposed either to a point-light animation sequence depicting a walking hen, or to a positionally scrambled walking hen (i.e. an animation in which exactly the same set of dots in motion as that employed for the walking hen was presented, but with spatially randomized starting positions). Chicks tested on day 1 (experiment 2) or on day 2 (i.e. after a period in the dark following exposure on day 1 (experiment 3)) proved able to discriminate the two animation sequences: males preferentially approached the novel stimulus, females the familiar one. These results indicate that discrimination was not based on local motion vectors, but rather on the temporally integrated motion sequence.

Abstract: It has recently been reported that predator inspection is more likely to occur when a companion (i.e. the mirror image of the test animal) is visible on the left rather than on the right side of mosquitofish Gambusia holbrooki. This very unexpected outcome could be consistent with the hypothesis of a preferential use of the right eye during sustained fixation of a predator as well as of a preferential use of the left eye during fixation of conspecifics. We measured the time spent in monocular viewing during inspection of their own mirror images in females of six species of fish, belonging to different families--G. holbrooki, Xenotoca eiseni, Phoxinus phoxinus, Pterophyllum scalare, Xenopoecilus sarasinorum, and Trichogaster trichopterus. Results revealed a consistent left-eye preference during sustained fixation in all of the five species. Males of G. holbrooki, which do not normally show any social behaviour, did not exhibit any eye preferences during mirror-image inspection. We found, however, that they could be induced to manifest a left-eye preference, likewise females, if tested soon after capture, when some affiliative tendencies can be observed. These findings add to current evidence in a variety of vertebrate species for preferential involvement of structures located in the right side of the brain in response to the viewing of conspecifics.

Abstract: Right hemisphere advantage in individual recognition (as shown by differences between response to strangers and companions) is clear in the domestic chick. Chicks using the left eye (and so, thanks to the complete optic decussation, predominantly the right hemisphere) discriminate between stranger and companion. Chicks using the right eye discriminate less clearly or not at all. The ability of left eyed chicks to respond to differences between strangers and companions stimuli is associated with a more general ability to detect and respond to novelty: this difference between left and right eyed chicks also holds for stimuli which are not social partners. The right hemisphere also shows advantage in tasks with a spatial component (topographical learning; response to change in the spatial context of a stimulus) in the chick, as in humans. Similar specialisations of the two hemispheres are also revealed in tests which involve olfactory cues presented by social partners. The special properties of the left hemisphere are less well established in the chick. Evidence reviewed here suggests that it tends to respond to selected properties of a stimulus and to use them to assign it to a category; such assignment then allows an appropriate response. When exposed to an imprinting stimulus (visual or auditory) a chick begins by using right eye or ear (suggesting left hemisphere control), and then shifts to the left eye or ear (suggesting right hemisphere control), as exposure continues. The left hemisphere here is thus involved whilst behaviour is dominated by vigorous response to releasing stimuli presented by an object. Subsequent learning about the full detailed properties of the stimulus, which is crucial for individual recognition, may explain the shift to right hemisphere control after prolonged exposure to the social stimulus. There is a marked sex difference in choice tests: females tend to choose companions in tests where males choose strangers. It is possible that this difference is specifically caused by stronger motivation to sustain social contact in female chicks, for which there is extensive evidence. However, sex differences in response to change in familiar stimuli are also marked in tests which do not involve social partners. Finally, in both sexes there are two periods during development in which there age-dependent shifts in bias to use one or other hemisphere. These periods (days 3-5 and 8-11) coincide with two major changes in the social behaviour of chicks reared by a hen in a normal brood. It is argued that one function of these periods is to bring fully into play the hemisphere most appropriate to the type of response to, and learning about, social partners which is needed at particular points in development. Parallels are discussed between the involvement of lateralised processes in the recognition of social partners in chicks and humans.

Abstract: Recent studies have revealed a variety of left–right asymmetries among vertebrates and invertebrates. In many species, left- and right-lateralized individuals coexist, but in unequal numbers (‘population-level’ lateralization). It has been argued that brain lateralization increases individual efficiency (e.g. avoiding unnecessary duplication of neural circuitry and reducing interference between functions), thus counteracting the ecological disadvantages of lateral biases in behaviour (making individual behaviour more predictable to other organisms). However, individual efficiency does not require a definite proportion of left- and right-lateralized individuals. Thus, such arguments do not explain population-level lateralization. We have previously shown that, in the context of prey–predator interactions, population-level lateralization can arise as an evolutionarily stable strategy when individually asymmetrical organisms must coordinate their behaviour with that of other asymmetrical organisms. Here, we extend our model showing that populations consisting of left- and right-lateralized individuals in unequal numbers can be evolutionarily stable, based solely on strategic factors arising from the balance between antagonistic (competitive) and synergistic (cooperative) interactions.

Abstract: We recorded preferential use of the left and right monocular visual field in black-winged stilts, Himantopus himantopus, during predatory pecking and during courtship and mating behaviour in a naturalistic setting. The stilts had a population-level preference for using their right monocular visual field before predatory pecking; pecks that followed right-hemifield detection were more likely to be successful than pecks that followed left-hemifield detection, as evinced by the occurrence of swallowing and shaking head movements after pecking. In contrast, shaking behaviour, a component of courtship displays, and copulatory attempts by males were more likely to occur when females were seen with the left monocular visual field. Asymmetric hemifield use observed in natural conditions raises interesting issues as to the costs and benefits of population-level behavioural lateralization in wild animals.