Social predispositions and underlying neural mechanisms in chicks (Gallus gallus domesticus)

The detection of animate creatures, being social companions, preys or predators, is fundamental for survival and social interaction among vertebrates. Social predispositions are a set of unlearnt rudimental knowledge about animate beings, helping animals to structure sensory experience immediately after birth. An animal can detect the presence of a living being relying on both dynamic as well as static cues. Immediately after hatching visually naïve chicks for instance, spontaneously prefer biological motion patterns as well as face-like stimuli. Similar evidence has been collected on human adults and infants. In the present work, we aimed to investigate both dynamic and static cues eliciting spontaneous social attraction in visually naïve chicks and the relative neural correlates using an immunohistochemical staining of the immediate early gene product c-Fos as a neuronal activity marker. In a first behavioural study, visually naïve chicks demonstrated to have an innate preference for visible speed changes, cues of self-propulsion (implying an internal energy source to the moving object). In a second study, we found an involvement of septum and preoptic area of the hypothalamus in the perception of this predisposed animate motion cues. Septum is an evolutionarily well-conserved part of the limbic system. Together with preoptic area, they are key nodes of the social decision-making network rich in sex-steroid hormone receptors and involved in various adult social behaviours. This is the first evidence of the involvement of these areas in the perception of elementary animate cues in unexperienced newborn animals. In a third study, we successfully replicated a previous work, finding chicks’ spontaneous preference for a naturalistic configuration of features (stuffed fowl) vs. an artificial one (similar fowl cut in pieces and reassembled scrambled on a box). Here we found a differential involvement of intermediate medial mesopallium in the perception of static configuration of features. Intermediate medial mesopallium is a telencephalic region known to be involved in learning the features of the imprinting object. As an interesting by-product of these studies, we found a recurrent asymmetry in c-fos expression in favour of the left hemisphere. Previous literature showed that light exposure of the egg in a critical embryonic time window stimulate selectively the left hemisphere of chicks and influence future brain and behavioural lateralisation. Intriguingly, the asymmetries we observed in our experiments were in chicks hatched from dark incubated eggs. We devised an experiment to understand how the spontaneous level of c-Fos in the two hemispheres could be modulated by embryonic light exposure. Preliminary results suggest a spontaneous higher c-fos expression in the left hemisphere regardless of embryonic light stimulation. These results adds to previous existing knowledge about the organisation of chicks brain immediately after hatching and pave the way for future studies on neural correlates of social predispositions in vertebrates.