FILOGENESI DELLA VISIONE TRICROMATICA: UNO STUDIO CON TRE SPECIE DI PRIMATI NON UMANI.

Trichromatic colour vision is rare within placental mammals, occurring only in Old World primates (Catharrine) and some New World primates (Plathirrine). Animals with this kind of vision can distinguish among mid-to long wavelengths of light (greenish to reddish colours) that are indistinguishable to dichromats. Because it is absent in other eutherian mammals, there has been considerable interest in the evolutionary origin of this trait. There are two different hypotheses in the literature about the selective pressure for the trichromatic colour vision in non-human primates. According to the first hypothesis (foraging hypothesis), trichromacy provides an important advantage for fruit eating species. Primates with trichromatic colour vision, in fact, may be more efficient in detecting red/orange food items against a background of green leaves in variably illuminated environments, than dichromatic individuals. Furthermore primates must rely on leaf foraging during periods when fruits are not available and young leaves, which are slightly redder in colour than mature leaves, could be discriminated from mature leaves along a red-green colour axis. The second hypothesis links trichromacy to the socio-sexual intraspecific communication and it suggests that the colour vision may be important for the perception of skin colour signalling, driving some species to develop reddish sexual skins. Our research focuses on the validity of the foraging hypothesis as the selective pressure favouring the evolution of trichromacy. From a phylogenetic point of view, three species of primates (Chlorocebus aethiops, Macaca nemestrina, Pan troglodytes) were observed in a foraging context. Three studies involved the observation of the primates behaviour in a social context; furthermore an observational study was conducted on pigtail macaques in social isolation from avoiding social food competition. Eight vervet monkeys, 8 macaques and 14 chimpanzees housed at Parco Natura Viva were the subjects of our research. In the social context, animals were employed with 10 sessions in which pairs of green and red coloured objects were given the three colonies of animals. Rewards were concealed within the coloured objects. In order to assess the colour preference we analysed the first choice between red and green objects and all the choices between the two colours. In the social isolation study, where the food competition is absent, macaques were employed in 18 sessions, in which trinary choices between red, green and white objects were given to individuals. In order to assess a preference for one colour we collected the frequency of the colour first choice. As lateralized behaviours might have an effect on object choices, an important variable to consider in these studies is the hand preference of the primates. In order to investigate the link between handedness and objects choices, hand preference of the primates were analysed. Furthermore we focused on the relationship between preferred hand and coloured object choices, in order to verify whether other mechanisms, such as emotional aspects, were involved in the colour choice. Results of these studies underline that vervet monkeys and pigtail macaques do not express a significant preference for one of two coloured objects, whereas chimpanzees seem to choose red coloured objects significantly more than the green ones. Besides, an individual hand preference was found only for some subjects of each species; however no significant hand preference was found at a group level. Our findings assess that pigtail macaques show a left hand preference in a condition of isolation, proving that the social context and food competition might affect the behaviour of individuals. Moreover, results do not show a link between behavioural lateralization and colour choices underline that further studies are necessary in order to investigate the implication of the emotions in colour preference. In conclusion, on the basis of the phylogeny, our findings seem to highlight a tendency to choose red colour objects from monkeys to apes. This may help achieving the plausibility of the foraging hypothesis as selective pressure influencing trichromatic colour vision. However, further studies are needed in order to make the picture of the evolution of human colour vision more accurate and complete.