Postnatal brain size decrease, visual performance, Learning, and discrimination ability of juvenile and adult American mink (Mustela vison : Carnivora : Mammalia)

The decrease in brain size to an amount of 20% during early Life from the j uvenile to the adult state in mink and other Mustela species is still poorl y understood and unresolved in its general biological and functional releva nce. The same holds true tor the decrease in brain cavity and the flattenin g of the cranial vault. Since the neocortex and other brain parts with high er integrative and associative sensory and motoric functions are especially involved, the question arises as to whether these size changes have any fu nctional consequences, i.e., are the functional capacities reduced concomit antly? This was tested for the visual system in 8 juvenile (4, 4) and 8 adu lt (4, 4) mink (Mustela vison energumenos) of wild descent using twofold-ch oice discrimination trials. After conditioning and testing for spontaneous side preferences, the individuals had to discriminate black dots of differe nt sizes against a white plate from 30 cm distance. Altogether, 16 000 indi vidual data-sets were statistically analysed for differences in visual perf ormance, in Learning velocity, and in discrimination ability. No difference s occurred between the juvenile and the adult group concerning Learning vel ocity. However, significant differences were found in discrimination abilit y with regard to age (juvenile mink performed better than adults) and sex ( females performed better than males). These results are discussed with rega rd to the importance of visually guided behaviour of the species, with the behaviour of juveniles and adults in general, and with the ontogenetic decr ease in mass of the central nervous tissue. According to this study, there is no indication of any functional impact of the ontogenetic reduction in b rain size on the capacity of the visual system.