Visual Recognition of Age Class and Preference for Infantile Features: Implications for Species-Specific vs Universal Cognitive Traits in Primates

Despite not knowing the exact age of individuals, humans can estimate their rough age using age-related physical features. Nonhuman primates show some age-related physical features; however, the cognitive traits underlying their recognition of age class have not been revealed. Here, we tested the ability of two species of Old World monkey, Japanese macaques (JM) and Campbell's monkeys (CM), to spontaneously discriminate age classes using visual paired comparison (VPC) tasks based on the two distinct categories of infant and adult images. First, VPCs were conducted in JM subjects using conspecific JM stimuli. When analyzing the side of the first look, JM subjects significantly looked more often at novel images. Based on analyses of total looking durations, JM subjects looked at a novel infant image longer than they looked at a familiar adult image, suggesting the ability to spontaneously discriminate between the two age classes and a preference for infant over adult images. Next, VPCs were tested in CM subjects using heterospecific JM stimuli. CM subjects showed no difference in the side of their first look, but looked at infant JM images longer than they looked at adult images; the fact that CMs were totally naïve to JMs suggested that the attractiveness of infant images transcends species differences. This is the first report of visual age class recognition and a preference for infant over adult images in nonhuman primates. Our results suggest not only species-specific processing for age class recognition but also the evolutionary origins of the instinctive human perception of baby cuteness schema, proposed by the ethologist Konrad Lorenz

Possible effects of periodontal inputs on the masticatory function

This study tested the hypothesis of whether the lack of tooth eruption would affect the masticatory function. For this purpose, we recorded the jaw movement trajectory in the three dimensions and electromyographic (EMG) activity of MAS and DIG muscles in freely behaving osteopetrotic (op/op) and normal mice. A masticatory sequence was divided into food intake and mastication periods, and 10 cycles in the latter stage were selected for analysis. Mean values of total cycle duration, closing phase duration and opening phase duration were obtained from the jaw movement and those of onset time, time duration and amplitude were obtained from the EMGs of the MAS and DIG muscles. The protruding phase coincided with the power stroke lacked in the op/op mouse. The total cycle duration and opening phase duration were longer in the op/op mouse than normal mouse. The pattern of MAS muscle activity in op/op mice was similar to that in normal mice while the duration of the DIG muscle was longer in op/op mice than normal mice. Our results indicate that the periodontal receptors play an important role in the completion of masticatory function