Ontogeny of action syntax in altricial and precocial rodents: Grooming sequences of rat and guinea pig pups

Behavior occurs as coordinated patterns of serial order, with rules of 'action syntax'. Grooming behavior of adult rodents provides several striking examples of action syntax rules. The most stereotyped of these is a 'syntactic chain' pattern, which organizes up to 25 facial strokes and licking movements into a predictable sequence. This pattern previously has been found to be emitted by diverse rodent species from all major suborders: myomorph, sciuromorph, and caviomorph. In this study, we compared the postnatal ontogeny of grooming syntax in two rodent species: rat versus guinea pig. Rats are relatively altricial at birth, whereas guinea pigs are precocial. A day-by-day study of the fine-grain structure of sequential patterns was carried out during the first three weeks after birth, using slow-motion videoanalyses and a choreographic notation system for scoring behavioral grooming sequences. The results showed that substantial action syntax rules exist in guinea pig grooming even on the day of birth. For guinea pigs, postnatal grooming syntax development was limited to minor increments in the strength of the syntactic pattern and in postural control. By contrast, for rat pups, action syntax did not begin to appear until the second postnatal week, and developed gradually into the third week. The development of rodent syntactic patterns in both species appeared to be independent of the maturation of the movements that composed the pattern. Our results indicate that action syntax rules develop as hierarchical entities independent from constituent movements.