Humans Crawl: Species Atypical Movement
Crawling is a form of species atypical movement for humans.
Striding, bipedal walking, and endurance running are the hallmark modes of human locomotion, yet humans locomote in a number of different ways. As young infants we start with a system that is more plastic and flexible than it seems, given the relative uniformity of adult forms of locomotion like human bipedalism. This plasticity is evident in the surprising number of different pathways to upright walking (Largo et al. 1985). In this piece we contrast the divergent pathways that humans and their relatively close neighbor, the chimpanzee, take in the ontogeny of adult forms of movement.
Factors Influencing the Ontogeny of Gait
The ontogeny of animal movement is constrained by factors both within and external to the organisms that both limit certain types of actions and facilitate others. These can be described as organismic constraints which are internal to the animal, environmental constraints that are external to the animal and that are relatively consistent across developmental time periods, and task constraints that pertain to any specific problem that animal is confronted with at a particular time period (Rosengren et al. 2003). At any one time point, these three different types of constraints interact to produce a particular action pattern or behavior.
A change in one constraint can have cascading influence on the emergence of different action patterns. For example, encouragement by health care professionals to have parents have their infants sleep on their backs to prevent sudden infant death syndrome (SIDS: Skadberg et al. 1998) had the unexpected consequence of delaying the onset of motor milestones such as creeping and crawling, in part because supine sleepers do not gain the same experience using their arm and torso muscles necessary for creeping and crawling as prone sleepers (Davis et al. 1998).
For the developing animal, organismic constraints for movement include the timing and pattern of growth, the organization and maturity of the musculoskeletal system, and the motivation to move from one place to another to gain food or obtain shelter. Environmental constraints include the fact that animals on earth are bound by gravity and the structure and characteristics of the surfaces that one can act on (e.g., a relatively flat horizontal ground or vertically rising trees). Task constraints include whether the animal is interested in seeking food, comfort, or safety. Although traditional models of the development of locomotion emphasized that maturation was the main driving force in the ontogeny of human gait, more current approaches emphasize the importance of the developing infants’ active exploration of the interaction of these different types of constraints (Rosengren et al. 2003; Thelen 1995).
Gait Ontogeny in Humans
Human infants enter the world with a relatively immature action system that provides ample opportunity to explore the interaction of constraints and in doing so develop a number of species atypical movement patterns. Over the first year of life they acquire the physical strength and coordination to effectively move through the environment. But the ways individual infants accomplish the act of locomotion is determined by their motivation, physical layout of their home (e.g., does it contain carpeted surfaces or hard tile floors, flat surfaces or stairs), and the goals of their caregivers in promoting exploration or optimizing safety. For this reason, young infants exhibit a wide range of atypical locomotor behaviors to get from one place to another including rolling, shuffling on their bottom, crawling on stomach, crawling on hands and knees, and crawling on hands and feet, prior to the acquisition of upright walking (Largo et al. 1985). The fact that upright gait is relatively uniform as the species-typical pattern of movement suggests that over the course of evolution the different types of constraints have led to a small number (e.g., walking and running) of highly efficient and effective patterns of movement.
Gait Ontogeny in Chimpanzees
For chimpanzees, infants cling to their mothers’ fur for safety, transportation, and feeding. These behaviors lead the infants to develop forelimb-dominated, suspensory grasping abilities. Like humans, infant chimpanzees are more variable in their locomotor modes than juveniles or adults, in part because their size permits it and in part because of differences in parenting styles. As offspring size increases (~ 3 years), mothers carry the offspring less frequently, and they transition to quadrupedal knuckle-walking. This species-typical behavior does not become the dominant mode of locomotion until after 5 years, when the chimpanzees are completely independent of their mothers (Sarringhaus et al. 2014).
This cross-species comparison provides evidence that the same three sets of constraints apply to different species. The transition to species-typical posture and locomotion will depend on organismic-, environmental-, and task-specific constraints. The fact that more limited forms of species-typical gait are found in adults suggests that these constraints interact over the course of development to select for efficient and effective patterns of movement.