Infants and Adults Are Sensitive to the Costs of Upright Versus Non-Upright Locomotion.

Variety in locomotor forms-upright (e.g., walking) versus non-upright (e.g., crawling)-reflects a "decision" about how to move. A decision about whether to move upright or non-upright reflects the costs incurred from getting into and moving in a particular posture-energy, stability, time, comfort, and so on. Are infants and caregivers sensitive to the immediate costs of upright and non-upright locomotion? We tested bout-level sensitivity to the costs of locomotor form in groups with different locomotor skills and body size (24 crawling and 24 walking infants and their caregivers). To encourage both short and long bouts, dyads played with toys clustered in a pile and toys dispersed around the room. Most critically, all groups showed bout-level sensitivity to the costs of upright and non-upright locomotion-they factored in their starting posture, typical mode of locomotion, and travel distance when deciding how to move. Walkers and caregivers only moved non-upright if they were already seated and traveled short distances; otherwise, they walked. Conversely, crawlers only moved upright if they were already standing and traveled short distances; if they were seated or moved long distances, they crawled. Moreover, across toy conditions, crawlers moved less than walkers and caregivers moved less than infants. Both crawlers and walkers displayed longer bouts when toys were dispersed, and caregivers mirrored their infants' activity. Thus, despite infants' propensity to move, babies-like adults-are indeed sensitive to the costs of upright versus non-upright locomotion. SUMMARY: Infants and caregivers showed bout-level sensitivity to locomotor costs by accounting for starting posture and travel distance when deciding whether to move upright or non-upright. Walkers and caregivers crawled when starting non-upright and traveling short distances; otherwise, they walked. Crawlers walked when starting upright and traveling nearby; otherwise, they crawled. Infants and caregivers moved more when toys were dispersed than when toys were clustered. Walkers moved more than crawlers, and infants moved more than caregivers. Despite infants' willingness to accumulate large amounts of seemingly gratuitous locomotion, infants, like adults, are sensitive to the varying, dynamic costs of different locomotor forms.