The costs of human locomotion: maternal investment in child transport.

This article investigates maternal investment in child carrying and presents a method for determining when it is energetically advantageous for a mother to carry her child rather than force her child to walk independently. I calculate maternal and child energy consumption while walking and develop correction factors to facilitate making these energy calculations for young children. In addition, I investigate the effect of maternal burdens in addition to the child and of external nutritional support on energy consumption. Since maternal energy is a finite resource, the "decision" to carry a child or force it to walk independently is especially important. This decision can be predicted from the body mass of the mother and child and the child's age. If the mother provides all of the child's nutrition, then the mother should choose to carry her child only when the energy usage of the mother carrying the child is less than the sum of the energy used when the mother and child walk independently. The critical velocity, when the two expenditures are equal, can then be determined. Several general hypotheses are also addressed. The critical velocity of a 60 kg mother with a 4-year-old child approximately equals the average walking speed of adult humans. For a lighter mother, the critical velocity is reached when her child is 3 years old, while for heavier mother this point is not reached until her child is 6 years old. The effect of burdens in addition to the child's mass is minimal. Nutritional support of the child by agencies other than the mother decreases the age at which the mother should force the child to walk independently. In some cases, especially for the lightest mothers, it is never in the mother's best energetic interest to carry her child.