Reproductive costs for everyone: How female loads impact human mobility strategies

Abstract

While mobility strategies are considered important in understanding selection pressures on individuals, testing hypotheses of such strategies requires high resolution datasets, particularly at intersections between morphology, ecology and energetics. Here we present data on interactions between morphology and energetics in regards to the cost of walking for reproductive women and place these data into a specific ecological context of time and heat load. Frontal loads (up to 16% of body mass), as during pregnancy and child-carrying, significantly slow the optimal and preferred walking speed of women, significantly increase cost at the optimal speed, and make it significantly more costly for women to walk with other people. We further show for the first time significant changes in the curvature in the Cost of Transport curve for human walking, as driven by frontal loads. The impact of these frontal loads on females, and the populations to which they belong, would have been magnified by time constraints due to seasonal changes in day length at high latitudes and thermoregulatory limitations at low latitudes. However, wider pelves increase both stride length and speed flexibility, providing a morphological offset for load-related costs. Longer lower limbs also increase stride length. Observed differences between preferred and energetically optimal speeds with frontal loading suggest that speed choices of women carrying reproductive loads might be particularly sensitive to changes in heat load. Our findings show that female reproductive costs, particularly those related to locomotion, would have meaningfully shaped the mobility strategies of the hominin lineage, as well as modern foraging populations.

Walking economy at simulated high altitude in human healthy young male lowlanders

“…gas exchange variables, HR, SpO 2 , and NIRS signals) were averages of the last 2 min of standing prior to starting walking. A single sample with an average final 1-min pulmonary V̇O 2 value (ml min −1 ) and V̇CO 2 (ml min −1 ) at each gait speed were used to obtain the energy expenditure (EE) during walking, based on the following equation (Brouwer, 1957; Masschelein et al, 2012): To calculate each particular C w , this equation can be transported as follows: The C w - v relationship can be mathematically described by the following equation (Abe et al, 2015; Wall-Scheffler and Myers, 2013): where the constants a, b, and c are determined by the least squares regressions with the actually observed C w values at each gait speed. A differential function of the original quadratic Eqn (2) of each individual can be described as follows: Then, the individual ES was determined at the gait speed when C w ′ ( v ) equaled zero, that is, the individual ES could be observed using the following equation: We recently reported that standing V̇O 2 amounted approximately 50% of the absolute V̇O 2 at the level gradient at 0.667 m s −1 under normoxia, indicating that a careful consideration should be necessary to calculate C w and ES by subtracting the standing metabolic rate (Abe et al, 2015).…”

The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation

“…Stern [46] raised concerns that we share about the costs of vertical oscillations and stride length, suggesting that specific stride lengths and levels of centre of mass oscillation may provide energetic efficiency. Work by WallScheffler and co-workers [69,70] suggests that a broad pelvis may provide the additional important benefit of increasing speed flexibility relative to locomotor cost, especially in short-legged individuals. Furthermore, Wall-Scheffler and co-workers have found that a wider pelvis reduces the cost of carrying an infant, which would have been an important energy drain for early female bipeds [69,71,72].…”

Size and Shape: Morphology's Impact on Human Speed and Mobility

“…Because females represent the energetic bottleneck for reproduction, and female fertility is sensitive to even minimal energetic perturbations [13,14,71], it is unlikely that females would bear the energetic burden of increasing their walking speed. Pregnant and lactating females are particularly vulnerable to energetic strain and are not likely to increase their speed; pregnant women show steep CoT curves as well as slower optimal and preferred speeds than when they are not pregnant [72,73].…”

“…Females would travel this distance in 3.18 hours, presuming they were walking at their unloaded optimal speed. Women walking unloaded may not be likely given that most ethnographic reports of women are that they are generally carrying children, food, and household items [25,30,75] and thus may be walking around their loaded optimum, which is significantly slower [72]. If males chose to slow down and walk at the female unloaded optimum, it would cost them 52 kJ more to travel this distance.…”

“…In reconstructions 2 Journal of Anthropology of mobility patterns and daily energy budgets [23,24], assumptions are generally made that all people are traveling at their optimal speed-the speed at which the metabolic cost of walking is at the lowest [19,[25][26][27][28][29][30]. Evidence supports this general assumption that people (and other animals) adjust their speed by numerous amounts of physiological input, including energetic [27,29,31,32], muscular [33,34], and thermoregulatory [35][36][37], so that the speed of travel is generally near this minimum.…”