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Belittling women and girls’ athletic ability is routine in American culture. “Throwing like a girl” and other phrases create a cultural norm of suggesting that women and girls’ physical process is severely lacking. Women are paid less in all fields, but athletes have the worst gender pay gap, with some data showing that men are paid 150 percent more than women. There is a growing body of evidence from people working in sports media, from people working with children’s physical education, and from people working in biomechanics, that athletic ability generally, and the value we give to certain kinds of athleticism has a strong cultural component: being encouraged to throw leads to better throwing abilities regardless of sex or gender identity.
As more funding has been moved into girls’ and women’s sports, women have begun winning all-gender competitions, particularly in competitions that have a strong endurance component. When we actually look at women and “women’s work,” we find that women move constantly and consistently across the lifespan, they walk in groups with people of varying ages and genders, and they are always carrying something. Furthermore, we know that women have been doing this not just for decades, not just for centuries, but for millennia. By understanding our evolutionary history, we get a special insight into these “new” victories, and why women’s endurance mobility has shaped our entire species.
Two crucial evolutionary approaches to women’s bodies (among many) are: looking cross-culturally at what humans seem to do regardless of culture (e.g., all cultures have protocols for teaching children), and experimentally testing how human variation might excel under conditions that exist in all human populations (e.g., energy consumption during load carrying). Both these pieces are vital because there are aspects of performance that might matter in the United States today but are rare among human populations cross-culturally (e.g., running quickly), and are probably not representative of characters that exist based on the Environment of Evolutionary Adaptedness (EEA). The EEA helps us to refocus our research questions to the ecosystem and behaviors of our evolutionary ancestors. Humans have incredibly variable phenotypes both within and between populations but the meaning of these patterns of variation make sense only within the context of the EEA. So, tying what people actually do universally with potential (reproductive or heritable) benefits of that activity help us better understand our evolution.
When we look at what women do across cultures, numerous features of their activity patterns are striking. In addition to daily walking across long distances, women also tend to walk with others while balancing numerous loads around their bodies. Children are only one load that women consistently carry cross-culturally. Women carry a wide range of toolkits and baskets, household goods, and food materials. While women do seem to walk slowly while carrying these loads, the loads themselves can be 20–40 percent of their total body mass, and they are more likely to be walking while carrying than doing almost any other daily task. It seems reasonable then that it is probably important to understand something everyone does all the time and also is most likely representative of our EEA. Given this environmental framework, what specific aspects of locomotion can help us understand human mobility from an evolutionary perspective?
Women aren’t just walking effectively when they are by themselves, they are walking effectively when they are walking with others, which helps groups maintain social cohesion and is a crucial aspect of humans’ success as a species.
Once we have established our EEA, modelling how much energy people use when doing these key tasks is one of the best ways to understand how humans might have evolved. Energetic methods of understanding locomotion bring people into a lab or have them walk around outside to measure their metabolic energy expenditure. Under typical workout-style cultural interpretations, people are interested in which activities use the most energy; when we are considering tasks evolutionarily, we look for which activities use the least amount of energy and which morphological elements can help us reduce energy even more—like long lower limbs or having a lower center of mass (COM). This is because when we save energy while walking―the task we do more than any other―we have extra energy for other tasks we must accomplish. From an evolutionary perspective that would be making gametes and, ultimately, building a fetus. We know that when women are able to reduce the amount of walking they do―for example, when a well is built in their town―they can also reduce the amount of time between pregnancies, which helps sustain the population and prevents it from going extinct. Similarly, providing women with help for their tasks―alternating who walks, who carries, and who makes the tools―increases the communal energy available to a reproductively active population.
When we’re looking at energetic models of human mobility throughout our evolution, we are thus interested in how women might be able to save energy doing locomotion that allows them to continue to gain access to food, water, and community, as well as to reproduce (the alternative being extinction). One option evolutionarily is to be smaller—this is because absolutely smaller bodies use absolutely less energy. Human females are smaller on average than males in any given human population for exactly this reason—females use less energy to walk overall, and thus can simply allocate any additional caloric intake to their fitness. But this comes with a trade-off. Even though you might use absolutely less energy, most small endotherms use relatively more energy to do specific tasks. The clearest example of this is maintaining body temperature—because small endotherms have a relatively high surface area to volume ratio, they lose heat at a high rate and so must have a higher metabolism to maintain the same body temperature as a large endotherm. This is the general framework of understanding why a mouse lemur must eat high quality insects, whereas a gorilla can sit around eating low quality grass—the mouse lemur must eat relatively more calories to maintain its mass and activities than a gorilla. This paradigm has generally been used to downgrade women’s bodies as susceptible to unfortunate trade-offs—women use relatively more energy to walk (or run) and therefore are less efficient at locomotion than men. Yet when women walk with loads—even the exact same load as men (which means the load is a relatively larger percentage of women’s mass since they are absolutely smaller)—they use both absolutely less energy and relatively less energy. This means that by every energetic measure, women are able to carry loads more effectively than men, which makes sense given that women carry loads universally. This might explain why women can outperform men during ultramarathons as well—the combination of efficiency and economy allows them to have increased endurance.
Understanding the basis for this potentially astonishing outcome takes us to another way of investigating locomotor mobility from an evolutionary perspective. This second approach looks at parts of the body preserved in the fossil record, such as bone shapes, sizes, and lengths, and measures these variables on living people. Then we investigate whether there are correlations between how people with certain body measures use their limbs. For example, do people with longer lower limbs gain some sort of advantage over people with shorter lower limbs? Does this matter if the limbs are absolutely longer, or just in proportion with overall stature? What is it about longer limbs that might convey a locomotor advantage? Is there something about women’s bodies that might convey an advantage, particularly while carrying loads?
Now, women are not a totally different species! I hope it is obvious that almost everything you might measure on a woman overlaps with the variation among men—variation occurs along a continuum. So, we are interested in averages and ranges of variation—women are on average smaller than men, but of course there are women who are bigger than the man by whom they are currently standing. This being said, on average, women have relatively wider pelves than men, at least in a few measures. While their lower limb lengths are similarly proportioned to men in overall length, women tend to have less variation in their lower limbs in a given population—that is their limb length tends to be more stable and less variable than men’s.
How do these patterns matter for mobility? And do they help us in explaining the patterns in energetics? It is true that people with relatively and absolutely longer limbs do have some advantages—they can cover more ground with a single stride, and they do seem to use less energy to perform that movement. What ends up happening when women pick up a load is that their extra pelvis width gives them a little bit of extra stride length. So, for a given lower limb length, women get a longer stride because they have a slightly wider pelvis to carry their lower limb through the stride cycle and forward a longer distance. This difference only pops up when women and men pick something up; when people are not carrying loads, stride length is driven entirely by lower limb length. Having that longer stride length while carrying loads allows women to cover more ground without taking more steps, thus saving them energy.
Women also carry their mass in a slightly different arrangement than men (on average), with more of their mass in their lower limbs (whereas men have more of their mass in their upper limbs). This means that women have a lower center of mass than men (all those different backpack choices at REI really do come in handy!). Having a lower center of mass increases a body’s stability, which is particularly important when carrying loads, especially loads that might stick out away from you, like a pregnant belly, a spear and tubers, or a toddler held on one hip. Many studies have shown that increasing stability reduces the metabolic cost of walking, because the small perturbations to remain stable can actually be quite metabolically costly; women’s low center of mass reduces the need to spend energy constantly restabilizing.
This increase in stability also appears to allow women to vary their walking speed, at least within a reasonable range, with no change in the metabolic cost to do so. When we consider human mobility from an ecological perspective, we often consider how people move between key locations for their success—between their home and where they will get food and water for the day. Interestingly, the relationship for endothermic mammals between walking speed and the metabolic cost to go a given distance is a u-shaped curve. If you can imagine a graph of this u-shape, with speed on the x-axis and cost to go to the place you need to get to on the y-axis, you will see that there is a speed at which you can walk to your destination for the least amount of energy. Most people will choose to walk at this speed when they are walking by themselves. It is also true that people with relatively wider pelves have a very broad base to this curve (very u-shaped), whereas people with narrower pelves have a more v-shaped curve. Smaller people also have a more u-shaped curve. This means that, on average, women can walk at a relatively wide range of speeds without any change to their metabolic cost for walking a given distance.
This saving of energy is a key part of being an excellent endurance athlete. Athletes who use less energy are often the ones who win races that combine speed and endurance. Because of their evolved body shape, women are the ones who can travel long distances for the smallest amount of energy. We can be confident then that women are excellent movers by any measure, and that the EEA which includes load carrying and sociality has selected for women’s particular morphology. Women aren’t just walking effectively when they are by themselves, they are walking effectively when they are walking with others, which helps groups maintain social cohesion and is a crucial aspect of humans’ success as a species.
We should also note that many of us are no longer in our EEA—we don’t walk to work, we don’t carry our babies, and we don’t gather our water from wells (or water holes!). Humans, of course, are much, much more than their bodies. This means that tossing your backpack to the women in your group might not be the best use of your social capital, but it does mean that coaxing a woman to be on your ultramarathon team is definitely a wise decision, especially when the race involves carrying your own gear. If you identify as a woman, know that you can economically walk in community with others―for example, racing to your next class, perusing produce at the farmer’s market, or marching together for reproductive rights! Our evolutionary history has shaped our morphologies and correlated mobility patterns but how we choose to spend our energy is strongly influenced by other cultural factors.