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Energy is the currency of life. As living beings, humans depend on constant infusions of energy to beat back the inexorable, disordering pull of entropy. Even when we are at rest, cell membrane proteins churn away, using ATP-sized chunks of energy to, among many other chores, maintain the electrochemical disequilibrium that keeps our nerves and muscles ready to sense and respond to our inner and outer environments.
The energy in our bodies can morph, as needed, between chemical, electrical, mechanical, and thermal forms; how we deploy these forms to do biological work is determined by finely evolved nutrient sensing pathways. These regulatory systems determine, for instance, when small aliquots of readily available energy should be used to move a tiny organelle across a cell, or when to make breast milk from mobilized glutofemoral fat, or when to initiate the energetically expensive cellular mechanisms that repair damage to our soma.
In addition to these internal processes, humans are constantly making behavioral decisions that channel energy to change the environment around us—including transferring some of our precious energy to affect the health and well-being of other individuals (a grandmother carrying an extra load for a pregnant mother, or a daughter caring for an aging parent).
Given their unique capacity to conceive and gestate new humans, females—across most of time and space—have shouldered the energetically costly lift of ensuring our species’ survival, despite also being the people who must maintain effective mobility while carrying the physical loads (children, food, fuel, water) that keep their families and communities functioning. Pregnant moms must fulfill the demands of a growing fetus, which include delivering a constant supply of nutrients and energy-rich molecules through the placenta, altering their maternal immune responses to avoid conflicts with their genetically distinct offspring, and having oxygen stripped from their red blood cells by covetous fetal hemoglobin. Mothers frequently breastfeed their babies for months after they are born, carry them on their bodies a large percentage of the time, and spend countless hours and calories caring for the needs of their developing human. All this, frequently, happens in the context of a multiplicity of ways women contribute time and energy to facilitate the lives of others.
Spending time and energy
Providing a nourishing environment for a developing fetus is an incredibly costly undertaking for a human mother. During this process, a single-celled zygote is serially replicating and differentiating into an entirely new set of metabolically active tissues (eventually trillions of cells!), all needing energy and nutrients—and adding to the work for mom of breathing, pumping blood, and clearing the body of wastes. Over the approximately 40 weeks of a pregnancy, mothers contribute about 85,000 kcal of energy to their growing baby, or an average of 300 kcal per day—some 15 percent of the recommended daily intake for adult women. Breastfeeding is even more expensive, with lactating moms transferring 650 kcal of energy per day to their babies. And none of these estimates include the extra energy used in transporting reproductive loads, such as a developing fetus or a frontally-slinged, unweaned infant, from place to place.
Of course, it’s not just the women who bear children who are generous with their energy transfers. As an adoptive mom, I didn’t have to use any of my energy stores to support a pregnancy; those energetic costs were paid by my daughter’s birth mom. That said, from the time our daughter joined our family at five months of age, to her (also taxing) adulting phase, my partner and I have committed a small solar farm’s worth of energy and time to the care, feeding, and carrying of this amazing person. Adoption can be thought of as an extreme form of “allomothering,” a term originated by Sarah Hrdy to capture the concept that people other than a birth mother can use their time and energy to help provision, or tote around, or physically care for, a child. This load sharing can take many forms, as women are often both the metaphorical and literal burden carriers for their families and communities.
In hunter-gatherer communities, an aunt might enhance a child’s early nutritional bounty by premasticating their food. Or a young child’s older sister might take a turn carrying her toddler brother while her mom joins a hunting trip. From rich ethnographies sensitive to the contributions of women, we know that Venezuelan Pumé women past their reproductive years frequently carry more than their share of the load back to camp on foraging trips—sometimes more than their own body weight in mangos—to ease the locomotor burden of the younger, reproductive-age women in the group.
In more industrialized contexts, women also do much work in aid of others. According to the International Labour Organization, women flood the zone in caring occupations, such as personal and healthcare workers, nonprofit professionals, and teachers (unsurprisingly, these jobs are consistently paid less than those dominated by men). And the situation is even more extreme outside of the realm of paid work. In her landmark book If Women Counted, economist Marilyn Waring laid bare the obliviousness of economic accounting systems to the enormous share of uncompensated energy women transfer to others in the form of housework and caring for children, elders, and sick friends and family. Women are also more likely than men to volunteer in their communities or help others in need.
Other-directed energy can have significant consequences for women. During pregnancy and breastfeeding, women experience potentially age-accelerating epigenetic changes, due to both the large energy and nutrient transfers to their fetus and infant, and the physiological tweaks (insulin resistance, lipid dysregulation) that favor their fetus at the expense of their own soma. If not reversed after weaning, these modifications can lead to dysfunction of the inflammatory and cellular senescence systems. How effectively a mother’s body returns to healthy functioning after a reproductive bout depends on the length of the inter-birth interval, and on the general health of the mother before and during the pregnancy; if the interval is too short, the mother’s regulatory and repair systems suffer, her overall health degrades, and preexisting issues can be exacerbated with subsequent pregnancies. Two factors important to the health of a gestating mother are how much energy she must expend on locomotion—often carrying other loads as well as the additional mass of the fetus—and whether she can regulate her body temperature within reasonable bounds.
Recouping time, energy, and quality of life
Women have a strong tendency to thrive when working cooperatively with others, frequently with other women seeking similar outcomes. Anthropologists have offered compelling arguments to explain how this tendency might have evolved as part of a cooperative reproduction pattern that likely set our particular hominin lineage a “baby step” ahead of contemporaneously evolving populations. Exploring these arguments involves understanding some of the physiological and energetic effects of reproductive load carrying in females of different ages, questions my collaborators and I have systematically investigated.
Studying the energetic consequences of moving while belly-loaded, we found that women across a large age range (18 to 64 years) could effectively carry reproductive loads up to 16 percent of their body mass, although their optimal walking speed (at which cost per distance was minimal) slowed down. In fact, this load-induced downshift, which was paralleled by the self-selected speeds our participants preferred, was such that their rate of energy use remained comparable to when they walked unloaded. We suspect this response allows women to simultaneously conserve overall energy use (cost per distance) while also ensuring that core body temperature remains within safe limits. In a later experiment, where young women carried different-sized loads in pack baskets, we found that self-selected walking speed went down in proportion to the extra load carried and in inverse proportion to the associated increase in body temperature. Thus, to modulate thermoregulatory changes that might damage their fertility and overall health, the speed of walking in females is likely tuned to the burdens they habitually carry and their reproductive role.
Our findings provide physiological proof of concept for the grandmother hypothesis of Kristin Hawkes and colleagues based on Hadza field data showing that older women had a critical role in reducing the time and energy loads for actively reproducing women. This intergenerational transfer of energy can be thought of as “distributed resilience” within the female network of a family or community group. Being able to behaviorally mitigate surges of energy use helps the females in a group remain in an optimal zone of physiological stress—on both sides of the exchange. Biology of aging researchers find that longevity mechanisms tend to be enhanced when our systems intermittently experience short periods of physiological stress, as long as it not too much stress, or for too long (a concept called hormesis). This means women in a group who strategically share their physical and psychological loads—both those buffered from extreme stress by the support and those who are pushing into their hormetic stress zone by offering the help—are healthier in the long run. An even longer payoff is seen when aging women are cared for by the younger women in their networks, often their daughters, bringing the intergenerational transfer of energy full circle.
Given that women outlive men in most societies, it is not surprising they have developed both culturally mediated and evolved solutions that allow them to persist individually despite sharing so much of their energy with others. That leaves open the question of quality of life for older women. Of course there are the intrinsic rewards of living a meaningful life in connection with others, but we know that later life vitality is often rooted in being independently mobile. How is a woman’s mobility impacted by a legacy of load-carrying?
We know that self-selected walking speed is considered a sixth vital sign by healthcare professionals assessing how well older people are functioning. We also know that, on average, women’s walking speed starts to slow down around age 50, declining to speeds associated with an increased risk of falling by age 70. But the walking speeds older women tend to use frequently do not represent the fastest speed of which they are capable, or even their energetically optimal speed. Instead, their brain’s central pattern generator in charge of regulating walking movements has likely settled into the slower oscillations that are optimal for walking while carrying the encumbrances of daily life.
My mother, a former high school drum majorette who bore six children within eight years, was a visibly slow walker at age 77; one of us would regularly hang back to scoot her along when out and about. Yet, at a small July 4 parade on a 90-degree day, she astounded us by filling in at the last minute to high step the tiny marching band down the six blocks of the parade route, baton in hand, driven by the insistent beat of the snare drum. Clearly, when her nervous system was provided with precisely timed inputs, my mother was capable of moving much faster and more effectively than usual. This begs the question, Are there ways to help older women ramp up their self-selected walking pace?
In fact, there are. For example, my research group tested whether a novel mobility trainer—a four-wheel, spring-loaded, upright pivoting frame that was designed to cue each step through timely proprioceptive feedback—could improve walking speed in women 55 and older. After only nine 30-minute training sessions over three weeks, we found that the women who walked using the mobility trainer had increased their stride length and walking speed, both compared to their baseline values and against a comparable group who walked the same amount without any device.
Inspired by such turnarounds in mobility, researchers are hard at work designing practices that help women relearn how to walk at a “younger” pace, with all the associated improvements in quality and quantity of life. By adopting these practices, women who have the time and opportunity might be able to upregulate their usual walking speed. And because women often pursue their exercise goals as part of a group of like-minded women, we see how the individual costs that women have paid as part of being the burden carriers for their family can be counteracted—at least to some extent—by the distributed resilience of their larger female network working to mutually reinforce better health and longevity outcomes.
At the same time, we must acknowledge that the long-term energetic equipoise women can experience operates most effectively within functional networks of support. Systemic racism and poverty lead to inequalities in care and underlying health that speed up aging processes, for both women and men. Supporting women and the families and communities they nurture is a whole-of-society imperative. For example, through interventions that connect custodial grandmothers with community resources, we see the power of this approach in improved family resiliency and caregiver health and wellness. It is within our power to create social support systems that help overburdened women have a little more time for recovery and connection, and a little less stress, so their cells can spend more time at the energetic sweet spot that maintains long-term integrity.
Women are the nexus of every society, stemming in part from their reproductive role, but also their magnificent ability to strategically distribute—and then reclaim—energy throughout their lifetime. Women most effectively navigate the energetic fluxes woven throughout their lives when they are supported by others: partners, friends, family, and communities. Of course, women having the capacity and opportunity to manage how they spend their individual energy resonates manyfold in the form of empowered offspring, healthy friends, and flourishing communities. In many ways, the legacy of women’s exquisite talent to cooperatively share energy is all around us.