Thursday, May 10th 2012
Trade Time and Energy So You Can Live Slow, Reproduce Fast
Who makes your food? Do you live alone and do everything yourself, or are you part of a partnership, roommate situation, or extended family where food is shared? Most likely, the more complicated your living situation, the more complicated the food allocation. Perhaps one person buys the food and another cooks it, or everyone shares in the acquiring and making but you don’t all like the same stuff, so sometimes you have to make an extra peanut butter and jelly sandwich for the kid who hates curry night.
Humans share both food and labor. To some extent, we allocate food based on status (“The Big Piece of Chicken” from Chris Rock’s Bigger and Blacker – warning, profanity (and a splash of sexism) ahead!):
[youtube http://www.youtube.com/watch?v=2Cn8sczZTZ4&w=420&h=315]
Maybe you’re the parent who always gets the big piece of chicken, or maybe you’re the roommate who always seems to have his leftovers stolen. The question is, why? What implications does this have for your own stress and energy stores, and ultimately your reproductive success?
The concept of pooled energy budgets
A series of papers over the last few years by Karen Kramer, Meredith Reiches, Peter Ellison and others raises exactly this question. In particular, they take on the paradox of slow human life histories – the fact that we take a long time to grow up, mature, be fully independent and have our own offspring – with fast reproduction – once we can have kids we are capable of having many overlapping, dependent offspring (Kramer and Ellison, 2010).
Consider that the age at first birth among many hunter-gatherer populations is in the late teens (Kramer et al., 2009), yet individuals in these populations often aren’t fully competent hunters and foragers until their thirties (Gurven et al., 2006; Kramer et al., 2009; Walker et al., 2002). Among industrialized populations, our average age at first birth is in the mid to late twenties (and here are CDC stats for the US only), but our average full financial independence equally late (that’s not to say there aren’t many teens and young adults fully independent from parents, but also many folks into their twenties and thirties are still getting loans, help with down payments, or gifts that help financially).
Kramer et al 2009 Figure 4 showing age at menarche and first birth among South American forager populations.
Gurven et al 2006 Fig 3b Age-dependency and hunting skill.
This gap would not do among non-human primates: most primates become independent foragers very early in their lives, and so what they collect, they eat. If they can’t get enough food to survive, they certainly won’t be allocating any of that energy towards reproduction, and so their reproductive success will be low or zero.
How do we handle the years between age at first birth and full independence? How do we take so long to mature, using up parental resources all the while, yet still manage to have a decent number of babies? Humans manage to circumvent our very slow and dependent juvenile period, and therefore have plenty of energy for reproduction, through transfers of energy (food, or in industrial societies money for food) and labor (especially childcare) between individuals.
When individuals hunt or forage, many are able to bring in more than they themselves will eat, and this gets shared among their family or community. Individuals in different reproductive states may even contribute more to their households to increase their indirect fitness, as has been found with grandmothers in Hadza foragers (Hawkes et al., 1997), and in suitors (Hawkes et al., 1997) or fathers (Marlowe, 2001), perhaps to offset potentially lower foraging capabilities of a pregnant or breastfeeding mother. And many increasingly agree that humans are a cooperatively breeding species (Hrdy, 2008), meaning that labor transfers of childcare can happen pretty often.
Hawkes et al 1997 Table 1. Time allocated to work among different age groups in Hadza men and women.
It’s great that we share, and that foraging and childcare behavior is responsive to one’s context and in the context of one’s family. This means that one’s social environment may play a significant role in the amount of energy and time you have for survival and reproduction (Reiches et al., 2009).
Why pooled energy budgets are cool
Understanding where our energy comes from – who has it easy and who has it hard in the daily labor of food acquisition – can help us understand a number of interesting things that vary among human populations. For instance, lighter juvenile workloads can contribute to faster juvenile growth. I’ve talked about the Pumé foragers before as an example of a population with relatively early reproduction. Kramer et al (2009) have shown that Pumé girls reach menarche and first birth earlier than most foragers (check out the first image in this post). These girls tend to have lower foraging expectations placed on them, which indirectly increases their energy budgets, as what would have gone towards foraging effort can now go towards maintenance and reproduction. This contributes to how they achieve faster growth and earlier reproductive maturity.
Understanding the nature of energy transfers can also help us understand cooperative breeding. In populations where pregnant and breastfeeding mothers’ foraging workloads are still high, are they getting respite from childcare to offset these costs? Who helps more: peers, fathers, maternal grandmothers or paternal grandmothers? What factors drive variation in allocare between populations? I wonder how varied these behaviors are even within a population: perhaps some mothers prefer foraging to childcare, or vice versa, and so find themselves doing more of one or the other. And I wonder if any of this could have an ultimate effect on reproductive success, depending on the environment and difficulty foraging.
But what I find really interesting is that some individuals may get more or less food through these energy transfers based on status, kinship, and friendship. That is, who you know, who your mother or father is, the kind of culture you come from and whether it treats some kinds of people better or worse, this may all have an ultimate impact on an individual’s energy budget. This is on top of the already documented impact of social environment on stress and health (e.g., Albert et al., 2008; Kiecolt-Glaser et al., 2010; Miller et al., 2002). So there are at least two very significant ways in which our social environment affects wellness and reproductive success: an energy-based mechanism and a stress-based mechanism, and they are not mutually exclusive paths.
Starting from the pooled energy budgets model resolves a lot of the bickering that can occur about which ecological factor is “most” important to reproduction. The reality is, no matter how much I and others love to talk about inflammation, stress, immune function and other stuff, all of these factors ultimately impact the body through allocation of resources – through energy. The pooled energy budget concept gives a space for all these factors, and a useful framework to understand them.
References
Albert MA, Ravenell J, Glynn RJ, Khera A, Halevy N, de Lemos JA. 2008. Cardiovascular risk indicators and perceived race/ethnic discrimination in the Dallas Heart Study. American heart journal 156(6):1103-1109.
Gurven M, Kaplan H, Gutierrez M. 2006. How long does it take to become a proficient hunter? Implications for the evolution of extended development and long life span. Journal of Human Evolution 51(5):454-470.
Hawkes K, O’Connell JF, Blurton Jones NG. 1997. Hadza women’s time allocation, offspring provisioning, and the evolution of long postmenopausal life spans. Current Anthropology 38(4):551-577.
Hrdy S. 2008. Evolutionary context of human development: the cooperative breeding model. In: Salmon C, Shackelford T, editors. Family relationships: an evolutionary perspective: Oxford University Press.
Kiecolt-Glaser JK, Gouin J-P, Hantsoo L. 2010. Close relationships, inflammation, and health. Neuroscience & Biobehavioral Reviews 35(1):33-38.
Kramer KL, Ellison PT. 2010. Pooled Energy Budgets: Resituating Human Energy Allocation Trade-offs. Evolutionary Anthropology 19:136-147.
Kramer KL, Greaves RD, Ellison PT. 2009. Early reproductive maturity among Pumé foragers: implications of a pooled energy model to fast life histories. American Journal of Human Biology 21(4):430-437.
Marlowe F. 2001. Male contribution to diet and female reproductive success among foragers. Current Anthropology 42(5):755-760.
Miller GE, Stetler CA, Carney RM, Freedland KE, Banks WA. 2002. Clinical depression and inflammatory risk markers for coronary heart disease. The American Journal of Cardiology 90(12):1279-1283.
Reiches MW, Ellison PT, Lipson SF, Sharrock KC, Gardiner E, Duncan LG. 2009. Pooled energy budget and human life history. American Journal of Human Biology 21(4):421-429.
Walker R, Hill K, Kaplan H, McMillan G. 2002. Age-dependency in hunting ability among the Ache of Eastern Paraguay. Journal of Human Evolution 42(6):639-657.
