Andy White Anthropology
  • Home
  • Research Interests
    • Complexity Science
    • Prehistoric Social Networks
    • Eastern Woodlands Prehistory
    • Ancient Giants
  • Blog
  • Work in Progress
    • The Kirk Project >
      • Kirk 3D Models list
      • Kirk 3D Models embedded
      • Kirk 2D images >
        • Indiana
        • Kentucky
        • Michigan
        • Ontario
      • Kirk Project Datasets
    • Computational Modeling >
      • FN3D_V3
    • Radiocarbon Compilation
    • Fake Hercules Swords
    • Wild Carolina >
      • Plants >
        • Mosses
        • Ferns
        • Conifers
        • Flowering Plants >
          • Grasses
          • Trees
          • Other Flowering Plants
      • Animals >
        • Birds
        • Mammals
        • Crustaceans
        • Insects
        • Arachnids
        • Millipedes and Centipedes
        • Reptiles and Amphibians
      • Fungi
  • Annotated Publications
    • Journal Articles
    • Technical Reports
    • Doctoral Dissertation
  • Bibliography
  • Data

The Dependency Ratio in Human Evolution

5/15/2015

3 Comments

 
As far as I know, humans are unique among animals in having an extended period between weaning and being able to subsist on their own.  We call this “childhood.”  The long period of post-weaning dependence provides our large brains with a lot of time to mature.  It also requires a lot of parental investment (in terms of time, energy, calories, etc.) and means that we would have to wait a long time between offspring if each one had to independent before the mother could have another.   We don’t do that, tending to have a shorter interval between subsequent births (the inter-birth interval, or IBI) than other great apes.  The long period of childhood dependence and the short IBI mean that, as a species, humans tend to have multiple, dependent offspring of different ages at the same time.  Speaking as a parent of multiple, dependent offspring of different ages, I can tell you that this is often no walk in the park.  This peculiar human strategy has a lot of costs.

Understanding when, how, and why this distinctly human reproductive strategy developed is a great evolutionary question.  Reducing the IBI increases the potential fertility of human populations, but also creates new demands on the energies of parents and families.  Human families today often offset those extra energy demands by getting help (evolutionary anthropologists call it “cooperative breeding”).  A new paper in the Journal of Human Evolution titled “When Mothers Need Others” by Karen Kramer and Erik
Otárola-Castillo tries to further our understanding of where cooperative breeding comes from, using an “exploratory model” to try to understand the selective pressures associated with the evolution of human-like patterns of reproduction and child-rearing.  The goal of the paper “is to develop a model to predict those life history transitions where selective pressure would have been strongest for cooperative childrearing” (pg. 5). 

Kramer and
Otárola-Castillo call their model the “Force of Dependence Model.”  Their model is a simple one, calculating “the net cost of offspring as a function of dispersal age, birth intervals and juvenile dependence” as a 3-dimensional surface (Supplementary Online Material  from Kramer and Otárola-Castillo 2015).  The authors use several different combinations of settings to represent a range of conditions from “ancestral” (juvenile independence at age 10, IBI of 6 years, and a dispersal age of 14) to “most derived” (juvenile independence at age 20, IBI of 3 years, and a dispersal age of 20).  Their graphs show that “net costs” within a domestic group (a mother and her offspring) are lower when offspring are spaced further apart and become independent at a younger age.  When offspring hang around past the age of juvenile independence, there is a net benefit to the domestic group as their productive capacities can be used to offset the drain of their younger siblings. The authors find that the strain points – where selective pressures for assistance would be greatest – occur in domestic groups with the most derived set of characteristics: late juvenile independence and a low IBI (lots of children who remain dependent for a long time).   

As I understand it, the “net cost” in this model more-or-less mirrors the dependency ratio (the ratio of consumers to producers) of a domestic group or family, something anthropologists have been interested in understanding for a long time.  The higher the ratio of consumers to producers, the higher the dependency ratio, and the higher the “cost” to each producer supporting the family.  The dependency ratio changes through the lifespan of a family in a patterned way: every domestic group that has children goes through a “pinch” period when the dependency ratio is highest, and the pinch period logically corresponds to the time when there are a lot of dependent offspring.  As I wrote in my 2013 paper in the Journal of Anthropological Archaeology (“Subsistence Economics, Family Size, and the Emergence of Social Complexity in Hunter-Gatherer Systems in Eastern North America,” available here):

“the duration and amplitude of the ‘pinch’ is affected by the rapidity of the addition of offspring and how quickly those offspring turn from consumers into producers.  The rapidity of addition of offspring will depend on factors such as fertility, infant and childhood mortality, and the number of wives. The productive potential of children will be affected by the presence and distribution of resources that can be procured by children and the foraging strategies that are employed to exploit those resources” (White 2013:128).

The main part of that paper used an agent-based model (ABM) to try to understand how the distribution of family size changes when the age at which children become producers (the “age of juvenile independence” in Kramer and
Otárola-Castillo’s model) decreases and there is an incentive for polygynous marriage.  In addition to the ABM, I used a simple spreadsheet model to show how the dependency ratio changed through the course of the developmental cycle of an individual family in cases where the age at production was low (8 years old) and where it was high (14 years old).  In this simple model, I used an IBI of 3, a dispersal age of 16 for females and 20 for males, and a female reproductive period spanning ages 20-35 years (giving a total fertility of 6 offspring).

The figure below compares Kramer and
Otárola-Castillo’s graphs from their cases with early and late juvenile independence (holding IBI at 3 and dispersal age at 14) with my data on changes in dependency ratio through the developmental cycle in cases with a single reproducing female and an age at production of 8 (top) and 14 (bottom).   My model data are the same as in my 2013 paper (Figure 5), but I have re-graphed them to make comparison with Kramer and Otárola-Castillo’s figure easier.  I have redrawn the graphs from Kramer and Otárola’s Figure 1 (third graphs from the left, top and bottom rows).   The dotted lines on the graphs of my results indicate a dependency ratio of 1.75, which is what I have generally used in my modeling efforts as a “typical” dependency ratio among hunter-gatherers (following Binford 2001:230).
Picture

My results showed the same pattern as Kramer and Otárola-Castillo’s:  the peak of the “pinch” comes earlier and is less severe when children become producers at an earlier age.  Even though our models have some differences (and some of the values of the parameters were different), the correspondence in results is notable. Compare, for example, when the amplitude of the “pinch” (peak dependency ratio in my results, greatest net cost in Kramer and Otárola-Castillo’s results; marked by stars) is greatest and the differences in amplitude between the early and late ages of juvenile contributions to subsistence.

The correspondence between my results and Kramer and
Otárola-Castillo’s is unsurprising.  The idea that the dependency ratio of a domestic group changes through the course of its developmental cycle in a somewhat predictable way is not new (and the idea that the “pinch” comes when you have lots of little kids running around at the same time won’t come as a revelation to anyone who has multiple children).   This is a phenomenon that has been studied for decades (e.g., Chayanov 1966; Donham 1999; Fortes 1958; Goody 1958) and recognized as a key aspect of how hunter-gatherers organize themselves (Binford 2001:229).

So where does this kind of work put us in terms of understanding the evolution of human reproduction, society, and family life?  I think it primarily puts us in a spot where we’re asking some good questions.  Going back to the issue of the origins of monogamous pair-bonding (which I touched on briefly in this post about birth assistance and this post about australopithecine sexual dimorphism), having a two person (male-female) unit forming the core of a domestic group would have a mitigating effect on the strain caused by a decrease in IBI (i.e., you’d be adding another producer into the equation).  If the appearance of male-female pair-bonding was associated with a sexual division of labor (which is I think what most of us would hypothesize), males and females would presumably be focused on procuring somewhat different sets of subsistence resources.  Offspring could be largely “independent” with regards to some of those resources but not to others – think about the difference between collecting berries and running down large game.  A sexual division of labor and an environment where relatively young children could make some contribution to their own subsistence (even if that contribution does not include the full range of resources that are exploited) would go a long way toward easing the “pinch” that comes from having more children spaced closer together.

When does this happen in human evolution?  Of course that’s a tough thing to get at directly.  I think if you took a poll, the winner would probably be “around the time our genus emerges” or “with Homo erectus.”  An increase in total fertility (coincident with a lowering of the IBI) would help explain the population growth that must have been part of the dispersal of our species out of Africa prior to 1.8 million years ago.  It would also fit nicely with the evidence for an increased exploitation of animal resources around that same time.  Maybe Glynn Isaac was right all along to propose the emergence of human-like central place foraging with home bases and a sexual division of labor at the beginning of the Lower Paleolithic?

But what if monogamous pair-bonding and a sexual division of labor appeared much earlier – with australopithecines or even some pre-australopithecine like Ardipithecus?  If those things came along with bipedal locomotion, would a decreased IBI and increased fertility have followed automatically?  Maybe not.  Perhaps those earlier hominids just didn’t have the wherewithal to exploit their environments like later hominids did – perhaps the diversity of the resource base they could exploit wasn’t great enough to really leverage a sexual division of labor until animal products became readily attainable.  That may have required a suite of anatomical adaptations for daytime exhaustion hunting (loss of body hair, skin pigmentation, greater body size, stiffer foot) and cognitive/behavioral adaptations for making and using stone tools to process carcasses.  The date of the “earliest” proposed use of stone tools continues to be pushed  back (now it’s at 3.3. million years ago), but as far as I know the density of stone tools and butchered animal bones that appears at about 1.8 million years ago is unlike anything that precedes it.

More modeling work will be required to really understand how changes in the dependency ratio might have articulated with changes in reproductive, social, and technological behaviors deep in human prehistory.  In order to understand what changes in reproduction might have meant in terms of social interactions, however, we’ll need a different grade of model than that used by Kramer and
Otárola-Castillo.  Of course I’m going to say that complex systems modeling is the way to go on this:  it will let us get past the limitations of deterministic inputs and help us understand how constraints, costs, and interactions would have played out within a society.   In order for “others” to help with raising and provisioning multiple dependents, those others had to have existed within these small-scale hominid societies and (again, speaking as someone involved in raising multiple small kids) there wouldn't have been some inexhaustible Plio-Pleistocene babysitting pool of “others” out there just waiting to step in and provide extra calories for a few years.  A different kind of modeling effort with broader scope will let us get at the group- and society-level contexts in which family-level changes in child-bearing and child-rearing would have played out. Stay tuned.
References

Binford, Lewis R.  2001. Constructing Frames of Reference: An Analytical Method for Archaeological Theory Building Using Hunter-Gatherer and Environmental Data Sets.  University of California Press, Berkeley.

Chayanov, A. V.  1966.  A. V. Chayanov on the Theory of Peasant Economy.  University of Wisconsin Press, Madison.

Donham, Donald L. 1999.  History, power, ideology: Central issues in Marxism and anthropology.  University of California Press, Berkeley.

Fortes, Meyer.  1958.  Introduction.  In The Developmental Cycle in Domestic Groups, edited by Jack Goody, pp. 1-14.  Cambridge Papers in Social Anthropology.  Cambridge University Press, London.

Goody, Jack.  1958.  The Fission of Domestic Groups among the LoDagaba.  In The Developmental Cycle in Domestic Groups, edited by Jack Goody, pp. 53-91.  Cambridge Papers in Social Anthropology.  Cambridge University Press, London.
ResearchBlogging.org
Kramer, K., & Otárola-Castillo, E. (2015). When mothers need others: The impact of hominin life history evolution on cooperative breeding Journal of Human Evolution DOI: 10.1016/j.jhevol.2015.01.009
3 Comments
Helga Vierich
5/15/2015 09:35:00 am

The dependency ratio relationship to individual work effort was confirmed in studies I did while working among subsistence farmers (slash and burn villages and pastoralists) in West Africa. I was one of a team of scientists studying household food production constraints in the semi-arid tropics, employed by the CGIAR institute ICRISAT (International Crops research Institute for the Semi-Arid Tropics). Generally, the higher the dependancy ratio (of children under 12, older and frail relatives, and women in the last stages of pregnancy) the longer the hours worked by able-bodied adults in the household, the larger the area of cultivated land, or the herd of livestock. Younger men with young families tended not to become completely separated economically from their parental generation until they were over forty. The extended family, and indeed the lineage (patril-ineages in these cases) were risk management organizations, and thus they were organized for extra work and food production directed by senior lineage heads, so that surplus production was put into storage in case of crop failure. This tended to distort the figures calculated for households headed by senior males: making for a much steeper incremental increase in work intensity and food production per person in larger households headed by senior people. We also found that the institution of polygyny appeared linked to these kinds of household compositions, further intensifying the effect, by raising both the dependancy ratio and the work output per person.

Hunter-gatherers in the Kalahari, which I studied before joining ICRISAT, tended to show increased frequency of gathering by women and changes in household composition through addition of adult siblings and older relatives, to balance out the work load, as the dependancy ratio increased. Among hunter-gatherers, camping group dependancy ratios tended to be remarkably constant for this reason, suggesting an evolutionarily significant functionality for the frequently observed flexibility and fluidity of camping group composition. It is also worth noting that this flexibility was greatly augmented by long term network extension through marriage between men and women, especially when the couple were only distantly related prior to marriage. It also pointed to the network extension and flexibility of camping group composition resulting from the addition of long term friendship and sharing networks. The sharing relationships, especially when they cross cut kinship networks, added to the overall flexibility, not only of camping party membership, but also to the range of localities where these parties could go.

This, to me, indicates that lineage systems developed in part to make up for the relatively more precarious position of people whose food supply was based on more fixed locations or access to owned herds of livestock. The relatively higher dependency ratios, attendant upon shortened birth spacing seen in the farming and pastoral groups in the Sahel, entailed more physical drudgery and risk.

Reply
Andy White
5/17/2015 11:42:58 pm

Hi Helga,

Thanks for the comment. I would be interested in having a look at the data you're talking about at some point. The connections between family size/structure and social networks is really interesting to me, and it would be great to have some ethnographic data (at whatever scale) that could be compared to modeling results. I'll be working more on these issues in the coming year.

Reply
Helga Vierich
5/18/2015 02:36:28 am

Well, we must keep in touch then! I have data on family size, residential groups size, larger networks (kin and friends) but it is all in hand written notebooks, although some of the West African data was coded and put on disk while I was at ICRISAT and the dependancy ratio data was published as in-house reports. I am going over the notebooks from the hunter-gatherer fieldwork again now, to tabulate and map out the networks. I did publish some of this in a joint paper with Susan Kent some years ago.

What I find most interesting is that the horticulturalists have ways of augmenting household labour supply when dependency ratios rise (additional wives) which tends to backfire, since it also adds more dependent children over the next decades, while the hunter-gatherers tend to just add another family to the camping party... so the flexible and fluid composition of the whole camping party (band) was the mechanism that kept dependency ratios low enough (to keep work load relatively low), and therefore it was not the dependency ratio within any one family (hearth sharing unit).

Reply



Leave a Reply.


    All views expressed in my blog posts are my own. The views of those that comment are their own. That's how it works.

    I reserve the right to take down comments that I deem to be defamatory or harassing. 

    Andy White

    Follow me on Twitter: @Andrew_A_White

    Email me: andy.white.zpm@gmail.com

    Enter your email address:

    Delivered by FeedBurner


    Picture

    Sick of the woo?  Want to help keep honest and open dialogue about pseudo-archaeology on the internet? Please consider contributing to Woo War Two.
    Picture

    Follow updates on posts related to giants on the Modern Mythology of Giants page on Facebook.

    Archives

    January 2023
    January 2022
    November 2021
    September 2021
    August 2021
    March 2021
    June 2020
    April 2020
    March 2020
    January 2020
    December 2019
    November 2019
    October 2019
    September 2019
    May 2019
    April 2019
    January 2019
    December 2018
    November 2018
    October 2018
    September 2018
    August 2018
    July 2018
    June 2018
    May 2018
    April 2018
    March 2018
    February 2018
    January 2018
    December 2017
    November 2017
    October 2017
    September 2017
    August 2017
    July 2017
    June 2017
    May 2017
    April 2017
    March 2017
    February 2017
    January 2017
    December 2016
    November 2016
    October 2016
    September 2016
    August 2016
    July 2016
    June 2016
    May 2016
    April 2016
    March 2016
    February 2016
    January 2016
    December 2015
    November 2015
    October 2015
    September 2015
    August 2015
    July 2015
    June 2015
    May 2015
    April 2015
    March 2015
    February 2015
    January 2015
    December 2014
    November 2014
    September 2014
    August 2014
    June 2014
    May 2014
    April 2014
    March 2014

    Categories

    All
    3D Models
    AAA
    Adena
    Afrocentrism
    Agent Based Modeling
    Agent-based Modeling
    Aircraft
    Alabama
    Aliens
    Ancient Artifact Preservation Society
    Androgynous Fish Gods
    ANTH 227
    ANTH 291
    ANTH 322
    Anthropology History
    Anunnaki
    Appalachia
    Archaeology
    Ardipithecus
    Art
    Atlantis
    Australia
    Australopithecines
    Aviation History
    Bigfoot
    Birds
    Boas
    Book Of Mormon
    Broad River Archaeological Field School
    Bronze Age
    Caribou
    Carolina Bays
    Ceramics
    China
    Clovis
    Complexity
    Copper Culture
    Cotton Mather
    COVID-19
    Creationism
    Croatia
    Crow
    Demography
    Denisovans
    Diffusionism
    DINAA
    Dinosaurs
    Dirt Dance Floor
    Double Rows Of Teeth
    Dragonflies
    Early Archaic
    Early Woodland
    Earthworks
    Eastern Woodlands
    Eastern Woodlands Household Archaeology Data Project
    Education
    Egypt
    Europe
    Evolution
    Ewhadp
    Fake Hercules Swords
    Fetal Head Molding
    Field School
    Film
    Florida
    Forbidden Archaeology
    Forbidden History
    Four Field Anthropology
    Four-field Anthropology
    France
    Genetics
    Genus Homo
    Geology
    Geometry
    Geophysics
    Georgia
    Giants
    Giants Of Olden Times
    Gigantism
    Gigantopithecus
    Graham Hancock
    Grand Valley State
    Great Lakes
    Hollow Earth
    Homo Erectus
    Hunter Gatherers
    Hunter-gatherers
    Illinois
    India
    Indiana
    Indonesia
    Iowa
    Iraq
    Israel
    Jim Vieira
    Jobs
    Kensington Rune Stone
    Kentucky
    Kirk Project
    Late Archaic
    Lemuria
    Lithic Raw Materials
    Lithics
    Lizard Man
    Lomekwi
    Lost Continents
    Mack
    Mammoths
    Mastodons
    Maya
    Megafauna
    Megaliths
    Mesolithic
    Michigan
    Middle Archaic
    Middle Pleistocene
    Middle Woodland
    Midwest
    Minnesota
    Mississippi
    Mississippian
    Missouri
    Modeling
    Morphometric
    Mound Builder Myth
    Mu
    Music
    Nazis
    Neandertals
    Near East
    Nephilim
    Nevada
    New Mexico
    Newspapers
    New York
    North Carolina
    Oahspe
    Oak Island
    Obstetrics
    Ohio
    Ohio Valley
    Oldowan
    Olmec
    Open Data
    Paleoindian
    Paleolithic
    Pilumgate
    Pleistocene
    Pliocene
    Pre Clovis
    Pre-Clovis
    Prehistoric Families
    Pseudo Science
    Pseudo-science
    Radiocarbon
    Reality Check
    Rome
    Russia
    SAA
    Sardinia
    SCIAA
    Science
    Scientific Racism
    Sculpture
    SEAC
    Search For The Lost Giants
    Sexual Dimorphism
    Sitchin
    Social Complexity
    Social Networks
    Solutrean Hypothesis
    South Africa
    South America
    South Carolina
    Southeast
    Stone Holes
    Subsistence
    Swordgate
    Teaching
    Technology
    Teeth
    Television
    Tennessee
    Texas
    Topper
    Travel
    Travel Diaries
    Vaccines
    Washington
    Whatzit
    White Supremacists
    Wisconsin
    Woo War Two
    World War I
    World War II
    Writing
    Younger Dryas

    RSS Feed

    Picture
Proudly powered by Weebly