LIFE HISTORY & REPRODUCTIVE STRATEGIES

While logic as well as observation tell us that populations generally achieve a balance btwn. births & deaths, and thus btwn. resource supply and demand, they don't tell us how this is achieved

Malthus suggested that, because the "flesh is weak," balance is achieved more often via mortality increase than fertility reduction

Others (esp. anthropologists) have argued that cultural regulation of population through fertility control is dominant means (a view well-represented in Bodley reading)

I want to focus on a third view, using theory of reproductive strategies developed in evolutionary ecology (see also reading by Bobbi Low)

Adaptation and Population Regulation

As we have seen, a key issue for ecological adaptation is level at which adaptive design is likely to occur

One argument holds that since exceeding carrying capacity threatens the persistence of the population (or even ecosystem), reproductive restraint evolves to limit threat of overpopulation

After all, if over-reproduction leads to resource depletion, then there will be fewer resources in future for one's descendants; hence it follows that less reproduction would actually increase number of surviving descendants, and provide an evolutionary advantage to fertility limitation

This view, which was popular among biologists until mid-1960's [Wynne-Edwards 1965], and remains popular with some social scientists today, sounds intuitively plausible

But it suffers from two serious weaknesses:

1) it conflates what's adaptive for the collectivity (the population) with what's adaptive for each member of the collectivity (individual, either as parents or as offspring)

2) it focuses on the very long-term outcome, which is not necessarily the way either natural selection or decision-making works

The key issue with 1) is that the adaptive benefits of increasing reproductive success (number of surviving offspring) go to those who do so, while costs (overpopulation, resource depletion) are divided among all N members of population -- a classic "public goods" problem (see "Concepts of Adaptation" lecture notes, section on "Levels of selection," for further discussion)

With regard to 2) some argue that a reproductively prolific strategy only "pays" (has a selective advantage) in short term, and as prolific strategy spreads, it will deplete resource base to point that everyone will be worse off (in terms of per capita resources or reproductive success) than before

True enough, but natural selection is rather myopic, and can't favor strategy that has lower fitness now just because it will have higher fitness in some very distant future generation (though it can certainly shape adaptations that pay after 1 or a few generations)

Some also argue that populations that overexploit resources will go extinct, and thus group selection would favor reproductive restraint (Wynne-Edwards); this is logically correct, but theory and data show that group selection generally much weaker or slower than individual selection (see Irons reading), so the latter will "win the race" (even, at the outer limit, a race to extinction)

Empirically, lots of evidence in nature of wasteful overproduction of young (go count the number of seeds produced by your favorite tree that rot on the ground each year); we know that in long run, a finite planet requires approximately zero population growth, yet under favorable conditions all species are capable of extravagant reproduction

In sum, little reason to believe that natural selection designs reproductive strategies to restrain reproduction in order to ensure survival of ecosystems or their constituent populations; whether cultural evolution or some other processes (rational forethought?) can overcome these limitations remains a more open question

Optimal Reproductive Rate

Although group selection against overpopulation is unlikely to prevail, it is still possible for natural selection to favor reproductive restraint; to understand how, we need to focus on fitness costs and benefits at the individual level

Since natural selection favors variants with higher reproductive success (RS), one might ask how it could ever favor fertility regulation

One possible reason is diminishing returns (in fitness) per unit of reproductive output

This can be due either to effects on parents (increased mortality), or to reduced survival or competitive condition of offspring

Let's consider just the second (offspring) effect, using a simple graphical model drawn from David Lack's ground-breaking work on clutch size in birds (Lack 1968)

Lack was a British ecologist who wanted to explain why birds of various species laid smaller clutches than physiologically maximum (as shown by egg-removal experiments)

Lack reasoned that given a finite amount of resources (food, time, etc.) a parent can invest in young, at some point the survival probability of young must begin to decline as total investment is divided among more and more dependents

In this simple model, there is an intermediate number of offspring that yields maximum RS to parent; this is more than is optimal for any one offspring, but less than the physiological maximum [see diagram of Lack Clutch-Size Model]

It basically comes down to a trade-off between quantity and quality: more offspring & smaller investment in each one, vs. fewer offspring & larger investment in each

The optimal point on this tradeoff will vary between species as a function of basic reproductive biology and behavior of each species, while the optimum within a species (e.g., diff. human populations), or even among individual parents, will vary as a function of local ecological and social constraints

!Kung Birth Spacing

These points are well illustrated in the case of !Kung San reproductive strategy

!Kung San (also known as Jhu//twasi) = hunter-gatherers in Kalahari desert of southern Africa (Botswana) studied by Richard Lee (1979), Nancy Howell (1979), and others

They have been characterized as an "affluent society" w/ plenty of food, leisure time, and healthy lives that last into old age (though all of these generalizations have been questioned)

One of the remarkable things about !Kung is that they have very wide inter-birth intervals (IBI) for a "natural fertility" (non-contracepting) population

Lee & Howell calculate that mean IBI is ca. 4 yrs for !Kung women living traditional foraging lifestyle (vs. 2 yrs among those settled at Bantu cattle posts)

One explanation = homeostasis (control population so don't exceed K or Cc); but this unlikely:

1. millions of mongongo nuts (staple food) rot on ground every year

2. harvest less than 1% of large herbivore biomass annually in their area, vs. 10-20% sustainable yield [Hill 1993:196ff]

Lee's explanation (Lee 1979, 1980): !Kung mothers carry their kids until they are 4 yrs old, and walk ~1500 km/yr, so carrying > 1 child too much work (as !Kung say, "Woman who gives birth to too many children has a permanent backache!")

Plausible, but why not 6 yr intervals? 8 yrs? Or (to really minimize workload) have no kids at all?

Since observed IBI leads to a population growth rate just barely above replacement rate, longer IBIs would apparently lead to demise of !Kung; but Lee's "backache" explanation doesn't predict any particular IBI, or why it would be at or above replacement level

Furthermore, !Kung women readily express desire for more children, and did not practice any contraception during study period

Nicholas Blurton Jones (1986, 1987) took a different tack: he formulated the birth-spacing problem using an optimization model, assuming goal = maximizing yield of surviving offspring (i.e., RS)

This is not meant to be conscious goal (though worth noting that is closer to expressed preferences of !Kung than is homeostasis or load-minimization); rather, envisioned as expected outcome of design by natural selection, regardless of what "proximate mechanisms" bring it about

Blurton Jones used Howell's data on individual life histories (IBIs, child deaths) to conduct direct test of this optimization model

Result = detailed confirmations of model predictions, of which simplest to explain = match btwn calculated optimal IBI (48 months) and modal IBI (most empirically common interval) [see !Kung IBI Graph]

This match is result of correlation btwn IBI and child mortality: if births are more closely spaced, offspring mortality rates increase so much that fewer children actually survive, just as Lack model assumes

Some additional hypotheses supported in Blurton Jones' study include:

1) child mortality not correlated with IBI for cattle-post women (much lower mobility costs)

2) death of unweaned infants leads to shortened IBI, but death of older child doesn't (because no effect on lactation, little effect on carrying cost to mother)

Conclude from these results that long IBIs are not evidence of population regulation (in fact, !Kung population growing slightly), but rather of what we might call "adaptive family planning"

Paradoxically, by spacing births widely, !Kung women are maximizing number of children they rear; higher birth rates, by disproportionately increasing mortality, would actually lead to decreased population

!Kung birth-spacing has multiple determinants; besides proximate physiological and behavioral ones (carrying effort, lactational amenorrhea), therer are also social and ecological ones:

1. Gender division of labor (females responsible for gathering & child care; no real cooperative child-tending or communal nursing)

2. Kid's reliance on parents for food and supervision (possibly related to ease of getting lost in bush) [Blurton Jones et al. 1994a, 1994b]

Thus, social and environmental constraints of !Kung combine to favor very low fertility in comparison to many other human populations, even other African hunter-gatherers like the Hadza

Demographic Transition

!Kung case suggests that variation btwn human populations in reproductive effort may be finely tuned to ecological constraints, and that cases of low reproductive effort need not be interpreted as cases of reproductive restraint, or even conscious decision-making or "cultural regulation" at all

However, would be foolish to generalize from a single case; are there any cases where we have good evidence of reproductive sacrifice that actually reduces parental RS?

One possible example is the so-called "demographic transition" (DT) = widely observed reduction in family size with socioeconomic modernization

Cause of DT is hotly debated by demographers, but most agree that it is associated with 2 things: increases in formal education (especially among women), & increases in wealth (or at least acquisition of skills which enhance competitive ability in labor market)

Thus, individuals higher on socioeconomic ladder are the ones voluntarily reducing reproductive output -- opposite of what might expect by simple Darwinian logic of converting additional resources into offspring (and opposite of what is found in most pre-industrial societies, where better-off families do have more kids)

There are 2 contrasting explanations for this I want to consider here:

First one is economic argument: increased wage rate leads to increased opportunity cost of having kids (that is, parents who earn more have more to lose by staying home to raise kids), while switch to wage-labor reduces economic advantage of kids (vs. family farm, where even fairly young children can contribute to household production)

While I think this type of argument plays some role in explaining DT, it is not enough:

1) Higher earning rate should allow hiring of caretakers (cf. wet nurses in European history, daycare & nannies today)

2) Kids (at least teenagers) could earn wages and make economic contributions to non-farm household (they certainly do in poorer households here and in the Third World)

3) If parents wanted to maximize wealth, they should have zero children in many (and not just modern industrial) economic systems; but even most yuppies want 1 or 2

Hence let's consider an alternative explanation: DT is result of changes in culturally-transmitted beliefs about relative importance of self (income, career) vs. family (marriage, children)

This explanation views DT as instance of cultural evolution rather than simply individual decision-making

Evolutionary question = Why are cultural norms & preferences that lead to reduced reproduction spreading among those with resources & education?

Note that those individuals who achieve prominence in industrial society generally do so by directing their energies toward careers (as opposed to their families, for example)

This is the "yuppie fitness-depression syndrome" -- overachievers sacrifice having kids for achieving high rank in various institutions: universities, corporations, entertainment, sports, etc.

This trade-off btwn success and reproduction is especially true of women, but increasingly of men as well (given crumbling of patriarchal division of labor)

Those who do make such fitness sacrifice have better chance of attaining positions of prominence (getting tenure, becoming an executive, getting an MD, etc.)

In turn, winners in career competition also have better chance of becoming influential, serving as role models (to grad students, lower-level employees, or via mass media)

This means they have disproportionate success in transmitting the values that made them successful to others, and these values can "swamp" competing values that might be spread primarily via parental transmission

Thus, behavior that reduces reproduction in order to advance career can increase in population through natural selection on cultural (not genetic!) variation

This example, if correct, shows how cultural evolution can diverge from biological evolution, and actually result in spread of traits that reduce the (genetic) fitness of their bearers (as discussed in previous lecture on cultural evolution)

This much more likely to happen if (as in this case):

1. cultural variants involved are not transmitted from parents
2. cultural influence competes directly with reproduction

(For further discussion, see Boyd & Richerson 2005:169ff)

Ecology and Mating Systems

Ethnographic evidence reveals an enormous variety of human mating & family systems cross-culturally, which raises an obvious question: Why have people developed so many different ways to regulate sexuality & raise kids?

One prominent hypothesis is that different systems are adapted to different ecological or economic conditions

Another is that certain cultural values or religious rules that strongly shape mating and family systems are not themselves adaptive, but spread for other reasons

If one or both views are correct, we should be able to match type of family system to 1) specific ecological/economic situations, and/or 2) cultural traditions

We can approach this issue by looking at each of 3 main forms of marriage

Monogamy is the dominant form in 2 primary situations:

1. Where resources are scarce, so that both parents needed to successfully raise offspring (e.g., traditional pattern among hunter-gatherers in resource-poor environments such as arctic, deserts)

2. Where laws or religious proscriptions prohibit polygyny (e.g., EuroAmerican society); no clear ecological reason why European prohibition of polygamy should have arisen and spread (though even here, mistresses, serial marriage, etc. lead to unofficial polygyny for many high-status or economically successful men)

Polygyny is culturally accepted (or even preferred) in great majority of human societies (Ethnographic Atlas lists it for 716 of the 863 societies therein = 83%), but given roughly equal sex ratios found in most human societies, it is limited to a minority of males in any given society at any one point in time

Polygyny is generally associated with very unequal distribution of resources among men, such that those with more resources have more wives

These resources are usually economic wealth (land, cattle, money, etc.), but can also be political power or influence (all of which are often correlated with age)

Example of first is found among East African cattle herders; here men must transfer stock to bride's kin in order to secure her in marriage; as men gain wealth throughout lives, successively convert part of it into "brideprice" and acquire additional wives

Some might condemn this as "buying and selling women," but that's incorrect: 1) brideprice only secures certain conjugal rights -- it's not a form of slavery; 2) women themselves often prefer polygynous marriage, because they would rather be 2nd or 3rd wife of wealthier, higher-status males than be sole wife of a poor or low-status man (see Low reading re "polygyny threshold" model)

Second kind of polygyny, involving differential power, is common among politically decentralized horticulturalists or hunter-gatherers living in relatively abundant circumstances (e.g., Amazonian horticulturalists like Yanomamo, etc.)

Logical opposite of polygyny is polyandry: 1 woman w/ 2 or more husbands

While polygyny is found in over 80% of known human societies, polyandry is a major option only in a tiny minority (Ethnographic Atlas lists 4/863 societies = 0.5%)

Equally interesting fact is that polyandry is almost always fraternal (co-husbands = brothers), whereas sororal polygyny (sisters married to same man) is not particularly common

This suggests that sharing a wife is rarely adaptive, at least from male perspective (females might gain some additional resources from second husband, but RS is generally limited by female reproductive rate, not by her number of mates); but if sharing must occur, better to do it with a brother, who is closest genetic relative of proper sex and age

Best documented case of human polyandry = Tibetan agriculturalists [reviews in Durham 1991 and Smith 1998]

Tibetan polyandry is always fraternal, and occurs in specific ecological & economic circumstances: family farming estates inherited by a set of brothers, who cooperate economically (e.g., one engaging in long-distance trade, another yak-herding, 3rd farming the family lands) and share a single wife

Tibetan data clearly indicate that monogamy is preferred if economic base is rich, but not if it requires dividing already small estates among more than one heir

Other societies have dealt with this by making one child (usually first-born son) the only heir -- a system termed "unigeniture"; but in the Tibetan case there seems to be enough payoff from cooperative labor that polyandry is economically & reproductively best option

In fact, limited data indicate that polyandrous marriage increases RS of the wife, and (given close relatedness to co-husbands) senior males don't sacrifice much in reproductive benefits, while junior males do better than if they were disinherited

Nevertheless, Tibetan households rarely include more than active 2-3 co-husbands; families with lots of sons send some of them to the local monastery to become Buddhist monks, or else the family estate is partitioned (sometimes in bitter disputes) after 4th or 5th co-husbands actually reach reproductive maturity [Levine & Silk 1997]

(Similarly, unmarried females, who are even more common, become nuns, or eke out a living at margins of society, perhaps supporting one or at most 2 kids)

Since benefits of fraternal cooperation and keeping estate intact should be fairly widespread in agricultural societies, economic explanation cannot explain why polyandry is so rare

One possible answer is that genetically-evolved reproductive selfishness exhibited by human males makes it very unattractive to them, and is major stumbling block to cultural evolution of polyandry:

Indeed, some data suggest conjugal dissolution more likely if any co-husbands feel they have no offspring (e.g., Nancy Levine's data shows men who left polyandrous marriages had fathered < half as many offspring as those who remained) [Levine & Silk 1997]

However, almost all polyandrous cultures cluster in S. Asia (certain Tibetans, Indians, and Sri Lankans)

This suggests that unique cultural features predisposed S. Asians to develop polyandry under the right social or ecological conditions

In sum, variation in human mating systems & family structure probably due to both ecological adaptation and non-adaptive but locally meaningful cultural rules (religion, etc.)

References

Blurton Jones, Nicholas G. (1986) Bushman birth spacing: a test for optimal interbirth intervals. Ethology and Sociobiology 7: 91-105.

Blurton Jones, Nicholas G. (1987) Bushman birth spacing: direct tests of some simple predictions. Ethology and Sociobiology 8: 183-203.

Blurton Jones, Nicholas, Kristen Hawkes, and Patricia Draper (1994) Differences between Hadza and !Kung children's work: Affluence or practical reason? In Key issues in hunter-gatherer research, ed. Ernest S. Burch, Jr. and Linda J. Ellanna, pp. 189-215. Oxford/Providence: Berg.

Blurton Jones, Nicholas, Kristen Hawkes, and Patricia Draper (1994) Foraging returns of !Kung adults and children: why didn't !Kung children forage? J. of Anthropological Research 50:217-248.

Boyd, Robert and Peter J. Richerson (2005) Not by Genes alone:  How culture transformed human evolution. Chicago: University of Chicago Press.

Durham, William H. (1991) Coevolution: genes, culture, and human diversity. Stanford: Stanford University Press.

Hill, Kim (1983) Male Subsistence Strategies and Time Allocation to Activities among the Ache Hunter-Gatherers of Paraguay. Ph. D. Dissertation, Dept. of Anthropology, Univ. of Utah.

Howell, Nancy W. (1979) Demography of the Dobe !Kung. New York: Academic Press.

Lack, David (1968) Ecological Adaptations for Breeding in Birds. London: Methuen.

Lee, Richard B. (1979) The !Kung San: Men, Women and Work in a Foraging Society. Cambridge, New York: Cambridge University Press.

Lee, Richard B. (1980) Lactation, ovulation, infanticide, and women's work: a study of hunter-gatherer population regulation. In Biosocial Mechanisms of Population Regulation, ed. M.N. Cohen, R.S. Malpass, and H.G. Klein, pp. 321-348. New Haven: Yale University Press.

Levine, Nancy E. and Joan B. Silk (1997) Why polyandry fails: sources of instability in polyandrous marriages. Current Anthropology 38:375-398.

Smith, Eric A. (1998) Is Tibetan polyandry adaptive? Methodological and metatheoretical critiques. Human Nature 9:225-261.

Wynne-Edwards, V.C. (1965) Self-regulating systems in populations of animals. Science 147: 1543-48.