The controversy over "Sociobiological thinking" and humans

Can we apply the concepts learned from the study of animal behavior to humans? 

Two basic complaints about human sociobiology

Scientific: Humans are too different from other animals in terms of the influences of culture, communication, and technology... so applying the concepts of animal behavior to humans tells us nothing about why humans do what they do (note: there is evidence that suggests otherwise)

Ethical: it is irresponsible to suggest that human behavior is, to some extent, a product of our genes and not the product of purely free thought. Additionally, any adequate tests of such ideas with regards to experiments that might demonstrate evolutionary/genetic causation underlying behaviors would be unethical.

In many respects, humans seems ill-equipped for survival.... what aspects of our phenotype contribute to our fitness?

The role of intelligence, behavior, culture, and technology in human evolution

 

Sociobiology and culture:

Is culture purely adaptive, is there no relationship to adaptiveness, or does the truth lie somewhere in between?

 

Benefits of cultural learning vs. genetic determinism or trial and error learning

Speed: Learning can spread rapidly between and within populations (inside a generation)

Higher average adaptive value: Cultural means are often a better indicator of optimal strategy than individual experiences.

Low costs: Cultural learning may be less costly if learning is risky or only a few opportunities to learn

 

Costs of cultural learning

Inertia: Can lead to non-adaptive behaviors if environmental conditions change... slows response relative to trial and error learning

Accumulated errors: Current wisdom may include false information that is universally accepted and never tested (e.g., the world is flat).

Rules of thumb: Can lead to secondary consequences that are not adaptive

Parasitic strategies: Cultural strategies may evolve that are maintained simply because they are successfully transmitted within a population (Dawkin's "memes").

Now discuss evidence for adaptiveness of human behavior

How?

1) Look for historical evidence:

Compare to closely related primates (our evolutionary kin).

Look at aboriginal societes

Inference from archaeological sites

 

2) Consider modern behavior

The problem of culture vs biology

Characteristic	Primate ancestors (Great apes)	Characteristics of Homo spp
Habitat	Forest habitat (arboreal & terrestrial)	Savannah habitat, near water
Diet	Mostly vegetarian	Meat and vegetables (omnivorous, high protein)
Locomotion	Semi-quadrupedal (agile hands)	Bipedal
Dispersion	Group living	Resource defense, often group territory
Dispersal	Male dispersal, females retained in kin groups	Female dispersal more typical (males hold territory)
Mating system	Female defense polygyny, no resource defense	Monogamy or Polygyny via resource defense
Reproductive signals?	Females advertise estrous (heat)	No female advertisement of estrous
Parental Care	No male parental care	Male parental care, mainly via provisioning of female
Intersexual cooperation	Little interaction, male dominance	Division of labor: male hunts meat, female gathers vegetables, sexes dependent on each other

 

Male strategies for increasing fitness	Female strategies for increasing fitness
Accrue and control resources needed by females	Direct investment in offspring care to increase survival
Select high quality, faithful mates	Select high quality mates (based on resource accruing potential)
Invest in parental care directly or via resources provided to mate	Conceal estrous to reduce attractiveness to other males
Guard paternity	Force males to consort and increase paternal investment
Increase female encounter rate (via polygyny, concubines, prostitutes, or rape)	Reduce male interference by climbing

 

Sexual selection in humans

Strong mate choice by both sexes despite monogamy

Male traits preferred by females: resource holding potential, physical strength, competitiveness, sexual attractiveness

Female traits preferred by males: indicators of reproductive condition and parental quality (fat storage in breasts and buttocks), mate fidelity

Relative occurrence of different systems

Monogamy: On a per family basis, the most common system (16% of societies have strict monogamy).

Polygyny: Allowed in a majority of societies (83%) but actually practiced by small fraction of individuals.

Polyandry: Rare (Tibetans: wife shares brothers).... an example of group/cultural selection? A high-altitude farming society with hard winters, only arable land is on alluvial fans with meltwater from glaciers; between fans is mountain desert. Limited number of farms/fan and no dispersal options. Minimum viable farm size, estate is handed to brothers jointly, they take one wife who is shared. Marriage rule prevents subdivision of the estate, promotes long-term survival for the family, limits population growth. Additional manpower does improve farm success and family reproductive success

Studies of aboriginal societies

Ecological factors affecting polygyny

High variance in male resources or wealth promotes polygyny,

Wealthy men may have more than one wife;

Females in polygynous marriages have same or higher RS compared to monogamous females (supports polygyny threshold model)

A comparative study of two South American tribes (Yanamamo and Saranahua) shows that the abundance of game affects the variance in hunter skill and the degree of polygyny. Siskind, J. 1973. Tropical Forest Hunters and the Economy of sex in Peoples and Cultures of Native & America, pp. 226-240, D.R. Gross, ed. Doubleday: New York.

A comparison of reproductive success between rich and poor Turkmen of Persia demonstrates rich men have more wives. Irons, W. Is Yomut social behavior adaptive? Sociobiology: Beyond Nature/Nurture. Barlow, G.W. and Silverberg, J., eds. AAAS Symposium Series.

 

Several mechanisms may maintain monogamy in human societies

Costs of polygyny

man pays bride's family for a wife, a strategy by parents of a daughter to insure that she will have monogamous marriage unless husband is extremely wealthy (67% of societies).

man provides service for bride's family or must perform tasks to prove his worthiness (11% of societies)... costs limit mating opportunites.

Societal enforcement

If husband lives with wife's family, monogamy is assured (Hopi Indians).

Laws against polygyny. A cultural imposition...note that serial polygamy (marriage, divorce, remarriage) is common

 

Foraging tactics

Human foraging strategies and optimal group size studies: Two examples

Bari fishermen

Uses marginal value theorem to determine optimum fishing time, given:

1 hour travel time to river

6 man hours to build a stone dam across river

Cumulative catch rate increases faster with more men

With 2 men, 1 hour travel, 3 hours dam building, and 4 hours fishing to get maximum rate of return of 35 fish, 8 hours total, gain rate is 2.24 fish/man-hour.

With 10 men, 1 hour travel, 0.6 hours dam building, and 1 hour fishing to get 41 fish, 2.6 hours total, gain rate is 1.67 fish/man-hour.

Although gain rate is higher with 2, it takes all day. With 10 men, there is time to fish a second or third spot (new dam each time but no travel) which would increase per individual gain and leave more time for other activities. Observed group size is 10-12.

From: Beckerman, S. 1983. Optimal foraging group size for a human population: The case of Bari fishing. American Zool. 23: 283-290.

Inuit Eskimos

Compares observed group sizes with net gain (kcal/man-hour) for 10 different kinds of prey/hunting styles.

Solitary hunting gives good fit to predictions of optimal gain (4 out of 5).

When optimal group size is greater than 1, observed group size is greater than the optimum.

Two hypotheses for larger group sizes

Conflict between potential joiners, whose alternative option is to forage solitarily with lower gain or to join, in which case his gain is better but the existing member's gain is reduced.

Kin selection: conflict between joiner and members is reduced if they are related; larger groups are optimal when members are kin.

Low levels of relatedness suggests kin selection not an issue; inability to control number of participants for distant hunts of large prey makes first hypothesis more likely.

From: Smith, E.A. 1985. Inuit foraging groups: Some simple models incorporating conflicts of interest, relatedness, and central place sharing. Ethol. and Sociobiol. 6: 27-47.

 

Dispersal systems

Among higher vertebrates (birds and mammals) it is commonly found that offspring of one sex disperse farther than the other sex; for social species, the sex that stays shows more cooperation than the dispersing sex.

Among birds, females disperse farther and males go short distances; in helper-at-the-nest species males typically stay and help.

Among mammals, males disperse farther, females often form kin groups.

Greenwood's "mating system" hypothesis

Where males employ resource defense, female-biased dispersal results (males are philopatric).

Acquiring a territory is essential for male reproductive success, this may be easier or less costly close to home.

Prior residency provides familiarity with the area; males can receive help from male kin, or males can inherit natal territory.

Resource defense and female-biased dispersal found in most birds, white-lined bats, chimps, gorillas, hunting dogs, and the monogamous pika.

Where males employ female defense, male-biased dispersal results (females philopatric).

In mammals with cooperative female kin groups, males defend female groups; male dispersal necessarily prevents inbreeding.

In a few birds (ducks and geese) with no resource defense but defense of single females (monogamous), female philopatry also found, so female grouping not required.

Human dispersal and inheritance rules similarly related to male strategy for gaining access to females.

Most common human strategy (85% of traditional societies) is female dispersal, males remain in natal village (patrilocal).

Associated with resource defense and male inheritance of resources (patrilineal).

When there is a resource to be defended, males will control and accumulate it and pass it down to sons.

Also associated with strict sexual mores so males are confident of paternity.

Types of societies: pastoral (animals are resource), agrarian (land), sedentary hunter-gatherers with cooperative defense of hunting grounds.

 

Male-biased dispersal found in remaining 15% of traditional societies, females remain with female kin (matrilocal).

Associated with matrilineal inheritance.

When there is no resource to defend or accumulate, males append themselves to stable female groups (female defense).

Also associated with lax sexual mores, low confidence of paternity, males may be nomadic.

Types of societies: some hunter-gatherer societies,in particular fishing societies.

Kin selection in humans

Humans are extremely conscious of degrees of relatedness.

Incest taboos ubiquitous.

An example - three types of cousins

Cross cousins (their parents are sister and brother).

Patrilineal parallel cousins (parents are brothers).

Matrilineal parallel cousins (parents are mothers).

In societies that practice sorroral polygyny (wives are sisters), matrilineal parallel cousins may not marry because r = 3/8 (can you do the math?), other types of cousins may marry (r = 1/8)...

Level of cooperation versus relatedness

In hunting societies, related males more likely to hunt together than unrelated males.

Level of interaction in Ye'kwana villages increases with increasing r even after controlling for proximity (Hames, R.B. 1979. Relatedness and interaction among the Ye'kawana: a preliminary analysis. In Evolutionary Biology and Human Social Behavior: an Anthropological Perspective (N.A. Chagnon and W. Irons, eds.) Duxbury Press, North Scituate, Mass.).