Date: Mon, 08 Apr 1996 16:27:20 -0600
Subject:  TREE article, part 2 of 3


Food sharing
	Food sharing is far more common among humans than among other
primates.  Among modern hunters, sharing varies with resource
characteristics: large, unpredictably acquired packages are shared
more widely than smaller items taken more predictably.  Most analysts
see this as risk-reduction: successful hunters give some of their
catch to the unsuccessful in anticipation of a time when their
respective fortunes are reversed.1,7,15-16  The free-rider problem
lurking here15,17 is veiled by ethnographers' use of the term
"reciprocity" to describe these transfers, absent any demonstration
of the back and forth exchanges of food between pairs of individuals
required for Trivers' reciprocal altruism.  In well-described cases,
some individuals consistently get shares even when they have not
given while others continue to provide shares even if they are not
repaid.17-18 

	Blurton Jones19 appealed to the economics of resource defense in
suggesting that this pattern might be better described as "tolerated
theft."  If foragers take resources unpredictably, so that their
successes are unsynchronized, in packages large enough that marginal
benefits diminish across portions for any consumer, then those who
have consumed less will value additional portions more.  If the cost
of contesting a portion is commensurate with its value to each
contestant, then those who have less will take more.  Acquirers have
no "property rights" in the food they capture.  If women forage to
feed themselves and their families, the small defendable fraction of
game animals plus the higher day-to-day probability of failure make
hunting the inferior strategy for them.20

Why do men hunt?
	The economics of defense create a potential social benefit to
foragers who acquire resources too expensive for anyone to
monopolize.  Since non-acquirers can expect to benefit, they have a
direct interest in monitoring and exploiting the success of acquirers
targeting such items.  A forager attracts favorable attention by
taking resources that many can expect to consume.18 

	This is an alternative to the long standing hypothesis that men hunt
to provision their wives and offspring.  Men's foraging may often be
mating rather than parenting effort.20  Because of the marked
differences in the defendability of various resource types foraging
can serve different goals: either feeding one's "own" or attracting
positive attention from potential allies and mates.  Those seeking
the former should pursue defendable resources, but favor companions
who target items that are widely shared.  Where men have more to gain
>from allies and additional mates than from contributing to the
welfare of wife and offspring, they should pursue widely shared foods
that earn them favorable social attention.  When additional mates are
scarce, mens's foraging patterns may be shaped by mate guarding.21 

Children's foraging and maternal tradeoffs 
  The children of foragers can be surprisingly adept at food
procurement.22  Mothers evidently adjust their own foraging tactics
to take advantage of this, playing off their children's capabilites
against their own in ways that maximize the rates they earn
collectively.23  Tactics adopted in any particular situation vary
greatly depending on both the array of resources available and
age-related differences in procurement capabilities.24-25   
 	Among non-human primates, juveniles feed themselves; thus
mothers can only exploit resources their youngest weanling can
acquire at a life sustaining rate.  Because human mothers supply food
to their offspring, they can rely on a wider array of resources,
including those that young juveniles cannot manage.  This practice
expands the range of habitats that people, or food sharing hominids
generally, can occupy.  
	It also has a cost: the more dependent children are on mother's
foraging, the larger the deficit they face when her attention is
diverted to a newborn.  Such costs can be offset by the efforts of an
adult helper.  Husband/father has long been cast in this role,
establishing the nuclear family as unit of common economic and
reproductive interest.  But, as reviewed above, men seek goals that
are often inconsistent with provisioning offspring.  Grandmother is a
more promising source of help for the weaned children of a nursing
mother.  The argument that nominates grandmothers as essential
helpers draws evolutionary links among food sharing, the use of
resources juveniles cannot handle adequately for themselves, and the
long post-menopausal lifespans that distinguish human life histories
>from those of other primates.20	

Human life histories
	 Humans differ from other primates in the timing and rate of
life-course events.  Not only do we have long post-menopausal
lifespans, but we also mature later and have generally higher levels
of fertility than chimpanzees.  Life history theory provides the
conceptual tools to investigate the evolution of these differences,
as well as the variation in patterns of development and fertility
among modern humans.26

	Hunter-gatherer birth spacing was among the first topics to be
addressed by human behavioral ecologists, and continuing work
indicates the importance of tradeoffs in parental investment in
explaining variation in patterns of fertility.22  The small but
growing number of forager demographies shows wide variation in
completed family sizes. Fertilities are lower where the character and
distribution of resources is associated with age and sex specific
foraging patterns that make children cost their mothers more.27-30 

	The proposition noted above, that mother-child food sharing depends
on grandmothers' involvement, elaborates the hypothesis offered by G.
C. Williams to explain the puzzle of post-menopausal lifespans. Since
mutations acting after the age of last reproduction are usually
assumed to have no effect on lifetime fitness, there should be no
"post reproductive" life.  Williams suggested that when extended
maternal care was crucial to offspring success, continued child
bearing by aging mothers might give lower marginal fitness gains than
re-allocation of that parental investment to children already born. 
The first quantitative tests of this hypotheis using demographic data
on Ache foragers do not support it.28  Alternative modeling31-32 and
further assessment of grandmothering patterns are both in order.

	Hill and Hurtado's28 descriptions of Ache fertility and mortality
patterns and comparisons with other populations illustrate the
analytical leverage provided by Charnov's33 theory of "invariants" to
explain life history evolution.  A few variables in this general
framework for explaining mammalian life histories capture enough
empirical variation to account for age at maturity among the Ache and
differences between the Ache and other hunter-gatherers.  Since the
theory links adult mortalities, growth rates, and reproductive rates,
it offers a new way to measure differences between the life history
patterns of humans and other living primates and pose hypotheses
about hominid lifehistories that may be tested in the fossil
record.26   




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