1. family and parenting
2. children

The Evolutionary Basis of Sex Variations in the Use
of Natural Resources: Human Examples
Kristen
Hawkes
University of Utah
unlike
People,
consume
sion
primates,
of
products
has seemed
the
of
have
other
labor"
each
foods
acquired
work.
other's
specialties,
This
human
distinctively
use.
resource
Ifmen
of human
creates
marriage
a social
different capacities
to serve family needs. Other distinctively
seem
this
to arise
from
by
When
others.
and men customarily acquire different foods and
the hallmark
economic
different
consume
regularly
forage for a living, women
people
economic
fundamental
unit
and women
that deploys
their
human patterns then
between
cooperation
divi
"sexual
the
sexes.
In
recent decades, the use of evolutionary theory to investigate and explain social
behavior across the livingworld has revealed pervasive conflicts of interest between
(aswell as within) the sexes. Application of these tools to human examples shows
the "sexual division of labor" to be the economic aspect of different and conflicting
reproductive
agendas
munities where
units
of
common
have different
patterns
of
for women
and men.
A
review
of
some
examples
from
com
people hunt and gather for a living illustrates that families are not
economic
interest.
As
with
other
primates,
reproductive goals and these differences
resource
males
and
females
shape sex differences
in
use.
are obvious
Sex differences
to even the most casual
among mammals
observer.
and explaining
tools for exploring
Evolutionary
theory provides
the variable
character and extent of those differences
by distinguishing
and parenting effort and exposing
the reasons why males usually
mating
females from com
gain more from competing with each other for mates,
resources
for
to offspring welfare
that contribute
(Low, this volume).
peting
Utah,
Please
address
to Dr. Hawkes,
correspondence
102 Stewart Bldg., Salt Lake City, UT 84112.
Dept.
A Journal of Interdisciplinary
and Environment:
Population
Volume
1996
18, Number
2, November
?
1996 Human
Sciences
161
Press, Inc.
of Anthropology,
University
Studies
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of
162
POPULATIONAND ENVIRONMENT
But standard scenarios of human evolution
postulate a key departure from
one
have made
the sexes much
that
would
mammalian
patterns,
general
more alike. In these scenarios, ancestral males hunt to supply food for their
units.
mates and offspring so that nuclear families become basic economic
in men's parental effort reduces the remainder avail
This sharp increase
able for mating competition.
has provided one
of contemporary
The ethnography
hunter-gatherers
for this argument. Men usually hunt and
of the main
lines of evidence
women
the
and children eat meat procured by men,
gather. Since women
"sexual division of labor" is assumed to indicate husbands and fathers sup
units
porting their households,
making nuclear families the basic economic
But closer study of food sharing patterns shows that
of human societies.
than to
their households
hunters often supply more food to people outside
in fun
their own families. The interests of nuclear family members
diverge
I review reasons to consider
the hypothesis
that men hunt
damental ways.
not for parental payoffs, but for mating advantages
instead.
com
After a brief description
of sexual selection
theory, I summarize
mon assumptions
and then report the
about hunting and human evolution,
from studies of the Ache of Eastern Paraguay and the
patterns emerging
In both cases hunting provides a substantial
Hadza of Northern Tanzania.
nutri
fraction of the diet. But a hunter's kill iswidely
shared, distributing
to women
other than his wife and children other than his
tional benefits
allocate most effort to support
well as to other men. Women
offspring?as
resources
that supply less for their
ing their children while men choose
is more widely
shared than plant
families than alternatives would. Meat
food. This is a reason for people to favor hunters as neighbors. The favor
and
from many who expect meat shares can give alliance
able attention
in resource
to hunters.
If so, the basic sex differences
mating advantages
use among other mammals may have been more
important in human evo
lution and
in ethnographic
variation
than generally
supposed.
SEXUAL SELECTION
his theory of sexual selection
(1871) to explain
developed
that, unlike other adaptations,
striking features of behavior and morphology
of survival; and that oc
to reduce rather than enhance
chances
seemed
that these features played
curred in only one sex, usually males. Observing
a role in mating, either in contests between males, or in attracting females,
that they were shaped by selection
he theorized
through effects on mating
Darwin
success.
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163
KRISTENHAWKES
reasons
the underlying
have since elaborated
Evolutionary
biologists
that
distin
variable
this
The
for such sexual asymmetries
volume).
(Low,
a
in the
difference
across
the
is
males
world
from
females
living
guishes
rich
few
size and number of gametes
eggs,
large
relatively
they produce,
rates
of
small
repro
sperm. Consequently,
resource-poor
potential
many
& Parker, 1992) are
1991 ;Clutton-Brock
duction
(Clutton-Brock & Vincent,
But
than
for
males.
females
each
for
lower
offspring has both a
usually
limited by the slower sex. In
mother and a father, so actual rates are always
rate of
females can usually reproduce at a potential maximum
primates,
about one offspring a year. Males, on the other hand, can potentially
repro
duce at a rate of (more than) an offspring a day. In a population with 100
of each sex, 100 babies could be born in a year, one to each
members
and
female,
they could all be fathered by a single male.
is an inevita
shows why mating competition
This thought experiment
in a way that it is not for females. The number of
for males
ble problem
increases
babies born is limited by the number of females. Any male who
the number fathered by others.
the number of babies he fathers decreases
in the popu
is a zero-sum game. Ifone of the males
Paternity competition
lation imagined above fathered all the babies, the 99 other males would be
sex ratios are near even (where selection
cut out entirely. When
usually
success than the female av
sets them), any male with higher reproductive
asymmetry
erage leaves less than that for other males. This fundamental
and
favors different capacities
between
the sexes explains why selection
is most
fitness
and females. Among
tendencies
females,
among males
ca
strongly affected by the number of successful offspring raised, favoring
to
to acquire and control resources that contribute
pacities and tendencies
fitness ismost strongly affected by
the welfare of offspring. Among males,
success
in competing with other males over sexual access to females.
HUMAN FAMILIES
to indicate substantial
role of men has been assumed
The economic
In the 1950s and
sex
in
in these
human evolution.
differences
reductions
three lines of evidence:
60s Sherwood Washburn
comparative
synthesized
to construct a power
and ethnology
archaeology,
paleolithic
primatology,
influential scenario of human evolution.
Nonhuman
primates rarely hunt or share food. Juveniles and adults of
role in the
adult males playing no "economic"
both sexes feed themselves,
record
lives of their mates and offspring. The archaeological
suggested an
our
That record
ancestors.
in the patterns of
hominid
important difference
fully
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164
POPULATIONAND ENVIRONMENT
about 2 million
years ago, with
boundary,
begins at the Plio-Pleistocene
stone tools and the bones of large animals, a combination Washburn
and
meat
read to indicate that males were
and
others
transporting
hunting
of contemporary
home to share. Ethnographic descriptions
hunter-gatherers
to fill out this picture. People who
seemed
forage for a living regularly
hunt and women
they carry home and share. Men generally
consume
so men seem to be family
the meat they acquire,
inference that
providers. These elements
persuasive
supported Washbum's
the initial evolutionary
hu
innovation that gave rise to other distinctively
man attributes was hunting by males to provision
their families (Washburn
& Lancaster, 1968).
were
new fossil evidence
Since these arguments
has
developed,
acquire food
and children
shown unexpected
variation
in the hominid
lineage and the long persis
tence of taxa that antedate
the archaeological
record (Foley, 1995; Klein,
of the archaeology
has challenged
the inferences of
1989). Reappraisal
and
home
bases
from
the
record
(Binford, 1981). Neverthe
hunting
early
essential
elements
of the scenario
less, absent a satisfactory
alternative,
it is
In particular
influential
1992, pp. 68-71).
(e.g., Diamond,
men
are
in
assumed
distinctive
the
that
among
making
widely
primates
to parental effort, therefore
substantial allocations
less to mating competi
tion?thus
the effects of sexual selection
(Lancaster & Lancaster,
blunting
men
it is assumed
that
and
1983). Among
hunt,
contemporary
foragers
so
to
do
their
wives
and
children.
support
they
cases to be described
The two ethnographic
these assump
challenge
tions. While
radical in light of conventional
wisdom
about hunter-gath
in light of general sex differ
erers, this is a challenge
easily anticipated
remain
the reasons
is not just that
for them. The possibility
is
variation
than
wider
but that the
appreciated,
ethnographic
commonly
are themselves
in these examples
illustrated
patterns
likely to be quite
common
but overlooked
because
the proposition
that men are providing
for their families has come to be one of those "known facts" that need no
ences,
and
test.
TWO ETHNOGRAPHIC CASES
Modern
and modern
environments
differ from those of the
people
behavior can shed light on ancestral patterns in the
past, but contemporary
same way that any particular
or set of
(and therefore unique) experience
can adjust expectations
observations
about others. Particular
instances ex
emplify
general
processes.
Where
people
hunt and gather
for a living, they
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165
KRISTENHAWKES
these
in human experience:
that have deep antiquity
face daily problems
so
see
to
what
an
view
such
problems,
provide
ethnographic
opportunity
local
of
features
whether
and
how
and
evaluate
lutions people
choose,
represents only a small frac
ecology
shape both. Any particular example
But each case offers a
variation.
of
tion of some
range
larger possible
If so, the
to see whether
variables are related in expected ways.
chance
systematic
relationships
provide the basis for hypotheses
terns elsewhere,
under different circumstances,
including
about likely pat
those of the past.
The Ache
the forests of
The Ache
1996) occupy
(Hill, 1983; Hill & Hurtado,
Basin. As is typical of
eastern Paraguay, an area just outside
the Amazon
birds, and
foragers, Ache men hunt, targeting a wide variety of mammals,
150 kg.
to
are
animals
the
up
they hunt, weighing
reptiles. Tapirs
largest
rare and the largest of the animals regu
are extremely
But tapir encounters
lipped peccary, have adult body sizes
larly taken, brocket deer and white
less than 40 kg. Men spend most of their hunting day away from women
their activities with other men (Hill
sometimes
and children,
coordinating
& Hawkes,
1983; Kaplan et al., 1990). Women
spend their days in com
et al.,
and children
(Hurtado,
1985; Hurtado
pany with other women
known foragers for
1985). The Ache are unusual among ethnographically
their frequent residential moves. On most days women,
carrying household
the hunting men.
forest
walk
the
and
children,
following
through
goods
to gather plant foods, honey
The women
stop often to rest and sometimes
or insects.
is set, nuclear families become
In the afternoon, when
camp
sets a fire for the night, and while
there is daylight she
visible. Each woman
may leave to gather food nearby. Women
spend little time in food acquisi
tion (Hurtado, 1985; Hurtado et al., 1985) and contribute a relatively small
fraction of the calories consumed
by Ache foragers (Hill et al., 1984), less
when
they are not nursing and have
they are nursing infants, more when
other children to feed (Hurtado, 1985; Hurtado et al., 1985). Men arriving
from a day of hunting settle at the fires of their wives,
they may
although
also leave again to collect plant foods nearby.
The association
of nuclear families with hearths suggests these might
be economic
units, with members
pooling the food they have acquired for
joint consumption.
Sharing among hearths is frequent, so systematically
nuclear fami
collected
quantitative data are required to determine whether
are
more
of the food acquired by their members
lies
(Kaplan,
consuming
1985a). Two salient points
1983; Kaplan et al., 1984; Kaplan & Hill,
First the overall amount of sharing
emerge from the record of observations.
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166
POPULATIONAND ENVIRONMENT
three quarters of the food
the nuclear family is very high. About
eats was acquired by someone
their own nuclear family.
outside
in the amount of sharing by re
there are systematic
differences
are so
that only men
type. The resources
take, game animals,
more
no
than
else.
The
distributed
that
members
anyone
get
widely
family
resources that women
most often take are less widely
nuclear
distributed,
units
family members
getting more than others. So families are economic
not for honey or game
for some resources but not for others?specifically
beyond
anyone
Second
source
animals.
Since Ache men not only hunt but also take (collect) all the resources
women
to test whether
the resource itself or the sex of the
do, it is possible
extent
determines
of
show that
the
the
acquirer
sharing. Partial correlations
resource
is
resource
its own
Each
has
type.
robustly patterned by
sharing
more
ones
matter
Men
of
the
it.
who
take
signature?no
sharing
acquired
less likely to go to their own family members
(Hawkes, 1991).
success
Men
rates (Hill & Hawkes,
have widely
varying
hunting
1983). The differences
persist from one year to the next (Kaplan, 1983),
reflecting skill, not just luck. The men who are more skillful hunters, and
therefore bring more meat for all, also spend more
time hunting than do
less successful
hunters. This pattern suggests they have more to gain from
time spent hunting than do less successful
hunters. But the shar
ing patterns spread the consumption
gains for their extra work across the
foraging group.
serve the
the narrowly shared collected
foods, women
By choosing
their children. Men, by choosing
game animals and the
goal of feeding
shared collected
foods, are serving a different goal. They are target
widely
resources
that many will consume.
them valuable
This makes
ing
neigh
so the better hunters they are. If other things are equal,
bors, increasingly
they should find more ready allies when disputes arise. Other men might
be more tolerant of their sexual overtures to other women. Women
for the
same reasons might be more
than
interested in traveling with these men
with those whose
foraging efforts brought them little.
The Ache sharing patterns and sex biases in resource choice challenge
the view that hunting is largely a kind of parental effort. Meat
is so widely
a
no
consume
more
shared that the wife and children
of
than
hunter
a
man
resources
to acquire
others. By hunting,
is choosing
that mostly go
to those outside his own family. The benefits he gets for doing so could be
attention
favorable
increase
his
from those outside
his family, which
Men
of enlisting
chances
in
allies and succeeding
competition.
mating
who are more successful
and
hunters do have more mating opportunities
success
&
Hill, 1985b).
higher reproductive
(Kaplan
additional
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167
KRISTENHAWKES
The Hadza
and
savannahs of East Africa present different foraging constraints
et
to
The
Hadza
(Blurton Jones
al., 1992) in
hunter-gatherers.
opportunities
sometimes months
residential bases for weeks,
northern Tanzania occupy
for nu
at a time. Dome shaped brush structures provide spatial distinction
adults are in camp, they spend
clear families. But during the day, when
and men
little time in or near their houses, women
joining other women
The
areas.
In all seasons most Hadza
Parties
leave
home early in the morn
forage together.
or
a
man
older
boy armed with bow and
by
ing, usually accompanied
arrows to guard against meetings with
strangers, most
likely pastoralists
in
The
the
foraging parties usually target
neighborhood.
tending their herds
in
a particular
resource, tubers at any time of the year, berries or baobab
and
et
infants
are
seasons when
al., 1989). Nursing
they
ripe (Hawkes
their mothers
children about 5 years of age and older often accompany
even on trips of long distance and duration (Hawkes et al., 1995). Although
Hadza youngsters are active foragers, and increasingly so as they get older,
rates fall below those of adults (Blurton Jones et al., 1989;
their acquisition
re
varies among
et al., 1995). Relative
Hawkes
juvenile effectiveness
joining
women
men
to sit
in public
activity
and children
not to maximize
their
sources, and women
adjust their resource choices
own personal acquisition
rates but those of the "team" that includes their
show that women
children
(Hawkes et al., 1995). Data as yet unpublished
less to their older
in their foraging patterns, attending
make adjustments
a
infant.
Older
children when
have
women,
"grandmothers,"
nursing
they
of
most energetically
expensive
spend more time foraging for tubers?the
of child bearing age (Hawkes et al.,
do women
the plant resources?than
are especially
data show that "grandmothers"
impor
1989). Unpublished
are
status of children whose mothers
tant in maintaining
the nutritional
infant. Women
share resources with foraging compan
nursing a younger
to claim shares
with
those who assemble
from
and
ions while
away
home,
on their return, but manage
to retain a substantial fraction for later house
status of
between
the nutritional
The correlations
hold consumption.
this
show
women
and their children and grandchildren
data)
(unpublished
are
women
that
differential
members,
consumption
confirming
by family
for their children and grandchildren.
providing
Mothers
and children with the addition of grandmothers may be seen
as consumption
units for an array of resources they gather. There are also
and their children
husbands
and wives
times when
regularly forage to
women
season
In
and girls carrying
the
form,
honey
family parties
gether.
the trees, smoking the
the containers,
boys and men pegging and climbing
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168
POPULATIONAND ENVIRONMENT
these trips are successful
the honey. When
bees, and extracting
large
amounts of honey are eaten by all before returning to camp. On arrival
others assemble quickly to claim shares. If the take has been large enough,
retain some for household
successful
consumption
honey collectors will
in a good honey year, a husband and wife
later. Sometimes
and especially
to carry off to trade with neighboring
accumulate
villagers
enough
use it to make a fermented drink.
in big game.
Most of the time Hadza men are hunters specializing
and
mark
the
smaller
end
of
the
range
they usually target.
impala
Warthogs
in this habitat ranging up to giraffe are all included,
The larger ungulates
to hunt. They also
leaving only elephants, who they say are too dangerous
carcasses
the
of all these
with
local
carnivores
for
compete
successfully
men
et al., 1988). Whenever
leave camp they are
animals
(O'Connell
may
who
armed with poison arrows
most days they devote time
tracking parties of
although
to
solid strike. In addition
women
and other children,
On
and vigilant for any hunting opportunity.
to
themselves,
usually
hunting,
by
specifically
a hunter reports a
men and boys form when
the time boys spend with
foraging parties of
In the late dry
they also hunt with age-mates.
season, when game animals are tied to the restricted points of surface wa
ter, men and older boys hunt at night from blinds set over water or along
game trails.
here sharpens a pattern that
The character of the hunting opportunities
for
less marked
the Ache.
is much
averages
among
Daily
acquisition
hunters are higher than those of Ache
hunters (hourly averages
Hadza
twice as high) but instead of the Ache failure rate of every fourth day or so
(Hill & Hawkes,
1990), the average Hadza hunter is suc
1983; Hawkes,
size
cessful only one day in thirty (Hawkes et al., 1991). It is the enormous
of Hadza prey that turns rare successes
into a high daily average. And this
the sharing radius. Not only other members
of the
prey size increases
hunter's camp, but residents of neighboring
camps come to claim shares.
Other men, as well as women
and children come to the kill site both to eat
and also to carry portions home.
in hunting success
The differences
hunters are even
among Hadza
even
those who have not
greater than among Ache hunters, but all men,
in a long time, take part in consumption.
been successful
and
Men, women
a
carcass
at
to
children assemble
claim shares. The claims often have an
in terms of the relative size of one's
edge of demand and are often couched
to those of others (Blurton Jones, 1987). Attempts
share compared
to refuse
a hungry crowd would be costly?even
would
clear
bene
be
though there
a large carcass. The technology
fits to those who could monopolize
for
meat
can
is
and
do
available
and
used. People
storing
readily
dry
widely
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169
KRISTENHAWKES
meat, some at the kill site and more on return home, frequently
trying to
to
to
mil
for
take
off
trade
accumulate
corr,,
tobacco,
marijuana,
enough
is rare because household
let, or other goods. But substantial accumulation
and visitors eat it as quickly as it is dried. Since indi
members,
neighbors,
or not they have provided
them
viduals successfully
claim shares whether
to store meat,
in the past, and since there is a ready technique
sharing
as a way to "bank" resources that would
cannot be explained
otherwise
lose their value.
By hunting big game the average Hadza hunter has a 97% chance of
home empty handed everyday. When
the meat
he is successful
coming
to others. An experiment
(Hawkes et al., 1991) showed one
goes mostly
way a man could contribute more, and more often, to his family's con
sumption. Men were paid to hunt and trap small animals to see what suc
cess they might have in this environment.
Results showed that they earned
a smaller but much more regular return than they earned specializing
in
a man
big game. Since small animals are mostly eaten by family members,
who
to take shares from any large car
them (while continuing
hunted
more
for
his
casses) would
provide
family than a man who did not. Men
could also collect plant food, something
they often do to feed themselves,
in big
to
much
collect
bring home. By specializing
they rarely
although
men
own
more
are
their
that
would
for
alternatives
game
provide
forgoing
families.
What benefits do they get instead? Data currently under analysis show
a pattern
that the wives and children of better hunters are better nourished,
same
to
But
data
that seems
the
contradict
claims.
the
initially
preceding
a
not
man's
do
show that family members' weight
track
hunting
changes
successes.
Instead children's nutritional status is correlated with the forag
to the
their mothers
make according
(and grandmothers)
ing adjustments
in
status
Women
differ
how
and
their
of
coresident
children.
age,
nursing
well they handle the day-to-day problems posed by the nutritional needs of
their children. Children of better hunters are generally
better nourished
because of their mothers
(and grandmothers).
FAMILIESAGAIN
families are visible groups, and monogamy
is the common
Nuclear
et al. (1979), assuming
pattern for both the Ache and the Hadza. Alexander
is fre
that men hunt to provide for their families, argued that monogamy
are
men
or
quent among foragers because, without
rarely
herding,
farming
able to support more
distin
than one wife. Alexander
and colleagues
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170
POPULATIONAND ENVIRONMENT
this "ecologically
guished
imposed" from the "socially
imposed" monog
in societies with marked wealth differences
but legal sanc
amy occurring
tions against polygyny.
Flinn and Low (1986) noted that hunter-gatherers
in
no
than
wealthier
Australia,
monogamous
Aboriginal
foragers elsewhere,
were
men
often polygynous.
that the greater "power" of some
They suggested
a problem
allowed
in Al
"socially
imposed polygyny." This exposed
exander's
that
is
the
If
reviewed
here.
compounded
by
typology
patterns
"social" and "ecological"
factors are independent,
variables
ecological
cannot explain
social behavior. But the examples
reviewed here, like the
whole
field of behavioral
show that the fitness-related
tradeoffs
ecology,
constraints
local
have
social
The
imposed by
prev
components.
ecological
alence of monogamy
as
among foragers like the Ache and Hadza depends
much on social variables as polygyny
does among some Australian
for
agers, as much as monogamy?or
among people depend
polygyny?does
ing on other subsistence
strategies.
If hunting
is the main arena for competition
a better
among men,
hunter may displace another
in contests for a particular wife, with day-to
day unpredictability
preventing him from being enough "better" than other
men to successfully
defend more than one wife at a time. According
to this
could
of women's
result, independently
monogamy
hypothesis
prefer
ences, from very high levels of mating competition
among men?not
just,
as is often assumed,
from men investing little inmating effort. Alternatively
women might prefer to marry better hunters if those men are more favora
to their chil
1993) and the favors extend
bly treated by others (Hawkes,
dren (Hill & Kaplan, 1988; Hawkes,
1990). Since that should lead to polyg
yny for the best hunters, a more complicated
pattern of female choice
would
be required to account for the data. Either way,
it is not the number
a man can support but the character of the mating competi
of dependents
that
tion, itself conditioned
1990, pp. 163-4),
by local ecology
(Hawkes,
a more
of marriage patterns (see also
emerges
likely primary determinant
Hurtado & Hill, 1992).
Studies of birds, other primates, and recent modeling
converge with
the patterns
here in showing
reviewed
that mating
competition
plays a
more prominent
role in shaping male
than recently supposed.
strategies
The work on birds is especially
em
since classic explanations
suggestive
the role of paternal care in promoting monogamy
in humans
phasizing
those long used to explain
the prevalence
of avian monogamy:
parallel
can help raise expensive
males
research
(Lack, 1968). Recent
offspring
shows strong mating competition
in many bird species where
previously
have
large paternal effort and attendant weak mating
competition
long
been assumed
(see review in Davies,
1991).
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171
KRISTENHAWKES
to the welfare
Adult male primate contributions
of infants and juve
niles have been assumed to depend on some possibility of parental payoff
to the males.
But recent work suggests that in many species caring males
earn mating
and not parenting
benefits as females prefer to mate with
carers (Smuts, 1985; Whitten,
1992). Very gen
1986; Smuts & Gubernick,
to investigate the way that a male's payoffs for mating
with
the frequency of competitive
from other
vary
competition
challenges
males show that this frequency dependence
has powerful effects. Selection
favors large allocations
to mating competition
even when male help could
make a big difference
in the number of offspring
raised by their mates
(Hawkes et al., 1995). In light of the results from these other lines of in
quiry the human patterns reviewed here are not surprising. Ethnographic
attention
to the choices open to individuals and the fitness related costs
and benefits
is relatively
recent. Better understanding
of
likely for each
how these vary with sex, age, and local ecology will improve the prospects
not only sex differences
for explaining
in resource choice but variation
in
other aspects of social behavior within and among human societies.
eral models
built
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