The ontogeny of prosocial behavior: foraging and food sharing ...

The ontogeny of prosocial behavior:
foraging and food sharing among Hadza
hunter-gatherer children
Alyssa N. Crittenden
Visiting Assistant Professor
Department of Anthropology
University of Nevada, Las Vegas

Routine food sharing is argued to
influence the evolution of:
• Life history
• Family formation
• Cooperation
• Economies of scale
Food transfers can have multiple functions
• Develop and maintain social bonds
• Reduce variance of food consumption –
provision others

Evolutionary Models
Kin Selection
An individual should be willing to share as long as the
cost is less than the benefit, times the coefficient of
relatedness between individuals C < B x r
(Hamilton 1964)
Reciprocal Altruism
Givers and receivers reverse positions on a systematic basis,
such that amounts received and given are correlated
(Axelrod 1984; Trivers 1971)

Very little is known the development of food sharing in
children and how it may influence the development
of prosocial behavior
Hadza juveniles are active food collectors and can produce
up to 50% of their daily caloric intake by the age of 5 years
(Blurton Jones et al. 1989, 1994)
They also share substantial portions of their collection with others
(Crittenden et al. 2010)

Current Study:
(1) How much do Hadza children forage and can they
offset the cost of their own care?
(2) Are Hadza children selecting sharing partners
based on kinship and/or reciprocity?
(3) Is there a significant caloric difference between the
amount shared with kin vs. non-kin?

~ Hadza Foragers ~
Approximately 1000 individuals (300 hunt and gather)
Northern Tanzania ~ Lake Eyasi
Camp membership is fluid ~ 30 residents
Residence pattern is variable ~ wifes kin OR husbands kin

Juvenile foraging
Previous research suggests that food collection by children is
constrained by ecological parameters and costs of collection
and processing
• Low predator pressure
• Access to drinking water
• Variability in terrain

In environments that are conducive to juvenile foraging,
young foragers can be avid and productive food collectors
Hadza of Tanzania
- Hawkes, OConnell & Blurton Jones (1995)
Current Anthropology 36:4, 688-700.
- Blurton Jones, Hawkes & OConnell (1997)
Uniting Psychology and Biology: Integrative Perspectives on Human
Development
Martu of Australia
- Bird & Bliege-Bird (2005)
Hunter-Gatherer Childhoods
Mikea of Madagascar
- Tucker & Young (2005)
Hunter-Gatherer Childhoods

Gangidape ~ April - May 2005 (wet season)
Siponga ~ October 2005 (dry season)
Total Sample: 65 days
• Children (1 - 17 years) n = 43
• Children who foraged (3 - 17 years) n = 34
Foraging Returns:
481 instances of food back to camp
(n 1 = 14 males, n 2 = 20 females)
Focal Follows:
- 13 follows of individual foragers
(n 1 = 6 males, n 2 = 7 females)
- Age range 6 - 14 years
- Ranged from 50 minutes to
5 hours in length, depending
on distance traveled

Ekwa - tuber
(Vigna frutescens)
Kongolobe - berries
Hornbill - bird
Embilipe - berries
Nguilabe - berries
Baalako - honey
Ogoyo - wild fig
Mangwala- legume

Weaver bird seasonal breeding and
Hadza exploitation of nestling birds
Genus Ploceus

Greg Downing
Hadza children are in direct foraging competition with baboons
- baobab pods & seeds
- figs
- drinking water

Calories Total Brought Foods Kilocalories Back Collected to Back Camp by to Children by Camp Children
(collapsed across all forays)

15%
Wet Season Calories Collected
19%
19%
15%
Baobab
Berries
Birds
Honey
Tubers
Fruit = 34%
Honey = 15%
Tubers = 19%
Birds = 32%
32%
Dry Season Calories Collected
10%
Fruit = 84%
Legumes = 1%
Birds = 5%
Small Mammals = 10%
1%
11%
8%
5%
4%
61%
Baobab
Berries
Birds
Drupes
Figs
Legumes
Small Mammals

⇒ Does age affect foraging return?
Age groups:
(1) 1 - 4 years (n = 2) = 67 kcal
(2) 5 - 9 years (n = 14) = 724 kcal
(3) 10 - 13 years (n = 11) = 634 kcal
(4) 14 - 17 years (n = 7) = 618 kcal
The mean values of calorie returns per foraging trip for
each age group were compared…
Age is NOT a significant predictor of foraging return
Wilcoxon rank sum p = .055, n 1 = 199 forays for age group 2, n 2 = 191 forays for
age group 3; p = .706, n 1 = 191 forays for age group 3, n 2 = 86 forays for group 4;
p = .143, n 1 = 199 forays for age group 2, n 2 = 86 forays for group 4

6000
5000
Kilocalories Collected
4000
3000
2000
1000
0
0 2 4 6 8 10 12 14 16 18
Age

Calorie Returns per Individual Foraging Trip
⇒
Does age affect food type?
Age IS a significant predictor of
the type of food collected
chi squared = 135.72, df = 24, p < .001

Kilocalories From Food Collected
140000
120000
100000
80000
60000
40000
20000
Total Kilocalories Collected, Split by Age
4 (14-17 yrs)
3 (10-13 yrs)
2 (5-9 yrs)
1 (1-4 yrs)
0
Baobab
Birds
Figs
Tubers
Honey
Berries
Small
Mammal
Drupes
Legumes

⇒ Does sex affect foraging return?
Mean return per trip =
males (n = 14) = 201 kcal
females (n = 20) = 281 kcal
Sex IS a significant predictor of foraging return ~
Female foragers bring back more food to camp
Wilcoxon rank sum p = .003, n 1 = 214 forays for males,
n 2 = 266 forays for females

Calorie Returns per Individual Foraging Trip
⇒
Does sex affect food type?
Sex IS also a significant predictor
of the type of food collected
chi squared =223.945, df = 8, p < .001

Kilocalories From Food Collected
140000
120000
100000
80000
60000
40000
20000
0
Total Kilocalories Collected, Split by Sex - Across Seasons
Female
Male
Baobab
Birds
Figs
Tubers
Honey
Berries
Small Mammal
Legumes

Focal Follows
Age DOES affect distance traveled ~
younger foragers stayed closer to camp
Wilcoxon rank sum test p = .005, n1 = 4 foragers < 10 years, n2 = 9 foragers > 10 years).
Average distance for males (n = 6 ) = 2.3 km
Average distance for females (n = 7 ) = 1.3 km
Sex of the child does NOT affect distance traveled
Wilcoxon rank sum test: p = .295

Average consumption by males (n = 6) = 2267 kcals
Average consumption by females (n = 7) = 667 kcals
Sex of the child DOES affect consumption while foraging ~
male foragers consume significantly more
Wilcoxon rank sum test: p = .014

Hadza juveniles are active foragers and routinely collect
a large portion of their daily requirements
Sex differences in collection and consumption may be linked
to differential foraging strategies and/or sex differences
in household responsibilities

Young male foragers target small mammals, birds, and honey.
This type of foraging is energetically demanding –
young male foragers may be feeding their own excess
energetic requirements

Young female foragers not only collect and prepare
food for other children, but also spend large amounts
of time caring for younger children in camp
There are greater demands on the time of young
female foragers

• Age does not appear to
significantly affect kilocalories back
to camp
• Age and sex differences in type of
food collected – follows adult
pattern of the sexual division of
labor among the Hadza
• Young female foragers bring back
more calories to camp
• Although there is no sex
difference in foraging distance,
male foragers consume more while
out foraging

Hadza juvenile foragers are able to self-provision and offset
the cost of their own care…
How do they distribute their foraging yield?

FOOD SHARING
2 camps (Gangidape = April 2005; Siponga = October 2005)
36 days
128 food transfers
n = 36 children in camp
n = 35 children participating in a food sharing event
(age 2 – 17 years; n 1 = 13 males and n 2 = 22 females)
n = 106 sharing dyads (n 1 = 54 kin and n 2 = 52 non-kin)
Food transfers range from 50kcal – 6,000kcal

Statistical analyses
Due to the unknown correlation structure among dyads
and because of possible non-Gaussian (non-linear)
responses, we used two non-parametric tests of statistical
significance:
(1) QAP (quadratic assignment procedure):
we constructed adjacency matrices, created null responses,
and iterated the process ~50,000 times
(2) Permutation testing:
simulations of the null distributions of the test statistic of
interest

Sex Differences in Food Sharing
NO difference between the amount of food that male and
female foragers share
(r = 0.0014, p = 0.503, two-sided permutation test on correlation)

Age Differences in Food Sharing
Age DOES predict total amount shared and frequency of sharing
correlation between increase in age and total amount shared
(r = 0.531, p = 0.0024, one-sided permutation test on correlation)
correlation between increase in age and frequency of sharing
(r = 0.608, p = 0.00006, one-sided permutation test on correlation)

Food sharing as a function of age

Partner Selection - kin selection or reciprocity?
Children who are genetically related tend to share
MORE food in HIGHER frequency
greater quantity (p = 0.00014, one-sided QAP on grand sum)
higher frequency (p ≈ 0; one-sided QAP on grand sum)
A higher degree of genetic relatedness
correlates with MORE food shared in
HIGHER frequency
(r = 0.141, p = 0.00016, one-sided QAP on correlation)

Partner Selection - kin selection or reciprocity?
Among genetically unrelated children, the
amount of food shared IS reciprocal
(r = 0.272, p = 0.036, one-sided QAP on correlation)
The frequency of sharing is NOT
reciprocal
(r = 0.226, p = 0.1101, one-sided QAP on
correlation)

Network graph showing observed meal sharing (arrows)
between children (circle = female, triangle = male)

Network graph showing observed meal sharing (arrows)
between children (circle = female, triangle = male)

Are Hadza juveniles selecting sharing partners based on
kinship or reciprocity?
Kinship first, reciprocity second
Is there a significant caloric difference between the
amounts shared with kin vs. non-kin?
More food, in greater frequency, is shared with kin

Young foragers not only self-provision, but also share
large portions of their yield with related and unrelated children.
These data support the hypothesis that other-regarding
preferences begin to strongly develop in middle childhood.
Middle childhood is the time when children begin to
incorporate the normative rules of the society in which they
live.

Acknowledgements
This research was funded by NSF, UC San Diego & Harvard University
Thank you to Frank Marlowe, my Tanzanian research assistants (Happy,
Golden, Ephraim, and Holiness), Jackie Benjamin (UCSD), and Dave Zes
(UCLA). Many many thanks to the Hadza for making this work so
enjoyable and for welcoming me into their lives…