The ontogeny of prosocial behavior: foraging and food sharing ...
The ontogeny of prosocial behavior: foraging and food sharing ... 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
- Page 2 and 3: Routine food sharing is argued to i
- Page 4 and 5: Very little is known the developmen
- Page 6: ~ Hadza Foragers ~ Approximately 10
- Page 12 and 13: In environments that are conducive
- Page 14 and 15: Ekwa - tuber (Vigna frutescens) Kon
- Page 17: Greg Downing Hadza children are in
- Page 21 and 22: 15% Wet Season Calories Collected 1
- Page 23 and 24: 6000 5000 Kilocalories Collected 40
- Page 25 and 26: Kilocalories From Food Collected 14
- Page 27 and 28: Calorie Returns per Individual Fora
- Page 29 and 30: Focal Follows Age DOES affect dista
- Page 31 and 32: Hadza juveniles are active foragers
- Page 33 and 34: Young female foragers not only coll
- Page 35 and 36: Hadza juvenile foragers are able to
- Page 37 and 38: Statistical analyses Due to the unk
- Page 39 and 40: Age Differences in Food Sharing Age
- Page 41 and 42: Partner Selection - kin selection o
- Page 43 and 44: Network graph showing observed meal
- Page 45 and 46: Are Hadza juveniles selecting shari
- Page 47: Acknowledgements This research was
<strong>The</strong> <strong>ontogeny</strong> <strong>of</strong> <strong>prosocial</strong> <strong>behavior</strong>: <br />
<strong>foraging</strong> <strong>and</strong> <strong>food</strong> <strong>sharing</strong> among Hadza <br />
hunter-gatherer children<br />
Alyssa N. Crittenden<br />
<br />
Visiting Assistant Pr<strong>of</strong>essor<br />
Department <strong>of</strong> Anthropology<br />
University <strong>of</strong> Nevada, Las Vegas
Routine <strong>food</strong> <strong>sharing</strong> is argued to<br />
influence the evolution <strong>of</strong>:<br />
• Life history<br />
• Family formation<br />
• Cooperation<br />
• Economies <strong>of</strong> scale<br />
Food transfers can have multiple functions<br />
• Develop <strong>and</strong> maintain social bonds<br />
• Reduce variance <strong>of</strong> <strong>food</strong> consumption –<br />
provision others
Evolutionary Models<br />
Kin Selection<br />
An individual should be willing to share as long as the<br />
cost is less than the benefit, times the coefficient <strong>of</strong><br />
relatedness between individuals C < B x r<br />
(Hamilton 1964)<br />
Reciprocal Altruism<br />
Givers <strong>and</strong> receivers reverse positions on a systematic basis,<br />
such that amounts received <strong>and</strong> given are correlated<br />
(Axelrod 1984; Trivers 1971)
Very little is known the development <strong>of</strong> <strong>food</strong> <strong>sharing</strong> in<br />
children <strong>and</strong> how it may influence the development<br />
<strong>of</strong> <strong>prosocial</strong> <strong>behavior</strong><br />
Hadza juveniles are active <strong>food</strong> collectors <strong>and</strong> can produce<br />
up to 50% <strong>of</strong> their daily caloric intake by the age <strong>of</strong> 5 years<br />
(Blurton Jones et al. 1989, 1994)<br />
<strong>The</strong>y also share substantial portions <strong>of</strong> their collection with others<br />
(Crittenden et al. 2010)
Current Study:<br />
(1) How much do Hadza children forage <strong>and</strong> can they<br />
<strong>of</strong>fset the cost <strong>of</strong> their own care?<br />
(2) Are Hadza children selecting <strong>sharing</strong> partners<br />
based on kinship <strong>and</strong>/or reciprocity?<br />
(3) Is there a significant caloric difference between the<br />
amount shared with kin vs. non-kin?
~ Hadza Foragers ~<br />
Approximately 1000 individuals (300 hunt <strong>and</strong> gather)<br />
Northern Tanzania ~ Lake Eyasi<br />
Camp membership is fluid ~ 30 residents<br />
Residence pattern is variable ~ wifes kin OR husb<strong>and</strong>s kin
Juvenile <strong>foraging</strong><br />
Previous research suggests that <strong>food</strong> collection by children is<br />
constrained by ecological parameters <strong>and</strong> costs <strong>of</strong> collection<br />
<strong>and</strong> processing<br />
• Low predator pressure<br />
• Access to drinking water<br />
• Variability in terrain
In environments that are conducive to juvenile <strong>foraging</strong>,<br />
young foragers can be avid <strong>and</strong> productive <strong>food</strong> collectors<br />
Hadza <strong>of</strong> Tanzania<br />
- Hawkes, OConnell & Blurton Jones (1995)<br />
Current Anthropology 36:4, 688-700.<br />
- Blurton Jones, Hawkes & OConnell (1997)<br />
Uniting Psychology <strong>and</strong> Biology: Integrative Perspectives on Human<br />
Development<br />
Martu <strong>of</strong> Australia<br />
- Bird & Bliege-Bird (2005)<br />
Hunter-Gatherer Childhoods<br />
Mikea <strong>of</strong> Madagascar<br />
- Tucker & Young (2005)<br />
Hunter-Gatherer Childhoods
Gangidape ~ April - May 2005 (wet season)<br />
Siponga ~ October 2005 (dry season)<br />
Total Sample: 65 days<br />
• Children (1 - 17 years) n = 43<br />
• Children who foraged (3 - 17 years) n = 34<br />
Foraging Returns:<br />
481 instances <strong>of</strong> <strong>food</strong> back to camp<br />
(n 1 = 14 males, n 2 = 20 females)<br />
Focal Follows:<br />
- 13 follows <strong>of</strong> individual foragers<br />
(n 1 = 6 males, n 2 = 7 females)<br />
- Age range 6 - 14 years<br />
- Ranged from 50 minutes to<br />
5 hours in length, depending<br />
on distance traveled
Ekwa - tuber<br />
(Vigna frutescens)<br />
Kongolobe - berries<br />
Hornbill - bird<br />
Embilipe - berries<br />
Nguilabe - berries<br />
Baalako - honey<br />
Ogoyo - wild fig<br />
Mangwala- legume
Weaver bird seasonal breeding <strong>and</strong><br />
Hadza exploitation <strong>of</strong> nestling birds<br />
Genus Ploceus
Greg Downing<br />
Hadza children are in direct <strong>foraging</strong> competition with baboons<br />
- baobab pods & seeds<br />
- figs<br />
- drinking water
Calories Total Brought Foods Kilocalories Back Collected to Back Camp by to Children by Camp Children<br />
(collapsed across all forays)
15%<br />
Wet Season Calories Collected<br />
19%<br />
19%<br />
15%<br />
Baobab<br />
Berries<br />
Birds<br />
Honey<br />
Tubers<br />
Fruit = 34%<br />
Honey = 15%<br />
Tubers = 19%<br />
Birds = 32%<br />
32%<br />
Dry Season Calories Collected<br />
10%<br />
Fruit = 84%<br />
Legumes = 1%<br />
Birds = 5%<br />
Small Mammals = 10%<br />
1%<br />
11%<br />
8%<br />
5%<br />
4%<br />
61%<br />
Baobab<br />
Berries<br />
Birds<br />
Drupes<br />
Figs<br />
Legumes<br />
Small Mammals
⇒ Does age affect <strong>foraging</strong> return?<br />
Age groups:<br />
(1) 1 - 4 years (n = 2) = 67 kcal<br />
(2) 5 - 9 years (n = 14) = 724 kcal<br />
(3) 10 - 13 years (n = 11) = 634 kcal<br />
(4) 14 - 17 years (n = 7) = 618 kcal<br />
<strong>The</strong> mean values <strong>of</strong> calorie returns per <strong>foraging</strong> trip for<br />
each age group were compared…<br />
Age is NOT a significant predictor <strong>of</strong> <strong>foraging</strong> return<br />
Wilcoxon rank sum p = .055, n 1 = 199 forays for age group 2, n 2 = 191 forays for<br />
age group 3; p = .706, n 1 = 191 forays for age group 3, n 2 = 86 forays for group 4;<br />
p = .143, n 1 = 199 forays for age group 2, n 2 = 86 forays for group 4
6000<br />
5000<br />
Kilocalories Collected<br />
4000<br />
3000<br />
2000<br />
1000<br />
0<br />
0 2 4 6 8 10 12 14 16 18 <br />
Age
Calorie Returns per Individual Foraging Trip<br />
⇒<br />
Does age affect <strong>food</strong> type?<br />
Age IS a significant predictor <strong>of</strong><br />
the type <strong>of</strong> <strong>food</strong> collected<br />
chi squared = 135.72, df = 24, p < .001
Kilocalories From Food Collected<br />
140000<br />
120000<br />
100000<br />
80000<br />
60000<br />
40000<br />
20000<br />
Total Kilocalories Collected, Split by Age<br />
4 (14-17 yrs)<br />
3 (10-13 yrs)<br />
2 (5-9 yrs)<br />
1 (1-4 yrs)<br />
0<br />
Baobab<br />
Birds<br />
Figs<br />
Tubers<br />
Honey<br />
Berries<br />
Small<br />
Mammal<br />
Drupes<br />
Legumes
⇒ Does sex affect <strong>foraging</strong> return?<br />
Mean return per trip =<br />
males (n = 14) = 201 kcal<br />
females (n = 20) = 281 kcal<br />
Sex IS a significant predictor <strong>of</strong> <strong>foraging</strong> return ~<br />
Female foragers bring back more <strong>food</strong> to camp<br />
Wilcoxon rank sum p = .003, n 1 = 214 forays for males,<br />
n 2 = 266 forays for females
Calorie Returns per Individual Foraging Trip<br />
⇒<br />
Does sex affect <strong>food</strong> type?<br />
Sex IS also a significant predictor<br />
<strong>of</strong> the type <strong>of</strong> <strong>food</strong> collected<br />
chi squared =223.945, df = 8, p < .001
Kilocalories From Food Collected<br />
140000<br />
120000<br />
100000<br />
80000<br />
60000<br />
40000<br />
20000<br />
0<br />
Total Kilocalories Collected, Split by Sex - Across Seasons<br />
Female<br />
Male<br />
Baobab<br />
Birds<br />
Figs<br />
Tubers<br />
Honey<br />
Berries<br />
Small Mammal<br />
Legumes
Focal Follows<br />
Age DOES affect distance traveled ~<br />
younger foragers stayed closer to camp<br />
Wilcoxon rank sum test p = .005, n1 = 4 foragers < 10 years, n2 = 9 foragers > 10 years).<br />
Average distance for males (n = 6 ) = 2.3 km<br />
Average distance for females (n = 7 ) = 1.3 km<br />
Sex <strong>of</strong> the child does NOT affect distance traveled<br />
Wilcoxon rank sum test: p = .295
Average consumption by males (n = 6) = 2267 kcals<br />
Average consumption by females (n = 7) = 667 kcals<br />
Sex <strong>of</strong> the child DOES affect consumption while <strong>foraging</strong> ~<br />
male foragers consume significantly more<br />
Wilcoxon rank sum test: p = .014
Hadza juveniles are active foragers <strong>and</strong> routinely collect<br />
a large portion <strong>of</strong> their daily requirements<br />
Sex differences in collection <strong>and</strong> consumption may be linked<br />
to differential <strong>foraging</strong> strategies <strong>and</strong>/or sex differences<br />
in household responsibilities
Young male foragers target small mammals, birds, <strong>and</strong> honey.<br />
This type <strong>of</strong> <strong>foraging</strong> is energetically dem<strong>and</strong>ing –<br />
young male foragers may be feeding their own excess<br />
energetic requirements
Young female foragers not only collect <strong>and</strong> prepare<br />
<strong>food</strong> for other children, but also spend large amounts<br />
<strong>of</strong> time caring for younger children in camp<br />
<strong>The</strong>re are greater dem<strong>and</strong>s on the time <strong>of</strong> young<br />
female foragers
• Age does not appear to<br />
significantly affect kilocalories back<br />
to camp<br />
• Age <strong>and</strong> sex differences in type <strong>of</strong><br />
<strong>food</strong> collected – follows adult<br />
pattern <strong>of</strong> the sexual division <strong>of</strong><br />
labor among the Hadza<br />
• Young female foragers bring back<br />
more calories to camp<br />
• Although there is no sex<br />
difference in <strong>foraging</strong> distance,<br />
male foragers consume more while<br />
out <strong>foraging</strong>
Hadza juvenile foragers are able to self-provision <strong>and</strong> <strong>of</strong>fset<br />
the cost <strong>of</strong> their own care…<br />
How do they distribute their <strong>foraging</strong> yield?
FOOD SHARING<br />
2 camps (Gangidape = April 2005; Siponga = October 2005)<br />
36 days<br />
128 <strong>food</strong> transfers<br />
n = 36 children in camp<br />
n = 35 children participating in a <strong>food</strong> <strong>sharing</strong> event<br />
(age 2 – 17 years; n 1 = 13 males <strong>and</strong> n 2 = 22 females)<br />
n = 106 <strong>sharing</strong> dyads (n 1 = 54 kin <strong>and</strong> n 2 = 52 non-kin)<br />
Food transfers range from 50kcal – 6,000kcal
Statistical analyses<br />
Due to the unknown correlation structure among dyads<br />
<strong>and</strong> because <strong>of</strong> possible non-Gaussian (non-linear)<br />
responses, we used two non-parametric tests <strong>of</strong> statistical<br />
significance:<br />
(1) QAP (quadratic assignment procedure):<br />
we constructed adjacency matrices, created null responses,<br />
<strong>and</strong> iterated the process ~50,000 times<br />
(2) Permutation testing:<br />
simulations <strong>of</strong> the null distributions <strong>of</strong> the test statistic <strong>of</strong><br />
interest
Sex Differences in Food Sharing<br />
NO difference between the amount <strong>of</strong> <strong>food</strong> that male <strong>and</strong><br />
female foragers share<br />
(r = 0.0014, p = 0.503, two-sided permutation test on correlation)
Age Differences in Food Sharing<br />
Age DOES predict total amount shared <strong>and</strong> frequency <strong>of</strong> <strong>sharing</strong><br />
correlation between increase in age <strong>and</strong> total amount shared<br />
(r = 0.531, p = 0.0024, one-sided permutation test on correlation)<br />
correlation between increase in age <strong>and</strong> frequency <strong>of</strong> <strong>sharing</strong><br />
(r = 0.608, p = 0.00006, one-sided permutation test on correlation)
Food <strong>sharing</strong> as a function <strong>of</strong> age
Partner Selection - kin selection or reciprocity?<br />
Children who are genetically related tend to share<br />
MORE <strong>food</strong> in HIGHER frequency<br />
greater quantity (p = 0.00014, one-sided QAP on gr<strong>and</strong> sum)<br />
higher frequency (p ≈ 0; one-sided QAP on gr<strong>and</strong> sum)<br />
A higher degree <strong>of</strong> genetic relatedness<br />
correlates with MORE <strong>food</strong> shared in<br />
HIGHER frequency<br />
(r = 0.141, p = 0.00016, one-sided QAP on correlation)
Partner Selection - kin selection or reciprocity?<br />
Among genetically unrelated children, the<br />
amount <strong>of</strong> <strong>food</strong> shared IS reciprocal<br />
(r = 0.272, p = 0.036, one-sided QAP on correlation)<br />
<strong>The</strong> frequency <strong>of</strong> <strong>sharing</strong> is NOT<br />
reciprocal<br />
(r = 0.226, p = 0.1101, one-sided QAP on<br />
correlation)
Network graph showing observed meal <strong>sharing</strong> (arrows)<br />
between children (circle = female, triangle = male)
Network graph showing observed meal <strong>sharing</strong> (arrows)<br />
between children (circle = female, triangle = male)
Are Hadza juveniles selecting <strong>sharing</strong> partners based on<br />
kinship or reciprocity?<br />
Kinship first, reciprocity second<br />
Is there a significant caloric difference between the<br />
amounts shared with kin vs. non-kin?<br />
More <strong>food</strong>, in greater frequency, is shared with kin
Young foragers not only self-provision, but also share<br />
large portions <strong>of</strong> their yield with related <strong>and</strong> unrelated children.<br />
<strong>The</strong>se data support the hypothesis that other-regarding<br />
preferences begin to strongly develop in middle childhood.<br />
Middle childhood is the time when children begin to<br />
incorporate the normative rules <strong>of</strong> the society in which they<br />
live.
Acknowledgements<br />
This research was funded by NSF, UC San Diego & Harvard University<br />
Thank you to Frank Marlowe, my Tanzanian research assistants (Happy,<br />
Golden, Ephraim, <strong>and</strong> Holiness), Jackie Benjamin (UCSD), <strong>and</strong> Dave Zes<br />
(UCLA). Many many thanks to the Hadza for making this work so<br />
enjoyable <strong>and</strong> for welcoming me into their lives…