7/29/2019 Payment for Labour in Monkeys
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Cooperative hunting, in which severalindividuals pursue prey but
only onemakes a capture, is central to theories
of human social and moral evolution13. Butamong other primates,
it is known onlyfrom the chimpanzee and a large-brainedneotropical
monkey, the capuchin47. Itprobably evolved through either
mutual-ism, in which two or more cooperatorsbenefit simultaneously,
or reciprocal altru-ism, in which one favour is repaid by
anoth-er8,9. We have found that brown capuchins(Cebus apella) share
rewards obtained by ajoint effort more readily than rewards
obtained individually. Even if hunting inthe field involves
selfish opportunism, this
food incentive will greatly enhance the per-sistence of
cooperation.
We made a pair of monkeys work forfood10, by placing two
transparent bowls infull view. Each bowl was accessible to
onemonkey by pulling the tray towards itselfusing one of two
protruding bars (Fig. 1a).Individual strength was periodically
testedto determine each individuals maximumpulling weight. We
investigated three condi-tions: solo effort (only one monkey
hadaccess to a pull bar and a baited cup, andthe tray was
counterweighted within thisindividuals pulling capacity);
cooperation
(both monkeys had pull bars, and the traywas counterweighted
such that the strengthof both was required, but only one bowlwas
baited), and mutualism (same as co-operation, but with both bowls
baited).
We put eight apple slices into the bowl(s)at the start of each
of four 10-min trials pertest. After pre-training, we did a
minimumof 24 cooperation, 8 mutualism and 8 solo-effort tests on
each same-sex pair (5 femaleand 2 male pairs) of unrelated adults
fromthe same social group, applying half the teststo each direction
within a pair. Thecapuchins successfully pulled in the tray in
a
mean (
s. e.) of 85.4
3.5% of solo-effort trials and 88.9 2.7% of mutualismtrials. The
success rate for cooperation trialswas substantially lower, 39.2
3.1%(paired comparison with mutualism: t14.06, P< 0.001; with
solo effort: t14.56, P< 0.001).
Capuchins will share attractive foodsspontaneously, even if
separated by a meshrestraint a pattern known as facilitatedtaking,
with the possessor approaching thedivider and dropping crumbs or
wholepieces while the partner reaches for thefood11. As the
possessor could monopolizethe food by avoiding the divider, both
par-ties play an active role. Facilitated taking isreciprocal
across individuals as well asacross time between any two
individuals12.
We measured the amount of sharing asthe number of times the
partners handreached through the mesh to pick up foodfrom the other
side, limiting ourselves tounambiguous videotaped behaviour.
Weignored mutualism tests, in which eachindividual had its own
food. Comparedwith solo-effort tests, significantly morepieces of
food were shared after successfulcooperation trials. Moreover, a
greater pro-portion of transfers after cooperation wereof a
tolerant nature (Fig. 1b).
Because the individual with the reward-ing bowl was invariably
motivated to pull,the helpers behaviour was decisive. We
found that the helper pulled two to threetimes more often in
cooperation trials if thepreceding trial had been successful than
if ithad been a failure (analysis of variance: F1,11 12.41, P
0.003, directional). This sug-gests either a stable motivation
across trialsor a causal connection between a share ofthe reward (9
out of 10 successful trialsresulted in food transfer) and
subsequentwillingness to pull.
We have shown that capuchins cooper-ate even if it is obvious
that only one ofthem, and which one, will be rewarded. Theincrease
in sharing following cooperation
may rest on psychological mechanisms ascomplex as mental
score-keeping ofservices13 and gratitude8, or as simple
asattitudinal reciprocity12. According to thelatter explanation, a
joint effort, and themutual coordination this entails, mayinduce a
positive attitude towards the part-ner, reflected in attraction and
social toler-ance. If this facilitates the sharing ofpay-offs, in
turn providing an incentive forcontinued cooperation, we have two
mech-anisms that together function as paymentfor labour and labour
for payment.Frans B. M. de Waal, Michelle L. BergerLiving Links,
Yerkes Regional Primate Research
Center, and Department of Psychology,
Emory University, Atlanta, Georgia 30329, USA
e-mail: [email protected]
1. de Waal, F. B. M.Good Natured(Harvard Univ. Press,
Cambridge, MA, 1996).
2. Whiten, A. in The Descent of Mind(eds Corballis, M. C. &
Lee,
S. E. G.) 173193 (Oxford Univ. Press, 1999).
3. Stanford, C. B. The Hunting Apes(Princeton Univ. Press,
Princeton, NJ, 1999).
4. Goodall, J. in Primate Behavior(ed. DeVore, I.) 425473
(Holt,
New York, 1965).
5. Boesch, C. & Boesch, H. Am. J. Phys. Anthropol. 78,
547573
(1989).
6. Perry, S. & Rose, L. Primates35, 409415 (1994).
7. Rose, L. Int. J. Primatol. 18, 727765 (1997).
8. Trivers, R. L. Q. Rev. Biol. 46, 3557 (1971).
9. Dugatkin, L. A. Cooperation Among Animals(Oxford Univ.
Press, 1997).
10.Crawford, M. P. Comp. Psych. Monogr. 14, 188 (1937).
11.de Waal, F. B. M.J. Comp. Psychol. 111, 370378 (1997).
12.de Waal, F. B. M. Anim. Behav. (in the press).
13.de Waal, F. B. M. Evol. Hum. Behav. 18, 375386 (1997).
brief communications
NATURE | VOL 404 | 6 APRIL 2000 | www.nature.com 563
Payment for labour in monkeysCapuchins will voluntarily share
treats with other monkeys that helped to secure them.
Trial
1 2 - 4
Piecesshared(mean)
0
6
8
10
12Cooperation
Solo
Rewarding cup Empty cup
Mesh partitiona
b
Figure 1 Cooperation and sharing. a, In a 144 60 60 cm
test chamber, two monkeys were divided by a mesh partition.
In
cooperation tests, the strength of both monkeys was required
to
pull in a tray with two transparent bowls. In all 4 trials per
test,
apple slices were placed in the same bowl. In solo-effort
controls,
the bar in front of the empty cup was removed and the
counter-
weight reduced. b, The rate of facilitated taking was higher
after
cooperation than after solo controls (ANOVA: F1,12 5.63, P
0.018, directional). Also, the mean ( s. e.) percentage of
food
collections taking place within reach and sight of the
possessor
was 64.84.5% after cooperation, compared with 57.94.8%
after solo controls (F1,12 10.44, P 0.0035, directional).
For
further details see www.emory.edu/LIVING_LINKS/
Figure 2A hopeful young capuchin watches an adult eating.
FRANSB.M.DEWAA
L
2000 Macmillan Magazines Ltd
