Hauser and Marler (1993) investigated the food-associated calls of rhesus
monkeys living on the island of Cayo Santiago off the coast of Puerto Rico.
When individuals discover food, they give one or more of five acoustically
distinct vocalizations. Three of these call types are given when high-quality,
rare food is discovered, whereas the two other call types are given when
lower-quality, common food is discovered. Playback experiments reveal that
these call types are classified on the basis of their putative referents and not
on the basis of acoustic morphology (Hauser 1998).
Given that rhesus monkeys are sometimes silent when they discover
food, experiments were conducted to provide a more precise quantification
of the necessary and sufficient conditions for call production as opposed
to suppression. A lone individual was presented with the same quantity
of either monkey chow (low-quality/common food) or coconut (highquality/rare food) and its behavior recorded. On approximately 50% of all
trials with individuals who were members of a social group, the food was
consumed, but there were no vocalizations. Paralleling our naturalistic
observations, males called less often in the context of a food discovery than
did females. However, there were no differences in call rate between highand low-ranking discoverers. Independently of rank or sex, individuals who
called obtained more food, and received less aggression, than individuals
who were silent and caught with the food by other group members. That is,
silent discoverers were chased and physically attacked, and, as a result,
obtained less food. However, individuals who were silent and never caught
at the food source obtained more food than anyone else. These results
suggest that rhesus monkeys have the capacity to commit lies of omission
and that there are measurable social costs and benefits to such deception.
What makes the rhesus case particularly interesting is that the dynamics
of this kind of deception change as a function of the discoverer’s group
status. All of the results reported above were obtained from individuals resident within a social group. When the same experiment is conducted on
peripheral males—individuals who have yet to join a social group—such
males never call and when they are caught with food are never attacked
(Hauser 1997). Rather, when members of a social group catch a peripheral
male, they supplant him from the food or chase him away without making
physical contact. Thus, peripheral males can get away with lies of omission
without paying the costs. One explanation for this difference between resident and peripheral males may be that only resident males are involved in
subsequent social interactions.
The rhesus food-call system satisfies the conditions for an analysis of lies
of omission and furthermore provides some evidence of an immediate punishment for those who commit them. Food calls are generally produced in
the context of finding food, and when individuals hear them, they approach.
Thus, there is a strong association between call and context, and the call
elicits a predictable response.When animals withhold information about the
discovery of food, they incur the costs of targeted aggression if caught but
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monkeys living on the island of Cayo Santiago off the coast of Puerto Rico.
When individuals discover food, they give one or more of five acoustically
distinct vocalizations. Three of these call types are given when high-quality,
rare food is discovered, whereas the two other call types are given when
lower-quality, common food is discovered. Playback experiments reveal that
these call types are classified on the basis of their putative referents and not
on the basis of acoustic morphology (Hauser 1998).
Given that rhesus monkeys are sometimes silent when they discover
food, experiments were conducted to provide a more precise quantification
of the necessary and sufficient conditions for call production as opposed
to suppression. A lone individual was presented with the same quantity
of either monkey chow (low-quality/common food) or coconut (highquality/rare food) and its behavior recorded. On approximately 50% of all
trials with individuals who were members of a social group, the food was
consumed, but there were no vocalizations. Paralleling our naturalistic
observations, males called less often in the context of a food discovery than
did females. However, there were no differences in call rate between highand low-ranking discoverers. Independently of rank or sex, individuals who
called obtained more food, and received less aggression, than individuals
who were silent and caught with the food by other group members. That is,
silent discoverers were chased and physically attacked, and, as a result,
obtained less food. However, individuals who were silent and never caught
at the food source obtained more food than anyone else. These results
suggest that rhesus monkeys have the capacity to commit lies of omission
and that there are measurable social costs and benefits to such deception.
What makes the rhesus case particularly interesting is that the dynamics
of this kind of deception change as a function of the discoverer’s group
status. All of the results reported above were obtained from individuals resident within a social group. When the same experiment is conducted on
peripheral males—individuals who have yet to join a social group—such
males never call and when they are caught with food are never attacked
(Hauser 1997). Rather, when members of a social group catch a peripheral
male, they supplant him from the food or chase him away without making
physical contact. Thus, peripheral males can get away with lies of omission
without paying the costs. One explanation for this difference between resident and peripheral males may be that only resident males are involved in
subsequent social interactions.
The rhesus food-call system satisfies the conditions for an analysis of lies
of omission and furthermore provides some evidence of an immediate punishment for those who commit them. Food calls are generally produced in
the context of finding food, and when individuals hear them, they approach.
Thus, there is a strong association between call and context, and the call
elicits a predictable response.When animals withhold information about the
discovery of food, they incur the costs of targeted aggression if caught but
122
W.T. Fitch and M.D. Hauser