evolution of different alarm calls and escape strategies in vervets. Thus, in
terms of the first condition for exploring lies of omission, vervets are ideally
suited. These three alarm calls are primarily heard in the context of predator encounters, and, in particular, snake alarm calls are given to snakes,
eagle alarm calls to eagles, and leopard alarm calls to leopards. When such
calls are heard, listeners respond in highly specific ways that are suited to
the style of predation. Two field observations suggest that, as in chickens,
the social context also mediates alarm-call production in vervets. First,
Cheney and Seyfarth (1988) noted that when lone vervets detect a predator, they remain silent. Second, low-ranking animals produce far fewer
alarm calls than do high-ranking animals. The difference in alarm-call rate
between high- and low-ranking animals is not due to differences in the
number of kin or in the probability of detecting a predator. Rather, the
observations suggest that low-ranking animals may actively suppress their
alarm calls, committing lies of omission.
To test the prediction that vervet monkey alarm calls are mediated by
social context, Cheney and Seyfarth (1985) conducted an experiment with
captive vervet monkeys. Using a human dressed up as a predator (a graduate student in a monkey mask and lab coat, carrying a net), adult females
were tested in the presence of their offspring or an unrelated but agematched infant, whereas adult males were tested in the presence of an adult
female or another adult male. Results demonstrated flexibility in alarm
calling in vervets, with both males and females producing calls in some circumstances and suppressing them in others. Females produced significantly
more alarm calls in the presence of their offspring than in the presence of
unrelated infants, and males produced more alarm calls in the presence of
adult females than in the presence of adult males; adult males were virtually silent in the presence of other males. For females, therefore, kinship
appears to play an important role in the mediation of alarm calls, whereas
for males, mating opportunities and intrasexual competition play a role.
As with studies of chickens, these experiments clearly show that both wild
and captive vervets have the capacity to withhold information. They do not,
however, allow us to assess the costs and benefits of such omissions. To
better assess the economics of withholding information, one would need to
perform similar experiments over a longer time course, assessing how individuals respond to those who have concealed information about predators,
food, and so forth. We now turn to such a study.
Many primate species produce distinctive calls in the context of food
(Wrangham 1977; Dittus 1984; Hauser and Wrangham 1987; Elowson et al.
1991; Benz 1993; Hauser and Marler 1993a and b). Like chickens, food commonly elicits such calls, and, in many species, the rate of call production
covaries with the quality of the food or the individual’s preference for a
particular food type. When individuals hear food-associated calls, they
typically orient and then approach the caller and sometimes call back with
the same call type.
3. Unpacking “Honesty”
121
terms of the first condition for exploring lies of omission, vervets are ideally
suited. These three alarm calls are primarily heard in the context of predator encounters, and, in particular, snake alarm calls are given to snakes,
eagle alarm calls to eagles, and leopard alarm calls to leopards. When such
calls are heard, listeners respond in highly specific ways that are suited to
the style of predation. Two field observations suggest that, as in chickens,
the social context also mediates alarm-call production in vervets. First,
Cheney and Seyfarth (1988) noted that when lone vervets detect a predator, they remain silent. Second, low-ranking animals produce far fewer
alarm calls than do high-ranking animals. The difference in alarm-call rate
between high- and low-ranking animals is not due to differences in the
number of kin or in the probability of detecting a predator. Rather, the
observations suggest that low-ranking animals may actively suppress their
alarm calls, committing lies of omission.
To test the prediction that vervet monkey alarm calls are mediated by
social context, Cheney and Seyfarth (1985) conducted an experiment with
captive vervet monkeys. Using a human dressed up as a predator (a graduate student in a monkey mask and lab coat, carrying a net), adult females
were tested in the presence of their offspring or an unrelated but agematched infant, whereas adult males were tested in the presence of an adult
female or another adult male. Results demonstrated flexibility in alarm
calling in vervets, with both males and females producing calls in some circumstances and suppressing them in others. Females produced significantly
more alarm calls in the presence of their offspring than in the presence of
unrelated infants, and males produced more alarm calls in the presence of
adult females than in the presence of adult males; adult males were virtually silent in the presence of other males. For females, therefore, kinship
appears to play an important role in the mediation of alarm calls, whereas
for males, mating opportunities and intrasexual competition play a role.
As with studies of chickens, these experiments clearly show that both wild
and captive vervets have the capacity to withhold information. They do not,
however, allow us to assess the costs and benefits of such omissions. To
better assess the economics of withholding information, one would need to
perform similar experiments over a longer time course, assessing how individuals respond to those who have concealed information about predators,
food, and so forth. We now turn to such a study.
Many primate species produce distinctive calls in the context of food
(Wrangham 1977; Dittus 1984; Hauser and Wrangham 1987; Elowson et al.
1991; Benz 1993; Hauser and Marler 1993a and b). Like chickens, food commonly elicits such calls, and, in many species, the rate of call production
covaries with the quality of the food or the individual’s preference for a
particular food type. When individuals hear food-associated calls, they
typically orient and then approach the caller and sometimes call back with
the same call type.
3. Unpacking “Honesty”
121