3.When rhesus hear such calls,how do they classify them? Are natural
categories constructed on the basis of the caller’s affective state,type of
food discovered,or some combination of factors?Based on a large sample of adult males and females,we first looked
at changes in call production as a function of food consumption and time
of day—a proxy for hunger level.Chow was placed into the dispensers
early in the morning and was finished by midday.Thus,rhesus did little
foraging from about 4:00 p.m.to 8:00 a.m.the next day.We therefore
assumed that they would be most hungry in the early morning and max-
imally satiated in the late afternoon.
In general,males produced food-associated calls less frequently than
females.If these calls function to recruit kin,this sex difference makes
sense.As in most mammalian societies,male rhesus monkeys leave their
natal groups on reaching sexual maturity,whereas females stay.Conse-
quently,groups consist of closely related females and distantly related
males.Other factors may also contribute to this pattern,such as sex dif-
ferences in arousal levels,social relationships with nonkin,and so forth;
at present it is not possible to determine which of these potential factors
is most important.In addition to a sex difference,we found that the rate
of food call production was highest in early morning before chow was
put out and declined rapidly thereafter.In particular,the rate peaked
before the peak in food consumption and dropped more rapidly than did
food consumption.This pattern suggests that call rate is positively cor-
related with hunger level.
To explore further the relationship between hunger level and vocal
production,we looked at call rate and an individual’s latency to arrive
and feed at a chow dispenser.While chow was placed into a dispenser,
one or more groups sat around the corral waiting to feed.During this
time,a number of animals called,apparently in anticipation of feeding.
Figure 6.1 plots the rate of food calling against latency to arrive and feed
(time elapsed from the placement of chow in the dispenser to feeding);
although all food call types are pooled here,most of them were coos.The
data set includes one to four focal samples each from twenty adult males
and females.Results indicate that as latency to feed increased,call rate
decreased;that is,individuals who called at high rates fed first.This
pattern is not accounted for by systematic individual differences.Indi-
vidual patterns of calling varied on a daily basis,as revealed by subject
480 (figure 6.1).On one day,this low-ranking female called at a high rate
and fed first,and on a second day,called at a low rate and fed relatively
late.Thus,and in parallel with the first set of analyses,call rate appears
to provide some information about the caller’s hunger level and moti-
vation to feed.80 Marc D.Hauser