AMER. ZOOL.,
14:303-315
(1974).
Exploration and Social Play in Squirrel Monkeys (Saimiri)
JOHN D. BALDWIN AND JANICE I. BALDWIN
Department of Sociology,
University of California,
Santa Barbara, California 93106
SYNOPSIS. Squirrel monkeys (Saimiri) have been studied in a variety of laboratory and
natural environments. The frequency and form of exploration and social play vary
considerably among different environments. For example, in some environments, young
monkeys have been observed to play for 3 hr per day; but in one natural environment,
not a single bout of social play was seen during a 10-week intensive study. Numerous
intermediate levels of play activity have been observed.
Whereas many theories of play make it appear that play is essential for the development of sexual behavior, integrated roles in troop structure, control of aggressive responses, social cohesion, etc., the data on squirrel monkeys indicate that social organization and many normal social behaviors can develop without social play. However, the
opportunity to play socially provides learning experiences that increase the variety of
each animal's behavioral repertoire and the subtly of social cues to which it can respond.
An adaptive modicum of competence can appear without social play, but the opportunity to play socially develops the competence of animals beyond that modicum.
(1967, 1968) conducted the first field study
on squirrel monkeys in 1965 on the llanos
Socially living groups of squirrel mon- of Colombia. Our research has included
keys (Saimiri) have been studied in an in- studies on a semifree-ranging troop in Florteresting variety of different environments. ida, a field study in Panama, a survey of 43
Several of these studies have dealt with troops in Panama, Colombia, Brazil, and
exploration and play, and taken together, Peru, and a laboratory study at the Maxthey show that the development of play Planck-Institute (Baldwin, 1969; Baldwin
varies among different environments. Ploog and Baldwin, 1971, 1972). Klein and Klein
and his colleagues at the Max-Planck-Insti- (personal communication) made observatute for Psychiatry in Munich have been tions on social play in squirrel monkeys
conducting diversified laboratory research during a 19-month field study on spider
for over a decade (Ploog et al., 1963, 1967). monkeys (A teles belzebuth) in the La MacaRosenblum (1968) has conducted labora- rena area of Colombia. Recently, Bailey
tory research on infant development and has been conducting observations on freemother-infant relationships. Thorington ranging troops on Santa Sophia Island in
the Amazon River in Colombia. Although
We would like to thank the following people for
has been only one target of research
play
their generous assistance in various aspects of our
in these studies, all of the studies
interest
work: Mr. and Mrs. Julio Arauz, Mr. Alberto Cadena, Mr. Frank DuMond, Dr. Sigrid Hopf, Dr. have provided valuable data on the relaC. W. Marinkelle, Dr. M. Maurus, Dr. Allan Mazur, tionship between play and the environment.
Dr. Martin Moynihan, Dr. Detlev Ploog, Dr. L. A.
It is already clear that there is considerRosenblum, Dr. Charles Southwick, and Dr. Richard
able variation in the play of squirrel monThorington.
Our studies were supported by a Public Health keys. For example, in some environments,
Service Fellowship (1-F1-MH-29, 131; BEH) from young squirrel monkeys have been reported
the National Institute of Mental Health, a small to play for 3 hr per day, but in one natural
grant (GS-1474) from the National Science Founda- environment not a single bout of social play
tion, Public Health Service grants (MN-16482-01)
and (1 ROS MH17882-01) from the National Insti- was observed during a 10-week, intensive
study. Numerous intermediate levels of play
tute of Mental Health.
INTRODUCTION
303
304
JOHN D. BALDWIN AND JANICE I. BALDWIN
activity have been observed. The differences
in play patterns covary with numerous complex ecological and social factors, such as
forest size, troop size, food abundance, adult
sociability, etc. The large number of interacting conditions that vary among environments demonstrates the complexity of factors that influence and are influenced by
play. In spite of the complexity, however,
certain patterns do emerge.
In this paper we will review the literature on exploration and social play in squirrel monkeys in order to provide descriptive
material on the frequency and style of play
in a variety of different environments. Also,
the role of social play in squirrel monkey
socialization and social organization will be
discussed. A subsequent paper (Baldwin
and Baldwin, 1975) will present an analysis
of the factors that appear to integrate the
diversity of data on squirrel monkey play.
METHODS AND DEFINITIONS
The methodology of observation, data
recording, and analysis used in the several
studies on squirrel monkeys has varied according to the observer and research setting. For the specific research strategies
used in the various studies, the reader is
referred to the methodology section of the
papers cited below.
Among the observers who have studied
exploration and play in squirrel monkeys
there appears to be a general concensus in
the definitions of terms, but quantitative
data have been collected according to differing criteria.
Although there have been few explicit
definitions of exploration or social play
for squirrel monkeys, most observers appear
to have used similar criteria. "Exploration"
tends to refer to nonsocial manipulation,
investigation or inspection of the physical
environment, although the term "social exploration" has been used to refer to the
earliest playful social contact among infants. The term "social play" clearly refers
to those social interactions that include
wrestling, chasing, sham-biting, jumping
on, pulling tails, carrying, steep leaps, and
other related activities. Although the term
"play" usually tends to refer to social play,
the term has often been used to include
both nonsocial exploration and social play.
In quantifying the amount of time that
individual squirrel monkeys spend in social
play, at least three different criteria have
been utilized, (i) The simplest type of measurement is a rough estimate of the time
that animals are "for the most part" active
in play, or the amount of time that play
seems to be the dominant activity. Thus,
short pauses in play are included in this
estimate. Most of the observers reviewed in
this article have at one time or another used
this type of rough estimate to quantify the
amount of play they were observing, (ii) A
more accurate means of measuring play is
to use a stop watch or other cumulative
time recorder to measure only the moments
when players are physically active in any of
the behaviors considered to be play. Short
pauses in play and moments when wrestlers
are inactive are not included. It has been
our experience that the second method
leads to estimates of duration of play that
are approximately 50 to 75% as large as
those from the first method, (iii) An alternative estimate of play has been used in
some of the later publications from the
Max-Planck-Institute. Only wrestling or contact play is measured during moments when
physical movement is visible (chasing play
is not included in these time recordings).
Hopf (personal communication) estimates
that this method produces measures 10%
smaller than the second method above.
It is not known what level of interobserver reliability exists among the different
research teams that have estimated duration
of play. Since most of the reports that describe play activity have used method number one for measuring the duration of play,
all estimates of play presented in this paper
are based on method (i), i.e., the total time
in which play is the dominant activity, including the short pauses in play.
THE DATA ON EXPLORATION
AND SOCIAL PLAY
Laboratory studies: Ploog et al. (1967)
In the early 1960's, careful observations
on squirrel monkey social behavior were
begun in the laboratories of the Max-Planck-
PLAY IN SQUIRREL MONKEYS (Saimiri)
Institute for Psychiatry in Munich. Ploog
et al. (1967) published the first detailed observations on the ontogeny of squirrel monkey behavior. The descriptions were based
on 1200 hr of observation on 10 animals
that were followed for varying periods of
time from birth (or shortly thereafter) up
to the third or fourth year of life. The
young animals lived in groups varying from
3 to 9 in size, in which there were 0, 1, or 2
other young animals with whom to play.
The sample consisted of 7 males and 3
females.
During the first 5 to 7 weeks of life, the
infant was very closely associated with its
mother. Between 3 and 16 days of age, the
infant began exploring the environment
by touching both objects and other animals
in its cage. By 17 to 29 days of age the infant climbed off the mother and broadened
its explorations. By the age of 5 to 7 weeks,
the infant made its first jumps and became
accustomed to being off the mother. Also,
the mother began weaning the infant at 5
to 7 weeks. If there were other animals in
the infant's group, they established play
relationships, but when no other young animals were present, the infant "teased" the
adults, who generally would not play with
it.
Ploog et al. (1967) identified several types
of peer play. The two main forms were (i)
contact play, with wrestling and scuffling,
and (ii) distance play with chasing. During
contact play one animal holds and shambites the second, while the second struggles
to get away. The two players of a dyad
often exchange roles frequently and quickly.
They roll around on the floor until one animal pulls away, and a pause occurs in play
until they jump on each other again to resume contact. During distance play, the
animals chase each other through the cage,
often switching between the roles of chasing and fleeing. They keep eye contact, but
they stay out of each other's reach. Several
times a day, young animals were observed
to engage in periods of social play that
lasted up to an hour each. Young animals
could spend as much as several hours a day
in active social play.
Other types of social play began to emerge
305
later in behavioral development. By the
first year of age, young males began play
fights with the colony's alpha male. With
the minimum of effort, the adult male controlled the yearling. As Winter (1968, p.
249) described, the play of the young male
and the adult male may be regarded as a
"fighting tournament." "The immature animal tries to attack the aduk male repeatedly whereas the adult male just seems to
defend itself." Occasionally real fights broke
out between the two. The young male began to be put in the role of Priigelknabe
(scapegoat) and was attacked whenever
there was a disturbance in the colony. Attacks directed to animals under 2.5 years
of age never led to injuries (Hopf, 1972).
Elements of sexual behavior appeared in
early play, and by 1 year of age, animals of
both sexes began to show adult-like sexual
patterns: pursuit, mounting, and thrusting
by the male, and courtship posturing, looking over the shoulder, and coquettish fleeing by the female. Ploog et al. (1963) described a play posture in which one animal
looked between its legs as an element of
sexual play. Animals could switch sex roles
during play and even long into adulthood.
Elements that parallel mother-infant interactions also occurred in play. One animal would carry its play partner in the dorsal-ventral position typical of mother-infant
carrying, though these bouts were usually
brief because the rider was usually thrown
off. Players also attempted to assume the
nursing position. Young females often carried small infants for prolonged periods on
their backs.
Near the end of the second year, the
young squirrel monkeys began to initiate
play less frequently. Often the playfulness
of the young animals was inhibited by the
rejection responses of the adults. The alpha
male dominated young males and adult females dominated young females. After the
second year, social play was uncommon
and was usually begun by the initiative of
younger animals.
Laboratory studies: Rosenblum (1968)
Rosenblum studied the development of
six infants (five of which were female) that
306
JOHN D. BALDWIN AND JANICE I. BALDWIN
were conceived and born in the laboratory.
Visual exploration of the environment began after the first several days of life. The
infant began to reach for objects in the environment and to explore manually by the
beginning of the third week. "This manual
exploration, while still clasping the mother
. . . rises sharply during the second month
and occurs at almost every opportunity at
that age" (p. 218). As manual exploration
began to decline in the third month, object
exploration and social play became more
common (see Fig. 1, 2). Nonsocial play, i.e.,
running, jumping, swinging, etc., developed
rapidly from the beginning of the second
month. Social play appeared in the beginning of the third month and remained at
a stable level for the remainder of infancy.
Social play consisted primarily of "infants
alternately chasing one another and occasional playful wrestling together . . ." (p.
230).
Llanos of Colombia: Thorington
(1967,1968)
In 1965 Thorington conducted the first
systematic field study on squirrel monkeys
in a 15-hectare forest on the llanos of Colombia. At the beginning of the study, the
troop of 18 animals consisted of 3 adult
males, 5 adult females, and 10 young, and
during the 10-week study 4 infants were
born. The troop tended to fragment into
subgroups of 5 to 8 animals during foraging
periods in which the animals looked for
dispersed fruits and insect foods. There was
usually a 1- to 2-hr midday rest period in
the heat of the day, and most activities subsided as the animals descended to low, dense
thickets. It was during these periods when
the adults were resting and the troop remained in one place that play among the
juveniles was most common. Play behavior
"principally involved hanging by the hind
60-1
Age (months)
FIG. 1. The development of manual exploration
while on the mother and object explorations made
while free of the mother in squirrel monkey infants.
(After L. A. Rosenblum, 1968, by permission of
Academic Press.)
PLAY IN SQUIRREL MONKEYS
(Saimiri)
307
30-,
Nonsocial play
Social play
20-
T
—1
10
Age (months)
FIG. 2. The development of social and nonsocial
play in squirrel monkey infants. (After L. A. Rosen-
blum, 1968, by permission of Academic Press.)
legs and wrestling, or mock biting and
chasing" (Thorington, 1968, p. 79). "All the
play observed took place in trees. Since the
troop was surprised very near the ground
on only one occasion, it is doubtful that
any play took place on the ground . . .."
"No vocalizations were observed to be given
by the juveniles as they played . . .." After
the midday rest period, the troop again fragmented into several subgroups for afternoon foraging. From Thorington's report
and from, personal observations on 12
troops on the llanos of Colobmia, we would
estimate that a young monkey in these
troops plays between 5 and 30 min per day.
chronized with the Florida rainy season,
and each year's cohort of players could be
discriminated for about the first 2 years
of life.
DuMond (1968) observed that the infant
began active exploration from the mother's
back in the second or third week. Social play
was common by the 8 to 10 week period. At
times it was active and rough, and it involved sexual elements even at that time.
Most social play tended to occur on the
ground, in clear areas where the animals
could run, hop on each other, wrestle and
roll. "When play occurs in the trees, it is
quite often in the form of a play pair hanging by their feet and arm wrestling" (p.
110). Arboreal play, "once initiated, quickly
turns into ground play as the animals tumble toward the ground while wrestling."
"The general play pattern . . . is usually
initiated by one animal hopping bipedally
to another monkey and grasping it on the
back with its hands. The second monkey
then twists around toward his 'attacker'
Seminatural environment: DuMond (1968)
In 1960, DuMond established a colony of
squirrel monkeys in a 1.6-hectare forest in
southern Florida. Provisioned with abundant food and protected from molestation,
the colony grew from 37 to 110 animals in
the first 6 years. Seasonal births became syn-
308
JOHN D. BALDWIN AND JANICE I. BALDWIN
and a wrestle pair forms which usually ends
a few seconds later with one pulling away
and scampering off, being chased by the
partner" (p. 129). When play became very
rough, the molested animal would begin to
vocalize. During the first 6 or 7 months,
mothers frequently retrieved their infants
from such situations. DuMond commented
that the posture of one animal looking between the legs at another appeared to be a
play-invitation posture rather than a form
of sex play, as Ploog et al. (1963) had interpreted it.
The young were independent of their
mothers by 1 year of age. Most social play
occurred among animals of the same age
cohort; however, play between animals of
different cohorts did occur. Also, play between young uakaris (Cacajao rubicundus)
and young squirrel monkeys was not uncommon. At times even young tamarins
(Saguinus fuscicollis) were observed to play
with the squirrel monkeys.
Young males showed interest in smelling
the genitalia of estrus females, but their
following and attempts to smell the females
were much briefer and less elaborate than
the complex smelling interactions between
adult males and adult females. By three
years of age, the females were adult and entered the reproducing female group. The
young males spent the years from 3 to 5
in a subadult male stage: They may have
been reproductively mature, but they were
not integrated into the adult male section
of the troop social organization.
Social play occurred at many times of
the day and in all seasons of the year. In
the heat of the day during the summers
there was a decrease in play that correlated
with increased resting in cool, shaded areas.
Seminatural environment: Baldwin (1969)
Our research on squirrel monkeys began
with a 1966-1967 study in the seminatural
forest established by DuMond in Florida.
In most ways our observations on play agree
with DuMond's, but having had the opportunity to spend many more hours making
observations on social behavior, we were
able to trace the ontogeny of play in greater
detail (Baldwin, 1969).
Infants began visual and tactile exploration during the first week of life, as other
observers have noted. However, in the seminatural environment, the infants did not
venture off their mothers' bodies until the
fifth week of life. This is 1 or 2 weeks later
than Ploog et al. (1967) and Rosenblum
(1968) observed in the laboratory. Perhaps
the relatively greater complexity of the
seminatural environment made early exploration more arousing or anxiety-inducing, and thus infants would be more
inhibited about engaging in early exploration.
In the seminatural environment, the first
social play was seen at about the eighth
week of life. This also is several weeks later
than the time that Ploog et al. (1967) noted
the onset of play in the laboratory. In the
seminatural environment "the first infantinfant interactions were very exploratory in
nature: the animals touched or gently
pulled at each other, often mouthing or
sham-biting any body part that was available. The interactions were short at first,
often only incidental to nonsocial exploration" (Baldwin, 1969, p. 51). Early play appeared to be gentle, but two lines of evidence suggested that it was very arousing
and perhaps sometimes painful to the animals involved. First, the players often gave
play peep and shriek vocalizations, and frequently individuals withdrew temporarily
from active interaction. Second, mothers
occasionally retrieved their infants from
such gentle-appearing play. During the
ninth and tenth weeks of life, infants became less vocal while playing; they played
more, they retreated or paused less during
play activities, and they returned quickly
to play if their mothers retrieved and removed them from play groups. Thus, infant
play patterns changed significantly during
the first 2 weeks as the infants showed decreasing signs of aversive experiences.
During the second, third and fourth
months of life, the format of infant social
play changed as the animals became more
coordinated and competent. The earliest
play exploration consisted of tentative
touching, pulling, and sham-biting. This
PLAY IN SQUIRREL MONKEYS (Saimiri)
309
"Occasionally a very persistent infant could keep
a juvenile engaged in play if there were no alternative play partners for the juvenile; but the incompatibility of the infant and juvenile styles of play
prevented a continuous play interaction. Infantjuvenile play usually progressed as follows: an
infant would approach a juvenile and grab or
tackle it; the two would tumble once or twice until
the juvenile had constrained the infant underneath
it in the dorsal-ventral position; they would lay
ijuicLly foi a few seconds (occasionally, the juvenile
would thrust at the infant for a few seconds at this
point) until the juvenile released its grasp and the
infant escaped. A persistent infant could cause this
pattern to be repeated many times; but the interaction was far from the continuous activity of playing age-mates" (p. 53-54).
quickly changed to include more complete
and close body contact: "first the infants
engaged in lying together while holding
and pulling each other with hands and feet;
then they progressed to rolling and tumbling play as their pulling and wrestling
became more vigorous" (p. 52). Dorsalventral holding was common in wrestling
play. With time, there was an increase in
the frequency of tumbling, jumping on,
tackling, and hopping around during play
at greater distances. Players often solicited
play "by lying on the side or shoulder or
looking between the legs" (p. 53).
During the first 3 months of play, infants
sometimes spent 1 or 2 hr per day in social
play; however, they often travelled together
or came near each other without engaging
in play activities. In this respect they had
weaker play habits than the older infants
and juveniles.
Play between the early infants and the
juveniles that were 1 year older was not
common. Infants often attempted to initiate
such play if juveniles came near them in
play periods, but the juveniles showed little
interest in playing with early infants.
During the second half of infancy (fifth
through eleventh month), peer group activities increased steadily as mother-infant separation became more complete. By the end
of infancy, the young animals travelled with
their peer group nearly all day—engaging
in foraging, resting, or play—although the
young often tried to be near their mothers
at night. Infant play had developed to include most of the juvenile patterns: chasing, hopping, presenting, sparring, mild
threatening, and rougher tumbling and
wrestling. As the late infants developed
50 -
40 -
cfd1
30 -
20 -
10
—i—
5-7
(Infant)
1
11-13
(Early Juv.)
1
1
17-19
25-27
(Mid Juv.)
(Late Juv.)
FIG. 3. The number of players of various age and
sex in randomly sampled social play groups (N =
272.)
30+
Months
310
JOHN D. BALDWIN AND JANICE I. BALDWIN
greater competence in the play patterns
typical of juveniles, they began to play
more frequently with the juveniles that
were 1 year older than they.
The young animals were classified as
juvenile between the ages of 11 and 30
months. As is shown in Figure 3, play activities reached a peak for the females during
the early juvenile period, and they peaked
for the males in the mid-juvenile period.
Sex differences in play activity had begun
to appear in late infancy, but they became
much more important in the juvenile period. For the females, play was the roughest
at the early juvenile stage, then became
progressively milder such that by the late
juvenile stage the females engaged only in
quiet play interactions. By the late juvenile
age, the females spent much time traveling
near or huddling with the adult females,
and they had become almost as passive as
the adult females. The juvenile males, on
the other hand, played progressively more
roughly and more aggressively throughout
the juvenile age period, although they
played less frequently after the mid-juvenile
period. Quiet play did occur among the
juvenile males, but it was prone to escalate
into rough activity. With increasing roughness, the animals showed an increasing tendency to avoid physical contact. Juvenile
play often consisted of "standing back from
or hopping around just beyond reach of
each other, as if somewhat hesitant to engage in contact; and more threats were
given during play, as if to keep others from
making contact" (p. 60).
By the mid-juvenile age, both males and
females began to show brief organized patterns of copulatory behavior.
"During play, from infancy on, both sexes engaged in thrusting behavior; but this sexual play
remained relatively infrequent, compared with
wrestling, chasing and other play forms, until the
mid-juvenile stage. By the mid-juvenile stage, the
males mounted and thrusted significantly more frequently during play than did the juvenile females.
It was apparently because the stronger, rowdier
animals seemed to have the advantage in reaching
the top position in dorsal-ventral holds that the
active males succeeded in thrusting more than the
females" (p. 62).
The juvenile males did not engage in any
sexual behavior other than thrusting; however, as the males matured during the subadult period, they developed much more
complex behavioral patterns that closely
approximated the adult consorting and copulatory activities.
By the time females reached 2.5 years of
age and entered their first active reproductive season, they had acquired most of the
passivity and social preference patterns typical of adult female squirrel monkeys. Also,
they had experienced many copulatory approaches from juveniles and subadult males.
At adulthood, the hormonally-induced
changes of the estrus cycle clearly began to
affect their behavior, too. The nonreceptive
adult female was usually intolerant of approaches by adult males, but her behavior
changed abruptly on those days when she
was in behavioral estrus, i.e., receptive.
There was no abrupt transition between
the juvenile male and the subadult male
stages. Between 2.5 and 4.5 years of age,
the males continued to spend progressively
less time engaged in play. The frequency of
rough, aggressive play declined the most
rapidly, and at 4 years of age males tended
to play quietly more than roughly. During
the subadult male phase, sexual play became increasingly complex.
"Although the young subadult males tended to
be rough in their general play and sexual play with
juveniles, often causing the juveniles to shriek and
run away, the older subadults were usually quite
gentle, especially when playing with the less rowdy
early juveniles or female juveniles. Occasionally
older subadult males 'consorted' with animals as
young as late infant: they would engage in many
of the adult-like precopulatory approach patterns,
then mount, gently holding the young animal, and
thrust until the least agitation of the partner caused
them to dismount. Such play often continued for
many minutes without the younger animal giving
vocalizations of discomfort or trying to leave the
subadult male. The early subadult sexual play interactions did not involve the 'consort' or precopulatory elements; the young subadult male merely
found a partner, mounted, and thrusted until the
partner could wrestle loose. It seems likely that
the subadult males learned to be gentle with their
partners in order not to disturb the juveniles and
hence to prolong the interaction" (p. 66).
It is interesting to note that the adult males
that succeed in copulating most frequently
during the mating season were not the very
PLAY IN SQUIRREL MONKEYS (Saimiri)
aggressive ones, but were the quieter males
(Baldwin, 1968).
The transition from subadult male to
adult male status was apparently a difficult
one. The adult males did not permit young
males to travel with the troop: they chased
them off every time they detected them near
the troop, and several young adult males
received serious wounds in fights with adult
males. In the 1966-1967 study, not one out
of the five developing young males succeeded in establishing himself in the adult
male subgroup. Follow-up observations
made in 1971 showed that subadult males
do eventually succeed in taking over and
even replacing original adult males in the
tix>op's adult male cluster. At present there
are insufficient data to evaluate to what degree the play fight experience of juvenile
and subadult males may benefit the individuals when the maturing males attempt
to gain entrance into the adult male subgroup.
Panama, Colombia, Brazil, and Peru:
Baldwin and Baldwin (1971)
In 1968 and 1969 we surveyed 31 separate troops of squirrel monkeys in Panama,
Colombia, Brazil, and Peru. The purpose of
the studies was to locate possible sites for
extended field work on squirrel monkeys
and to note possible variations in ecology
and behavior. Troops were observed in the
following locations: eight troops in small
forests in Panama, twelve troops in the
small to moderate-size forests on the llanos
of Colombia, four troops in natural areas
of the rain forests of Amazonia, and seven
troops in areas of Amazonia that had been
altered by human activities.
In the small forests of Panama and the
llanos of Colombia, troop size varied between 10 and 35 animals, but in the natural
forests of Amazonia, troop size varied from
120 to 300 or more individuals. In altered
Amazonia, troop size appeared to range
from 20 to 80 animals, though good estimates were difficult to obtain.
The frequency of social play varied considerably among the troops in the different
locations. In the small troops of Panama
311
and the llanos of Colombia, play was not
nearly so common as in the large troops of
natural Amazonia. It is difficult to make
estimates of the time that individuals spent
in play, because of the limited contact that
we had with each troop. However, our observations in conjunction with Thorington's (1968) data suggest that the young
animals in the small troops of Panama and
the llanos may play between 5 and 30 min
per day. Our subjective impression from
spending 5.9 hr with the large troops in
natural Amazonia was that they played
about half as much as the seminatural troop
in Florida: this would imply about 1 to 2 hr
per day of play per individual.
An interesting correlation with the frequency of play was the fact that the adults
of the large troops maintained closer individual distances and interacted more frequently than the adults of the small troops.
The comparative data from the 31 study
sites suggested the following hypothesis
(Baldwin and Baldwin, 1971): Young animals in small troops have fewer potential
play partners and hence less opportunity
to play than animals in large troops; this
restricted experience with play gives diem
less opportunity to learn strong habits for
social activity, and thus animals maturing
in small troops tend to be less social as
adults than animals that grow up in large
troops with ample opportunity to play. In
other words, the young monkeys in small
troops have few age mates with whom to
play, and this apparently creates a low probability that there will be two or more animals ready to play at any point in time. In
large troops with many infants and juveniles, there is usually a subset of young animals ready to play during many hours of
the day. The higher frequency of play in
large troops provides the maturing monkeys with many and varied social experiences in conjunction with the positive reinforcers that are intrinsic in social play. Thus,
the monkeys in large troops are reinforced
for maintaining close individual distances
during social play, they develop larger repertoires of social interaction patterns and
they emit these social behaviors at higher
frequencies as long as the behaviors remain
312
JOHN D. BALDWIN AND JANICE I. BALDWIN
reinforcing.
The data from the 31 study sites in South
America consistently supported the hypothesis that increases in social play experience
lead to decreases in the average individual
distances maintained by animals during
the infant, juvenile, and adult years. In
other words, the peer group that plays together stays together. Play increases the reinforcers for maintaining close proximity
and strengthens the habits of interacting
socially.
Barqueta, Southwestern Panama:
Baldwin and Baldwin (1972)
Between December 19, 1970, and February 25, 1971, we observed two troops of
squirrel monkeys consisting of 23 and 27 animals in a natural forest on Hacienda Barqueta in Southwestern Panama. The squirrel monkeys habituated to our presence
within 2 weeks and could be observed from
distances of 3 to 15 m for long periods of
time. In a total of 261 hr of observations on
the squirrel monkeys during all hours of
their waking day, not one instance of social
play was observed among any of the animals
in either troop.
Presumably, the lack of play at the Barqueta study site was due to the dearth of
foods preferred by the squirrel monkeys.
The monkeys spent 95% of each hour of
their 14-hr waking day engaged in foraging
and in traveling between foraging areas.
This pattern of daily activity left little time
for any social interactions, including the
two social activities that are most common
for squirrel monkeys in other environments,
resting together and playing. During the
5% of the time when the animals were not
foraging or traveling, however, the young
animals came into many situations that
would have led to play in the other environments studied, but not once did the animals
play. In spite of the lack of play, the Barqueta troops were very cohesive and the animals maintained very close individual distances. The earlier hypothesis that playing
together was a necessary precondition for
staying together in dose, integrated troops
was clearly inadequate.
The following evidence shows a great
difference between the non-playing squirrel
monkeys at Hacienda Barqueta and the
playful squirrel monkeys at other study
sites. The fact that the two Barqueta troops
consisted of only 23 and 27 members respectively places them near the small end
of squirrel monkey troop size (which varies
from 10 to 300 or more animals). In general, the young monkeys in small troops
tend to have few play partners and to engage in less play than monkeys in larger
troops (Baldwin and Baldwin, 1971). However, animals in small troops do play. At
Barqueta there were also opportunities for
play. There were 5 infants and 7 juveniles
in the main study troop of 23 animals.
These young animals were frequently within
5 m of one or more age-mates. The two
main food sources at Barqueta often attracted the whole troop into an area of
180 m2. The fruits of the palm Scheelea
and the cactus-like plant Achmea both grew
in tight clusters that forced the monkeys to
come into very close contact as they spent
15 to 120 sec locating and picking suitable
fruits. The animals regularly came into
physical contact, often crawling over each
other's bodies, without a single playful interaction. Playful infant and juvenile squirrel monkeys in other environments often
began bouts of social play when they came
equally close.
At Barqueta during the periods between
bouts of foraging when the monkeys did
rest, they occasionally rested within arm's
reach of each other. Yet, on only six occasions did one animal reach out and touch
another. All of these touches were very brief;
none led to a bout of social play, and one
evoked a mild threat from the touched animal. Playful infants and juveniles in other
environments very often became drawn into
play under such circumstances. In short, the
opportunities for play (i.e., the stimuli of
the environmental setting and social cues)
were present at Barqueta, but the animals
never showed playful responses. If the monkeys were playful in other seasons of the
year, one might expect that their play habits
would appear with at least some moderate
frequency under these favorable social cir-
PLAY IN SQUIRREL MONKEYS (Saimiri)
cumstances.
Loy (1970) studied rhesus monkeys on
Cayo Santiago Island before, during, and
after a 22-day period when very little food
was available. The starvation period was so
severe that 6 of the 69 animals died. The
frequencies of all behaviors except foraging
dropped significantly during the 22-day period. The frequency of social play decreased
from 2.54 to 0.15 bouts per hour. The fact
that play did not decrease to zero under
these severe starvation contingencies suggests that more factors beside temporary
food deprivation must be operating to explain the total absence of play at Barqueta.
If food were abundant in other seasons at
Barqueta, and the monkeys played then,
we doubt that a temporary deprivation period would reduce play to zero bouts in
261 hr of observation.
Exploration behavior at Barqueta was
somewhat less frequent than in other environments we have studied, but it is difficult to quantify the amount of time spent
in exploration. Infants and juveniles at
Barqueta showed explorative tendencies at
various times during their daily traveling.
Most of this explorativeness was directed
to investigating things in the environment,
like tree holes, abandoned squirrel nests,
wasp nests, sources of food, broken branches,
birds, insects, etc.
The young squirrel monkeys often traveled near each other in the troop. The infants still nursed from their mothers and
they spent less time foraging for fruits and
insects than did the adults. This gave them
more time to explore and/or interact, but
no investigatory or play behavior was directed towards another monkey. Nor did
the young rest together, cuddle together, or
show other friendly interactions.
Despite the absence of play and a reduced
amount of exploratory behavior, the troops
at Barqueta appeared to be cohesive and
stable. The troops seldom fragmented and
animals often traveled at close individual
distances without signs of uneasiness or
tension. Agonistic interactions were somewhat less frequent than in other environments: chases and displays occurred 40 to
313
45 times a day, and no fights were ever observed. Copulations were observed, though
they consisted of very brief and simple episodes of mounting and thrusting, without
any of the consort activities seen in other
environments. The fact that 85% of the
adult females were accompanied by infants
indicates that the reproductive success rate
was normal for the species.1 When comparing the behavior of the adults at Barqueta
to adults in other environments, the most
outstanding features are that their social behaviors were very simple and infrequent.
However, a simple, basic repertoire can be
adequate to allow survival.
The Barqueta data suggest that squirrel
monkeys can survive, reproduce, and maintain cohesive troops even in the absence of
social play. There are other developmental
routes to "normal" adult behavior besides
the socializing experiences of social play;
several of these are described and discussed
elsewhere (Baldwin and Baldwin, 1973).
Hopf (1972, personal communication) also
presents data that indicate that social play
experience is not crucial to social development. Infants raised in all-adult laboratory
groups, without peers to play with, had
minimal play experience but showed no
marked differences from animals raised with
peers and with social play experience. Hopf
points out that there are numerous socializing factors that all make overlapping contributions to social development. These
factors combine so differently for each maturing individual that each animal develops
its own unique personality and role in the
group structure. Presumably, the individuals raised without social play are exposed
to enough other socializing factors that they
can develop within the limits of what is perceived as "normal."
observations in several natural and seminatural
environments indicate that 85% of the adult females in a troop usually bear one infant per year:
on the llanos of Colombia (Thorington, 1968; Baldwin and Baldwin, personal observation), on Santa
Sophia Island near Leticia, Colombia (Bailey, personal communication), and in a seminatural environment in Florida (Baldwin, 1969).
314
JOHN D. BALDWIN AND JANICE I. BALDWIN
SUMMARY
The frequency and form of exploration
and social play in squirrel monkeys vary
considerably across different environments.
The onset of active exploration away from,
the mother's body can occur as early as the
third week of life in the laboratory (Ploog
et al., 1967), or as late as the fifth or sixth
week in more complex environments (Baldwin, 1969). The onset of social play can begin as early as the fifth week in the laboratory (Ploog, et al., 1967; Hopf, 1972), or in
the 8- to 10-week period in other environments (Baldwin, 1969), and in some environments, such as Barqueta, social play apparently never begins (Baldwin and Baldwin, 1972, 1973). Also, the amount of social
play that young monkeys engage in can vary
from 0 to over 3 hr per day (Baldwin and
Baldwin, 1971, 1972).
Many theories of play make it appear
that social play is essential for the development of sexual behavior, integrated roles in
the troop structure, control of aggressive
responses, social cohesion, etc. (Beach, 1947;
Welker, 1961; Harlow and Harlow, 1965;
Marler and Hamilton, 1966; Loizos, 1967;
Dolhinow and Bishop, 1970; Suomi and
Harlow, 1971). The Barqueta data, however, indicate that social organization and
many normal social behaviors can develop
in squirrel monkeys without social play. On
the other hand, the comparative data on
squirrel monkeys indicate that the opportunity to play socially provides learning experiences that increase the variety of each
animal's behavior repertoire and the subtlety of social cues to which it can respond.
An adaptive modicum of competence can
develop without play, but the opportunity
to play develops the competence of animals
beyond that modicum. Briefly stated, (i) social play is not necessary for the development and/or learning of an adaptive modicum of social interaction patterns and troop
cohesion, but (ii) the opportunity to play
provides socializing experiences in which
young animals can develop more complex,
varied social interaction patterns and
stronger habits for engaging in frequent,
overt social exchanges.
The causes and effects of the variability
in exploration and play are numerous and
complex. A subsequent paper will present
data that suggest a multifactor explanation
of exploration and social play in squirrel
monkeys (Baldwin and Baldwin, 1975).
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