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Development of activity patterns social interactions and exploratory behavior in infant tufted capuchins (Cebus apella).

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American Journal of Primatology 35:255-270 (1995)
RESEARCH ARTICLES
Development of Activity Patterns, Social Interactions,
and Exploratory Behavior in Infant Tufted Capuchins
(Cebus apella)
GAYLE BYRNE AND STEPHEN J. SUOMI
Laboratory of Comparative Ethology, National Institute of Child Health and Human
Development, NIH Animal Center, Poolesville, Maryland
Early organization of activity states was studied in 17 tufted capuchin
(Cebus upella) infants from birth to 11 weeks of age. Development of exploration and interactions with mothers and other group members were
studied in 14 of these infants up to the age of 1 year. Activity profiles
changed from 3 to 8 weeks as infants began to move off mothers and
explore their environments. From 2 to 6 months time with mothers decreased; time alone increased correspondingly. Time spent with other
group members did not vary significantly over the first year. By 7-9
months capuchin infants spent more time alone or with other group members than with mothers, although weaning was still not completed by the
end of the first year. Simple environmental exploration began in the 2nd
month and reached stable levels by 4 months. Complex manipulation of
food and objects first began a t 3-4 months and increased to stable levels
in the second half of the first year. Some preliminary differences were
evident between infants living in indoor cages and those living in indoor/
outdoor runs. Infants in cages spent less time in dorsal contact with mothers, and less time in social play and proximity to other animals than those
in runs. Instead, infants in cages spent more time alone and engaged in
more manipulation of food. Some measures of social and exploratory behavior showed a high degree of variability which may be usefal in exploring individual differences in infant temperament or reactivity.
0 1995 Wiley-Liss, Inc.*
Key words: capuchin, Cebus, development, activity, social behavior
INTRODUCTION
Capuchin infants are faced with a somewhat unique developmental challenge.
Comparatively altricial a t birth, Cebus develop at a slower rate than many other
nonhuman primates [e.g., Elias, 1977; Watts, 19901, and go through an extended
period of maternal dependency and juvenescence. Developmental milestones such
as weaning and physical maturity occur relatively late in life in Cebus [Fragaszy,
1990; Robinson et al., 19871, and lifespan itself is prolonged, as animals may live
Received for publication January 7, 1994; revision accepted July 19, 1994.
Address reprint requests to Dr. Gayle Byrne, NIH Animal Center, P.O. Box 529, Poolesville, MD 20837.
0 1995 Wiley-Liss, Inc. *This article is a US Government
work and, as such, is in the public domain in the United States of
America.
256 I Byrne and Suomi
into their 40s in captivity [Harvey et al., 19871. Capuchin infants grow into highly
adaptive, manipulative adults, with comparatively large brains [Stephan, 1972;
Bauchot, 19821. As adults in the wild, capuchins engage in intricate, extractive
foraging methods in searching for a wide variety of food items. The extended
juvenile period provides time in which to learn many of the complex skills necessary for self-sufficiency; juveniles do not exhibit fully adult foraging patterns until
later than 2 years of age [Fragaszy et al., 19911. The complex manipulative tendencies of capuchins carry over into captivity as well, where they exhibit a greater
propensity for object manipulation and tool use than any other nonhuman primates except great apes [Parker & Gibson, 1977; Torigoe, 19851.
Previous studies of development in the tufted capuchin (Cebus apella) have
characterized the development of activity states, manipulatory, and locomotor
abilities in early infancy, and the development of social relations with group members in the first few years. For example, Fragaszy [19891 reported that major
changes in organization of activity states around 8-9 weeks denoted the completion of major maturational events, concomitant with increases in locomotion and
manipulation of the environment. Independent locomotion increases at about 3
months of age, although considerable time may be spent with mothers through 6
months, and weaning is not completed until well into the 2nd year [EscobarParamo, 1989; Fragaszy, 1990, Fragaszy et al., 1991; Welker et al., 19871. Welker
et al. [1990] found that from 6 to 12 months the majority of social interaction
shifted from mothers to other group members, with special emphasis on peers and
dominant adult males. The purpose of the present study was to further characterize the normative course of development of tufted capuchin infants from birth to 1
year of age, with respect to early activity states, social interactions with mothers
and group members, and the development of independence and exploratory activity. A secondary purpose was to identify preliminary effects of different types of
housing on these measures, as well a s sources of individual differences that might
afford some insight into infants’ behavioral styles or temperament patterns.
METHODS
Subjects
The subjects were 17 tufted capuchin (Cebus apella) infants (13 males, 4 females) from 7 social groups housed at the NIH Animal Center in Poolesville, MD.
Data collection took place over a period of about 3.5 years. There were some minor
changes in group composition over this time due to births, deaths, and movement
of adult males between groups. A listing of group composition towards the end of
the study is presented in Table I.
Housing
One of the seven social groups was housed in a n indoor-outdoor run (two
connecting areas, each 2.5 x 3 x 2.5 m); one group was housed in a run from
October to May and in a n outdoor corncrib (4.2 m high, 5 m in diameter) from May
to October; the remaining 5 groups were housed in indoor cages (.9 x 1.8 x 1.7 m)
with wire mesh floors. All groups had 3-4 various manipulable objects present in
their cages a t all times. Purina monkey chow was provided twice daily and fresh
fruit was provided 3 times weekly; water was available ad lib. The indoorloutdoor
runs and corncrib had woodchip bedding covering the floors, into which sunflower
seeds were scattered daily. Group size in runs was on average larger than that in
cages; thus, the runs were both physically and socially more complex than the
cages.
Activity Patterns in Cebus Infants I 257
TABLE I. Composition of Study Groups*
Adult
Group
1
2
3
4
5
6
7
Juvenile
Males
Females
4
5
2
2
1
2
2
1
2
2
2
1
2
2
Males
Females
Housing
8 (6)
3 (2)
4 (2)
1(0)
1
2 (1)
1
1
2 (1)
1
1
0
0
0
Rudcorncrib
Run
Indoor cage
Indoor cage
Indoor cage
Indoor cage
Indoor cage
*Adult = over 6 years, Juvenile = 0-6 years. Number in parentheses indicates number of
infants included in the present study, if different from total number of juveniles.
Procedure
1) Activity states. Observation of infant activity states was carried out for 17
infants (13 males, 4 females) on a schedule that was a modified version of the one
used by Fragaszy [1989], and that adopted her classification of infant activity
states. Activity states were recorded on the day of birth, day 4, week 1, and every
week up until week 11. On these days 3 separate hours of 1-minute scan samples
were performed in which infant state (asleep, nursing, alert-quiet, alert-active,
drowsy) and infant position (dorsal on mother, ventral on mother, other contact/
proximity to mother (within 15 cm), away from mother) were recorded on a checksheet. Observations took place a t 1100, 1300, and 1500 hours.
2) Videotape observations. Ten minute videotape sessions were recorded 3
times/week between 0930 and 1100 hours for 13 infants (9 males, 4 females) up to
the age of 13 months (4-week periods) and one additional male infant up to the age
of 11months. These subjects had all provided data for the activity state measures
as well. Video data for the remaining 3 infants were excluded from analysis; one
infant from Group 1 died at the age of 4 months, and the other 2 were less than 9
months of age when the data were analyzed.
The videotapes were scored for occurrence of the behaviors listed in Table I1
with a keyboard event recorder. The same observer scored all of the videotapes;
intraobserver reliability (percentage of agreements) was evaluated a t 90% a t the
onset of the study by successive scorings of the same samples. A recheck performed
at the end of the study produced a mean reliability score of 88%over all behaviors.
D a t a Analysis
All analyses were performed using the SYSTAT statistical software package.
An alpha level of .05 was adopted in interpreting results of all analyses. Percentage measures were submitted to arcsin transformations to normalize the data for
use in ANOVAs, and Tukey HSD tests were used to identify significant mean
differences when the overall F was significant.
1) Activity state data. The percentage of samples per day in which infants
were scored in each activity state and position over the 11weeks was subjected to
repeated measures ANOVAs.
Bout lengths of sleeping, waking (excluding drowsy), and nursing were estimated from the scan sample data as the number of consecutive 1-minute scans
scored in the same state. Bouts of less than 3 minutes occurring a t the beginning
or end of a n hour’s observations were excluded. The frequency of changing from
one state to another (excluding nursing), longest sleeping and waking bouts, and
258 / Byrne and Suomi
TABLE 11. Behaviors Scored From Videotape Samples
Mother reject infant”
Infant “tantrum’”
Mother approach infant”
Mother leave infant“
Infant approach mother”
Infant leave mother”
Dorsal contact with mother
Ventral contact with mother
Other contact with mother
Proximity to mother (within 15 cm)
Nursing
Mother groom infant
Proximity to other group member (within 15 cm)
Contact with other group member
Groom by other group member
Alone (out of proximity to any animal)
Tactile/oral environmental exploration (nonfood)
Eatinghactile exploration of food
Complex food/object manipulation (repetitive pounding, rubbing of food/objects)”
Self play
Social play
Out of sight
Total time with mother (all contact, proximity, grooming)
Total time with others (proximity, grooming, contact, social play)
Total exploration (tactile/oral plus eating)
“Behaviors scored a s frequency measures; all other behaviors were scored a s duration measures and converted to a percentage of time infants were in sight.
number of sleep-waking transitions within each hour were also estimated from
changes in successive scores on the checksheets. Mean bout lengths, frequency of
state changes, sleep-waking transitions and longest sleeping and waking bouts
were also subjected to ANOVAs across the 11 weeks.
2) Videotape samples. Data from each infants’ videotape sessions were uploaded from the keyboard event recorder to a PC. The total duration of each behavior within each sample was converted to a percentage of time in sight. Approaches and leaves between mothers and infants and incidences of complex object
manipulation were counted as frequency measures. Weekly mean values for each
behavior were obtained, and monthly means for each behavior were calculated
from the weekly means. Scores for month 1 to 13 were subjected to repeated measures ANOVAs with housing condition (runs vs. cages) as a between groups factor.
One infant had not yet reached the age of 13 months; data for his last 2 months
were filled in with mean values over all infants.
An “approach index” ranging from - 100 to 100 was calculated by subtracting
the percentage of all leaves that were initiated by infants from the percentage of
all approaches that were initiated by infants. This index was positive if infants
were primarily responsible for maintaining proximity to mothers, and negative if
mothers were responsible Ccf., Hinde & Atkinson, 19701.
The age a t which selected behaviors were first seen in infants’ repertoires was
recorded ad lib during both activity state sessions and videotape samples, in order
to examine individual differences in appearance of these behaviors. Individual
differences in absolute levels of selected behaviors collapsed over the first year
Activity Patterns in Cebus Infants / 259
TABLE 111. Activity State and Position Data*
SL
NUR
AQ
AA
DR
DOR
VEN
OTH
AW
53.6
51.5
18.9"
10.9"
9.0"
14.8b
6.7
4.8
10.3
14.3
62.2"
70.4
34.9"
26.0b
1.7
0.0
0.0
0.0
48.4
44.9
43.9
42.4"
38.0b
26.6"
32.1
30.5
28.5
22.3b
18.5
9.2
9.6b
6.3'
4.4b
4.4
3.9
3.2'
3.2
3.4
2.4
2.2
21.6"
26.3b
30.0'
27.0d
24.0
25.0
21.7
18.1'
18.3
16.gd
15.8
4.0
5.1
9.4"
18.0"
25.7b
39.2b"
39.6d
45.3
44.7
55.8'
59.4d
16.0"
13.3b
9.6"'
7.7b
6.0d
4.4'"
2.gd
2.2
2.5
1.8'
1.5
79.2
82.1
84.3"
85.8
83Bb
76.9
77.6'
62.0
61.1b
52.9"
50.8
19.9"'
16.5d
14.7"
13.1
11.2b
14.7
7.3
6.0'
6.2
4.5d
3.8"
0.0
0.1
0.3
0.4
1.2
4.1"
5.5b
10.0"'
10.3M
13.0'
14.6d
0.0
0.0
0.0
0.2"
1.8
4.0b
9.3"'
20.3b
19.7'
28.8
30.6
Day
1
4
Week
1
2
3
4
5
6
7
8
9
10
11
*Raw percentage of samples spent in each activity state (SL = sleep, NUR = nurse, AQ = alert-quiet, AA =
alert-active, DR = drowsy) and position (DOR = dorsal contact, VEN = ventral contact, OTH = other contact/
proximity to mother, AW = away from mother) over the first 11 weeks. Data points within each column with
matching superscripts differ significantly from each other in posthoc tests after they have been urcsin trunsformed. Measures are not corrected for out-of-sight time and thus may not add to 100%.
were examined by calculating the range and coefficient of variation (standard
deviatiodmean) for those behaviors over all infants.
RESULTS
Activity State Data
Table I11 summarizes the significant differences among weeks in infant activity state and position data. Major changes in activity profiles took place over the
first 6-8 weeks. By week 6 sleep had declined from high initial values during the
first 4 weeks; sleep continued to decrease gradually over weeks 5 to 10. Time spent
in alert-quiet peaked in weeks 2 to 4; thereafter time spent awake was more often
in a n alert-active state, increasing steadily as motor control and coordination
improved. Nursing was highest on day 1 and decreased significantly from weeks 2
to 4 and from weeks 3 to 7. Time spent in drowsy was initially high, but decreased
steadily over weeks 1 to 7.
Differences in activity states were mirrored to some extent by changes in
position over time. The percentage of samples in which infants were carried dorsally increased from day 1 to week 3 as ventral carrying declined. Dorsal contact
then decreased significantly from week 5 to 9; during this period infants began to
spend time in other contactlproximity with mothers, and to spend time away from
mothers. Time off and away from mothers continued to increase gradually over
weeks 7 to 11, as infants began to explore their environments and actively engage
in social interactions with others. Ventral contact was associated mainly with
nursing and by weeks 5-8 had decreased significantly from high values in the first
few days.
Mean estimated sleeping and waking bout lengths increased significantly over
the first 7-8 weeks (Table IV). Nursing bout lengths decreased from high initial
values on the first few days to weeks 3-4, and decreased again from weeks 3 to 9.
The estimated number of state changes infants went through in a n hour of
observation decreased from weeks 5 to 8. The number of sleep-waking transitions
260 / Byrne and Suomi
TABLE IV. Bout Length Data From Activity State Samples*
SL
AWK
NUR
STCH
s-w
8.4
7.9
6.5
6.8
4.1"
2.8"b
12.9
15.2
8.2
8.6
24.4
22.7
29.0
22.2
6.7
6.3
7.4"
8.2
9.8
9.5
14.3"
13.2
14.8
12.0
12.6
6.9
8.0
9.5
11.2"
14.8b
23.9"
24.0
27Ab
28.2
34.4
35.4
2.6
2.5"
l.gbd
1.5"
1.3
1.2
1.2
1.2
15.7
15.8
15.1
15.3
14.6"
14.2b
12.8
10.9"
10.8
9.6b
10.2
8.9"
8.1b
6.9
6.0"'
4.6bd
3.2"
2.9
2.4
3.1
lBd
1.6
26.6
29.5"
32.4b
42.4"'
47.0bd
55.6"
56.8
56.0
58.1d
56.6
60.0
23.9
21.2
22.5
22.9
24.9
25.1
27.5
25.8
26.4
20.8
23.8
LW
LS
Day
1
4
Week
1
2
3
4
5
6
7
8
9
10
11
l.ld
1.1
1.o
*Mean bout lengths (SL = sleep, AWK = awake, NUR = nurse) and state change measures (STCH = number
of state changes, S-W = number of sleep-wake transitions, LW = longest waking bout, LS = longest sleeping
bout) over the first 11 weeks. Data points within each column with matching superscripts differ significantly
from each other in posthoc tests.
infants went through per hour decreased steadily from peak scores during the first
weeks to weeks 4-5, and again from weeks 5 to 10. The longest observed sleeping
bout did not change significantly over the 11 weeks of observation; the longest
waking bout increased significantly from 30 successive 1-minute samples in week
2 to 42 in week 4, and then to 56 by week 6, when it was not uncommon for infants
to be awake for the entire hour.
Video Samples
1. Changes over time. The earliest changes in behavior occurred a s infants
began to get off mothers (Fig. 1).Mothers and infants first began separating in
month 2. Approaches to infants by mothers increased significantly from months 2
to 3; leaves by mothers and approaches and leaves by infants all increased significantly from months 2 to 4. Mothers were initially responsible for maintaining
proximity to infants; responsibility shifted to infants in month 4 (Fig. 1).
In the first 4 months dorsal contact with mothers, ventral contact, and nursing
all decreased significantly (Fig. 2, Table V; Note-although analyses were performed on arcsin transformed data, values depicted in the figures and tables are
raw percentage measures). Non-carrying contact with mothers peaked in months
3 and 4; proximity to mothers increased significantly from month 2 to 3 and from
month 3 to 5.
From months 4 to 12 there were steady decreases in time spent in dorsal
contact with mothers (Fig. 2) and in total time with mothers (Fig. 3, Table V). The
frequency of approaches to mothers and leaves by mothers kept increasing over
this time. There were no significant changes over the first year in mothers' rejections of infants, infants' tantrums, or grooming received from mothers.
Proximity and contact with other group members showed relatively high values in the first few months of life (Table VI). There were no significant changes
over the year, however, in grooming or total time spent with others (Fig. 3 , Table
VI). Play with others increased over months 2 to 7.
Tactileioral exploration of the environment first appeared in month 2 and
Activity Patterns in Cebus Infants I 261
Mother-Infant
Mother-Infant Approaches
Leaves
By infant
0
By mother
T
0 1 2 3 4 5 6 7 8 9 101112130
1
l
l
Age (months)
40
1
30
-
l
l
l
l
l
l
l
l
l
l
2 3 4 5 6 7 8 9 I0111213
Age (months)
Infant responsible for proximity
20 X
$
.-C
10 -
L
0 -
u
P
8
Q
-10 -
-30
-
Mother responsible for proximity
-40
I
I
I
I
I
I
1
I
I
I
I
I
1
Fig. 1. Frequency of mother-infant approaches (top left) and leaves (top right) and approach index (bottom)
over the first year.
increased significantly from month 2 to 3 and from month 3 to 4 (Fig. 4,Table VII).
Exploration of food increased from months 3 to 5. Complex manipulation of objects
(pounding, rubbing) first began in month 3, and increased from month 4 to 8. Over
months 3 to 7 there were gradual increases in self play (Table VII), and steady
262 / Byrne and Suomi
o Dorsoi contact
Ventral contoct
I
L
0
v Other contact
4
a
;
I
1
0
1
*O]
0
15
2
3
4
5
6
7
8
9 1 0 1 1 1 2 1 3
i
o Proximity
Nursing
Grooming
v
L
0
10
c
C
a,
I
0
0
P\
T
l
l
l
l
l
l
i
l
1
2
3
4
5
6
7
8
9
l
l
l
l
10111213
Age (months)
Fig. 2. Percent of time spent in interactions with mothers over the first year.
increases in time spent alone (Fig. 3). By 7 to 8 months most of these behaviors had
reached stable levels.
2. Housing effects. There were significant housing effects in several measures of interaction with mothers. Approaches and leaves by infants, leaves by
mothers, and proximity to mothers all continued to increase in cages after the
point where these measures leveled off in runs; these measures were significantly
higher in cages than in runs in some later months (Table VIII). The overall fre-
Activity Patterns in Cebus Infants I 263
TABLE V. Interactions With Mothers*
~
~~
Month
1
2
3
4
5
6
7
8
9
10
11
12
13
DC
VC
CON
PR
GRM
NUR
Total
81.2
79.4"
61.gb
44.5"'
31.8bd
24.2""
18.0
17.7
13.1d
11.6
10.6
5.6"
8.4
17.9"
14.1b
4.7
3.1"
1.9'
2.0
2.0
2.0
2.7
1.7
1.5
1.1
1.6
0.1"
2.3"b
5.6b
5.3"
4.0d
2.4'
2.3
2.2
2.2d
3.0
1.6
1.8
1.2
0.1
1.2"
4.4"b
6.3
8.2'
6.2
5.4
6.7
6.3
7.6
7.8
9.0
6.6
1.2
1.1
1.3
1.0
2.1
0.7
1.0
1.6
1.8
1.0
1.2
1.2
1.3
5.6"
3.0'
2.4
2.1
1.8"
2.0
1.7
1.8
2.0
1.6
1.6
1.0b
1.3
100.0
98.0"
77.Sab
60.2"
48.1bd
35.5"
28.7d
30.2
26.1
24.9
22.7
18.6'
19.0
*Raw percentage scores from videotapes over the first year of life (thirteen 4-week periods).
DC = dorsal contact, VC = ventral contact, CON = other physical contact, PR = proximity,
GRM = grooming, NUR = nurse. Data points within each column with matching superscripts differ significantly from each other in posthoc tests after they have been arcsin transformed.
With mother
0
With others
4
Alone
,+
,*. -4 / +
+
t
4'
,'
0
0
:o
0
0
..-
Fig. 3. Percent of infants' time spent with mothers, with other group members, and alone over the first year.
quency of infant tantrums, although low, was significantly higher in cages (.08/10
minute session) than in runs (.Ol); (F = 5.631,1,12 df, P < .05). Overall percent of
time in dorsal contact was higher in runs (35.2%)than in cages (26.4%)(F = 5.616,
1,12 df, P < .OW. Time in ventral contact, conversely, was higher in cages than in
runs in the first 2 months of life; ventral contact and nursing decreased signifi-
264 I Byrne and Suomi
TABLE VI. Interactions With Others*
Month
1
2
3
4
5
6
7
8
9
10
11
12
13
PR
CON
GRM
Play
Total
27.6
29.6"
23.7
16.3
15.3"
18.9
17.9
14.3
19.3
18.7
18.3
16.1
15.6
0.5"
2.1
5.0"
2.5
2.2
1.5
2.6
1.7
2.9
5.2
4.5
2.0
4.1
0.9
1.8
1.6
0.9
0.2
0.5
1.0
0.3
0.9
0.5
1.0
0.4
0.9
0.0
0.2"
1.5
1.3
2.2
1.8
3.5"
5.2
2.6
3.4
5.0
5.4
4.4
29.0
33.7
31.7
20.9
19.8
22.6
24.9
21.5
25.7
27.8
28.8
23.8
24.9
*Raw percentage scores from videotapes over the first year of life (thirteen 4-week periods).
PR = proximity, CON = physical contact, GRM = grooming. Data points within each column
with matching superscripts differ significantly from each other in posthoc tests after they have
been arcsin transformed.
cantly in the first 3-5 months in cages but did not vary significantly over the first
year for infants in runs (Table VIII).
Some measures of interaction with other animals were significantly higher in
runs than in cages. Infants in runs spent 3.9% of time overall in social play as
compared to 1.3% for those in cages (F = 11.07,1,12 df, P < .01). Total time spent
with others was 29.6% for infants in runs and 20.5% for infants in cages (F =
5.288, 1,12 df, P < .05). Time spent alone, in contrast, was higher in cages (45.7%)
than in runs (39.3%) (F = 5.391, 1,12 df, P < .05); self play was higher in cages in
months 9 and 10 than in runs (Table VIII).
Total environmental exploration (tactile/oralplus eating) reached stable levels
in runs by month 4; however, tactile/oral manipulation continued to increase in
cages over months 4-10. As a result, in month 10 total exploration was higher in
cages than in runs (Table VIII). Manipulation of food was also higher in cages
(13.2%) than in runs (8.5%)(F = 10.934, 1,12 df, P < .01).
Individual Differences
The mean age of first occurrence of selected behaviors is listed in Table IX,
along with the range of ages over which these behaviors were first seen. This range
was small for some behaviors (e.g., 4 weeks for tactile/oral exploration, 3 weeks for
the age a t which alert-active scores exceeded alert-quiet scores). For behaviors
related to growing infant independence from mothers, the range was about 7-8
weeks (e.g., for getting off of mothers, separating from mothers, being alone).
Measures of increased exploration and independence (e.g., social play, complex
manipulation of objects/food, spending more than 50% of time alone) showed a
larger range of 13-18 weeks.
Table X lists the range of subjects' absolute levels of selected behaviors over
the first year and the coefficient of variation for those behaviors over subjects.
Considerable variability was seen in scores for complex object manipulation, proximity to mother, and social and self play over the first year. Lowest variabilities
were seen in total time spent with mothers, time spent alone and total exploration
over the year.
Activity Patterns in Cebus Infants I 265
c
Y
E
0
1
2
3
4
5
6
7
8
9 10111213
Age (months)
2o
15
1
Complex Object
Manipulation
0
1
2
3
4
5
6 7 8
9 10111213
Age (months)
Fig. 4. Exploratory behavior over the first year. Tactileioral exploration and eating (top) are recorded as
percentage of observation time. Complex object manipulation (bottom) is recorded as absolute frequency of
poundingirubbing of food or objects.
DISCUSSION
A major shift in Cebus activity patterns occurred from 3 to 8 weeks, when
infants began to stay awake longer and increased motor control enabled them to
venture off mothers and actively explore their physical and social environments.
266 I Byrne and Suomi
TABLE VII. Exploratory and Solitary Behaviors*
Month
1
2
3
4
5
6
7
8
9
10
11
12
13
TAC
Eat
OBJ
SP
Alone
0.0"
3.2ab
7.a b c
17.0'
16.6
20.2
20.5
19.7
19.6
21.6
20.8
18.1
18.0
0.0
0.3
3.6"
6.6b
13.6"
11.8
12.1
14.7
15.3b
11.6
16.3
13.8
16.2
0.0
0.0
0.2
0.3"
2.7
2.9
3.5
5.9"
5.7
3.6
5.9
3.8
6.4
0.0
0.0
0.1"
0.4b
0.8"
1.1
1.3b
1.7
1.4
1.7
1.4
1.3
0.8
0.0
0.9"
12.7"b
30.6b"
41.0d
50.6'
55.6
53.9d
57.4
56.7
56.6
62.2
62.2
"Raw scores from videotapes over the first year of life (thirteen 4-week periods). TAC =
tactileioral exploration, OBJ = complex object manipulation, SP = self play. Data points
within each column with matching superscripts differ significantly from each other in posthoc
tests; if percentage measures, means differ after they have been arcsin transformed.
From birth to week 5, sleep was the predominant state. Time spent awake was
primarily in alert-quiet up to week 4;from week 6 on infants spent more time in
the alert-active state than in any other, and began to dismount from mothers
regularly. These results concur with Fragaszy's [19891 report of significant
changes in the organization of activity a t about 8 weeks. In the present study, the
decrease in the number of state changes from 5 to 8 weeks and the lengthening of
waking bouts over this period also support a reorganization of activity patterns
reflecting increasing behavioral persistence and infant attention span.
Most infants first began leaving mothers at about 6-8 weeks. This timing
concurs with that seen in other studies of captive groups [e.g., Fragaszy, 1990;
Fragaszy et al., 19911. At 3 months infants began spending increasing amounts of
time alone, out of proximity to any group member.
Dependence on mothers was still prolonged in comparison to related species
such as squirrel monkeys, [e.g., Boinski & Fragaszy, 19891. By the end of the first
year, most infants were still nursing and were still carried dorsally when upset.
Informal observations suggested possible weaning conflicts (tantrums, increased
attempts to nurse) in two infants, both in cages, towards the end of the first year.
However, these and most other infants were not completely weaned until well into
their 2nd year. Nevertheless, by 6-7 months infants spent more time alone than
with their mothers, and this pattern held for the rest of the year.
In the first months of life infants were extremely attractive to other group
members, and were frequently sought after and contacted by juveniles and adult
males. Instances of allomaternal carrying were seen in all agehex classes, but were
most frequent in juveniles and adult females. Previous findings suggest a shift in
emphasis from mothers to other group members after 6 months [Welker et al.,
19901. For example, Welker et al. [19871reported that adult males did not interact
much with infants under 5 months of age. In the present study, however, this shift
was not apparent; all age classes, including adult males, were active participants
in interactions with infants from birth. Instead, with the onset of independent
locomotion, time spent with mothers began to shift to time spent alone, often in
extensive exploration of the environment. Social groups in Welker's reports were
on average larger than in the present study, housed in areas roughly equivalent to
.OO
.03
1.01
1.19
1.70
1.65
1.38
1.24
1.42
1.33
1.12
1.23
1.04
.OO
.30
1.54
1.62
.84
1.18
1.30
1.01
.74
1.20
.90
.43
.76
1
2
3
4
5
6
7
8
9
10
11
12
13
.02
.19
.95
1.57
1.43
1.74
1.55
1.41*
1.51*
2.30
1.40
1.32
1.42
Run
.OO
.03
1.05
1.78
2.22
2.54
2.76
3.81
4.44
3.36
3.27
3.42
3.21
Cage
ML
.01
.19
1.56
3.07
1.81
1.94
1.98
1.68*
1.42*
2.58*
1.91*
2.24*
1.86*
Run
.OO
.02
.96
3.06
4.95
4.82
4.17
6.65
8.28
7.15
8.38
8.48
8.80
Cage
AM
.OO
.35
2.22
3.16
1.42
1.55
1.88
1.30
.86*
1.61
1.65*
1.44*
1.20*
Run
.OO
.01
1.09
2.66
4.39
4.00
2.91
3.97
5.28
5.05
6.17
6.31
6.58
Cage
LM
0.2
1.9
5.0
6.1
6.0
3.4
4.8
2.9*
3.6
5.4
6.2
8.1
4.6
Run
0.0
0.3
3.6
6.5
11.2
10.0
6.2
11.6
9.9
10.5
10.0
10.5
9.7
Cage
MPR
5.8*
3.0*
3.2
2.0
1.7
2.3
1.8
2.6
3.8
2.0
1.6
1.0
1.5
Run
34.0
28.8
6.6
4.5
2.3
1.6
2.4
1.4
1.1
1.3
1.4
1.3
1.6
Cage
vc
1.0
1.0
1.0
0.6*
0.7*
1.1
1.0
0.9
0.6
0.0
0.0
0.2
0.5
Run
0.0
0.0
0.1
0.3
1.1
1.3
1.7
2.5
2.5
3.1
1.9
1.7
0.6
Cage
SP
19.8
10.0
22.2*
17.8
19.2
20.0
18.0
17.4
18.4
21.9
22.2
0.0
3.8
Run
0.0
2.3
4.9
10.5
15.1
21.4
21.2
22.0
22.5
26.0
19.4
11.5
15.0
Cage
TAC
0.0
4.0
13.4
29.5
29.1
31.2
30.4
32.6
28.7
26.6*
33.5
32.5
31.6
Run
0.0
2.8
8.7
16.2
32.1
33.2
36.2
37.5
43.9
42.5
42.4
31.0
38.7
Cage
EXPL
'Raw scores from videotapes over the first year of life (13 4-week periods). MA = mother approach infant, ML = mother leave infant, AM = infant approach mother,
LM = infant leave mother, MPR = proximity to mother, VC = ventral contact, SP = self play, TAC = tactile/oral exploration, EXPL = total exploration (tactileioral
plus eating).
*Significant mean differences between runs and cages during that month (P < ,051.
Cage
Run
Month
MA
TABLE VIII. Housing Differencest
268 / Byrne and Suomi
TABLE IX. Age in Weeks of First Occurrences of Selected Behaviors
Tactile/oral explor.
Alert active > quiet"
Off mom, holding on
Off mom, in proximity
Mom and infant separate
Carried by othersb
Infant alone
Eat/explore food
Social play
Self play
Complex object manip.
50 % alone'
Alone > with momd
Mean
Range
Difference
4.2
5.6
5.5
6.6
7.4
7.5
7.5
6.9
8.7
10.9
15.4
17.5
22.1
3 -7
4-7
4-11
4-12
4-12
2-20
4-12
4-14
6-19
7-15
9-26
10-28
15-38
4
3
7
8
8
18
8
10
13
8
17
18
23
"Age a t which alert active scores exceeded alert quiet scores in activity state scoring.
infant in Group 2 was never observed to be carried by another individual. The earliest
case (at week 2) was due to kidnapping of a n infant by a juvenile male, and is not counted as
a voluntary separation of mother and infant. The next earliest incidence of carrying by others
was a t 4 weeks of age.
'Age at which infant was consistently alone for 50% of the observation time.
dAge at which infant was consistently alone more than with its mother.
TABLE X. Individual Differences in Levels of Selected Behaviors
Collapsed Over the First Year of Life*
Proximity to mother
Nursing
Dorsal contact
Total with mother
Proximity to others
Alone
Self play
Social play
Tactileioral explor.
Complex obj. manip.
Total explor.
Range
Coeff. of var.
2.2-17.3
.9-4
14.4-39.6
33.0-53.8
6.3-30.1
34.1-53.2
.2-1.7
2-5.6
6.9-20.4
Z8.3
18.1-30.7
.69
.45
28
.13
.34
.14
.55
.63
23
.71
.15
*Range scores are raw percentages of observation time, except for complex object manipulation, which is frequencyil0 minute period.
the indoorloutdoor runs. Increased social density andlor presence of established
matrilines in such groups may have affected infants' social scores in relation to
other animals.
Housing Effects
Housing in the present study was necessarily confounded by differences in
group composition. On average there were many more animals available in runs
than in cages, both in number and type. Approaching and leaving mothers and
proximity to mothers were all higher in cages than in runs, perhaps reflecting a
forced proximity in the smaller area. However, this effect was limited to mothers;
infants in cages spent less time with others and more time alone than infants in
runs. The difference in time spent with others was due to differences in proximity
Activity Patterns in Cebus Infants 1 269
and social play. Three of the six infants living in cages had only adults present in
their groups when they were born, and opportunities for play were limited. Contact
and grooming with others did not differ despite the differing number of animals
available in cages vs. runs.
For infants in cages, some relationships with other group members may have
been more intense; if total time spent with other animals is divided by the number
of animals available, scores for infants in cages were higher than those in runs.
Although not scored specifically, the most extensive cases of allomaternal care
were seen in cages; in two cases adult females in cages frequently carried other
females’ infants over a period of several months. I n one such case the allomother
also nursed the infant regularly and slept with it.
Self play and manipulation of food were higher in cages than in runs; data
from our laboratory report higher levels of manipulation of food and objects in
cages than in runs throughout the colony (Byrne and Suomi, unpublished manuscript). This difference may reflect a housing situation which is conducive to playing with food and objects, rather than other animals.
Finally, dorsal contact was higher in runs than in cages; early ventral contact
was conversely higher in cages. Increased spatial area in runs required locomotion
over longer distances, which may dictate the more efficient, dorsal style of carrying.
Individual Differences
Within these generalities there were wide individual differences in the levels
of some behaviors and timing of developmental changes. Some differences may be
related to housing, but some may be indicative of real differences in infant temperamentlreactivity. In particular, variability in complex manipulation was quite
high. Pounding of foodlobjects, for example, appeared as early as 9 weeks in one
infant and not until 6-7 months in another; frequencies of performing this behavior were highly variable among individuals. Ongoing studies examine the stability
of these differences and how they relate to infants’ reactivity patterns in mildly
stressful situations.
Future studies will follow infants as they mature, correlating early activity
with later measures of independence and exploration. Relations with specific age1
sex classes of other group members will be examined more closely. Finally, sex
differences in development are also of interest; the large proportion of males in our
sample made analyses of sex differences problematic. Sex differences will be evaluated as more infants are born into the colony.
CONCLUSIONS
1. Major changes in Cebus infant activity patterns take place from 3 to 8
weeks, as increases in waking states and improved motor control enable infants to
venture off mothers and actively explore their physical and social environments.
2. Over the first 4 months of life infants begin to gradually leave mothers,
interact with other group members, and explore their environments. From months
6 to 12, a s time alone increases, time with mothers continues to decrease. Time
spent with other animals remains stable over the first year.
3. Infants living in indoor cages show increased levels of approaches and leaves
to mothers, but spend less time with other group members and more time alone.
Self play and manipulation of the nonsocial environment is also higher in cages
than in runs.
270 / Byrne and Suomi
ACKNOWLEDGMENTS
We thank Georgina Slavoff, Kristin Abbott, and David Goldstein for assistance
in data collection during parts of this study. We also thank 3 anonymous reviewers
for their comments on a n earlier version of this paper. The first author was supported by a n Intramural Research Training Award from the National Institute of
Child Health and Human Development.
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~~~
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