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Behavior of an all-juvenile group of rhesus monkeys.

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Behavior of an All-juvenile Group of Rhesus Monkeys
JAMES LOY AND KENT LOY
Caribbean Primate Research Center,' School of Medicine, University of
Puerto Rico, Repto. Univ., Culle 1 0 , Sun German, Puerto Rico 00753
K E Y WORDS
Macaca mulatta
ganization . Matriline.
.
Juvenility
. Behavior .
Or-
ABSTRACT
The behavior and organization of a group of 33 juvenile rhesus
monkeys removed from a single freeranging group were found to parallel closely
intact rhesus social groups. Dominance ranking among the juveniles was 95%
predictable from prior knowledge of mothers' ranks. Grooming, sitting close together, sitting touching, and play were found to occur much more frequently
between matrilineal kin than expected from random selection of partner. Some
estrous behavior was observed, including two instances of peri-menstrual estrus.
One lengthy homosexual interaction between two two-year-old females was observed. The basis of much of the behavior and organization shown by the juveniles was found to be the matrilineal kin bonds brought from their natal group.
Among Cercopithecoid monkeys, the developmental period occurring between nutritional weaning and sexual maturity is
commonly referred to as the juvenile stage.
During this period, individuals have been
described as expanding their social relationships, becoming increasingly integrated into the social group, and learning adult
behavior patterns (Jay, '65). Although recognized as a period of critical importance
to individual development and social organization, relatively little detailed information is available regarding juvenility
in monkeys.
This paper reports on the behavior of a
unique assortment of juvenile rhesus monkeys (Macaca mulatta). Thirty-three yearling and two-year-old monkeys were removed from a single free-ranging social
group on Cay0 Santiago and observed in a
corral at the La Parguera, Puerto Rico primate colony. Thus, the interactions within
a single (though broad) ontogenetic stage
and among animals from a natural social
unit were observed away from the direct
influences of animals of other developmental stages.
The value of the study group was greatly
increased by the availability of information
regarding the behavior and organization of
their natal group on Cay0 Santiago. These
data were gathered in 1968-1969 by Dr.
E. A. Missakian, who kindly provided unpublished information.
AM. J. PHYS. ANTHROP.,40: 83-96.
The unique history and composition of
our study group allowed us to test current
theories regarding the predictability of
dominance relationships, and to analyze
juvenile grooming, play and sexual behavior.
METHODS
Animals
On November 18, 1971, 21 rhesus monkeys (1 1 males and 10 females) were transported to La Parguera from the Cay0 Santiago, Puerto Rico primate colony. All of
these monkeys were from Cay0 Santiago
Group A, and all had been born during the
1970 birth season. (On Cay0 Santiago, the
birth season extends from January through
mid-July, with March and April as the peak
birth months [Koford, '651 .) These 21 animals were placed in a one-half acre, outdoor corral and observations were begun on
November 19, 1971.
On December 3, 1971, 12 more monkeys
from Cay0 Santiago Group A arrived at La
Parguera and were introduced into the
corral with the earlier arrivals. Six of the
new monkeys were males and six were females, and all were born during the 1969
birth season.
The monkeys were individually marked
1 The Caribbean Primate Research Center was supported by contract NIH-71-2003 from the NINDS, PHS, HEW
until June 30, 1972. Since July 1, 1972, the Center h a s
been supported by contract NIH-DRR-71-2003 from the
Division of Research Resources of NIH, PHS, HEW.
83
84
JAMES LOY AND KENT LOY
by chest and thigh tattoos, and ear-notches. Matrilineal kin of all the study group
animals were known from census records
at Cay0 Santiago. Ten of the two-year-olds
had siblings in the yearling age-class. Ob-
servations on the group of 33 were begun
on December 6, 1971. Figure 1 presents
the study group genealogies.
During the study the monkeys fed ad
libitum on either Gravy Train dog chow or
YEAR OF BIRTH
'52 '63 '64 '6,s '66 '67 '66 '6F '60
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'
'
'bl 'b,t '68 '6?
'66 '66
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'6.9 '7.0
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I
Y
2
4
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Fig. 1 Study Group Matrilines. This figure shows the genealogies present during Dr. E. A.
Missakian's study of Cay0 Santiago Group A in 1968-1969. The relative rankings of the genealogies were presented in Missakian ('72). Only those monkeys born in 1969 and 1970 were in the
present study group. (d) and (r) indicate respectively animals that had died or had been removed from Group A prior to the removal of the entire group from Cayo Santiago.
BEHAVIOR OF JUVENILE RHESUS MONKEYS
Wayne Monkey Diet. Bananas and oranges
were occasionally provided. Water was
available at all times.
OBSERVATIONS
The juveniles were typically observed on
Monday-Friday mornings. Between November 19th and December 3, 1971, 22.2
hours of observation were made on the
group of 21 yearlings. Between December 6,
1971, and January 21, 1972, 95.3 hours
were spent watching the entire group of 33.
Thus the yearlings were observed for 117.5
hours and the two-year-olds for 95.3 hours.
On January 25, 1972, the group was divided into two parts of approximately equal
size and composition preparatory to a longterm study of the effects of castration and
ovariectomy. (AU animals were intact during the present study.)
While initially shy of the observers, the
monkeys, already habituated on Cay0 Santiago to human presence, soon allowed
close observation. Observations were made
within the corral by both authors following
a schedule which prevented an overlap of
information. Prose notes were taken, and
the data were categorized, tabulated and
plotted on matrices at the end of each day.
Numerous checks revealed a high degree
of inter-observer agreement in the description and interpretation of rhesus monkey
behavior. Positive identification of individuals was made in all interactions reported
in this paper unless specifically noted otherwise.
Visibility within the corral was excellent
due to a lack of cover and obstructions.
The observers continually scanned the
study group for social interactions, rather
than focusing on individual monkeys for
long periods. It is our assumption that the
differences in total observation time for individual monkeys were negligible. However, since this assumption was not tested,
we will limit ourselves in this paper to informal inferences from the observed behaviors.
RESULTS
Dominance relationships
Both Kawai (’65) and Kawamura (‘65),
studying different groups of Japanese macaques (Macaca fuscata), concluded that
the position of an individual in a hierarchy of agonistic dominance was strongly
85
influenced by the ranking of that monkey’s
mother in relation to other adult females.
Kawai theorized that offspring-rank is determined by mother’s-rank via the protectiveness of monkey mothers, who typically
rush to the aid of squabbling offspring.
Thus an infant supported in its encounters
with other monkeys by a high-ranking
mother would have a high “dependent
rank.” A second type of rank called “basic
rank” was also hypothesized by Kawai.
“Basic r a n k is the order established when
animals interact aggressively on a one-toone basis, with the outcome of the encounter being unaffected by other members of
the society. Basic rank was viewed as based
on physical strength and size, as well as
upon kinship in some unknown fashion.
Sade (’67) demonstrated that among freeranging rhesus monkeys, the rank of offspring among their peers closely parallels
the rankings of their respective mothers.
Marsden (‘68) reported that in captive
rhesus monkeys, induced changes in the
adult female dominance hierarchy produced rises or drops in offspring-rank paralleling the changes in the mother’s rank.
Sade (‘67) presented some data indicating that the rank of offspring persists even
after the loss of the mother. He suggested
that dominance hierarchies are established
early in life and persist for several years
“either because the first few fights set precedents which cannot be easily broken, or
because the offspring continue to associate with their mothers or older monkeys
who rank near their mothers, or because
of both reasons” (’67:113).
The present study group provided a
unique test of the relationship of motherrank to offspring-rank. During her 19681969 study, Missakian determined the rank
ordering of the adult female genealogy
heads for Cay0 Santiago Group A (Missakian, ’72, and personal communication).
Using her data, we were able to predict
the rank orders with the two ageclasses
represented in our study group. Missakian’s genealogical rankings are shown in
figure 1.
Dominance ranks for the study group
monkeys were based on observations of the
following types of interaotions.
1. Fights. A fight is an attack of any
intensity followed by a flight of any intensity. This definition follows Sade (’67), as
86
JAMES LOY AND KENT LOY
does our catalog of attack and flight behavior patterns, with two exceptions. In
the present study i t was not considered a
definite indication of subordinate rank
when an attacked monkey either “glanced
away” from or “presented to its attacker.
In interactions of these sorts, the attacking
monkey was usually awarded a “probable”
win.
2. Unprovoked submissive gestures. For
example, it was considered a good indication of subordinate rank if monkey “A”
grimaced at monkey “B”, who was walking
by, without overtly threatening the first
animal.
3. Displacements. One animal moved
away from the approach of another prior
to any noticeable directed threat from
the latter monkey (spatial equilibration;
Chance, ’56).
Interaction type 2 was considered as
strong an indicator of rank as type 1, and
the totals for both are combined in figure
2 under the category “fights.” Type 3 was
considered to be a slightly less reliable
measure of dominance, since it depended
upon the observers’ interpretation of the
motivation prompting movement. Occasionally, some confusion would arise as to
the winner of an agonistic interaction.
Often when this occurred, the interaction
was scored as a “probable” win for one
monkey or the other, and was also considered a less than completely reliable indicator of dominance. Only dyadic encounters
were utilized in determining hierarchial
positions.
Testing the predictability of
offspring-rank
During the study, 1303 dominance interactions were recorded (fig. 2). Of that
number, 727 were “within age-class” interactions (either between yearlings or between two-year-olds). Of the “within ageclass” dominance interactions, 686 (94.4%)
featured as the dominant animal that monkey predicted from mothers’ ranks to be
higher ranking (table 1). Looked at differently, of a total of 276 possible “within
age-class’’ dyadic dominance relationships
(e.g., A is dominant to B), data were gathered on 229 dyads. Of those 229 dyads, 219
relationships (95.6%) were in the order
predicted from knowledge of the mothers’
ranks (table 2).
The dominance hierarchies within the
two age-classes were thus highly predictable from knowledge of mothers’ ranks,
and were also quite stable during the period immediately following removal from
the natal group (in other words, immediately following the uniform removal of
direct maternal influence). This suggests
that for rhesus monkeys the mother’s rank
is the major factor determining the offspring’s “basic r a n k within its ageclass.
Sade (’67) has indicated that these hierarchies remain remarkably stable up to sexual maturity for rhesus monkey males
and throughout adulthood for females.
Reversals of predicted relationships
Figure 2 contains the dominance interactions observed during the study. Among
the two-year-olds, two unpredicted dominance relationships were discovered. Female 260 out-ranked female 277, and
female 235 out-ranked male 241. Eight unpredicted relationships were found among
the yearlings: female 400 over male 355;
female 331 over male 383; female 388 over
male 383; male 330 over male 383; male
330 over male 301; male 330 over female
388; male 335 over male 383; and male
301 probably dominant to female 367.
There were other unpredicted dominance
encounters not included in the above relationships. (1) Female 386 possibly displaced female 366 once. However, 366 defeated 386 definitely four times, and
“probably” three other times, and 366 was
considered dominant. (2) Female 386 defeated male 399 once. However, 399 defeated 386 fourteen times (plus 1 “probable” and 1 displacement) and 399 was
considered the dominant. (3) The relationship between females 333 and 386 was
unclear. Each won one fight over the other.
(4) Likewise, the ranking between females
333 and 354 was unclear, since each displaced the other once. ( 5 ) Although male
330 possibly displaced female 366 once,
366 “probably defeated 330 once. 366 was
considered the probable dominant monkey.
(6) Female 379 “probably” defeated male
383 once, but 383 definitely defeated 379
once and 383 was considered the dominant animal.
When the unpredicted relationships are
totalled by sex, it is found that in two cases
a male unpredictably out-ranked a female,
”
BEHAVIOR O F JUVENILE RHESUS MONKEYS
a
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A
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t
87
JAMES LOY AND KENT LOY
88
TABLE 1
Dominance predictability: Observed interactions
Fights
2-year-olds
Yearlings
Combined
1
Displacements
153
366
519
Probables
13
47
60
62
86
148
Total
Unpredicted
228
499
727
Predicted
%
%
4.8
6.0
5.6
95.2
94.0
94.4
Within ageclass interactions.
TABLE 2
Dominance predictability: Observed dominance relationships
Possible
dyads
2-year-olds
Yearlings
Combined
1
Observed
relationships
66
210
2 76
61
168
229
Unpredic ted
1
Predicted
%
%
3.3
4.8
4.4
96.7
95.2
95.6
Within ageclass relationships.
in three cases a male out-ranked another
male, in one case a female out-ranked
another female, and in four cases a female
unpredictably out-ranked a male.
ity of rising or falling in rank remains unclear.
Sibling dominance
Dominance relationships were observed
Reversals and genealogy
for nine of the ten sibling pairs in the
Two instances were recorded of all mem- group. In seven of these nine, the twobers of a genealogy attaining higher domi- year-old monkey was the dominant (1 case
nance than predicted. (1) Female 260 was of male over female, 1 case of male over
dominant to female 277,and 260s yearling male, 3 cases of female over male, and 2
sister 400 was likewise unpredictably dom- cases of female over female).
inant to 277’s yearling brother 355. It is
Yearling male 389 had risen over his
possible that since Missakian’s study there two-year-old brother 230 by the end of the
has been a rise in rank of LH (mother of study, although 230 was initially the dom260 and 400)over GS (mother of 277,355). inant. Very few instances of younger
Such a rise would explain the dominance brothers rising in rank over older brothers
alignment observed in the study group. have been reported for rhesus monkeys.
(2) Several members of the 090 genealogy Sade (‘72) describes two such cases, and
rose in rank. Female 235 was unpredict- Missakian (‘72)reports on the rise of a
ably dominant to male 241 (037 geneal- five-year-old male over both his ten-yearogy). 235’s yearling brother 330 was un- old and six-year-old brothers. To our knowlpredictably dominant to his cousins, male edge, 389 is the youngest male known to
301 and female 388,and to male 383.Male have risen in rank over an older brother.
Yearling female 400 was observed to
301 was dominant to female 367, and female 388 was dominant to male 383. It cause her two-year-old sister 260 to cower
is probable, but not demonstrable, that the a few days after the study group was
rank changes of any of these monkeys may formed. This was the only dominance interhave affected those of their relatives.
action observed between 400 and 260. The
Four yearlings were unpredictably dom- rise of 400 over 260 was not unexpected.
inant to yearling male 383, who had no Sade (‘72) reports that among female rhematrilineal kin in the study group. Six re- sus siblings, rank order is inversely related
versals were recorded in which the unpre- to age by the time all of the monkeys are
dictably subordinate monkey had three adults.
relatives in the group. The relationship beInter-age-class dominance relationships
tween the numbers of relatives present, or
Dominance relationships between yearthe absence of relatives, and the probabil-
BEHAVIOR OF JUVENILE RHESUS MONKEYS
lings and two-year-olds were not predictable, since the rule that “mother’s rank
determines rank of offspring” is applicable
only within age-classes. However, certain
trends regarding inter-age-class dominance
were discovered.
Observed dominance interactions between yearlings and two-year-olds totalled
576. The older monkeys defeated, probably
defeated, or displaced the yearlings in
question 334 times. In general, two-yearold monkeys tended to be able to (1) defeat yearlings in all genealogies ranked
below their own, (2) occasionally defeat a
yearling of their own genealogy whose
mother out-ranked their own, and (3) occasionally defeat a yearling belonging to the
genealogy ranked immediately above their
own (fig. 2). These were only trends, however, and individual variability was great.
Yearlings tended to be able to defeat
two-year-olds belonging to genealogies
more than one rank below their own genealogy. Again, individual variability in yearling dominance over two-year-olds was
great. Four yearlings were observed to defeat two-year-olds in the genealogy ranked
immediately above their own. Female 400
repeatedly defeated female 277, who was
also unpredictably defeated by 400’s older
sister 260. Yearling female 388 and yearling males 301 and 330 (all cousins) all
out-ranked two-year-old male 241 in the
genealogy above their own. Female 235
(sister of 330 and cousin of 301 and 388)
also unpredictably defeated male 241. It
is conceivable that the yearlings’ ability to
defeat the unrelated two-year-olds in the
four intances above was related to the unpredicted rise in dominance of their twoyear-old relatives.
Grooming
Grooming and kinship
During the study, 1753 grooming episodes were seen (fig. 3). Of that total, 323
episodes (18.4% ) were self-grooming and
1430 were inter-monkey grooming. Grooming between matrilineally related monkeys
(who comprised 6.6% of the total pair
combinations of the group) accounted for
491 episodes, or 34.3% of total inter-anima1 grooming. Thus related monkeys were
involved in grooming at a frequency which
was over five times greater than that expected from random selection of grooming
partner. Sade (‘65) reported that within a
89
free-ranging rhesus group on Cay0 Santiago, related pairs (15% of the total possible grooming combinations) accounted
for 62% to 64% of observed grooming episodes. The fact that the ratios between
percentage of related pairs and percentage
of related grooming are remarkably similar for the present study (0.19) and for
Sade’s (’65) study (0.24) strongly indicates
the normalcy of the grooming behavior in
the corral group, and suggests as well the
possibility of a mathematical constant between the number of related monkeys in a
group and the amount of inter-relative
grooming which will occur.
Grooming and dominance
Mid-ranking and high-ranking monkeys
tended to receive more grooming than they
gave to other animals. This trend was reversed among the low-ranking members of
the group (table 3). In his review of grooming behavior, Sparks (‘67) mentions evidence of the inverse relationship between
dominance and grooming in several primate species.
N u m b e r of grooming partners
It has been suggested that the juvenile
period is characterized by an increase in
the number of social relationships (Jay,
’65). We decided to utilize grooming data
to determine whether or not the two ageclasses represented in our study group
differed in the number of social contacts
per monkey. Sade (‘65:5) has argued that
grooming is the single activity “best suited for demonstrating relations between
animals of different sex and ages.”
It was found that the two-year-old monkeys were grooming and being groomed by
a larger number of individual monkeys
than were the yearlings (table 4). There
seemed to be no difference between males
and females (age-classes lumped) in the
mean number of monkeys from whom they
received grooming. However, two-year-old
females tended to have slightly more individual groomers than like-aged males (table 4). Females groomed greater numbers
of other monkeys than males (ageclasses
lumped). This trend occurred in both age
groups (table 4).
Measures of spatial relations
Two measures of inter-monkey spacing
were used to determine further the strength
90
JAMES LOY AND KENT LOY
GROOMEE
266
370
334
246
381
387
37I
277
316
260
400
366
272
o=
377
230
W 369
P
399
0 366
383
g
f9 231
331
273
333
242
367
241
314
301
388
231
330
379
335
Fig. 3 Grooming Interactions. Matrix cells contain the totals of observed grooming episodes. Interactions between matrilineal relatives are shown within the heavy black lines. Episodes of self-grooming appear along the diagonal.
of the matrilineal kinship bonds. Animals
were recorded “sitting close” (i.e., close
enough to touch one another, but not in
actual physical contact) and “sitting
touching.” Both are non-agonistic activities and presumably reflect either a mutual
attraction between animals or else a oneway attraction coupled with tolerance for
the presence of the attracted monkeys. Sitting touching was exclusive of other activities, such as grooming, which by definition
involve physical contact between animals.
A total of 658 “sitting close” dyads were
recorded. (In those occasional instances
in which a large number of monkeys sat
close, the clusters were broken down into
dyads for analysis.) Of that total, 224
dyads (34.0% ) involved related monkeys.
This figure is over five times greater than
expected, based on random selection of
“sitting close” partners.
Observed “sitting touching” dyads totalled 359. Of that number, 160 dyads
(44.6% ) involved related animals. This
figure is 6.8 times larger than expected by
chance.
BEHAVIOR OF JUVENILE RHESUS MONKEYS
TABLE 3
Grooming and dominance
~~~~
~
Dominance ranks 1
Episodes groomee
High
Mid
Low
1.14
1.30
0.67
0.88
0.77
1.50
Episodes groomer
Episodes groomer
Episodes groomee
1 Study group divided into thirds (the 11 highest-ranking, the 11 mid-ranking, and the 11 lowest-ranking
monkeys).
TABLE 4
Grooming relationships
Mean no. of indi- Mean no. of individuals who
viduals groomed
groomed subject
by subject
Males
Females
Sexes combined
2-year-olds
12.7
14.2
13.4
10.8
14.3
12.6
Males
Females
Sexes combined
Yearlings
9.2
8.6
8.9
7.5
11.5
9.4
(ages combined)
All males
10.4
8.7
(ages combined)
All females
10.7
12.6
Play
Play and kinship
Social play was frequently observed
among the juveniles. Play consisted primarily of mutual chasing, wrestling, and
mouthing (see Southwick et al., ’65). Play
typically appeared to be a pleasurable experience for all participants, but in several
instances, vigorous wrestling erupted into
aggression with the dominant monkey
biting and/or pursuing the subordinate.
Loizos (‘67) reviewed play among primates,
including theories regarding the nature
and function of play. We will direct our attention to the frequency of play in different juvenile age-sex classes and to the
selection of play partners.
A total of 479 play dyads were recorded
in the study. Play occurred most frequently
between pairs of monkeys, although larger
play groups were not unusual. Play inter-
91
actions involving more than two monkeys
were divided into multiple dyads for analysis. Eighteen play groups of three animals
and six groups of four animals were recorded. These observations accounted respectively for 11.3% and 7.5% of the total
play dyads.
Related monkeys were involved in 127
dyads (26.5% of the total). There is little
doubt that related monkeys were seeking
each other out as play partners, since the
observed frequency is slightly over four
times greater than the figure expected
from random selection of partners. The remaining 352 dyads (73.5% of the total) involved unrelated pairs.
Age-sex differences in play
Table 5 lists the raw frequencies of play
interactions by the age-sex classes involved.
Even assuming some differential observation, certain trends are clear. The age-sex
class most active in play was the yearling
males, followed by two-year-old males and
yearling females, with the two-year-old females being least playful. Lindburg (’71)
notes that among wild rhesus monkeys in
India, older juvenile females play much
less than like-aged males. The combined
males showed a much higher mean frequency of play (4 1.7 episodeslmonkey)
than combined females (1 5.6 episodes/
monkey). Combined yearlings showed more
play (mean of 37.9 episodeslmonkey) than
combined two-year-olds (1 3.6 episodes/
monkey).
Yearling males and females played with
all available age-sex classes, Two-year-old
males played with all age-sex classes except two-year-old females, and the latter
played only with yearlings of both sexes
(table 5). Observations following the study
period have shown that play does occur,
although apparently rarely, between twoyear-old males and females, and among
two-year-old females.
Sexual behavior
Single mounts
Although single mounts are often dominance (rather than sexually) motivated,
they are included under the heading “sexual behavior” since the motor patterns involved are essentially sexual in derivation.
Single mounts occurred between almost
all age-sex classes. Two-year-old females
92
JAMES LOY AND KENT LOY
TABLE 5
Play
Two year
male
Two year
female
Yearling
male
Yearling
female
Total
1
Mean play
episodes/
monkey
120
20.0
15
43
7.2
65
589
53.5
61 *
206
20.6
I
2 year
female
-
0
Yearling
male
-
-
Yearling
female
-
-
28
1
212"
-
1
I
1 Dyads involving animals of like agesex class contributed t w o episodes to the class total. Those subtotals which were doubled in calculating the grand total for the category are marked with an asterisk.
Class totals are summed left to right for two-year-old males, down and to the right for two-year-old females and yearling males, and down for yearling females.
were not seen to mount yearling females,
and yearling females were not observed
mounting males of any age.
Of the 230 total single mounts observed,
2 13 were between monkeys whose dominance relationships were known. Of the
latter total, 183 mounts (85.9%) featured
the dominant monkey as the mounter.
Mounts between related monkeys accounted for 12.6% of the total observed single
mounts.
Estrous behavior
Carpenter ('42) was the first to describe
the behavioral characteristics of estrus in
free-ranging rhesus' monkeys. Our behavioral definititions closely follow Carpenter,
plus the few refinements presented in
another publication (Loy, '71). A seriesmounting is equivalent to a mating, and
is defined as at least two mounts, closely
spaced temporally, and connected by
grooming, sitting close together or sitting
touching. Copulation refers to a seriesmounting which ends in ejaculation. Records were kept of sexual swellings and sex
skin coloration. The latter was judged by
eye on a five-point scale from zero (no
color) to four (bright red).
Sexual swelling, color, and menarche
During the study, four females, all twoyear-olds, showed sexual swelling and reddening of the sex skin (fig. 4). Undoubtedly
these were the first swellings and coloration for all four monkeys. Van Wagenen
('66) has reported that these two phenom-
ena are reliable immediate precursors of
the first vaginal bleeding, and that in the
laboratory, rhesus females reach menarche
between one and one-half to two and onehalf years.
Of the four study females mentioned
above, three showed probable vaginal
bleeding 36-43 days after their swellings
were initially noticed (fig. 4). The fourth
female was initially recorded as swollen
only 19 days before the study ended, and
no vaginal bleeding was observed.
Series-mountings
Twenty-two male-to-femaleseries-mountings were observed (fig. 4). Two were between two-year-old males and females, one
between a two-year-old male and a yearling
female, five between yearling males and females, and 14 between the same yearling
male and two-year-old female pair. Average number of mounts per series was 5.6,
with a range from 2 to 18.
In no instance was a series-mounting
observed to end in the motor patterns characteristic of ejaculation (Loy, '71), or in
the female clutching reaction (Zumpe
and Michael, '68). No female was seen
with a vaginal plug of coagulated semen.
These results were not surprising, since
Conaway and Sade ('65) reported that freeranging rhesus males on Cay0 Santiago do
not reach sexual maturity until about
three and one-half years of age.
The juvenile series-mountings were awkward and amateurish in comparison to
adult behavior. Female posturing, male
BEHAVIOR OF JUVENILE RHESUS MONKEYS
FEMALES
II IA
sw
277
260
272
273
242
238
400
388
367
Fig. 4 Sexual Behavior. Data are presented for all two-year-old females and for the three
yearling females who were series-mounted by males. Vertical dashed lines indicate missed days
of observation. Symbol key: SW ( A ) indicates the presence of swelling of the perineal area;
SM ).(
indicates a day with a t least one male-to-female series-mounting; and M ( 0 ) indicates
possible menstrual bleeding. Sex skin color ( S S C ) is the mean of the face, posterior thighs,
and pen-anal skin. Values for SSC range from zero (no color) to two (light red).
94
JAMES LOY AND KENT LOY
body orientation and grasping of the fe- in rhesus monkeys was found to be characmale’s waist and ankles were almost always terized by much play, expansion of social
done correctly. In most cases, it was not interaction spheres and the development of
possible to determine whether or not the skill in motor patterns (such as copulamale had an erection during a mount, al- tion) characteristic of adult monkeys.
though several mounts were seen that The juveniles in the study group were obwere definitely accompanied by an erec- viously immature, physically and behaviortion. Males seemed rarely to achieve intro- ally. In many ways, however, their behavmission, and although pelvic thrusts were ior and organization paralleled those of
often given, the male seemed usually to be intact rhesus groups. Data on agonistic
patterns, dominance, grooming and spatial
“searching” for the vagina.
As noted above, yearling male 399 was relationships all bear strong resemblance
seen series-mounting two-year-old female to reports from intact groups. The key to
242 on 14 occasions. These matings oc- these parallels is a behavioral base comcurred between December 28, 1971 and mon to both the study group and intact
January 15, 1972, and were accompanied free-ranging social units, the matrilineal
genealogy.
by a peak in 242s sex skin color (fig. 4).
Sade (’65, ’67, ’68, ’72) has outlined in
Two examples of peri-menstrual estrus
were seen (fig. 4). Male 399 series-mount- detail the interactions within a rhesus
ed female 242 on January 15th, two days monkey social group, and has emphasized
before 242 was seen with probable men- the importance of the mother-offspring
strual bleeding. Two-year-old male 230 bond and the matrilineal genealogy to
series-mounted two-year-old female 235 on rhesus behavior and organization. With
the second day of probable menstruation the development of mother-infant specificon January 21st. Loy (‘70a) discussed peri- ity, the infant monkey is linked to its mamenstrual estrus among rhesus monkeys, triline and, to a large extent, its future
behavior is determined. The infant develincluding its possible hormonal basis.
ops close relationships with mother, sibHomosexual behavior
lings and other matrilineal kin, and its
Between December 23, 1971 and Jan- kin ties then serve as the basis for its inuary 11, 1972, two-year-old female 272 tegration into the remainder of the social
was observed to mount two-year-old female group.
The strength of the matriline is demon242 fifty-one times. Most of these mounts
were in the form of series, with the indi- strated by its adaptability. Sade has shown
vidual mounts separated by grooming, or its importance to a medium-sized rhesus
the females sitting close together or sitting group, and Missakian (‘72) has presented
in body contact. Many of the mounts were similar data for a group with over 230
posturally identical to male-to-female members. Loy (‘70b) has shown that the
mounts; i.e., mounter grasps mountee’s matriline (and behavior centered on the
waist with her hands and mountee’s ankles matriline) remains stable during periods
or calves with her feet. However, on many of extreme food shortage.
The most significant result of the pres“mounts,” 272 would sit on 242’s lower
back and scoot forwards and backwards ent study was the discovery of a stable,
while she (272) yawned repeatedly. The viable social organization among the imlatter type of “mount” possibly functioned mature members separated from a naturas an auto-erotic stimulus for the mounter. ally occurring rhesus monkey social group.
It is of interest that the homosexual be- Upon separation from their natal group,
havior described above coincided with 242s the juveniles did not fall into behavioral
sex skin color peak and her most intense chaos, but rather maintained that organiheterosexual mounting activities. Beach zation and those relationships which they
(‘68) has presented evidence indicating had known as a segment of the larger
that in several mammalian species, an unit. This study demonstrated that the efestrous female is an effective sexual stim- fects of the matriline were strongly seen
as early as the juvenile stage of developulus for female conspecifics.
ment. Months after being separated from
DISCUSSION
their natal group, monkeys only 20-30
As reported by other workers, juvenility months of age were able to interact in an
BEHAVIOR OF JUVENILE RHESUS MONKEYS
orderly and predictable fashion because of
genealogically determined relationships
which formed the core of their interaction
systems.
ACKNOWLEDGMENTS
The authors are indebted to Dr. Clinton
H. Conaway, Scientific Director of the
Caribbean Primate Research Center. Dr.
Conaway originally proposed the castration-ovariectomy study for which the juvenile group was assembled. Dr. D. S. Sade
kindly provided the juveniles from Cay0
Santiago. Thanks go to Dr. Elizabeth A.
Missakian for her permission to use unpublished data, to Miss Jeanie DeRousseau
for her assistance in securing data stored
at Cay0 Santiago, and to the staff of the
La Parguera Primate Colony for their assistance throughout the study. Drs. Conaway, Sade and Gershon Berkson critically
reviewed the manuscript. Any errors remaining in the paper are attributable
solely to the authors.
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