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The development of sexual dimorphism in the bony pelvis of the squirrel monkey.

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The Development of Sexual Dimorphism in the
Bony Pelvis of the Squirrel Monkey
PHILIP D. GINGERICH
Peabody Museum of Natural History, Y a l e University,
N e w Haven, Connecticut 06520
ABSTRACT
Bony pelves of mature squirrel monkeys are sexually dimorphic.
They can be sexed confidently using the ratio of pubis length divided by ischium
length, together with a measure of the relative width of the superior pubic ramus.
Pelves of immature squirrel monkeys are not sexually dimorphic. Pelves of immature monkeys of both sexes resemble pelves of mature males morphologically.
Sexual dimorphism in the pelves of squirrel monkeys is the result of female
pelves acquiring morphological features at puberty which differentiate them
from mature male pelves and from immature pelves of both sexes, and is not
the result of male pelves being altered from immature pelves of the female type
as reported for mice arid rats.
Sexual dimorphism of the bony pelvis
has previously been described in mature
specimens of many genera of primates,
including man (cf., van den Broek, '11;
Washburn, '42; Schultz, '419; Davivongs,
'63; Wettstein, '63; Leu tenegger, '70;
Black, '70).
The development of this dimorphism of
male and female pelves in man has received some attention. Thomson (1899)
believed from his study of eight human
fetal pelves that pelvic sexual dimorphism
is evident before birth. More recently
Morton and Hayden ('41), Morton ('42),
and Crelin ('69) concludecl from studies
of large samples that the pelves of newborn and immature individuals are sexually indistinguishable and that pelvic
sexual dimorphism is acquired at puberty.
Coleman ('69) presents a detailed study of
morphological changes in the human pelvis
during growth as revealed by x-rays, and
summarizes the literature on this subject.
The only previous work on the development of sexual dimorphism of the bony
pelvis in a non-human primate is a study
on an old world monkey, the macaque, by
Polacek and Novotny ('65) in which they
conclude that sex differences in the pelvis
develop with sexual maturity as in man.
This paper is the first report describing the
development of sexual dimorphism of the
bony pelvis in a new world monkey.
ANAT. REC., 172: 589-596.
MATERIALS AND METHODS
Skeletal pelves of 54 squirrel monkeys
(Saimiri sciureus) in the collection of Prof.
G. E. Erikson, Brown University, were examined. The sex of 37 of these was recorded when the specimens were collected,
13 mature pelves were sexed easily by inspection, 4 immature pelves were sexually
indeterminate. Of those of known sex 24
were male, 26 female.
Four characters were chosen to describe
the pelvic sexual climorphism : the length
of the pubis divided by the length of the
ischium (pubis/isc:hium ratio; 100 x pubis/ischium ratio = "ischio-pubic index"
of some writers), the width of the superior
ramus of the pubis relative to ilium length,
the shape of the inferior border of the
ischio-pubic ramus, and the subpubic
angle. The differences between male and
female with respect to each of these characters is evident in the photographs of
typical adult male and female pelves in
figures 6-9.
Figure 1 shows a squirrel monkey pelvis
with various landmarks indicated. Measurements of the lengths of the ischium
(AB), pubis (AC). and ilium (AD) were
made following the method of Schultz
('30). The point A, representing the point
of fusion of the ischium, pubis, and ilium,
was located on fused mature specimens
Received April 15, '71.Accepted Nov. 1, '71.
589
590
PHILIP D. GINGERICH
D
+ MALE
Y
9
FEMALE
0
INDETERMINATE
x1m7Y
./
\
,
,
P
,
W i d t h of s u p e r i o r
r a m u s of pubis
,195
\
\
UNFUSED
FUSED
\
O L
4
2
3
Ilium length (crn)
Fig. 2 Relationship between pubis length and
ilium length. Upper line and equation represents
females, lower line and equation represents males.
1.04 ? 0.09 are the upper and lower 95% confidence limits on the male allometric constant.
Inferior border
of ischio-pubic rarnus
Fig. 1 Pelvic measurements used in this study.
A, point of fusion of ischium, pubis, and ilium;
B, point where axis of ischium crosses ischial
tuberosity; C, upper end of pubic symphysis; D,
most distant point on iliac crest.
at the intersection of the inner edge of the
lunate surface of the acetabulum with a
straight line prolonging the lower part
of the ventral border of the ilium downward. The width of the superior ramus
of the pubis was measured at its narrowest
point. The shape of the inferior border of
the ischio-pubic, or conjoined ramus was
described simply as convex, flat, or concave. The subpubic angle is formed by the
inferior borders of the left and right ischiopubic rami. It was measured with a simple
device incorporating a protractor. Fusion
of the three coxal bones was noted. Growth
is not necessarily completed with fusion
of the pelvic bones, but by this time characteristic geometry of the pelvis is determined. Monkeys with fused pelves are here
described as mature, monkeys with unfused pelves as immature.
The lines plotted in figures 2-4 and their
equations are derived from straight lines
fitted to the logarithms of the measurements (see Gould, '66, for a discussion
of this method applied to problems of rela-
Fig. 3 Relationship between ischium length
and ilium length. Upper line and equation represents males, lower line and equation represents
females. 0.76 0.06 are the upper and lower 95%
confidence limits on the male allometric constant.
1
+ MALE
FEMALE
0
INDETERMINATE
'f
'I
UNFUSEO
G/
I
FUSED
,
1
2
3
Ilium length(cm,)
4
Fig. 4 Relationship between the width of the
superior pubic ramus and ilium length. Upper line
and equation represents males, lower line represents females.
591
PELVIC DIMORPHISM IN THE SQUIRREL MONKEY
spect to ilium length (table 1). There is
some discrimination to sex by each of these
ratios but they are not as effective as the
pubis/ischium ratio. The graphs of figures 2 and 3 show how pubis length and
ischium length develop relative to ilium
length. In males both pubis length and
RESULTS
ischium length are related approximately
Pubis/ischium ratio. The most fre- isometrically to development of the ilium.
quently used sexual discriminant of bony In females however the pubis lengthens
pelves is the pubis/ischium ratio : the relatively faster than the ilium. The female
length of the pubis divided by the length ischium lengthens relatively more slowly
of the ischium. Table 1 gives the mean, than the ilium. Calculation of pubis/
range, and standard deviation of the pubis/ ischium ratios compounds these differences
ischium ratios for each category under and increases the power of discrimination
study. The mature pelves are clearly di- between male and female pelves.
morphic (male mean = 0.67, female
Comparison of the pubis/ischium, pubis/
mean = 0.86, t = 9.58 with 39 degrees of ilium, and ischium/ilium ratios of imfreedom, the probability is less than 0.001 mature pelves with those of the mature
that both samples come from a mono- pelves (table 1) reveals that all of the immorphic population). Most of the mature mature pelves have ratios falling more
pelves in this sample are discriminated to nearly within the male ranges than within
sex using the pubis/ischium ratio. There is the female ranges. This is most clearly
however, some overlap in tlhe ranges of seen in the best sexual discriminant, the
the pubis/ischium ratios of mature male pubis/ischium ratio, The relative proporand female pelves. The pubis/ischium tions of pubis length, ischium length, and
ratio by itself is therefore not completely ilium length in all the juveniles are clearly
reliable in discriminating sex in squirrel those of a mature male pelvis.
monkey pelves.
Superior pubic ramus. The second charPubis/ilium and ischium /ilium ratios acter useful in distinguishing mature male
were calculated to see how each component pelves from female pelves is the width of
of the pubis/ischium ratio varies with re- the superior ramus of the pubis relative to
tive growth). The statistical techniques
employed are outlined in Mmpson, Roe,
and Lewontin ('60), and Anderson ('58).
The discriminant function was calculated
by hand and checked with a !standard program in the Yale Computer Center.
TABLE 1
Observed range, mean, arid standard deviation of the pubis length/ischium length,
pubis length/ilium length, and ischium length/ilium length ratios
ibr the pelves i n the study sample
N
Range
Me a n
SD
Pubis/ischium ratio
Fused male pelves
Fused female pelves
Unfused male pelves
Unfused female pelves
Unfused indeterminate
17
24
7
2
4
0.56-0.73
0.760.95
0.60-0.66
0.57-0.62
0.59-0.65
0.67
0.86
0.62
0.60
0.63
0.05
0.07
0.04
Pubis/ilium ratio
Fused male pelves
Fused female pelves
Unfused male pelves
Unfused female pelves
Unfused indeterminate
17
24
7
2
4
0.37-0.44
0.42-0.55
0.40-0.44
0.4S0.42
0.35-0.41
0.41
0.48
0.42
0.42
0.39
0.02
0.03
0.01
17
24
7
2
0.55-0.66
0.48-0.62
0.66-0.70
0.67-0.74
0.60-0.63
0.61
0.56
0.68
0.70
0.61
0.03
0.03
0.02
Ischium/ilium ratio
Fused male pelves
Fused female pelves
Unfused male pelves
Unfused female pelves
Unfused indeterminate
4
-
0.03
-
0.03
c
0.01
592
PHILIP D. GINGERICH
TABLE 2
Observed range, m e a n , and standard deviation of t h e ratio of t h e w i d t h of t h e
superior r a m u s of t h e p u b i s l i l i u m length
Fused male pelves
Fused female pelves
Unfused male pelves
Unfused female pelves
Unfused indeterminate
N
Range
Mean
SD
17
24
7
2
4
0.10-0.15
0.06-0.10
0.14-0.17
0.14-0.15
0.12-0.15
0.12
0.08
0.15
0.15
0.13
0.01
ilium length (table 2, cf., figs. 6, 8 ) . The
male ramus is in general much more robust than the female ramus. Plotting the
width of the superior ramus of the pubis
against ilium length (fig. 4) shows that in
males the width increases approximately
linearly as the ilium increases. In the mature females however, the data suggest that
the width of the ramus decreases absolutely with age. Comparison of the mean
width of the superior pubic ramus of the
mature females with the mean of the immature females yields a probability of 0.06
that they represent a single population
(mature female mean = 0.27 cm, immature female mean = 0.32 cm, t = 2.00
with 24 degrees of freedom).
With respect to the second discriminating character, width of the superior ramus
of the pubis, the immature pelves of both
sexes are of the male morphological type.
Shape of the ischio-pubic ramus. The
third distinguishing character, shape of
the inferior border of the ischio-pubic
ramus, is most discriminating in large mature pelves. Being a non-metric character,
it is somewhat less objective than the two
characters described above. In the larger
pelves the inferior border of the ischiopubic ramus is convex in males, whereas
this border is concave in females (figs.
6, 8). In many cases this character can
only be described as flat, and therefore not
discriminating; however, in no case did a
fused male pelvis have a concave border,
in no case did a fused female pelvis have
a convex border, and in 12 of the unfused
pelves the border was convex. In the remaining one unfused pelvis (sexually indeterminate) the border was flat. Thus the
immature pelves of both sexes are of the
male morphological type with respect to
this third character.
Subpubic angle. The fourth character,
subpubic angle, could only be measured in
0.01
0.01
0.01
35 pelves, the remaining pelves being disarticulated or in some other way unsuitable
for the measurement. The inferior borders
of the ischio-pubic rami are generally somewhat curved, especially so in male pelves,
and this measurement is difficult to take
consistently, Where it could be measured
i t exhibited a great deal of variation
(fig. 5 ) . As a general rule the mature female pelves tend to have a larger subpubic
angle than the mature male pelves (female
mean = 51", male mean = 44", standard
deviation = 5" in each case; compare figs.
7, 9). Because of the difficulty of measurement and the great variance in the results
this character is not very useful in discriminating between the sexes in mature
pelves, nor is it useful in typing the immature pelves.
Discriminant function. To facilitate
classification of male and female pelves
the discriminant function
Z = X i - 6.31 Xp
-
0.12
was constructed (Anderson, '58) based on
the two best sexual discriminants. The variables XI and X, are the pubis length/
ischium length ratio and the width of the
superior pubic ramus/ilium length ratio respectively for the pelvis in question. If Z is
I
FEMALE
.
I
0
+
INDETERMINATE
.
**-.+
8
+
m
I
/
.
i
2
,
+
- * *
+
4
I
I
I
UNFUSED
. .
..
,
+
FUSED
Ilium length (cm,)
3
4
Fig. 5 Relationship between subpubic angle
and ilium length.
PELVIC DIMORPHISM IN THE SQUIRREL M0NKTi:Y
TABLE 3
Mean and standard deviation of the discriminant
function Z for each category studied
Fused males peves
Fused female pelves
Unfused males pelves
Unfused female pelves
Unfused indeterminate
N
Mean
SD
17
24
7
2
4
-0.21
t0.26
-0.45
- 0.44
- 0.31
0.08
0.11
0.06
0.08
0.09
positive the pelvis is female, if Z is negative the pelvis is male. The calculated probability of error in allocating a fused pelvis
is less than 0.03. All pelves in the study
sample described here are sexed correctly
by this discriminant function. Values of
the discriminant function Z are given in
table 3 for each category studied. The unfused pelves all fall within the male range
of the discriminant function.
Comparison of the immature male and
female pelves illustrated in figures 10-13
with the mature male and female pelves
in figures 6-9 shows the uniform presence
of a relatively shorter pubis, wider pubis,
and convex inferior border of the ischiopubic ramus in mature males and immature pelves of both sexes, and the relatively longer, narrower pubis and concave
border of the ischio-pubic rarnus characteristic of mature female pelves.
DISCUSSION
In the sample of squirrel inonkey pelves
studied here it is not possible to distinguish
unfused male from unfused female pelves.
Also, in this sample it is not possible to
sex all fused pelves using oiily the pubis/
ischium ratio. There is some overlap in
the ranges of variation of this ratio for
males and females, contrary to the case
in the samples studied by Leutenegger
('70) and Black ( ' 7 0 ) where a large separation was noted in the ranges of the
pubis/ischium ratio for males and females. However, by combining the pubis/
ischium ratio with the relative width of
the superior pubic ramus it is possible to
sex every mature specimen in this sample.
With respect to each of the discriminating characters (pubis/ischium ratio, width
of the superior pubic ramus, and shape of
the inferior border of the ischio-pubic
ramus) the unfused or immature pelves
exhibit precisely the morphology char-
593
acteristic of the fused pelves of mature
males. Prior to fusion the female pelves
diverge from this to establish the female
pelvic type. This alteration of female pelves
is reasonably correlated with puberty. At
this time the pelves of females are modified
from the generalized functional morphology present in males and juveniles and become specialized for bearing offspring.
Leutenegger ('70) has demonstrated that
among new world monkeys the squirrel
monkey has the largest newborn head size
relative to the size of the birth canal of
the mature female. The pelvic characters
listed above as being sexually discriminating are precisely the characters associated
with expansion of the birth canal. The relatively large size of the newborn's head requires specializations of the mature female
pelvis not found in immature and male
pelves. It is presumably adaptive to maintain a relatively short, robust, convex pubic
bone except when giving birth to a large
fetus is required.
Crelin ( ' 6 0 ) , and Bernstein and Crelin
('67) concluded, after a series of experiments on the development of the bony
pelvis in normal and gonadectomized mice
and rats, that in the absence of gonads
both male and female mice and rats develop a pelvis of the isogametic female
type, which is consistent with the general
pattern of sexual self-differentiation in the
absence of gonads (Witschi, '59). They
showed that development of a male-type
pelvis is dependent upon the presence of
testicular androgen. and further stated that
sexual dimorphism of the bony pelvis is
the result of the male pelvis acquiring features during postnlatal development that
cause it to differ from that of the female.
Quite possibly a castrated male squirrel
monkey would develop a bony pelvis of the
isogametic female type as reported for
mice and rats (Crelin, '60; Bernstein and
Crelin, '67). However, the bony pelves of
normal immature squirrel monkeys of both
sexes resemble mo~rphologicallythe pelves
of mature males. The presence of androgen
in the male squirrel monkey should therefore be interpreted as necessary to maintain an already male pelvis morphology
during development, rather than to cause
its differentiation from an earlier femaletype morphology. The development of
594
PHILIP D. GINGERICH
sexual dimorphism in the bony pelvis js
the result of the female pelvis acquiring
features during postnatal development
that cause it to differ from that of the male.
ACKNOWLEDGMENTS
I thank Prof. G. E. Erikson, Department
of Anatomy, Brown University for allowing me to study his collection of new world
monkeys and for his comments on the
manuscript. I also thank Drs. E. S. Crelin,
D. R. Pilbeam, and F. Ankel, Yale University, for their comments and assistance.
LITERATURE CITED
Anderson, T. W. 1958 A n Introduction to Multivariate Statistical Analvsis. John Wilev and
Sons, New York.
Bernstein, P., and E. S. Crelin 1967 Bony pelvic
sexual dimorphism i n the rat. Anat. Rec., 157:
517-526.
Black, E. S. 1970 Sexual dimorphism in the
ischium and pubis of three species of South
American monkeys. Jour. Mamm., 51: 794-796.
Coleman, W. H. 1969 Sex differences in the
growth of the human bony pelvis. Am. J. Phys.
Anthrop., 31: 125-151.
Crelin, E. S. 1960 The development of bony
pelvic sexual dimorphism i n mice. Ann. N. Y.
Acad. Sci., 84: 479-512.
1969 The development of the bony
pelvis and its changes during pregnancy and
parturition. Trans. N. Y. Acad. Sci., 31: 10491058.
Davivongs, V. 1963 The pelvic girdle of the
Australian aborigine: sex differences and sex
determination. Am. J. Phys. Anthrop., 21:
443-455.
-
Gould, S.J. 1966 Allometry and size in ontogeny
and phylogeny. Biol. Rev., 41: 587-640.
Leutenegger, W. 1970 Beziehungen zwischen
der Neugeborenengrijsse und dem Sexualdimorphismus am Becken bei simischen Primaten.
Folia primat., 12: 224-235.
Morton, D. G. 1942 Observations of the development of pelvic conformation. Am. J .
Obstet. Gynec., 44: 799-819.
Morton, D. G. and C. T. Hayden 1941 A comparative study of male and female pelves in
children with a consideration of the etiology
of pelvic conformation. Am. J. Obstet. Gynec.,
41: 485-495.
Polacek, P., and V. L. Novotny 1965 Sex differences in the bony pelvis in growing
macaques. Folia Morphol. (Prague), 13:
145-157.
Schultz, A. H. 1930 The skeleton of the trunk
and limbs of higher primates. Human Biol., 2:
303-438.
1949 Sex differences in the pelves of
primates. Am. J. Phys. Anthrop., 7: 401-423.
Simpson, G. G., A. Roe and R. C . Lewontin 1960
'Quantitative Zoology. Harcourt, Brace and
World, New York.
Thomson, A. 1899 The sexual differences of
the foetal pelvis. J. Anat. Physiol. London, 33:
359-385.
van den Broek, A. J. P. 1911 Uber Geschlechtsunterschiede im Becken bei Primaten. Arch.
Anat. Physiol. anat. Abt., 1911: 163-184.
Washburn, S. L. 1942 Skeletal proportions of
adult langurs and macaques. Human Biol., 14:
444-472.
Wettstein, E. B. 1963 Variabilitat, Geschlechtsunterchiede und Altersveranderungen bei
Callithrix jacchus L. Morph. Jahrb., 104:
185-271.
Witschi, E. 1959 Age of sex-determining mechanisms in vertebrates. Science, 130: 372-375.
PELVIC DIMORPHISM IN THE SQTJIRREL MONKEY
Philip D. Gingerich
PLATE 1
EXPLANATION OF FIGURES
6 Pelvis of mature male. Lateral view. X 2/3.
7 Pelvis of figure 6. Ventral view.
8 Pelvis of mature female. Lateral view.
9
x 3/4.
Pelvis of figure 8. Ventral view.
10
Pelvis of immature male. Lateral view.
11
Pelvis of figure 10. Ventral view.
x
12 Pelvis of immature female. Lateral view.
13
1.25
x 1.25.
Pelvis of figure 12. Ventral view.
595
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