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Brain weight-body weight relationships in 12 species of nonhuman primates.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 5637-81 (1981)
Brain Weight-Body Weight Relationships in 12 Species
of Nonhuman Primates
RODERICK T. BRONSON
Department of Comparative Pathology, Harvard Medical School, New
England Regional Primate Research Center, Southborough, Massachusetts
01772
KEY WORDS
Brain weight, Body weight, Primates, Allometry
ABSTRACT
Necropsy data from a Primate Center were used in a study of the
brain weight-body weight relationships of 1 2 species of nonhuman primates. The
sample sizes ranged from six Cercopithecus aethiops to 163 Macaca mulatta. By
plotting mean brain-mean body weight of each species on log-log paper, it was
shown that the straight line fitting the plots of all species had a slope of 0.72.
Slopes for three species of the genus Macaca, and for six species of the family Cebidae, were 0.61 and 0.81 respectively. Coefficients of determination of the three
lines were greater than 0.90. Two species of the family Cebidae, Saimiri sciureus
and Aotus trivirgatus, had equivalent body weights, but the former had a 30%
larger brain than the latter. The results suggest that brain-body weight scaling
characteristics of primate species can be studied effectively using necropsy data.
Some statistically significant discrepancies between these and published data,
however, show that more data are required to describe these characteristics with
greater certainty.
How brain weight varies with body weight
between species is optimally studied in log-log
plots of mean brain weight-mean body weight
of large samples of individuals of each species
(Jerison, 1976; Bronson, 1979). Thus species
within a taxonomic class, plot along a line with
slope of 0.67 (Jerison, 1976). Classes differ in
that one class line, e.g., the Mammalia line,
may be shifted above another, e.g., the Reptilia
line (Jerison, 1976). How brain size varies with
body size within and between taxonomic orders, families, and genera is not known, probably because the data have not been available.
Brain-body weight scaling within species has
been studied. Mean brain weight-mean body
weight, for samples of dog breeds and cat
breeds, have been plotted on intraspecies lines
with slopes of 0.27 and 0.67 respectively (Bronson, 1979).
This is a report of the brain-body weight relationships of 12 nonhuman primate species, for
which data were available, from the pathology
files at the New England Regional Primate Research Center. Mean brain weight and mean
body weight for each species were calculated,
and log-log plots of the data were analyzed.
These data will be compared to those collected
by Bauchot and Stephan (19691, from which
they differ significantly in some respects.
0002-948318115601-0077502.00
Some real, and artifactual, reasons for variability in brain weight-body weight and relationships between them, will be considered.
MATERIALS AND METHODS
The monkeys for which weight data were
studied, had been maintained in captivity for
variable periods, and had died from a wide variety of natural and experimental diseases. The
body and organ weights were recorded by
many different people over a decade, on periodically adjusted scales, as part of routine necropsy procedure. The species, probable age,
sex, brain weight, and body weight were recovered from each pathology report. Data were accepted into the study without regard to the
pathologic diagnosis. Age was usually not
known, since monkeys were caught in the wild.
Weight data from monkeys known to be infants or juveniles, because they were born in
captivity, or because their body weights were
clearly too low to be appropriate for those of
adults, were excluded from the analysis.
The mean brain weight and mean body
weight, and standard deviations for each sex,
and for both sexes of each of 12 species, were
0 1981 ALAN R. LISS, INC.
~
_
_
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Received February 17.1981; accepted June 22,1981
78
R.T. BRONSON
T A B L E 1. Mean brain and body weights for 12pnmate species
Female
1 Saguinus oedipus
2 Aotus trivirgatus
3 Saimiri sciureus
4 Cebus albifrons
5 Cebus apella
6 Ateles geoffroyi
7 Ateles paniscus
8 Cercopithecus aethiops
9 Macaca fasciculans
10 Macaca mulatta
11 Macaca arctoides
12 Papio anubis
47: 9.7 t 0.6
140: 17.2-1- 1.7
58: 23.1 -1- 1.8
21: 60.7 2 5.9
10: 62.6 2 6.4
10:107.7 f 10.1
17:101.2 t 11.5
6: 66.1 k 8.9
36: 61.9 f 7.9
132: 85.2 -1- 8.6
25: 99.8 f 9.8
5:148.0 2 9.7
47: 370k 85
143: 6 3 0 f 153
57: 5 8 0 f 85
21: 1,470 f 300
11: 1,810 f 260
11: 4,680 f 1,920
15: 5,220 t 2,210
6: 3,220 k 550
44: 3,360 f 1,150
163: 4,880 f 2,350
27: 7,450 -t 3,770
7:12,900 k 3,160
0.5
Male
Body_weight, gm
n:
X t o
Brain wzight. g m
n:
X t o
Body weight, gm
n:
X + o
I
5
58: 3 7 0 2 66
116: 720 2 180
33: 7 2 0 5 150
45: 1,770 k 640
7: 2,770 f 650
4: 4,760 f 1,620
6 5,040 k 2,730
10: 5,230 k 1,020
8: 4,740 t 1,680
125: 5,210 t 2,950
13: 7,810 t 4,010
13:16,060 3,770
+
10
Brain wAight, gm
n:
X-+o
59: 9.2 k 0.6
120: 17.2 & 1.5
32: 25.0 k 2.4
45: 65.2 ? 8.9
7: 70.8 -C 6.2
4:107.6 +. 16.5
6:106.5 % 12.4
10: 66.3 ? 5.6
7: 70.0 ? 11.6
93: 90.7 k 12.1
9:101.6 ? 12.0
15:174.7 ? 17.4
50
MEAN BODY WElGHT g x103
Fig. 1. Log-log plots of mean brain weight-mean body
weight data for 12 species of primates. The numbers refer t o
species listed in Table 1. The ellipse around each point
represents one standard deviation in each dimension. Four
lines are fitted to the appropriate points by the method of
least squares; their formulae appear in Table 2. The average
mammal line is from Jerison (1976). and is equivalent to the
line fitting data for domestic cats la) and dogs (b) IBronson,
1979).
79
BRAIN-BODY WEIGHTS IN PRIMATES
calculated. To analyze how brain weight varied
with body weight for various groups of primates, power curves were fitted to the data by
the method of least squares.Thecurves had the
general formula, y = axb, where y = brain
weight, x =body weight. The coefficient of determination (r2),was calculated for each curve.
The groups so analyzed were: (1)males of 12
species; (2)females of 1 2 species; (3) animals of
both sexes of 12 species; (4) each sex and combined sexes of s ecies of the enus Macaca
(mulatta, fascicufks, arctoide5; ( 5 ) each sex
and both sexes of six species of the family Cebidae (Cebus albifrons, Cebus apella, Ateles
paniscus, Ateles geoffroyi, Saimiri sciureus,
and Aotus trivirgatus); and (6) individuals of
each of three species, M. mulatta, C. albifrons,
and C. apella. These were chosen arbitrarily as
examples of species represented by large, intermediate, and small sample sizes.
RESULTS AND DISCUSSION
The weight data are presented in Table 1.
The data for males and females combined are
plotted on log-logpaper in Figure 1. The ellipse
around the point for each species in Figure 1
represents the standard deviations in both dimensions. The power curve fitting data for 12
species, and for Macaca and Cebidae, are
straight lines in the figure. Formulae and coefficients of determination for these and other
lines are presented in Table 2. The Carnivora
line in the figure is from Bronson (1979).
Variability in body weight is greater than
variability in brain weight in most species
studied. This was especially pronounced in
such species as macaques, which are prone to
obesity (Walike et al., 1977).Variation in body
weight may also be due to disease factors. Saguinus oedipus, for example, often lose considerable weight before death (Chalifoux and
Bronson, 1981). Inadvertent inclusion of data
from immature animals also increases variability, and may seriously bias the data, since immature animals have larger brain weight-body
weight ratios than adult animals.
A consequence of the large variability in
body weight, is that power curves fitted to
brain weight-body weight data for individuals
of the same species have almost zero slope, and
very low r2 values (Bronson, 1979). This finding, repeated for three primate species, is presented in Table 2. Clearly intraspecies brainbody weight scaling cannot be studied usefully
from individual animal data.
In studying brain-body weight scaling of
species within a taxonomic group, four potential sources of error must be considered. First,
the location of the point for each species is subject to error as a result of the intraspecies variation in both parameters. This effect can be severe if the sample is small.
Second, the validity of the line fitting a
group of species depends on the number of species. When small numbers are studied, the effect of inclusion or exclusion of a single species
can be severe, particularly if it plots at one end
of and away from the regression line fitting the
data. For example, if mean brain and body
weight for nine Tupaia glis, 3.1 k 0.5 gm and
129 k 19.7 gm respectively, are included in the
regression analysis of the 12 species studied
here, the slope of the line is 0.81 instead of 0.72.
A third source of error is expected when the
groups plotted have similar body weights.
This is especially true when large standard de-
TABLE 2. Formulae ofpower curves, y = axb, fitting brain weight fy) body weight (x) data from
various samples of primate species
Sample
a
b
Coefficient
of determination,r2
0.20
0.21
0.17
0.72
0.7 1
0.73
0.91
0.91
0.91
0.43
0.51
0.45
0.61
0.59
0.61
0.90
0.95
0.71
0.12
0.16
0.08
0.12
34.40
7.76
26.5
0.81
0.77
0.85
0.67
0.11
0.28
0.12
0.91
0.92
0.94
12 species of primates
M & F
F
M
~3 species of Macaca
M & F
F
M
6 species of Cebidae
M & F
F
M
Average mammal (Jerison, 1976)
92 M M. mulatta
17 M & F C. aaella
61 M & F C. aibifrons
-
0.06
0.48
0.06
80
R.T. BRONSON
viations are also present. Thus the Primates
line, including species ranging in body weight
from 370 gm to 16,060 gm, is less affected by
individual animal variation than the Macaca
line.
The fourth source of error results from inclusion of both male and female animals of sexually dimorphic species, or of adults and juveniles. The mean body weight of a sample consisting primarily of female or juvenile animals
might be considerably less than that of one
consisting of male animals. Since brain-body
weight scaling of females was similar to that of
males of the three taxonomic groups studied
(Table Z), sexual dimorphism was probably not
a source of error here.
In spite of the uncertainties intrinsic to this
analysis, some general conclusions about
brain-body weight scaling in primates can be
drawn safely. First, scaling within the order
Primates, the family Cebidae, and the genus
Macaca, occurs along lines with slopes approximating those of taxonomic classes, 0.67, rather than that of breeds of dogs, 0.27 (Bronson,
1979). This finding for the genus Macaca contradicts that of Sholl (1948),and the assumption by Pilbeam and Gould (1974)that scaling
within a genus occurs along lines with slopes of
between 0.20 and 0.40. Sholl studied various
ways of fitting curves to brain-body weight
data of individual macaques of several species
and ages, including what must have been
juveniles, judging from the weights. He found
that the line fitting log-logplots had a slope of
around 0.25. This line was probably an artifact
of summation of data for which the interspecies line would have a slope of around 0.67, and
for which the intraspecies line would have virtually no slope, as we have seen.
Another interesting finding is that S. sciureus has a 30%larger brain than A. trivirgatus,
even though the two species have similar body
weights. A likely explanation for the weight
difference is that S. scuireus has a much larger
occipital lobe than A. trivirgatus (Fig. 2). How
this might relate to neurological or behavioral
differences is unclear. This pair of species
seems a reasonable one in which to explore the
poorly defined concept of “encephalization”
(Holloway,1968;Krompecher and Lipak, 1966;
Pilbeam and Gould, 1974).That Suimin’isdiurnal, and Aotus is nocturnal, might make comparisons difficult, however.
Other conclusions about these data are more
tentative, because of uncertainties arising
from the four sources of error previously mentioned. Of greatest interest is the difference in
Fig. 2. Skulls of Sairniri sciureus (top),and Aotus triuir.
(bottom). The distance from the posterior edge of the
foramen magnum t o the occipital pole is 1.4 cm in the former. 0.9 cm in the latter. X 1.29.
gatus
slopes for the macaques and the Cebidae.
Whether it is real or artifactual can be resolved
with certainty only by including more data and
more species. There appears to be no statistical
test available to test the significance of differences in slopes of two regression lines fitted to
data points that themselves represent the
means of individual data points.
In comparing the data presented here with
those collected by Bauchot and Stephan (1969)
for the same species, the validity of the difference in the slopes for Cebidae and Macaca become still less certain. Whereas mean brain
weight for none of the species differed significantly between the two samples when tested
by Student’s t test, body weights for five species did: C. apella, A. geoffroyi, A. paniscus, C.
aethiops, and M. mulatta. The mean body
weight of all these, other than A. geoffroyi, was
lower in the published samples, even after excluding those individuals stated to be immature, or whose weights clearly indicated they
were. In A. geoffroyi, Bauchot and Stephan
(1969)reported very high mean body weights,
7,159 gm, n = 98, compared to that of this sam-
BRAIN-BODYWEIGHTS IN PRIMATES
ple. There is no clear explanation for the discrepancy. Significantly, if the scaling line of
the Cebidae is recalculated using this value the
slope is 0.71, not 0.81.
This study, like a previous one (Bronson,
1979),has shown that brain-body weight scaling within taxonomic groups can be studied effectively by plotting mean weights of large
samples of necropsy specimens. In the future,
as more data are collected, it may be possible
to map many primate species onto a log-log
plot with a reasonable degree of certainty. At
that time, scaling characteristics of species
within families and genera can be looked at
again.
ACKNOWLEDGMENTS
This work was supported by NIH grant NO.
RR00168, from the Division of Research
Resources, Bethesda, Maryland.
81
LITERATURE CITED
Bauchot. R. and Stephan H. (1969)Encephalization et niveau evolutif chez les simiens. Mammalia 33:225-275.
Bronson, RT (1979) Brain weight-body weight scaling in
breeds of dogs and cats. Brain Behav. Evol. 16227-236.
Chalifoux, LV, and Bronson, RT (1981) Colonic adenocarcinoma associated with chronic colitis in cotton top marmosets, Saguinus oedipus. Gastroenterology 80:942 -946.
Holloway, RL (1968) The evolution of the primate brain:
Some aspects of quantitative relations. Brain Res. 2121 172.
Jerison, H J (1976) Paleoneurology and the evolution of
mind. Sci. Am. 23490-101.
Krompecher, ST, and Lipak. J (1966) A simple method for
determining CerebraliGtion brain weight and intelligence.
J. Comp. Neurol. 122113-120.
Pilbeam, D, and Gould, SJ (1974) Size and scalingin human
evolution. Science 186892-901.
Shall, D (1948) The quantitative investigation of the vertebrate brain and the applicability of allometric formulae to
its study. Roc. R. Sac. Lond. Ser. D. 135243-358.
Walike, BD, Goodner, CJ, Keorker, DJ, Chideckel, EW, and
Kolnasy. LW (1977) Assessment of obesity in pigtailed
monkeys (Macacu nemestrinu). J . Med. Primatol. 6 1 5 1 162.
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