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The Brain of the Horse: Weight and Cephalization Quotients

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The horse is a common domestic animal whose anatomy has been studied since the XVI century. However, a modern neuroanatomy of this species does not exist and most of the data utilized in textbooks and reviews derive from single specimens or relatively old literature. Here, we report information on the brain of Equus caballus obtained by sampling 131 horses, including brain weight (as a whole and subdivided into its constituents), encephalization quotient (EQ), and cerebellar quotient (CQ), and comparisons with what is known about other relevant species. The mean weight of the fresh brains in our experimental series was 598.63 g (SEM ± 7.65), with a mean body weight of 514.12 kg (SEM ± 15.42). The EQ was 0.78 and the CQ was 0.841. The data we obtained indicate that the horse possesses a large, convoluted brain, with a weight similar to that of other hoofed species of like mass. However, the shape of the brain, the noteworthy folding of the neocortex, and the peculiar longitudinal distribution of the gyri suggest an evolutionary specificity at least partially separate from that of the Cetartiodactyla (even-toed mammals and cetaceans) with whom Perissodactyla (odd-toed mammals) are often grouped. © 2013 S. Karger AG, Basel.
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Original Paper
Brain Behav Evol 2014;83:9–16
DOI: 10.1159/000356527
The Brain of the Horse:
Weight and Cephalization Quotients
Bruno Cozzi a Michele Povinelli a Cristina Ballarin a Alberto Granato b
a Department of Comparative Biomedicine and Food Science, University of Padova, Legnaro , and
b Department of
Psychology, Catholic University, Milan , Italy
Introduction
The horse is a very common, widely distributed, well-
researched domestic animal whose affinity with humans
dates as far back as the beginning of civilization. As an
example of its diffusion, according to the Italian National
Register there are presently 439,585 equines (381,694 of
which are horses) in Italy. The statistics report that each
year slightly fewer than 100,000 horses are slaughtered for
meat consumption in Italy (horse meat is a very popular
specialty in several European countries, including Italy,
France, and Germany).
The horse was the first animal to receive a dedicated
exhaustive textbook of anatomy after the human [Ruini,
1598], and modern veterinary anatomy (late XVII century
to the end of the XX century) is based on the horse as a
model. Knowledge on the functional structure of the cen-
tral nervous system of the large herbivores mostly dates
back to more than 100 years ago, when – in the wake of the
discoveries on the structure of the human and rodent cor-
tex – several researchers analyzed also the brains of the
large domestic animals [Chauveau and Arloing, 1879;
Zimmerl, 1909; Ellenberger and Baum, 1912; Sisson, 1914].
A few quite preliminary electrophysiological studies were
performed in the middle of the XX century [Breazile et al.,
1966]. However, the steady interest in equine anatomy and
the present growing attention to animal welfare notwith-
standing, scientific information on the neuroanatomical
and neurophysiological basis of horse behavior is absent
or scarce. As Nieuwenhuys et al. [1998] stated, ‘A modern
neuroanatomy of Ungulates is not existent.’
Key Words
Horse brain · Brain weight · Encephalization quotient ·
Cerebellar quotient
Abstract
The horse is a common domestic animal whose anatomy has
been studied since the XVI century. However, a modern neu-
roanatomy of this species does not exist and most of the data
utilized in textbooks and reviews derive from single speci-
mens or relatively old literature. Here, we report information
on the brain of Equus caballus obtained by sampling 131
horses, including brain weight (as a whole and subdivided
into its constituents), encephalization quotient (EQ), and cer-
ebellar quotient (CQ), and comparisons with what is known
about other relevant species. The mean weight of the fresh
brains in our experimental series was 598.63 g (SEM ± 7.65),
with a mean body weight of 514.12 kg (SEM ± 15.42). The EQ
was 0.78 and the CQ was 0.841. The data we obtained indi-
cate that the horse possesses a large, convoluted brain, with
a weight similar to that of other hoofed species of like mass.
However, the shape of the brain, the noteworthy folding of
the neocortex, and the peculiar longitudinal distribution of
the gyri suggest an evolutionary specificity at least partially
separate from that of the Cetartiodactyla (even-toed mam-
mals and cetaceans) with whom Perissodactyla (odd-toed
mammals) are often grouped. © 2013 S. Karger AG, Basel
Received: September 6, 2013
Returned for revision: October 1, 2013
Accepted after second revision: October 18, 2013
Published online: December 4, 2013
Bruno Cozzi
Department of Comparative Biomedicine and Food Science
University of Padova, Viale dell’Università 16
IT–35020 Legnaro (Italy)
E-Mail bruno.cozzi @ unipd.it
© 2013 S. Karger AG, Basel
0006–8977/13/0831–0009$38.00/0
www.karger.com/bbe
Cozzi /Povinelli /Ballarin /Granato
Brain Behav Evol 2014;83:9–16
DOI: 10.1159/000356527
10
Comparative investigations on the mammalian neo-
cortex [Hof et al., 1999] emphasize a lack of data on the
cytoarchitecture of Artiodactyla and point out the ab-
sence of an evident layer 4 in the cortical columns. We
presently have no idea of the ‘quality’ of the presumed
‘frontal’ cortex of the horse and are therefore at a loss
when defining this area, its extension, and the reciprocal
relationship with intracortical bundles and thalamic af-
ferents. This is also due to the fact that the distribution of
neural markers (including calcium-binding proteins) in
the cortex is relatively well known in several orders of
mammals but not in the horse [Hof et al., 1999, 2000].
Thus, current textbook definitions of the areas and lobes
of the horse brain are acceptable only as a general topo-
graphical reference but are not based upon reliable mor-
phological or functional criteria.
Studies on brain size generally group species of different
orders and apply a number of indexes to assess the evolu-
tionary position of each group. The most widely accepted
method of comparison remains the encephalization quo-
tient (EQ), proposed by Jerison [1973]. However, even in
general reviews and research articles on the subject, the
horse is seldom represented; in an interesting study dedi-
cated to 86 species of Ungulates (i.e. Artiodactyla and Pe-
rissodactyla), the horse is absent and the only equine con-
sidered is the zebra [Pérez-Barberia and Gordon, 2005]. In
a recent comprehensive analysis of encephalization data in
mammals [Boddy et al., 2012], 6 species of Perissodactyla
were considered as a whole group, but no further detail was
given and no comparison was made with other closely re-
lated hoofed mammals. Even the updated study by Zilles
et al. [2013] on cortical folding, when considering the brain
of the horse, refers to a study from 1913 [Brodmann, 1913].
This article starts with very basic, still largely unre-
solved questions: how large is the brain of the horse?
(Older literature is summarized in table1 .) If a cephaliza-
tion index is considered, what is the position of the horse
in the general mammalian distribution? To address these
challenges, we analyzed the relevant past literature as a
starting point and then harvested a large number of horse
brains at the slaughterhouse. For the latter brains, we con-
sidered the total weight, the relative weight of the single
cerebral vesicles, the EQ and the cerebellar quotient (CQ),
the relation to data from other mammalian species, and
statistical aspects of the data we obtained.
Materials and Methods
Brain Sampling
For the present study, we sampled a total of 131 equine brains,
105 (64 females and 41 males) of which were collected at local ab-
attoirs and 26 of which were removed in the necropsy room of the
Department of Comparative Biomedicine and Food Science of the
University of Padova at Legnaro, Italy. The horses were all mixed
breed. The brains collected in the necropsy room were photo-
graphed and, after fixation, subdivided into telencephalon, dien-
cephalon, mesencephalon, cerebellum, pons, and medulla oblon-
gata. The spinal cord was transected from the brain at the level of
the occipital foramen. Subdivision of the brain into its constitutive
parts was performed consistently by one of the authors (B.C.) to
avoid bias in subsequent sampling sessions.
At the slaughterhouse, animals were treated according to the
European Community Council directive (86/609/EEC) concerning
animal welfare during the commercial slaughtering process, and
they were constantly monitored under mandatory official veteri-
nary medical care. The cause of death of the horses sampled in the
necropsy room was related to fatal illnesses of various natures but
not involving the central nervous system. Age was determined
based on official documentation available at the moment of slaugh-
tering (for the animals sampled at the abattoirs), on the documenta-
tion presented by the owners, or by direct reading of teeth wear and
tear (for the animals sampled in the necropsy room). All of the an-
imals whose brains were sampled in the necropsy room were adults.
Determination of Brain and Body Weight
Brains removed at the slaughterhouse (n = 105) were weighed
with a precision scale. The dura mater was removed during extrac-
tion of the brain. The arachnoid was frequently broken during sub-
traction of the dura. The pia mater was generally left in place since
its painstaking elimination was rather impossible to achieve work-
ing at the slaughterhouse without risk of damaging the brains.
Body weight was determined for each horse by the staff of the
slaughterhouse.
Brains removed in the necropsy room (n = 26) were immersed
in formalin for 2 weeks and stored at 4
° C to allow hardening and
proper fixation. Such short-term immersion in formalin deter-
mined an increase in brain weight due to penetration of the fluid.
Comparison between fresh and formalin-fixed brain weights
yielded the following conversion formula: Bw
fresh = Bw
fixed /1.104,
where Bw is brain weight. The weight of the brain and its parts de-
Table 1. Brain weight (g) as reported in the literature
Brain weight 650 652 577 650 600 680 630 400 700 588
Reference Chaveau and
Arloing [1879]
Vaughan
[1892]
Zimmerl
[1909]
Sisson
[1914]
Seiferle
[1988]
Barone and
Bortolami [2004]
Dyce et al.
[2010]
Shultz and
Dunbar [2010]
The Brain of the Horse: Weight and
Cephalization Quotients
Brain Behav Evol 2014;83:9–16
DOI: 10.1159/000356527
11
rived from the principal components (telencephalon, diencepha-
lon, mesencephalon, pons, cerebellum, and myelencephalon) was
calculated based on formalin-fixed brains after careful dissection.
EQ and CQ
The weight of the brain was related to body weight to obtain
the EQ, calculated with the formula EQ = E
i /0.12P 2/3 , where E
i
and P are the mean weight of the brain and body, respectively
[Jerison, 1973]. We maintained the value of the exponent (2/3 or
0.67) originally indicated by Jerison [1973]. However, we are
aware that recent studies suggest that a slightly higher value
(0.75) would best fit all mammals [Boddy et al., 2012]. The EQ
in this study was calculated using only data from fresh brains
(n = 105). The data thus obtained were then compared to reports
in the literature for other species ( table2 ). When choosing the
species for comparison, we deliberately included most domesti-
cated mammals and primates. We also calculated the individual
EQ of each adult horse, using their specific brain and body
weights. To calculate the CQ, we applied the formula CQ = Cb
vol /
(0.145M
b 0.978 ), proposed by Maseko et al. [2012], in which Cb
vol
is the volume of the cerebellum (Cb
vol × 1.04 = Cb
mass × 0.96
[Weaver, 2005]) and M
b is the brain mass (=brain weight). Since
the weight of each brain component was determined on fixed
specimens, we applied the conversion formula to obtain the
weight of fresh tissue (see above).
Nomenclature of the Cortex
The nomenclature of the equine cortical gyri and fissures is still
incomplete or not universally accepted. For the present study we
used the international nomenclature [International Committee on
Veterinary Gross Anatomical Nomenclature, 2012].
R e s u l t s
Weight of the Brain
The general features of the brain of the horse are de-
picted in figure 1 . The weight of the horse brain reported
in a series of widely recognized reference texts and articles
is summarized in table1 . There is a general consensus
[with the exception of Ellenberger and Baum, 1912] that
the weight of the brain of the horse is between 600 and
700 g. The number of animals (if more than one) consid-
ered by each study is seldom reported.
In the present study, young prepubertal animals aged
less than 2 years (n = 28) had a brain weight of 578.17 g
(SEM ± 11.89) and a mean body weight of 456.1 kg (SEM
± 25.84). Adult horses older than 2 years (n = 77) had a
mean brain weight of 606.07 g (SEM ± 9.39) and a mean
body weight of 535.22 kg (SEM ± 15.42).
The mean brain weight of the horses, based on a total
of 105 animals and reported in table2 , was 598.63 g (SEM
± 7.65), with a mean body weight of 514.12 kg (SEM ±
15.42). Values grouped for sex in the two age classes are
also reported in table2 . The mean weight of the brains
fixed in formalin (n = 26) was 660.4 g (SEM ± 13.46).
The absolute weight and relative percentages of the tel-
encephalon, diencephalon, mesencephalon, cerebellum,
pons, and medulla oblongata are summarized in table3 .
Age Sex n Body
weight, kg
SEM Brain
weight, g
SEM EQ
Total 105 514.12 15.42 598.63 7.65 0.78
F 64 546.96 22.03 608.76 9.51 0.76
M 41 445.43 16.42 574.02 15.04 0.82
<2 years 28 456.10 25.84 578.17 11.89 0.81
F 11 449.39 41.86 587.52 18.20 0.83
M 17 460.43 33.83 572.11 15.92 0.80
>2 years 77 535.22 18.32 606.07 9.39 0.77
F 53 567.21 24.37 613.17 10.82 0.74
M 24 464.58 16.60 590.39 18.30 0.82
Table 2. Brain and body weight (data
from fresh tissues)
Table 3. Absolute and relative weight (g) of brain divisions (data from formalin-fixed brains)
Brain
weight
(n = 26)
Telencephalon
(n = 26)
Diencephalon
(n = 11)
Cerebellum
(n = 26)
Brainstem
(n = 26)
Mesencephalon
(n = 11)
Pons
(n = 11)
Medulla
oblongata
(n = 26)
weight % weight % weight % weight % weight % weight % w eight %
Mean 660.40 496.30 75.14 34.49 5.10 75.82 11.52 55.36 8.42 18.21 2.74 13.01 1.95 23.72 3.61
SEM 13.46 10.49 0.39 2.20 0.19 1.37 0.16 1.09 0.15 0.80 0.16 0.63 0.11 0.48 0.06
Cozzi /Povinelli /Ballarin /Granato
Brain Behav Evol 2014;83:9–16
DOI: 10.1159/000356527
12
Felis catus
Panthera leo
Lynx lynx
Panthera tigris
Canis I. familiaris (small)
Canis I. familiaris (large)
Vulpes vulpes
Equus caballus (own data)
Equus caballus (literature)
Bos taurus (own data)
Camelus bactrianus
Loxodonta africana
Ovis aries
Capra h. aegagrus
Sus scrofa
Homo sapiens
Hylobates sp.
Gorilla gorilla
Pan troglodytes
Macaca mulatta
Logarithm of body weight
1
1
Logarithm of brain weight
4 5 6 7
4
3
2
Fig. 2. Logarithmic plot of the brain and
body weight of several mammals. The solid
black line represents the expected values
for the equation E
e = E
i /0.12P 2/3 following
Jerison [1973]. The predictable brain
weight for a given body weight should ide-
ally fall along the line. Values above the line
represent a brain mass higher than expect-
ed for a given body weight. The sources of
the data are listed in table4.
a
b
c
d
Fig. 1. Macroscopic images of the fresh ( a ) and formalin-fixed ( b–
d ) brain of an adult horse after removal of the dura mater. a Dorsal
view.
b Lateral view. c Sagittal view. d Basal view. Numbers refer
to the main sulci: 1 = sulcus cruciatus (centralis); 2 = sulcus coro-
nalis; 3 = sulcus ansatus; 4 = sulcus obliqus; 5 = sulcus suprasylvius
caudalis; 6 = sulcus ectomarginalis (ectosagittalis); 7 = sulcus mar-
ginalis (sagittalis); 8 = sulcus endomarginalis (endosagittalis); 9 =
sulcus genualis; 10 = sulcus calcarinus; 11 = sulcus proreus; 12 =
fissura sylvia; 13 = sulcus rhinalis caudalis; 14 = sulcus obliquus.
Letters refer to the main external gyri: a = gyrus proreus; b = gyrus
precrociatus (precentralis); c = gyrus marginalis; d = gyrus ecto-
marginalis (ectosagittalis); e = gyrus occipitalis; f = gyrus sylvius
caudalis; g = gyrus sylvius rostralis; h = gyrus olfactivus lateralis;
i = gyrus cinguli; j = bulbus olfactorius; k = lobus piriformis; l =
hypophysis (glandula pituitaria), surrounded by persisting frag-
ments of the dura mater. All scale bars = 3 cm.
The Brain of the Horse: Weight and
Cephalization Quotients
Brain Behav Evol 2014;83:9–16
DOI: 10.1159/000356527
13
EQ and CQ
The EQ value for all 105 horses was 0.78. The EQ for
animals older than 2 years (n = 77) was 0.77. Subdivisions
based on sex are reported in the last column of table2 .
These data are summarized in table4 , together with data
for the other mammals used for comparisons. Figure 2
shows a logarithmic plot of brain and body weight for
several mammals. The distribution of individual EQ val-
ues in adult horses (n = 77) is reported in figure 3 a. The
EQ relative to each age in adult horses (n = 77) is report-
ed in figure 3 b, together with the mean body weight. The
CQ (n = 26) was 0.841.
The raw data on brain weight, body weight, and EQ
are available as supplementary material (online supple-
mentary table1; for all online suppl. material, see www.
karger.com/doi/10.1159/000356527).
Discussion
Our study reports information from a large number of
animals regarding the macroscopic anatomy of the brain
of the horse, its absolute and relative weight (including
details of the weight of its constituent parts), the value of
its EQ, and its position in relation to other mammals.
The domestic horse possesses a large and extremely
convoluted brain, with a very complicated pattern of sul-
ci and gyri. Consequently, immediate recognition of the
landmarks commonly used in primates is generally dif-
ficult. Although a discussion of nomenclature and iden-
tification of the gyri is beyond the scope of the present
study, we emphasize the complexity of the external brain
surface of this species and the general longitudinal ar-
rangement of the sulci and relative gyri. In our series of
brains, the cruciate sulcus ( S. centralis ) was thin, short in
the medial-lateral direction, and more rostral than in pri-
mates and carnivores, and its identification was challeng-
ing in the majority of animals. The endomarginal sulcus
was easier to identify.
The average (fresh) brain weight of an adult (age >2
years) middle-sized horse was slightly over 600 g (i.e.
606.07 g). As expected, younger animals had somewhat
lighter brains. In her seminal paper on the evolution of
the horse brain, Edinger [1948] noted that although pro-
genitors of the present-day Equidae were smaller, their
brain was proportionately larger. However, even if the
brain of Equus caballus is proportionately smaller than
that of its pre-Eocene ancestors, the present average value
of slightly over 600 g remains one of the largest in terres-
trial mammals.
Comparison of the equine EQ value with those report-
ed for other mammals places the horse in a position close
to other species of similar size ( fig.2 ; table4 ). Its location
thus shows that the brain of the horse is smaller than ex-
pected according to the equation of Jerison [1973]. How-
ever, based on data reported in the literature for similar
species, the brain of the horse has a weight similar to (or
slightly higher than) what has been described for other
0
EQ
a
5
10
15
20
25
Horses (n)
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
0
0.2
0.4
0.6
0.8
1.0
Age (years)
b
1.2
EQ
Body weight (kg)
100
–100
300
500
700
900
1,100
1,300
1,500
1234 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
n = 4
n = 8
n = 7
n = 9
n = 6
n = 13
n = 12
n = 8
n = 2
n = 2
n = 2
n = 1
n = 1
n = 1
n = 1
Fig. 3. a Histogram showing the frequency distribution of EQ of adult horses (n = 77). b Histogram showing the mean EQ (±SEM) of
adult horses (n = 77) subdivided by age (the digits within the bars represent the number of individuals); the solid line represents the
mean body weight relative to each age.
Color version available online
Cozzi /Povinelli /Ballarin /Granato
Brain Behav Evol 2014;83:9–16
DOI: 10.1159/000356527
14
hoofed animals of corresponding body weight. We are
aware that the position of the horse brain in the graph
may vary greatly according to the data considered for oth-
er species. Here, we report the result of over 100 individ-
ual measures, but in many references comparisons were
made using a single brain for relatively rare species; thus,
variations may be vast. Within our experimental cohort,
several adult horses showed an EQ that was much higher
than average, reaching values approximating what was
observed in the domestic cat, or even the elephant and
primates ( fig.3 a). Variations in EQ were minimal in the
different ages ( fig.3 b) and apparently reflect fluctuations
in body weight. In fact, a sharp increase in body weight in
animals aged 11 and 12 years (n = 2 + 2) resulted in a
lower EQ, as expected.
Analysis of the weight of each brain division showed
that the telencephalon occupies 75% of the whole enceph-
alon. The cerebellum is the second largest component,
and the CQ of 0.841 is in the range given for insectivores,
Megachiropterans, and primates [Maseko et al., 2012],
but it is well below those for cetaceans and both African
and Asian elephants. The absolute weight (and the rela-
tive percentage values) of the brain divisions that we re-
port for the horse are different from what has been re-
ported for the zebra E. burchelli [Reep et al., 2007]. Our
data suggest that the cerebellum is larger in the horse than
in the zebra. Minor differences with the zebra are evident
also in the other brain divisions.
If we accept that brain and body size grow allometri-
cally in vertebrates [Shultz and Dunbar, 2010], our data
Table 4. Brain weight (g), body weight (kg), and EQ in selected mammals
Species Brain weight Body weight Reference EQ
Carnivora – Felidae
Felis catus 37 5.5 Schultz and Dunbar [2010] 1.00
Lynx lynx 70 18 0.85
Panthera leo 224 161 0.63
Panthera tigris 279 163 0.78
Carnivora – Canidae
Canis lupus 68 135 7 59 Seiferle [1988] 1.55 0.74
Vulpes vulpes 43 8 Schultz and Dunbar [2010] 0.90
Arctiodactyla – Suidae
Sus scrofa 180 125 Schultz and Dunbar [2010] 0.60
Arctiodactyla – Bovidae
Bos taurus 445 550 Seiferle [1988] 0.55
Ovis aries 130 50 0.80
Capra hircus 95 37.5 Zimmerl [1909] 0.71
Arctiodactyla – Camelidae
Camelus bactrianus 518 594 Xie et al. [2011a, b] 0.61
Proboscidea – Elephantidae
Loxodonta africana 4,927 3,185 Shoshani et al. [2006] 1.67
Perissodactyla – Equidae
Equus caballus 588 399 Schultz and Dunbar [2010] 0.91
599 514 this study 0.78
Primates – Cercopithecidae
Macaca mulatta 88 7.8 Schultz and Dunbar [2010] 1.86
Primates – Hylobatidae
Hylobates sp. 97.5 5.7 Schultz and Dunbar [2010] 2.55
Primates – Hominidae
Pan troglodytes 382 46 Schultz and Dunbar [2010] 2.48
Gorilla gorilla 471 105 1.76
Homo sapiens 1,300 1,400 70 Miller and Cosellis [1977] 6.62
The Brain of the Horse: Weight and
Cephalization Quotients
Brain Behav Evol 2014;83:9–16
DOI: 10.1159/000356527
15
apparently endorse the evolutionary trend. Although we
cannot rule out that in some taxa changes in EQ are due
exclusively to changes in body size, the value of the EQ as
a general indicator still makes it a useful tool for interspe-
cies comparison [for a critical revision of the EQ, see the
original work of Jerison, 1973, and more recently Boddy
et al., 2012]. Some (but not all) mammalian taxa show
strong evidence for a specific macroevolutionary increase
in brain size, possibly explained by the social brain hy-
pothesis that couples changes in sociality and relative
brain size [Pérez-Barberia and Gordon, 2005; Shultz and
Dunbar, 2005; Dunbar and Shultz, 2007; for a critical re-
view, see Healy and Rowe, 2007]. Analysis of ‘individual’
carnivores (i.e. musteloids, bears, and small felidae),
whose life habits are quite solitary [Finarelli and Flynn,
2009], contradicts the social hypothesis of brain evolution.
Several questions regarding the cortical development
of the horse remain unexplained. Although a detailed dis-
cussion of the gyrification patterns and cortical thickness
is beyond the scope of this article, we noted intensive
(mostly longitudinal) folding of the horse cerebral cortex.
The preponderance of longitudinal gyri in horses was re-
ported by Todd [1982], while the intense gyrification of
Ungulates and its many possible causes were described by
Pillay and Manger [2007]. In Cetartiodactyla, increased
gyrification accompanies an increase in brain size [Zilles
et al., 2013]. However, in their review on brain size and
cognitive performance, Chittka and Niven [2009] sug-
gested that larger animals should be under ‘selective pres-
sure to minimize expensive neural tissue that isn’t need-
ed’. Analysis of gyrification patterns shows that the high-
ly gyrencephalic Ungulates have a thinner cortex and
possibly fewer afferents and efferents [Pillay and Manger,
2007], a fact confirmed by Zilles et al. [2013]. Explana-
tions of the apparent contradiction between higher fold-
ing (increased gyrification) and a thinner cortex include
easier mechanical ‘buckling’ of fibers [Pillay and Manger,
2007] and reduction in the size of the ‘gyral’ window
[Prothero and Sunsten, 1984]. Mammals with extensive
gyrification include, among others, cetaceans (especially
members of the Delphinidae family), whose degree of so-
cialization is hardly analogous to that of the horse. More-
over, the highly social bottlenose dolphin Tursiops trun-
catus possess a very convolute and relatively thin cortex
[Morgane and Jacobs, 1972].
The overall value of the EQ and cortex analyses as pre-
dictors of cognitive capacities is doubtful, at least in pri-
mates [Deaner et al., 2007], for which absolute brain size
is a better correlate factor. Furthermore, the relationship
between EQ and body mass is unfavorable for the largest
species within each group [Radinsky, 1978]. Our data
place E. caballus among the species with the highest val-
ues in the graph. We also emphasize the importance of
interspecies variations: a discrete number of horses have
an EQ in the range of that of the cat and the elephant and
close to the inferior limits of those of primates. Quantifi-
cation of the intense gyrification of the horse brain [Zilles
et al., 2013, based on data from Brodmann, 1913] suggests
a gyrification index of 1.99, among the highest in mam-
mals [Manger et al., 2012]. The gyrification index factor
(2.94) of another Perissodactyla, the wild zebra E. burchel-
li, is even higher [Pillay and Manger, 2007]. The presence
of evolutionary factors promoting selective development
of the telencephalon may also influence adaptive shaping
of the cortical columns. Based on the general concept that
the specific organization of the brain must (or should)
provide some improvement in function for the species,
the factors that promoted the evolution of the horse cor-
tex remain to be explained, considering differences with
the hoofed Cetartiodactyla in the organization of the cor-
tical columns. The complex control of gait in quadrupe-
dal locomotion may be a factor to contemplate, if we take
into account the relationship of brain evolution and ‘fit-
ness’, as described by Finlay et al. [2011].
Finally, we emphasize that some of the features dis-
played by the CNS of the horse deserve further attention.
The absolute weight (though not the position on the evo-
lutionary EQ scale), the gyrification pattern, and the lam-
inar organization of the cortex warrant additional studies.
Known dogmas in neuroscience, including current state-
ments on the uniqueness of human brain complexity, de-
serve continuous scrutiny [for a critical appraisal, see
Lent et al., 2012], and comparisons with large-brained
domestic species may contribute. The use of murine
models to investigate brain functions may be question-
able [Manger et al., 2008] and perhaps limitative in the
long run [Bolker, 2012]. The study of the brain of the
horse may contribute to expanding contemporary views
on the organization of large CNSs and suggest valid trans-
lational models that may contribute to reducing the num-
ber of laboratory animals in biomedical research. In fact,
horse brain tissue sampled at the slaughterhouse may be
used in culture, thus following the 3Rs principle [Russell
and Burch, 1959]. From an ethical point of view, we also
have a moral obligation to understand the complex brain
of one of the oldest domesticated species, and to answer
some of the question raised by the general public concern
regarding the welfare of farm animals and the need to
protect their psychic and physical integrity.
Cozzi /Povinelli /Ballarin /Granato
Brain Behav Evol 2014;83:9–16
DOI: 10.1159/000356527
16
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Copyright:S.KargerAG,Basel2014.ReproducedwiththepermissionofS.KargerAG,
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... In a last few years, our group analyzed the brains of large ungulates, including the horse (Cozzi et al. 2014a(Cozzi et al. , b, 2017b, the bovine (Peruffo and Cozzi 2014;Ballarin et al. 2016;Graïc et al. 2018;Corain et al. 2020); the pig (Minervini et al. 2016); the sheep (Peruffo et al. 2019) and the giraffe (Graïc et al. 2017). The cytoarchitectonics of these mammals indicates a substantial difference from primates and rodents, with a consistent reduction of L4 and different targets of thalamic afferent within the cortical column (for details and reference see Cozzi et al. 2017b;Peruffo et al. 2019). ...
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... With domestication dating back to ∼3.500 BC, the domestic horse (Equus caballus) has become a close companion to human beings through farm work, war, sports, and leisure. With its complex gyrified (Zilles et al., 2013;Cozzi et al., 2014) and voluminous brain, its distinct cognitive skills and predictive behavior in a controlled environment (Brubaker and Udell, 2016;Roberts et al., 2017), its accessibility for neurological examination and neurophysiological testing (Pickles, 2019;Rijckaert et al., 2019), its compliance to perform controlled exercise and its long lifespan, the horse has regained attention as a natural model for ethological, neuroanatomic, and neuroscientific studies (Cozzi et al., 2014;Roberts et al., 2017;Johnson et al., 2019). ...
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... As an interesting side finding, the relationship between body weight and brain weight was non-linear logarithmic relationship. Therefore, results support previous evaluations across species by confirming their hypothesis of brain-body weight interrelation for the first time within a cohort of equids (Jerison, 1973;Cozzi et al., 2014;Minervini et al., 2016). ...
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... This stratification reflects the process of differentiation and migration of the germinal neuroepithelium cells that will eventually form the cortical columns of the neocortex (Gilbert, 23 ), here organized in five layers as typical of large herbivores (Cozzi et al., 17 ; van Kann et al., 49 ). However, here we note that adult Cetartiodactyla and Perissodactyla, contrarily to rodents and primates, are generally considered to be endowed with a five-layered cortex, for further discussion see (Cozzi et al.,16 ,17 ). ...
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... Some studies have focused on the brain of domestic animals (or on its sub-structures: hypothalamus, substantia nigra, sculum, cerebellum…), including bovine [6,14], ovine [7,15,16], horse [9,17] and pigs [18][19][20][21]. These studies are mainly focused on the description of innervation systems, distribution of specific neurotransmitters [22,23] and functional connectivity [24]. ...
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... The body weight and the brain weight is almost 5 to 6 times more in elephant than the buffalo and so the value of EQ is more in elephant. Chen et al. (2009) in Bactrian camel, Cozzi et al. (2014) in horse, Ballarin et al. (2016) in domestic Bos taurus and Minervini et al. (2016) in industrial pig (Sus scrofa) studied the ratio between brain mass to body mass and reported their EQ as 1.3, 0.78, 0.56 and 0.60, respectively. However, Jerison (1973) reported that in this system, a mammal with a brain/body ratio with an EQ value equal to 1.0 is considered to have average EQ. ...
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The incidence of dystocia in goats has been reported about 7% (Abdul-Rahman et al., 2000) to 8.23 % (Mehta et al., 2002). Fetal causes of dystocia are more common than maternal causes in goat (Abdul-Rahman et al., 2000) and sheep (Taha et al., 2005). Among dropsical conditions, the hydrallantois is more common compared to hydramnios (Hafez, 1993) and is frequently reported in bovines (having twins), rarely in mares (Milton et al., 1989) and less frequently in small ruminants. Hydrallantois is excessive accumulation of fluid in the allantoic sac in uterus, usually occurs within 5 to 20 days in advance pregnancy. Goats suffering from hydrallantois are usually presented in their second stage of labor with a history of sudden enlargement of the abdomen after mid gestation (Purohit, 2006). The present report documents a case of hydrallantois associated with fetal anasarca and its successful management in a non-descript doe.
... The body weight and the brain weight is almost 5 to 6 times more in elephant than the buffalo and so the value of EQ is more in elephant. Chen et al. (2009) in Bactrian camel, Cozzi et al. (2014) in horse, Ballarin et al. (2016) in domestic Bos taurus and Minervini et al. (2016) in industrial pig (Sus scrofa) studied the ratio between brain mass to body mass and reported their EQ as 1.3, 0.78, 0.56 and 0.60, respectively. However, Jerison (1973) reported that in this system, a mammal with a brain/body ratio with an EQ value equal to 1.0 is considered to have average EQ. ...
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... The body weight and the brain weight is almost 5 to 6 times more in elephant than the buffalo and so the value of EQ is more in elephant. Chen et al. (2009) in Bactrian camel, Cozzi et al. (2014) in horse, Ballarin et al. (2016) in domestic Bos taurus and Minervini et al. (2016) in industrial pig (Sus scrofa) studied the ratio between brain mass to body mass and reported their EQ as 1.3, 0.78, 0.56 and 0.60, respectively. However, Jerison (1973) reported that in this system, a mammal with a brain/body ratio with an EQ value equal to 1.0 is considered to have average EQ. ...
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Chapter
Pure cerebellar diseases result in ataxia of the limbs, but unlike in humans, overt weakness is not evident, distinguishing cerebellar disease from other causes of spinal ataxia [wobblers], although the slow onset of movements can appear as weakness. Cerebellar ataxia is alterations in the force, speed and range of movement of body parts, notably the limbs and neck, that is seen as excessive [hypermetric], reduced [hypometric] or a combination [dysmetric] of altered joint movements. Other signs include ataxia of the neck with wide, swinging, head excursions, jerky head bobbing, an intention tremor of the head but not the body and limbs, and an abnormal menace response. Vestibular signs may accompany the cerebellar syndrome. Acquired, post‐natal cerebellar disease alone is not common, the more frequent syndromes being acquired in utero insults, and congenital (hypoplasia, atrophy) and delayed (abiotrophy) genetic disorders.
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Here we report the unusual presence of thalamic reticular neurons immunoreactive for tyrosine hydroxylase in equids. The diencephalons of one adult male of four equid species, domestic donkey (Equus africanus asinus), domestic horse (Equus caballus), Cape mountain zebra (Equus zebra zebra) and plains zebra (Equus quagga), were sectioned in a coronal plane with series of sections stained for Nissl substance, myelin, or immunostained for tyrosine hydroxylase, and the calcium-binding proteins parvalbumin, calbindin and calretinin. In all equid species studied the thalamic reticular nucleus was observed as a sheet of neurons surrounding the rostral, lateral and ventral portions of the nuclear mass of the dorsal thalamus. In addition, these thalamic reticular neurons were immunopositive for parvalbumin, but immunonegative for calbindin and calretinin. Moreover, the thalamic reticular neurons in the equids studied were also immunopositive for tyrosine hydroxylase. Throughout the grey matter of the dorsal thalamus a terminal network also immunoreactive for tyrosine hydroxylase was present. Thus, the equid thalamic reticular neurons appear to provide a direct and novel potentially catecholaminergic innervation of the thalamic relay neurons. This finding is discussed in relation to the function of the thalamic reticular nucleus and the possible effect of a potentially novel catecholaminergic pathway on the neural activity of the thalamocortical loop.
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The macroscopic anatomy of the cerebrum of the bactrian camel (Camelus bactrianus) is detailed herein for the first time. Many features vary significantly from most previously described ungulate species including its large size, sulci pattern of the cerebral hemispheres and well developed sular cortex. In this study, six brains of adult bactrian camels were examined and the mean values of the cerebrum length were about 11.23 cm; width 8.67 cm; height 5.77 cm; weight of the brain 518.3 g and the sulci of the cerebrum, in general, were similar to that of the Chinese Qin-tibetan yak and Chinese water buffalo. The bactrian camel had the large temporal lobes, the well-developed olfactory bulbs and the prominence of the sulci and gyri. The results are beneficial for further research on the comparative neuroanatomy and developmental neurology.
Article
Design-based stereological methods (the Cavalieri principle and vertical sectioning) for estimation of volumes and surface areas were applied to fixed Bactrian camel brains. Brains from 10 subjects were hemisected. Cerebrum hemispheres and cerebellum halves from both sides were sliced systematic randomly for Cavalieri estimates of volume and vertical sectioning estimates of cortical surface area. Weights and linear dimensions were also recorded. The differences between left - right hemispheres and cerebellum halves did not show statistical significance (P>0.05). For cerebrum (both hemispheres combined) the average volume was 446 cm3 and cortical surface area was 1264 cm2. One thirds of this surface was hidden within sulci. Cortical volume was 222 cm3 with arithmetic mean thickness of 2.3 mm. The cerebellum occupied 38cm3 with a cortical surface of 396cm 2 of which 70% was hidden in fissures.
Chapter
The lampreys represent the most primitive group of presently living vertebrates. They are water inhabitants with elongated, eel-like bodies which lack paired fins (Fig. 10.1). In contrast to amphioxus, the head of the lamprey bears a number of special sense organs (nose, eyes, ears). The information gathered by these organs is relayed over the cranial nerves to centres in the enlarged rostral part of the CNS. There is a single nasal orifice high on top of the head and slightly behind this opening; a patch of pigment-free skin marks the position of the well-developed third or pineal eye. The animals lack jaws, having instead a large disc-shaped sucking mouth with many horny teeth. Many, but not all adult lampreys are predacious. The predacious varieties attach themselves to fish using their sucking mouths; then they produce a wound by rasping movements of a tongue-like structure which bears numerous sharp denticles. Finally, the lamprey ingests the blood and tissue fragments of its prey.
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The available data on relative brain size in fossil carnivores and ungulates provide no evidence for relatively larger brains in carnivores than in ungulates. Relative brain size of archaic ungulates was similar to that of modern basal insectivores and lower than that of contemporary ancestors of modern ungulates. Later archaic carnivores had brains similar in relative size to those of contemporary modern carnivore ancestors. The wide range of EQs seen among modern carnivores and ungulates suggests that caution should be used in attributing significance to differences in mean EQs of small fossil faunal samples. Elucidation of the biological significance of differences in relative brain size remains an outstanding problem.
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We report morphological data on brains of four African, Loxodonta africana, and three Asian elephants, Elephas maximus, and compare findings to literature. Brains exhibit a gyral pattern more complex and with more numerous gyri than in primates, humans included, and in carnivores, but less complex than in cetaceans. Cerebral frontal, parietal, temporal, limbic, and insular lobes are well developed, whereas the occipital lobe is relatively small. The insula is not as opercularized as in man. The temporal lobe is disproportionately large and expands laterally. Humans and elephants have three parallel temporal gyri: superior, middle, and inferior. Hippocampal sizes in elephants and humans are comparable, but proportionally smaller in elephant. A possible carotid rete was observed at the base of the brain. Brain size appears to be related to body size, ecology, sociality, and longevity. Elephant adult brain averages 4783g, the largest among living and extinct terrestrial mammals; elephant neonate brain averages 50% of its adult brain weight (25% in humans). Cerebellar weight averages 18.6% of brain (1.8 times larger than in humans). During evolution, encephalization quotient has increased by 10-fold (0.2 for extinct Moeritherium, ∼2.0 for extant elephants). We present 20 figures of the elephant brain, 16 of which contain new material. Similarities between human and elephant brains could be due to convergent evolution; both display mosaic characters and are highly derived mammals. Humans and elephants use and make tools and show a range of complex learning skills and behaviors. In elephants, the large amount of cerebral cortex, especially in the temporal lobe, and the well-developed olfactory system, structures associated with complex learning and behavioral functions in humans, may provide the substrate for such complex skills and behavior.
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Cortical folding is a hallmark of many, but not all, mammalian brains. The degree of folding increases with brain size across mammals, but at different scales between orders and families. In this review we summarize recent studies that have shed light on cortical folding and discuss new models that arise from these data. Genetic analyses argue for an independent development of brain volume and gyrification, but more recent data on the cellular development of the cortex and its connectivity highlight the role of these processes in cortical folding (grey matter hypothesis). This, and the widely discussed tension hypothesis, further tested by analyzing the mechanical properties of maturing nerve fibers, synapses, and dendrites, can provide the basis for a future integrative view on cortical folding.