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Proc. R. Soc. B
doi:10.1098/rspb.2012.1212
Published online
High morphological variation of vestibular
system accompanies slow and infrequent
locomotion in three-toed sloths
Guillaume Billet1, *, Lionel Hautier2, Robert J. Asher2,
Cathrin Schwarz1,3, Nick Crumpton2, Thomas Martin1
and Irina Ruf1
1
Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Rheinische Friedrich-Wilhelms-Universität
Bonn, Nussallee 8, 53115 Bonn, Germany
2
Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK
3
Department of Palaeontology, Geozentrum, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria
The semicircular canals (SCs), part of the vestibular apparatus of the inner ear, are directly involved in the
detection of angular motion of the head for maintaining balance, and exhibit adaptive patterns for locomotor
behaviour. Consequently, they are generally believed to show low levels of intraspecific morphological
variation, but few studies have investigated this assumption. On the basis of high-resolution computed tomography, we present here, to our knowledge, the first comprehensive study of the pattern of variation of the
inner ear with a focus on Xenarthra. Our study demonstrates that extant three-toed sloths show a high level
of morphological variation of the bony labyrinth of the inner ear. Especially, the variation in shape, relative
size and angles of their SCs greatly differ from those of other, faster-moving taxa within Xenarthra and
Placentalia in general. The unique pattern of variation in three-toed sloths suggests that a release of selection
and/or constraints on their organ of balance is associated with the observed wide range of phenotypes. This
release is coincident with their slow and infrequent locomotion and may be related, among other possible
factors, to a reduced functional demand for a precise sensitivity to movement.
Keywords: inner ear; shape; variability; constraint; Xenarthra; semicircular canals
1. INTRODUCTION
The semicircular canal (SC) system constitutes a major part
of the organ of balance in the inner ear of vertebrates and is
primarily specialized for sensitivity to rotational acceleration/deceleration of the head [1–3]. Accordingly, its bony
morphology presents diverse morphological patterns correlated to the modes of locomotion and agility [1,2,4–6].
A common assumption is that, notably owing to adaptive
constraints, the morphology of the mammalian inner ear,
especially the SC, shows low levels of intraspecific variation
[7–11]. However, possible fluctuations of these levels of
intraspecific variation have never been widely investigated
or compared with locomotor behaviours.
Extant sloths in the genera Bradypus (three-toed) and
Choloepus (two-toed) offer an opportunity to address this
question. They are unique among extant placentals in exhibiting an activity pattern with very infrequent and slow
motion. Their daily trips rarely exceed 100 m [12], and
average speed does not exceed 0.5–0.6 km h – 1 [13–15].
A few studies have previously investigated a possible
relationship between these species’ vestibular system morphology and their slow behaviour. As long ago as 1907,
Gray [16] noted an irregular shape and small size of the
SC in one specimen attributed to Bradypus tridactylus and
hypothesized that this ‘may be in some way related to the
sloth’s clumsy and slow movements’. More recent studies
have focused on the size of the SC in sloths [5,17], but
their previously reported irregular shape has not been
closely investigated to date.
Our study explores SC morphology in sloths and asks
whether their irregular shape is part of a larger pattern of
variability. Indeed, Darwin ([18], p. 455) hypothesized
that ‘an organ, when rendered useless, may well be variable,
for its variations cannot be checked by natural selection’.
Links between reduced function, relaxed natural selection
and greater variation has been acknowledged by more
recent studies [19] and demonstrated to characterize certain vestigial traits of insects [20]. More distantly related
but still relevant examples of reduced selection and
increased variation include certain birds and fishes, which
show greater sperm morphological variation coincident
with a release of sexual selection [21,22]. In addition,
reduced competition in Garter snakes may have increased
variation heritability of colour patterns [23]. Accordingly,
one would expect to find among sloths a substantial
degree of morphological variation in their SC, because
significant travelling distances, speed and agility are not
part of their locomotor repertoire. In this context, sloths
constitute an ideal case to address Darwin’s hypothesis.
Using a large dataset of inner ear bony labyrinths, we
compare the pattern of variation exhibited by the SC in
sloths (especially three-toed sloths) to that of faster-moving
species in order to test the possibility that lethargy in sloths
* Author for correspondence: ([email protected]).
Electronic supplementary material is available at http://dx.doi.org/
10.1098/rspb.2012.1212 or via http://rspb.royalsocietypublishing.org.
Received 25 May 2012
Accepted 6 July 2012
1
This journal is q 2012 The Royal Society
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2 G. Billet et al.
Vestibular variation in sloths
(c)
PSC
ASC
(a)
Bradypus
variegatus
(d)
(e)
(i)
( j)
(f)
(g)
(h)
LSC
Tamandua
tetradactyla
(l)
(k)
(m)
cochlea
(n)
Dasypus
novemcinctus
(o)
(p)
(q)
(r)
(b)
5 mm
Sciurus
vulgaris
(s)
(t)
(u)
(v)
(w)
Figure 1. Bony labyrinth morphology and variation pattern in selected xenarthrans and Sciurus. (a) Ventral view (anterior to the
top) of the skull of Bradypus variegatus ZFMK 60157 (see the electronic supplementary material). (b) Ventral view (anterior to
the top) of virtually reconstructed transparent basicranium and bony labyrinths of same specimen. (c) Ventrolateral view
(anterior to the right) of isolated left bony labyrinth of same specimen. (d)–(w) Views of bony labyrinths showing profile
of anterior and lateral SCs and illustrating greater SC variation in B. variegatus (d)–(h), than in Tamandua tetradactyla
(i)–(m), Dasypus novemcinctus (n)–(r) and Sciurus vulgaris (s) – (w). In D. novemcintus, (n)–(p) correspond to subspecies mexicanus and (q) and (r) to subspecies novemcintus; in S. vulgaris, (s) corresponds to subspecies infuscatus, (t) and (u) to fuscoater
and (v) and (w) to vulgaris (see the electronic supplementary material). (d)–(w) not to scale. ASC, anterior semicircular canal;
PSC, posterior semicircular canal; LSC, lateral semicircular canal.
corresponds to their patterns of SC variation. Our results
demonstrate that three-toed sloths depart from the low
levels of morphological variation of the SC system observed
in other xenarthrans and placentals. Echoing Darwin’s
hypothesis, we discuss the probability of a relaxed selective
pressure on the balance organ as a partial cause explaining
the persistence of these varying SC morphologies, and tentatively relate it to three-toed sloths’ slow behaviour. This
hypothesis, amenable to further testing, suggests that even
highly conservative organs such as the inner ear may exhibit
substantial morphological variation when functional
demand is decreased.
2. MATERIAL AND METHODS
We investigated the shape of the bony labyrinth of the inner ear
by high-resolution computed tomography in a total of 65 adult
specimens: 33 extant xenarthran skulls representing nine
genera and 14 species, and 32 other skulls for two wellsampled species outside Xenarthra (the red squirrel Sciurus
vulgaris and European mole Talpa europaea) for outgroup comparison (see some selected specimens in figure 1; and see the
electronic supplementary material). Species and subspecies
identification is based on collection data, geographical origin
and cranial anatomy [24–26]. The bony labyrinths were
imaged using high-resolution computed microtomography
(mCT). Three-dimensional reconstruction and visualization
of the bony labyrinth were performed using stacks of digital
mCT images with AVIZO v. 6.1.1 software (Visualization
Sciences Group). We then assessed the morphological variation from values and ratios of linear and angular
measurements taken directly from virtual endocasts of bony
Proc. R. Soc. B
labyrinths, which reflect the geometrical attributes of the
membranous semicircular ducts [2,27].
Measurements were taken on the SC as well as on other
parts of the bony labyrinth, such as the cochlea, in order to
look for distinctive variation levels between different inner
ear subsets and especially to address the assumption that
high variation would be more concentrated on the SC in
sloths. The following linear and angular measurements
were taken with the software POLYWORKS v. 10.0.5 (InnovMetric Inc.) on different parts of bony labyrinth endocasts
(see the electronic supplementary material, figure S1):
—
—
—
—
—
—
—
—
—
—
—
—
height (Coc H) of cochlea in profile,
width (Coc W) of cochlea in profile,
number of cochlear canal turns (TC),
length of cochlea (Coc L),
cube root of entire inner ear volume (R3Vol),
mean thickness of SC ducts (SC D),
length and width diameters of SC arc (ASC L, ASC W,
PSC L, PSC W, LSC L, LSC W),
SC radii (ASC R, PSC R, LSC R; R ¼ half the average of
the arc length and width diameters),
inner perimeter of area enclosed by SC (ASC P, PSC P,
LSC P),
vestibule width (VW) ¼ maximal distance in dorsal view
between anterior and posterior ampullae,
vestibule height (VH) ¼ shortest distance from the line
joining anterior and posterior ampullae (VW) to the
posteroventralmost point of the fenestra vestibuli, and
angles ASC/PSC, ASC/LSC, PSC/LSC, cochlea/LSC;
with ASC, PSC and LSC being the respective
abbreviations for anterior, posterior and lateral SCs.
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Vestibular variation in sloths
G. Billet et al.
3
standardized average absolute deviation
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0
*
R1A
R1B R1C
R2A
R2B
R2C
semicircular canals
R3
characters
*
R4
R5
R6
R7
other inner ear parts
Figure 2. Amount of intraspecific morphological variation in the bony labyrinth of the inner ear. Bar graph (red, Bradypus variegatus; green, Tamandua tetradactyla; purple, Dasypus novemcinctus; yellow, Sciurus vulgaris; blue, Talpa europaea) shows the
values of the standardized average absolute deviation for each measured linear character (ratios R1 –R7; see §2) in each species.
Asterisks (*) denote the absence of data for Tal. europaea (see §2).
Several ratios were calculated from these measurements
in order to standardize the data and compare proportions
of different inner ear parts (see the electronic supplementary
material, dataset S1). Ratios are:
— R1. ASC L/ASC W (R1a), PSC L/ PSC W (R1b), LSC
L/LSC W (R1c);
— R2. ASC P/ PSC P (R2a), ASC P/LSC P (R2b), PSC P/
LSC P (R2c);
— R3. SC D / mean SC R;
— R4. Coc H/Coc W;
— R5. Coc L/R3Vol; and
— R6. VW/VH.
Also TC noted as R7 in figures and datasets (see details of
earlier-mentioned abbreviations).
Principal component analysis (PCA) and the calculation of
variation indices allowed us to visualize and quantify the
amount of variability in our sample. PCA (see the electronic
supplementary material, dataset S2) was performed in PAST
v. 2.06 [28] on a set of 54 specimens (excluding Tal. europaea)
and 10 variables: angle ASC/PSC, angle ASC/LSC, angle
PSC/LSC, R1a, R1b, R1c, R2a, R2b, R2c and R3 (normalized
by using correlation matrix).
Standardized average absolute deviation (SAAD) and
standardized ranges of variation (SRV) were calculated
from the ratios (figure 2 and the electronic supplementary
material, dataset S1 and figure S2). The SAAD measures
the average absolute deviation from the mean exhibited in a
sample for a considered variable, and it is here divided
through the mean to make it comparable across species
with different values. The SRV is the range of variation
divided by the mean. For the angles, the average absolute deviation and range of variation have been calculated (see the
electronic supplementary material, figure S2). For all these indices, high values indicate a high range or degree of variation in
the respective measurement. The significance of the differences
of variation levels between best-sampled species was assessed
with Levene’s tests [29] using PAST v. 2.06 (see the electronic
supplementary material, dataset S2).
Proc. R. Soc. B
Some measurements could not be performed in Tal.
europaea concerning the angle made between the ASC and
other SCs, owing to the overly high planar deviation of the
former, and concerning R3 and R5, which were not possible
to estimate confidently owing to insufficient precision on the
thickness of structures on the reconstructed endocasts due to
low scan resolution.
3. RESULTS
Data from our best-sampled species indicate that threetoed sloths (Bradypus variegatus) present a significantly
higher level of variation than red squirrels (S. vulgaris)
and European moles (Tal. europaea) for almost all measured
bony labyrinth characters (Levene’s test, p , 0.05). Threetoed sloths also exhibit significantly higher variation than
nine-banded armadillos (Dasypus novemcinctus) for eight
out of 10 characters of the SC but not for the cochlea
(Levene’s test, p , 0.05). Conversely, non-sloths do not
show significantly different levels of variation for most of
the SC and cochlear characters (Levene’s test, p . 0.05).
The calculated SAAD demonstrates that, in comparison
with the nine-banded armadillo, southern tamandua
(Tamandua tetradactyla), red squirrel and European mole,
three-toed sloths always exhibit the highest measured
intraspecific variation, i.e. highest SAAD values, for each
investigated character of the bony labyrinth (figure 2). For
the SC characters, the three-toed sloths show an amount
of variability always twice or more than that of most other
taxa. Other structures such as the vestibule (also related to
balance with the detection of linear acceleration/deceleration) and the cochlea (related to hearing) are also variable
in three-toed sloths. This variation mostly concerns the
vestibule and cochlear canal coiling shape (R6, R4 and
R7). By contrast, three-toed sloths show a less variable
scaled cochlear length (R5) with a range of variation
that is similar to that in red squirrels (see the electronic supplementary material, figure S2). More generally, three-toed
sloths exhibit less variation in their cochlear characters than
in their SC characters. That is, the mean SAAD of SC
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4 G. Billet et al.
Vestibular variation in sloths
3 Tol_m
Brad_v
2
Eup_s
Brad_v
Pri_m
Brad_v
Chl_t
Brad_v
Brad_v
Cho_d
–2
Cho_h
–1.5
Cho_h
Das_n 0.5
Cho_d –0.5
PC1 (41.6%)
1.5
Tam_t Mym_t
Brad_v
Brad_v
1 Das_h
Das_n
Cha_v
Das_n
Das_n
Brad_t Das_n
Das_n
Cho_h
Cyc_d
Tam_t Tam_t
Mym_t
Tam_t
–2
Tam_t
–3
PC2 (18.4%)
Figure 3. Shape differentiation of the bony labyrinth of xenarthrans on the first two axes (60% of the among group variance) of
the PCA performed on a set of 10 variables on the SCs and 54 specimens (axes 3 and 4 available in the electronic supplementary material, dataset S2). Data points are labelled as follows: squares, sloths; crosses, armadillos; triangles, anteaters; circles,
red squirrels. The area shaded in red delimits variation exhibited across individuals of B. variegatus (Brad_v); purple indicates
D. novemcinctus (Das_n); green indicates Tam. tetradactyla (Tam_t); and yellow indicates S. vulgaris. Dashed and solid lines
delineate xenarthran subordinal or familial groups (Folivora, Vermilingua and Dasypodidae) and the species S. vulgaris.
Other taxonomic abbreviations are B. tridactylus (Brad_t), Choloepus didactylus (Cho_d), C. hoffmani (Cho_h), Cyclopes didactylus
(Cyc_d), Myrmecophaga tridactyla (Mym_t), Dasypus hybridus (Das_h), Chaetophractus vellerosus (Cha_v), Euphractus sexcinctus
(Eup_s), Chlamyphorus truncatus (Chl_t), Priodontes maximus (Pri_m), Tolypeutes matacus (Tol_m).
characters is much higher than the mean SAAD of cochlear
characters (see the electronic supplementary material,
dataset S1). Such a difference in variation between the SC
and cochlea is not apparent for other taxa except for red
squirrels, which show, in contrast to Bradypus, very low
levels of variation for both regions. The analysis of the
SRV mostly parallels the SAAD results but also demonstrates that the less well sampled two-toed sloth Choloepus
hoffmani also presents substantial variation for some SC
characters. Although we had access to only three specimens
for this species, it nevertheless showed a large SRV of the
anterior and lateral SC shape, much higher than for nonsloth species (see the electronic supplementary material,
figure S2).
The PCA allowed us to examine the morphological
space defined by the overall SC morphological variation
and enabled us to include in our sample additional xenarthran taxa represented by fewer specimens. The first
principal component is positively correlated with angle
ASC/LSC, angle LSC/PSC, R1b, R1c and R2a; negatively
with the angle ASC/PSC, R1a, R2b, R2c and R3. The
second axis is positively correlated with angle ASC/PSC,
angle LSC/PSC, R1a, R1b, R1c and R3; negatively with
the angle ASC/LSC, R2a, R2b, R2c. MANOVAS indicate
a significant morphological differentiation between the
different suborders (Wilk’s l-test: value ¼ 0.003185, F ¼
24.57, p , 0.001). This analysis shows a large, random
and scattered distribution of the specimens of three-toed
Proc. R. Soc. B
sloths in the morphological space representing the SC
shape (figure 3). There is no clear cluster among the
seven specimens of B. variegatus (on PCA graphs with
PC1–PC2 and PC3–PC4; figure 3 and see the electronic
supplementary material) that would support recognition
of taxonomic subsets of this species in our sample. The
species B. variegatus occupies a morphological space of
the SC shape larger than that of any other species sampled,
and even larger than that of all specimens of armadillos or
anteaters combined.
Therefore, our results demonstrate that the variation of
the bony labyrinth in B. variegatus is very high and that it
is especially apparent in their SC. Most of this variation
concerns the overall, irregular shape of the SC, and the
relative size, thickness and angles delimiting, each SC
(figure 2). Such variation is clear from a visual inspection
of the endocasts (figures 1 and 4). Interestingly, the high
level of intraspecific variation among three-toed sloths
is not apparent intra-individually. That is, our data do
not indicate appreciable left-right asymmetry (see the
electronic supplementary material, figure S3).
4. DISCUSSION
(a) Inner ear variation pattern compared with
other placentals
Our analyses demonstrate that three-toed sloths
(B. variegatus) present a much higher level of intraspecific
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Vestibular variation in sloths
(a)
(b)
PSC P / LSC P
= 1.03
PSC P / LSC P
= 1.66
(d)
2 mm
43.14°
87.14°
ASC P / PSC P
= 0.82
5
(c)
ASC P / PSC P
= 1.21
(e)
95.86°
G. Billet et al.
ASC L/W = 0.93
ASC L/W = 1.53
( f)
127.79°
LSC L/W = 0.65
LSC L/W = 1.25
Figure 4. Bony labyrinth reconstructions showing the extreme cases in three-toed sloth B. variegatus of the (a), ratio of PSC P/
LSC P (R2c); (b), ratio of ASC P/PSC P (R2a); (c), ASC aspect (R1a); (d), angle ASC/LSC; (e), angle ASC/PSC; ( f ), LSC
aspect (R1c). Values in grey and black represent, respectively, the lower and higher extremes of the respective character. ASC,
anterior semicircular canal; PSC, posterior semicircular canal; LSC, lateral semicircular canal; P, inner perimeter; L, length
diameter; W, width diameter (see §2).
variation in the structure of their SC compared with
other xenarthrans (D. novemcinctus, Tam. tetradactyla) and
non-xenarthran placentals (S. vulgaris, Tal. europaea).
Additionally, data for other well-documented mammals
[8,27,30–33] support the fact that SC variation in threetoed sloths is unusually high. Neither large variation in SC
shape nor differences in the relative sizes of each SC, as
found in three-toed sloths (figures 1 and 4), have, to our
knowledge, previously been documented (although sample
sizes in previous studies rarely permit good characterization
of intraspecific diversity). Another striking example concerns
the 448 range of variation of the angle between the anterior
and lateral SCs observed in B. variegatus (figure 4d). Our
sample (n ¼ 7) spans a range of variation for this angle comparable to that observed among multiple genera from various
placental orders [7], and much higher than the variation
exhibited by D. novemcinctus, Tam. tetradactyla, S. vulgaris
(see the electronic supplementary material, figure S2; data
on this angle could not be collected for Talpa—see §2),
Homo sapiens, Monodelphis domestica, Mus musculus and
Cavia porcellus [27,31,34,35]. Hence, we regard high variation in the SC of three-toed sloths to be a remarkable
exception to the conservatism exhibited by other mammals.
The varying morphologies of other inner ear regions in
three-toed sloths might also depart from phenotype
observed in other mammals (see §3 and data on cochlear
coiling variation in [8]), but to a lesser extent than the SC,
as shown in our sample.
(b) Constraints and selection on semicircular canal
morphological variation
Considering the observed high morphological variation of
their inner ear, and most specifically of their SC, we
would expect three-toed sloths to exhibit a marked release
of the processes normally limiting vestibular variation.
Specifically, processes limiting the extent and directions
of morphological variation in living organisms can be of
several kinds [36 – 39]. Constraints that bias the production of variant phenotypes should be distinguished
Proc. R. Soc. B
from selection that limits the persistence of the produced
variants [39]. The pattern of selection of a morphological trait is theoretically influenced by its functional
architecture, i.e. the complex relationships uniting a morphological trait with its function(s) [40]. Morphological
variations in the SC system can dramatically change the
animal’s temporal response dynamic (i.e. the behaviour
of the cupulae with regard to time during a rotation)
and sensitivity to motion [1,2], and this is probably the
case in three-toed sloths. In particular, the high variability
of SC relative proportions in B. variegatus (figures 2
and 4) should lead to great differences in relative sensitivity of canals among specimens. Moreover, the high
variation in angles between the anatomical planes of the
SC (figure 4) suggests varying directions of maximal
sensitivity [2,41]. However, these morphological and
functional variants of the inner ear in B. variegatus do
not lead to apparent perception impairment nor are
they pruned by natural selection. The persistence of
such varying sensitivity patterns of the SC in three-toed
sloths thus suggests that their functional architecture
is not associated with a high level of selection on their
shape. On the other hand, absence of left-right asymmetry in our sample of sloths, as usual in other mammals
[7], indicates that intra-individual variation of the inner
ear is limited, possibly in order to avoid vertigo pathophysiology [27], or as a result of a constraint at the level of
individual development, which remains to be determined.
Therefore, we hypothesize that relaxed selective pressure
on the SC has permitted the persistence of the observed
variants in three-toed sloths. The level of genetic and/or
developmental constraints on the production of these
variants might also depart from that of other placentals.
(c) Phenotypic variation in sloths
The relative roles of selection and constraint in determining the degree of inner ear variation are not yet
established. Whether a release of these limiting processes
is restricted to certain organs such as the SC or applies to
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6 G. Billet et al.
Vestibular variation in sloths
most of the phenotype of three-toed sloths constitutes a
critical issue. Unfortunately, the overall genetic and phenotypic variability of sloths, especially of Bradypus, is not
well-known [24,42], but some data are available.
Concerning B. variegatus, the great outlier of our
study, Anderson & Handley [42], previously noted variability in skull shape as a function of geographical
provenance for this species. While authors usually point
out the overall high variability of this three-toed sloth
species, they also emphasize the lack of clearly differentiated forms, which has until now impeded the
recognition of different subspecies [42,43]. Similarly,
our results do not show clearly recognizable subsets in
our sample for this species. It is otherwise noteworthy
that the recognition of different subspecies, or even
species, in B. variegatus would not be a sufficient explanation for their high SC variation, because this variation
exceeds that observed in entire families of armadillos or
suborders of anteaters (encompassing six different
genera of Dasypodidae, and three of Vermilingua in our
sample; figure 3).
In addition, substantial variation in the axial skeleton of
extant sloths has also been demonstrated [44–46]. However, in our dataset, xenarthran taxa that do share a high
level of vertebral thoracolumbar variability with sloths
[44], such as D. novemcinctus, show a very low variation in
the configuration of their SC, suggesting no association
between high variation in these anatomical regions.
Interestingly, new data on the timing pattern of cranial
suture closure in Xenarthra indentify both extant sloths
genera as outliers, exhibiting very high variation [47].
This phenomenon might parallel that of the inner ear in
sloths and, together with previous evidence, suggests
that sloths could show high variation for many parts of
their phenotype. The nesting of sloths within Xenarthra
and Atlantogenata might already endow them with a
relaxed level of constraint in patterning several characters
as notably illustrated by the high variation of axial skeleton and dental eruption pattern reported in these
clades [44,48,49]. However, as mentioned earlier, sloths
(especially three-toed sloths) clearly constitute outliers
within this clade for showing high level of intraspecific
phenotypic variability of their SC.
(d) Sloth semicircular canal variation and slow
locomotion
We proposed earlier the hypothesis of a release of selective
pressure on the SC in three-toed sloths to explain at least
partially their morphological variation. The most plausible reason for such a released selective pressure on
their SC morphology lies in their reduced activity pattern.
Such high variation of the SC has not been detected in
faster-moving xenarthrans, nor has it been documented
in other fast-moving mammals. Furthermore, while slow
locomotion and suspensory behaviour are likely to have
evolved convergently in both extant genera of sloths
[50 –53], both display a rather highly variable configuration of the SC, even if this is much more distinctive in
Bradypus than in Choloepus in our sample (figure 3 and
the electronic supplementary material, figure S2). This
latter fact coincides with the observation that three-toed
sloths move less and more slowly than two-toed sloths
[12,15]. According to our interpretation, the infrequent
Proc. R. Soc. B
and slow motion of three-toed sloths could make the
demand for precise detection of head angular movements
relatively less important than in faster-moving animals. It
is also possible that the input of the motion signal is so
weak for them that a difference in the morphology of
the SC does not make any significant difference for the
output message sent to the brain.
Contrasting with these ideas however, Jones & Spells
[54] suggested that the sensitivity required to detect
slow head movements can be very demanding and
increased for this purpose. On the one hand, fairly thick
SC ducts, like those of most three-toed sloths, might correspond to some increase in sensitivity [54,55]. On the
other hand, the general small size of the SC (radius) relative to body mass in sloths [5] and the small area they
enclose do not apparently represent an increase in sensitivity [2,54,55]. Although these different possibilities are
not yet fully elucidated, tolerance of varying locomotor
sensitivity (e.g. directionality and relative sensitivity
between canals) in three-toed sloths can still be associated
with their peculiar behaviour. According to this hypothesis, slow and infrequent motion would render the wide
range of morphological variation of the SC architecture
more accessible to three-toed sloths. For two-toed
sloths, further investigation on their pattern of SC variation is required to determine whether the same
hypothesis could apply to them.
Numerous implications and prospects result from our
discovery. It highlights the need for further research on
the implications of SC variability on the functions of balance, which would help determine the effect of functional
architecture on selection patterns for the inner ear. It also
calls for further investigation detailing the pattern of variation of other anatomical structures in sloths in order to
circumscribe more precisely the roles of selection and
constraint. Moreover, this potential link between a
decreased functional importance of the organ of balance
and part of its phenotype is apparent at the species
level, but not at that of the individual. In other words,
the investigation of such variation requires quantification
of size and shape across individuals, as does recognition of
an apparent release of axial skeleton conservatism [44].
This is of primary importance as the investigation of
variation patterns is a promising but poorly explored
field in mammals.
Our study shows very high variation of a phenotype
previously believed to be highly conservative, and
suggests that this variation has been influenced by a
reduced level of selection on the sensory modalities of
balance and agility to which it is linked. This idea corroborates another of Darwin’s [18] predictions on the
evolution of organs with decreased functional demand.
The research was funded by the Alexander von Humboldt
Foundation (Germany; G.B.), Sidney Sussex College
(Cambridge, UK; L.H.); Leverhulme Trust (UK; R.A. and
L.H.). We thank R. Schellhorn, A. Schwermann (Universität
Bonn), E. Weber (Universität Tübingen), R. Hutterer
(Museum Koenig, Bonn), R. David (Muséum National
d’Histoire Naturelle, Paris), R. Lebrun, S. Jiquel
(Université de Montpellier), K. Lin, M. Lowe and A. Heaver
(University of Cambridge) for their generous help with
sampling and computed tomography scanning of specimens
and/or for helpful advice. Many thanks to two anonymous
reviewers whose comments helped us to substantially
improve the manuscript. G.B. and L.H. initiated the project;
Downloaded from http://rspb.royalsocietypublishing.org/ on June 17, 2017
Vestibular variation in sloths
G.B., L.H. and I.R. designed the research plan; L.H., R.J.A.
and N.C. collected CT data from Cambridge and
Montpellier; G.B., C.S. and I.R. collected CT data from
Bonn and Tübingen; I.R., C.S. and N.C. reconstructed the
3D data; G.B. made the measurements; G.B. and L.H.
analysed the data; G.B., L.H., I.R., R.J.A., T.M., C.S. and
N.C. discussed the results and wrote the manuscript.
REFERENCES
1 Sipla, J. & Spoor, F. 2008 The physics and physiology of
balance. In Sensory evolution on the threshold: adaptations in
secondarily aquatic vertebrates (eds J. G. M. Thewissen &
S. Nummela), pp 227 –232. Berkeley, CA: University
California Press.
2 David, R., Droulez, J., Allain, R., Berthoz, A., Janvier, P. &
Bennequin, D. 2010 Motion from the past. A new method
to infer vestibular capacities of extinct species. C. R. Palevol.
9, 397–410. (doi:10.1016/j.crpv.2010.07.012)
3 Jeffery, N. & Cox, P. G. 2010 Do agility and skull architecture influence the geometry of the mammalian
vestibulo-ocular reflex? J. Anat. 216, 496 –509. (doi:10.
1111/j.1469-7580.2010.01211.x)
4 Silcox, M. T., Bloch, J. I., Boyer, D. M., Godinot, M.,
Ryan, T. M., Spoor, F. & Walker, A. 2009 Semicircular
canal system in early primates. J. Hum. Evol. 56,
315 –327. (doi:10.1016/j.jhevol.2008.10.007)
5 Spoor, F., Garland Jr, T., Krovitz, G., Ryan, T. M., Silcox,
M. T. & Walker, A. 2007 The primate semicircular canal
system and locomotion. Proc. Natl Acad. Sci. USA 104,
10 808–10 812. (doi:10.1073/pnas.0704250104)
6 Ryan, T. M. et al. 2012 Evolution of locomotion in Anthropoidea: the semicircular canal evidence. Proc. R. Soc. B
279, 3467–3475. (doi:10.1098/rspb.2012.0939)
7 Cox, P. G. & Jeffery, N. 2008 Geometry of the semicircular canals and extraocular muscles in rodents,
lagomorphs, felids and modern humans. J. Anat. 213,
83–596. (doi:10.1111/j.1469-7580.2008.00983.x)
8 Ekdale, E. G. & Rowe, T. 2011 Morphology and
variation within the bony labyrinth of zhelestids (Mammalia, Eutheria) and other therian mammals. J. Vert.
Paleontol. 31, 658–675. (doi:10.1080/02724634.2011.
557284)
9 Lange, S., Stalleicken, J. & Burda, H. 2004 Functional
morphology of the ear in fossorial rodents, Microtus arvalis and Arvicola terrestris. J. Morphol. 262, 770 –779.
(doi:10.1002/jmor.10277)
10 Sánchez-Villagra, M. & Schmelzle, T. 2007 Anatomy
and development of the bony inner ear in the woolly
opossum, Caluromys philander (Didelphimorphia,
Marsupialia). Mastozoologı́a Neotrop. 14, 53–60.
11 Lindenlaub, T. & Burda, H. 1993 Morphometry of the
vestibular organ in neonate and adult African mole-rats
Cryptomys species. Anat. Embryol. 188, 159–162.
(doi:10.1007/BF00186249)
12 Sunquist, M. E. & Montgomery, G. G. 1973 Activity
patterns and rates of movement of two-toed and threetoed sloths (Choloepus hoffmanni and Bradypus infuscatus).
J. Mammal. 54, 946– 954. (doi:10.2307/1379088)
13 Adam, P. J. 1999 Choloepus didactylus. Mamm. Species
621, 1 –8. (doi:10.2307/3504332)
14 Brattstrom, B. H. 1966 Sloth behavior. J. Mammal. 47,
348. (doi:10.2307/1378148)
15 Britton, S. W. & Kline, R. F. 1939 Augmentation of
activity in the sloth by adrenal extract, emotion and
other conditions. Am. J. Physiol. 127, 127 –130.
16 Gray, A. A. 1907 The labyrinth of animals, vol 1. London,
UK: Churchill.
17 Walker, A., Ryan, T. M., Silcox, M. T., Symons, E. L. &
Spoor, F. 2008 The semicircular canal system and
Proc. R. Soc. B
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
G. Billet et al.
7
locomotion: the case of extinct lemuroids and lorisoids.
Evol. Anthropol. 17, 135 –145. (doi:10.1002/evan.20165)
Darwin, C. 1859 The origin of species by means of natural
selection or the preservation of favoured races in the struggle
for life. London, UK: John Murray.
Walker-Larsen, J. & Harder, L. J. 2001 Vestigial organs as
opportunities for functional innovation: the example of
the Penstemon staminode. Evolution 55, 477 –487.
(doi:10.1111/j.0014-3820.2001.tb00782.x)
Crespi, B. J. & Vanderkist, B. A. 1997 Fluctuating
asymmetry in vestigial and functional traits of a haplodiploid insect. Heredity 79, 624 –630. (doi:10.1038/hdy.
1997.208)
Birkhead, T. R., Pellat, E. J., Brekke, P., Yeates, R. &
Castillo-Juarez, H. 2005 Genetic effects on sperm
design in the zebra finch. Nature 434, 383 –387.
(doi:10.1038/nature03374)
Calhim, S., Immler, S. & Birkhead, T. R. 2007 Postcopulatory sexual selection is associated with reduced
variation in sperm morphology. PLoS ONE 2, e413.
(doi:10.1371/journal.pone.0000413)
Westphal, M. F., Massie, J. L., Bronkema, J. M., Smith,
B. E. & Morgan, T. J. 2011 Heritable variation in garter
snake color patterns in postglacial populations. PLoS
ONE 6, e24199. (doi:10.1371/journal.pone.0024199)
Moraes-Barros, N., de Silva, J. A. B. & Stenghel
Morgante, J. 2011 Morphology, molecular phylogeny,
and taxonomic inconsistencies in the study of Bradypus
sloths (Pilosa: Bradypodidae). J. Mammal. 92, 86–100.
(doi:10.1644/10-MAMM-A-086.1)
Wetzel, R. M. 1985 The Identification and distribution of
recent Xenarthra (¼Edentata). In The ecology and evolution of armadillos, sloths, and vermilinguas (ed. G. G.
Montgomery), pp 5–21. Washington, DC, USA:
Smithsonian Institution Press.
McBee, K. & Baker, R. J. 1982 Dasypus novemcinctus.
Mamm. Species 162, 1–9. (doi:10.2307/3503864)
Bradshaw, A. P., Curthoys, I. S., Todd, M. J.,
Magnussen, J. S., Taubman, D. S., Aw, S. T. & Halmagiy,
G. M. 2010 A mathematical model of human semicircular canal geometry: a new basis for interpreting vestibular
physiology. J. Assoc. Res. Otolaryngol. 11, 145 –159.
(doi:10.1007/s10162-009-0195-6)
Hammer, Ø., Harper, D. A. T. & Ryan, P. D. 2001 PAST:
paleontological statistics software package for education
and data analysis. Palaeontol. Electron. 4, 9.
Van Valen, L. 2005 The statistics of variation. In Variation: a central concept in biology (eds B. Hallgrı́msson &
B. K. Hall), pp 29–47. Boston, MA: Elsevier Academic.
Welker, K. L., Orkin, J. D. & Ryan, T. M. 2009 Analysis
of intraindividual and intraspecific variation in semicircular canal dimensions using high-resolution x-ray
computed tomography. J. Anat. 215, 444–451. (doi:10.
1111/j.1469-7580.2009.01124.x)
Ekdale, E. G. 2010 Ontogenetic variation in the bony
labyrinth of Monodelphis domestica (Mammalia: Marsupialia) following ossification of the inner ear cavities.
Anat. Rec. 293, 1896 –1912. (doi:10.1002/ar.21234)
Spoor, F. & Zonneveld, F. 1998 Comparative review of
the human bony labyrinth. Yearb. Phys. Anthropol. 41,
211 –251. (doi:10.1002/(SICI)1096-8644(1998)107:27)
Gunz, P., Ramsier, M., Kuhrig, M., Hublin, J.-J. & Spoor,
F. 2012 The mammalian bony labyrinth reconsidered,
introducing a comprehensive geometric morphometric
approach. J. Anat. 220, 529–543. (doi:10.1111/j.
1469-7580.2012.01493.x)
Calabrese, D. R. & Hullar, T. E. 2006 Planar relationships of the two semicircular canals in two strains of
mice. J. Assoc. Res. Otolaryngol. 7, 151–159. (doi:10.
1007/s10162-006-0031-1)
Downloaded from http://rspb.royalsocietypublishing.org/ on June 17, 2017
8 G. Billet et al.
Vestibular variation in sloths
35 Curthoys, I. S., Curthoys, E. J., Blanks, R. H. I. &
Markham, C. H. 1975 The orientation of the semicircular canals in the guinea pig. Acta Otolaryngol. 80,
197 –205. (doi:10.3109/00016487509121319)
36 Maynard Smith, J., Burian, R., Kauffman, S., Alberch,
P., Campbell, J., Goodwin, B., Lande, R., Raup, D. &
Wolpert, L. 1985 Developmental constraints and evolution. Q. Rev. Biol. 60, 265 –287. (doi:10.1086/414425)
37 Arnold, S. J. 1992 Constraints on phenotypic variation.
Am. Nat. 140, 85–107. (doi:10.1086/285398)
38 Klingenberg, C. P. 2011 Evolution and development of
shape: integrating quantitative approaches. Nat. Rev.
Genet. 11, 623 –635. (doi:10.1038/nrg2829)
39 Pearce, T. 2011 Evolution and constraints on variation:
variant specification and range of assessment. Philos.
Sci. 78, 739– 751. (doi:10.1086/664568)
40 Walker, J. A. 2007 A general model of functional
constraints on phenotypic evolution. Am. Nat. 170,
681 –689. (doi:10.1086/521957)
41 Hullar, T. E. & Williams, C. D. 2006 Geometry of the semicircular canals of the chinchilla (Chinchilla laniger). Hear.
Res. 213, 17–24. (doi:10.1016/j.heares.2005.11.009)
42 Anderson, R. P. & Handley Jr, C. O. 2001 A new species
of three-toed sloth (Mammalia: Xenarthra) from
Panama, with a review of the genus Bradypus. Proc.
Biol. Soc. Wash. 114, 1 –33.
43 Hayssen, V. 2010 Bradypus variegatus (Pilosa: Bradypodidae). Mamm. Species 850, 19–32. (doi:10.1644/850.1)
44 Asher, R. J., Lin, K. H., Kardjilov, N. & Hautier, L. J.
2011 Variability and constraint in the mammalian vertebral column. J. Evol. Biol. 24, 1080– 1090. (doi:10.
1111/j.1420-9101.2011.02240.x)
45 Hautier, L. J., Weisbecker, V., Sánchez-Villagra, M. R.,
Goswami, A. & Asher, R. J. 2010 Skeletal development
in sloths and the evolution of mammalian vertebral patterning. Proc. Natl Acad. Sci. USA 107, 18 903 –18 908.
(doi:10.1073/pnas.1010335107)
46 Buchholtz, E. A. & Stepien, C. C. 2009 Anatomical
transformation in mammals: developmental origin of
Proc. R. Soc. B
47
48
49
50
51
52
53
54
55
aberrant cervical anatomy in tree sloths. Evol. Dev. 11,
69–79. (doi:10.1111/j.1525-142X.2008.00303.x)
Rager, L. & Sánchez-Villagra, M. R. 2011 Heterochrony in
cranial suture closure of recent and fossil Xenarthra. In
Program and Abstracts of the 9th Annual Meeting of
the European Association of Vertebrate Palaeontologists, 14–19
June, Heraklion, Crete, Greece (eds A. van der Geer &
A. Athanassiou), p. 49. See: http://www.eavp.org.
Asher, R. J., Bennett, N. & Lehmann, T. 2009 The new framework for understanding placental mammal evolution.
Bioessays 31, 853–864. (doi:10.1186/1741-7007-6-14)
Ciancio, M. R., Castro, M. C., Galliari, F. C., Carlini,
A. A. & Asher, R. J. 2012 Evolutionary implications of
dental eruption in Dasypus (Xenarthra). J. Mamm.
Evol. 19, 1 –8. (doi:10.1007/s10914-011-9177-7)
Delsuc, F. & Douzery, E. J. P. 2009 Armadillos, anteaters
and sloths (Xenarthra). In The timetree of life (eds S. B.
Hedges & S. Kumar), pp. 475– 478. Oxford, UK:
Oxford University Press.
Gaudin, T. J. 2004 Phylogenetic relationships among
sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence. Zool. J. Linn. Soc. 140, 255 –305.
(doi:10.1111/j.1096-3642.2003.00100.x)
Nyakatura, J. A. In press. The convergent evolution of
suspensory posture and locomotion in tree sloths.
J. Mamm. Evol. (doi:10.1007/s10914-011-9174-x)
Pujos, F., Gaudin, T. J., De Iuliis, G. & Castelle, C.
In press. Recent advances on variability, morphofunctional adaptations, dental terminology, and evolution
of sloths. J. Mamm. Evol. (doi:10.1007/s10914012-9189-y)
Jones, M. G. & Spells, K. E. 1963 A theoretical and comparative study of the functional dependence of the
semicircular canal upon its physical dimensions.
Proc. R. Soc. Lond. B 157, 403 –419. (doi:10.1098/rspb.
1963.0019)
Muller, M. 1999 Size limitations in semicircular duct systems. J. Theor. Biol. 198, 405–437. (doi.org/10.1006/jtbi.
1999.0922)