Downloaded from http://rspb.royalsocietypublishing.org/ on June 15, 2017
rspb.royalsocietypublishing.org
Experimental evidence for frictionenhancing integumentary modifications
of chameleons and associated functional
and evolutionary implications
Eraqi R. Khannoon1,2, Thomas Endlein2, Anthony P. Russell3
and Kellar Autumn4
Research
Cite this article: Khannoon ER, Endlein T,
Russell AP, Autumn K. 2014 Experimental
evidence for friction-enhancing integumentary
modifications of chameleons and associated
functional and evolutionary implications.
Proc. R. Soc. B 281: 20132334.
http://dx.doi.org/10.1098/rspb.2013.2334
Received: 9 September 2013
Accepted: 29 October 2013
Subject Areas:
biomechanics, behaviour, evolution
Keywords:
chameleon, setae, friction, biomechanics,
convergent evolution
Author for correspondence:
Eraqi R. Khannoon
e-mail: [email protected],
[email protected]
1
Faculty of Science, Department of Zoology, Fayoum University, Fayoum 63514, Egypt
Centre for Cell Engineering, University of Glasgow, Joseph Black Building, University Avenue,
Glasgow G12 8QQ, UK
3
Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta,
Canada T2N 1 N4
4
Department of Biology, Lewis and Clark College, 0615 Palatine Hill Road, Portland, OR 97219-7899, USA
2
The striking morphological convergence of hair-like integumentary derivatives of lizards and arthropods (spiders and insects) demonstrates the
importance of such features for enhancing purchase on the locomotor substrate. These pilose structures are responsible for the unique tractive abilities
of these groups of animals, enabling them to move with seeming ease on overhanging and inverted surfaces, and to traverse inclined smooth substrates.
Three groups of lizards are well known for bearing adhesion-promoting
setae on their digits: geckos, anoles and skinks. Similar features are also
found on the ventral subdigital and distal caudal skin of chameleons. These
have only recently been described in any detail, and structurally and functionally are much less well understood than are the setae of geckos and anoles. The
seta-like structures of chameleons are not branched (a characteristic of many
geckos), nor do they terminate in spatulate tips (which is characteristic of
geckos, anoles and skinks). They are densely packed and have attenuated
blunt, globose tips or broad, blade-like shafts that are flattened for much of
their length. Using a force transducer, we tested the hypothesis that these
structures enhance friction and demonstrate that the pilose skin has a greater
frictional coefficient than does the smooth skin of these animals. Our results
are consistent with friction being generated as a result of side contact of the
integumentary filaments. We discuss the evolutionary and functional implications of these seta-like structures in comparison with those typical of other
lizard groups and with the properties of seta-mimicking synthetic structures.
1. Introduction
The adhesive pads of gekkotan (Gekkota) and anoline (Dactyloidae) lizards have
been well studied [1–6]. The adhesive setae of gekkotans evolved convergently
multiple times [7,8], and those of gekkotans and anoles interact reversibly with
the locomotor substratum to promote attachment [7,9] under the control of specific
patterns of governance [5,10]. The squamate adhesive system, particularly that of
geckos, has attracted the attention of evolutionary and functional morphologists
[1,7,8,11–16], and those interested in biomimetics [9,17–22].
Studies of geckos are now beginning to reveal how the entire adhesive complex may have arisen [8]. Indications are that early phases in the elaboration
and modification of the subdigital integument may have been associated
with the enhancement of frictional interaction between the feet and the locomotor substratum [23]. Such modifications appear to occur in anatomically
localized regions at which interaction between the substratum and the integument is particularly intimate [23]. It has been postulated that the development
& 2013 The Author(s) Published by the Royal Society. All rights reserved.
Downloaded from http://rspb.royalsocietypublishing.org/ on June 15, 2017
body to both confirm earlier morphological findings [39–42]
and obtain force measurements of the interaction between the
integument and a test surface. We devised a means of testing
the interactions between these surfaces using a probe designed
to measure frictional interactions at a scale commensurate with
the dimensions of the integumentary outgrowths.
2. Material and methods
(a) Gross morphology and histology
(b) Scanning electron microscopy
Skin samples from spirit-preserved specimens of Ch. africanus
Chamaeleo calyptratus, Calumma parsonii, Furcifer verrucosus and
Furcifer oustaleti were examined. Samples were removed from
the following areas: dorsal and ventral skin of the manūs and
pedes in the area of the fused and free portions of the digits;
dorsal, lateral and ventral skin of the tail at the proximal and
distal ends of that appendage; dorsal and ventral regions of the
antebrachium and crus. Samples were placed in histological
tissue baskets and surface debris was removed by immersion
in distilled water in a sonicating bath. Samples were then cellophane-stripped using adhesive tape to remove remaining
surface debris and loose epidermis. They were then fully dehydrated by immersion (for 30 min each) in 85% ethanol, 100%
ethanol (twice), 25% hexamethyldisalizane (HMDS), 50%
HMDS, 75% HMDS and 100% HMDS (twice). They were then
placed on an absorbent surface (a Kimwipe) in a fume hood
for 10 min to allow the remaining HMDS to evaporate, left to
air dry overnight, and then affixed to aluminium scanning electron microscopy (SEM) stubs with carbon tape, and sputter
coated with gold alloy (20 nm thick gold/palladium film, 82%
Au/18% Pd) using a Polaron SC7640 Sputter Coater. Skin
samples were examined using a Zeiss EVO60 SEM. Images
were acquired at 2048 1536 pixel resolution and saved in
TIFF format. Additional samples of Ch. calyptratus were dehydrated by immersion in 100% ethanol (2 30 min), placed in
specimen capsules containing 100% ethanol and critical-point
dried, mounted on aluminium stubs using copper double-sided
tape and silver conductive paint, gold/palladium (20 nm thick
film) coated using a Polaron SC515 SEM coating system, and
viewed on a JEOL6400 SEM. Images were acquired using
Olympus Scandium Universal SEM Imaging software.
(c) Friction force measurements
We measured friction forces on the ventral and dorsal sides of the
hands and feet of two living specimens of Ch. calyptratus. To test
small areas of skin from the ventral and dorsal surface of the
hands and feet, we restrained the specimens as follows. We
lined an open box with bubble wrap and placed each animal
on the cushioning lying on its left side. Using broad (5 cm
Proc. R. Soc. B 281: 20132334
The gross morphology of the manus and pes of spirit-preserved
specimens of Chamaeleo africanus was examined using a Zeiss
stereo microscope (Zeiss Stemi DV4). Specimens of Ch. africanus
were captured and sacrificed for histological examination. Manūs
and pedes were fixed in 10% neutral buffered formalin or in
Bouin’s fluid [43]. 10 5 mm patches of skin were excised,
washed overnight in distilled water, dehydrated in an ethanol
series from 70 to 100%, cleared in xylene, and infiltrated with (two
or three changes) and embedded in, paraffin wax (melting point
56–588C). Seven to nine micrometre thick histological sections
were cut with a manual rotary microtome (Erma Type RA–303).
Haematoxylin and eosin staining was employed [43]. The sloughing
cycle of living individuals was recorded so that the shedding cycle of
the skin could be documented and related to the histological data.
2
rspb.royalsocietypublishing.org
of regionally and zonally specialized outer epidermal spinules
on the ventral surface of the feet and digits served as the precursors for the more highly elaborate setae, and that these may
have had functional relationships with the substrate that
were later exapted as adhesive modifications [13,23].
Because of the phenomenal adhesive capabilities that
geckos (and anoles) exhibit, attention has been almost exclusively devoted to understanding the form and function of
fully expressed, adhesion-inducing setae [1,15,24–28]. Van
der Waals forces and specific forms of shear-based frictional
interaction are responsible for setal adhesion [5,9,18,19,22,29].
The possible friction-enhancing role of the potential precursors,
however, has received only theoretical attention [23]. It is noteworthy, in this regard, that synthetic microfibrillar materials
have been developed that dramatically enhance friction,
permitting strong attachment under zero normal load [30,31].
Our search for a vehicle to assist in understanding the
functional transition from friction-enhancing epidermal outgrowths to adhesion-inducing setae led us to a group of
lizards that would evidently benefit from enhanced frictional
interaction with the locomotor substratum: chameleons. Independent acquisition of functional characteristics (analogues)
in distantly related lineages can be powerfully instructive in
the interpretation of functional transitions. Chameleons are
known for their prowess in negotiating narrow, and often
steeply inclined, perches [2,32,33] and for moving slowly
but assuredly on trees and bushes [33 –36]. To do this, they
employ uniquely configured zygodactylous feet [37] and a
prehensile tail [33,38]. Herrel et al. [33] examined grasping
in chameleons and found that arboreal species have a more
powerful grip than do terrestrial species, and have hands
and feet with relatively greater surface area than their terrestrial counterparts. They suggested that habitat use drove the
evolution of autopodial morphology through its effects on
performance. Enhanced gripping strength was attributed to
the mechanical configuration of the feet and tail [33], with
increased surface area promoting increased contact and
positively affecting frictional and adhesive forces.
Lange [39] reported the presence of ‘epithelial fibre hairs’
on the ventral digital surfaces of Chamaeleo vulgaris, and
these observations were subsequently verified [40–42], and
the form of the microornamentation on the digits and subcaudal scales described. Spinner et al. [42] noted that the form and
configuration of both the digits and the seta-like derivatives
[39–41] of chameleons are morphologically different from
those of gekkotans and anoles, and suggested that they may
function differently in locomotion. They proposed that the
structure of the feet and the tail, their mode of mechanical
interaction with branches (the typical support surfaces), and
the nature of the seta-like outgrowths pointed towards a friction-enhancing rather than an adhesion-inducing interaction.
The non-directional orientation of the seta-like structures of
chameleons was posited to be favourably disposed to the
enhancement of frictional interactions, with the perpendicular
shafts and flexible tips of the epidermal outgrowths probably
able to induce frictional interactions in all directions.
Based upon previous suggestions [39–42], we tested the
hypothesis that the integument of the ventral surfaces of
the manūs and pedes, and that of the distal subcaudal area,
of chameleons enhances frictional interactions with surfaces
they contact. To do so, we examined the ventral manual,
pedal and caudal integument of chameleons and compared
these to the features of the integument of other regions of the
Downloaded from http://rspb.royalsocietypublishing.org/ on June 15, 2017
3
D
M
P
proximal–distal
normal and lateral force
F
k
bac
two- dimensional
force
transducer
rubber bands
manual
xyz-translation
stage
eed
f
rce
t
fo
Figure 1. Force measurement set-up. The experimental animals were restrained using broad rubber bands, with one limb exposed to the spherical force transducer
probe. The force transducer was mounted onto a motorized stage controlled by a computer. The normal force was predefined using feedback loops, whereas the
friction force was measured as the probe was moved in a proximal – distal or lateral– medial direction (see arrows in inset). Three areas (P, proximal; M, middle and
D, distal) of the ventral and dorsal skin of the feet were probed. (Online version in colour.)
wide), stretchable rubber bands, we gently immobilized the torso,
limbs and tail, ensuring that blood flow was not compromised. The
box was mounted on a manual micromanipulator to enable
positioning of the selected skin area relative to the force measurement probe (figure 1). Each animal was subjected only briefly
(5–10 min) on any given day to force measurements, in order to
minimize stress. The animals remained very calm in most cases
and allowed measurements in the micro-Newton range to be accurately taken. On the few occasions when the animal moved and
affected the force recording, those data were discarded, and the
animal was released immediately and not tested again until the following day. We gathered only data about the frictional properties
of the skin from animals in the resting phase (stage one) of the
sloughing cycle, so as to minimize the effects of contamination
and wear. We used a smooth glass bead (diameter of 4 mm)
glued to a custom-built two-dimensional force transducer as a
force measurement probe. The transducer was sensitive in both
the normal and lateromedial directions and was moved by a
motorized two-dimensional translation stage (Physik Instruments,
Germany) controlled by a custom-written LABVIEW program
(National Instruments, USA), which allowed us to move the transducer with precision and to employ force-feedback loops to control
the normal load that the probe delivered to the skin. We used
normal loads of 1, 5, 10 and 20 mN while measuring the friction
forces of the probe against the skin of the animal. For each
normal load, the probe travelled a distance of 1 mm towards the
distal end of the hand/foot (see inset in figure 1). To test for any
directionality of interaction, we also probed the hands and feet
mediolaterally (see inset in figure 1). We conducted all observations at four different locations: the proximal, mid-point and
distal end of the ventral surface of the hand/foot (see inset in
figure 1) and the dorsum of the hand/foot. Each trial was repeated
three times and the results were averaged. For each trial, we
extracted the average friction force per average normal force,
resulting in four values for each trial (friction force at the four
normal loads); we then moved to an adjacent spot on the animal’s
skin and repeated the procedure. Adjacent areas of skin in the same
general vicinity were tested, each representing a separate trial.
For each bout of data acquisition, a localized area was tested to
minimize the amount of time that the animal was restrained.
Different areas were probed on different occasions to build up
the dataset for the areas being investigated.
To test whether the original surfaces of the hand influenced frictional interaction, we measured friction forces for certain spots
before and after altering the surface structure of individual scales
of prepared skin samples from the ventral areas of the hands
and feet. One individual of Ch. calyptratus was sacrificed by
intraperitoneal injection of sodium pentabarbitone and a small
piece of ventral manual skin was glued to a microscope slide and
positioned beneath the force transducer using a micromanipulator.
To enable the probing of single scales, we used a small glass bead
(diameter of 500 mm) and a probe travel distance of only 0.1 mm
side-to-side. We used constant normal loads of 1, 2 and 5 mN.
After obtaining measurements from this scale, we marked the
spot next to the scale with a marking pen and transferred the
sample to a stereomicroscope. After locating the probed scale, we
carefully scraped the scale surface with a scalpel blade to remove
the outer epidermal covering. We then placed the sample beneath
the probe again and repeated the force measurement. Each
measurement was repeated twice and these two values were averaged. In total, seven individual scales from the ventral skin of the
manus were tested in this way.
(d) Statistical analysis
We used customized routines and the statistical toolbox of
MATLAB, v. 7 (Mathworks, USA) to analyse and statistically compare results. We tested the data for normal distribution using the
Lilliefors test. When comparing two samples against each other,
we used either the Student’s t-test or the Mann – Whitney U-test
as parametric and non-parametric tests, respectively. Test results
are either stated in the text or are indicated in the figures.
3. Results
(a) Gross morphology and histology (figure 2a,b)
The digits of chameleons are opposable (figure 2a) and are
grouped into zygodactylous clusters, with I, II and III opposing
IV and V in the manus, and I and II opposing II, IV and V in the
pes. They are fused basally but separate distally and bear strong,
curved claws. The ventral and dorsal surfaces of the manus and
pes are clad in juxtaposing tubercular scales, separated by
narrow hinge regions. Histological images reveal that the ventral tubercles carry straight-shafted, seta-like structures on the
Oberhäutchen layer (figure 2b). The selected representative
stage, stage six of the sloughing cycle, shows the replacement
seta-like structures forming in the renewing layers of the epidermis. They follow the general pattern of integumentary renewal
through the sloughing cycle [44]; thus, the inner Oberhäutchen
layer carries the derivatives of the replacement generation.
Proc. R. Soc. B 281: 20132334
restrained animal
motorized xz-translation stage
rspb.royalsocietypublishing.org
lateral
Downloaded from http://rspb.royalsocietypublishing.org/ on June 15, 2017
(a)
I
II
4
(b)
cla
ns
Obi
bi
mi ai
IV
pai
(c)
(d)
200 mm
(e)
200 mm
( f)
50 mm
(g)
2 mm
5 mm
Figure 2. (a) Manus of Ch. africanus showing the zygodactylous configuration. (x6). (b) Histological section at stage six of the sloughing cycle of the manus showing
the new seta-like derivatives of the inner generation and their mould (arrows) in the clear layer of the outer epidermal generation. Layers of the inner generation: ns
(new setae); Obi (Oberhautchen); bi (b-layer); mi (mesos layer); ai (a-layer); pai ( presumptive a-layer cells); sg (stratum germanitivum cells) (x1250). Histological details for the pes are identical. (c,d) SEM images of the ventral and dorsal scales of the manus of Ch. calyptratus at the same magnification—the pilose
nature of the outer epidermal layer of the ventral scales (c) is clearly evident. (e) Carpet-like appearance of the filamentous epidermal derivatives covering the
manual scales of F. verrucosus. ( f ) Magnification of (e) showing the epidermal derivatives with their tapered distal ends. (g) Longer and denser epidermal derivatives of the ventral manual surface of Ch. africanus. (Online version in colour.)
(b) Scanning electron microscopy (figures 2c–g and 3)
We investigated the ventral areas of the manus, pes and tail and,
as controls, skin from the dorsum of these same areas. SEM imaging reveals that the ventral surface of the hands and feet
(figure 2c–g) carry short seta-like structures that resemble
setae found on subdigital scales of anoline lizards [2,23]. They
form a continuous pile on the ventral scales. Their shafts taper
gradually and the distal tips may be rounded and slightly
globular or more attenuated. They attain maximum lengths of
13–14 mm in Ch. Africanus, 8–9 mm in Ca. parsonii, 14–16 mm
in Ch. Calyptratus, 7.5–8.7 mm in F. verrucosus and 10–11 mm
in F. oustaleti. For species examined by us and Spinner et al.
[42], reported length dimensions of the longest epidermal outgrowths are very similar. The diameters of these seta-like
structures range from 1.3 to 1.8 mm at the base and 0.7 to
1 mm at the tip. They are densely packed, with approximately
800 000–900 000 filaments mm22. Such structures cover all
scales in the area encompassed by the fused digits and are
also present on the distal, short, free parts of the digits. On
the prehensile tail (figure 3a–d), the scales of the lateral and
ventral surfaces bear similar pilose surfaces, with outgrowths
of comparable length to those of the manus and pes of each particular species. Generally, on each tubercular scale, the seta-like
structures are the longest at the most elevated part of the dome
and gradually decrease in length and become more tapered distally as the hinge region is approached, ultimately grading into
spines and spinules on the lowest points of relief of these scales
(figure 3e). On the tail, the same pattern is evident on the ventral
and lateral scales of the distal region. An identical pattern of
distribution and form of these structures is evident in all
of the species of chameleon examined. Other regions of the
body exhibit scales that lack these elongated filamentous structures, and are essentially smooth or bear only minute spinules
(figures 2d and 3f ). Our observations thus indicate that the epidermal filaments are restricted in their distribution and occur
only on areas that routinely directly contact the locomotor substratum. These results accord with those presented by Müller &
Hildenhagen [41] and Spinner et al. [42].
Proc. R. Soc. B 281: 20132334
sg
V
rspb.royalsocietypublishing.org
III
Downloaded from http://rspb.royalsocietypublishing.org/ on June 15, 2017
(a)
5
(b)
(d)
5 mm
(e)
Proc. R. Soc. B 281: 20132334
(c)
2 mm
1 mm
( f)
2 mm
5 mm
Figure 3. (a,b) SEM images of the ventral distal caudal scales of Ch. africanus showing the densely packed seta-like derivatives with globose distal tips.
(c,d) Epidermal derivatives with tapered distal ends on the ventral distal caudal scales of Ca. parsonii. (e) Peripheral region of a ventral distal caudal scale
(right) and the adjacent hinge region (left) of Ch. calyptratus showing the gradation from seta-like derivatives to shorter spinules. ( f ) Ventral proximal
caudal scales of F. verrucosus showing the absence of filamentous derivatives and the presence of small spinules.
(c) Frictional properties of the skin
After confirming the presence of seta-like structures on the
ventral skin of the manus and pes and the distal portion of
the tail, we measured the frictional forces generated by
these structures on the ventral surface of the manus, and
the frictional forces generated by the epidermis on the
dorsal surface of the manus (figure 1). There were no significant differences between outcomes for the directions in
which we probed the same location ( proximal– distal
versus mediolateral directions: Student’s t-test: t ¼ 21.5,
d.f. ¼ 29, p ¼ 0.14), nor between the different locations that
we tested on the ventral manual scales ( proximal versus
medial, Student’s t-test: t ¼ 1.03, d.f. ¼ 8, p ¼ 0.66; proximal
versus distal: t ¼ 2.5, d.f. ¼ 9, p ¼ 0.06; medial versus distal:
t ¼ 0.9, d.f. ¼ 9, p ¼ 0.76). Thus, we pooled the data for the
ventral scales. Figure 4a shows the frictional forces exhibited by the ventral and dorsal surfaces (i.e. in the presence and
the absence of the hairs) under varying load. For each
load, the ventral surface showed higher frictional forces than
the dorsal one (for 1 mN, p ¼ 0.0127; 5 mN, p ¼ 0.0030;
10 mN, p ¼ 0.0024; 20 mN, p ¼ 0.0030: Mann–Whitney Utests), with a strong relationship of frictional force to the applied
load. However, as we found that the signal-to-noise ratio of our
force transducer did not allow for precise measurements under
the smallest load (1 mN), we excluded those measurements
rspb.royalsocietypublishing.org
10 mm
from the pooled data. The average frictional coefficient
(m ¼ Fx/Fz) for the pooled measurements of the ventral
manual scales was significantly greater than that for the
dorsal manual scales (m ¼ 0.21 + 0.06 and m ¼ 0.10 + 0.08,
respectively; independent t-test: t ¼ 3.52, d.f. ¼ 25, p ¼ 0.0016;
figure 4b), indicating enhanced frictional capacity associated
with the seta-like structures.
The ventral and dorsal manual and pedal skin bear scales
with a similar tubercular form, although the dorsal scales
lack the seta-like structures. To test whether the differences
in frictional capacity of the ventral and dorsal manual skin
relate to a potential difference in the relative softness of the
skin in these locations, and to test the hypothesis that frictional attributes of the ventral manual integument are
owing solely to the presence of the seta-like structures that
it bears, we tested the same sample of scales (from the ventral
side of the hand) before and after scraping away the outer
epidermal layer, thus removing the seta-like structures. The
frictional forces of the scales with intact outgrowths were
greater than those generated once they had been removed.
Whereas the highest normal load used (5 mN) yielded a significantly different ( paired t-test: t ¼ 23.5, d.f. ¼ 7, p ¼ 0.0096)
frictional coefficient (m ¼ Fx/Fz ¼ 0.22 + 0.022 before scraping
and 0.19 + 0.038 after scraping; figure 4c), the lower normal
loads (1 and 2 mN, respectively) showed only a trend towards
Downloaded from http://rspb.royalsocietypublishing.org/ on June 15, 2017
(a)
*
**
**
**
(b)
6
**
dorsal
ventral
7
rspb.royalsocietypublishing.org
0.40
8
0.35
5
Fx/Fz
Fx(mN)
6
4
0.30
0.25
3
2
1
0
0.15
1
5
10
20
ventral
dorsal
Fz (mN)
(c)
0.35
n.s.
n.s.
**
before scraping
0.30
Fx/Fz
after scraping
0.25
0.20
0.15
0.10
1 mN
2 mN
5 mN
Figure 4. Box and whisker plots displaying the results of friction force measurements. The median value is represented by a line, and the 25th and 75th percentiles
by the box. The plotted ‘whiskers’ extend to the adjacent values, which are the most extreme data values that are not outliers. Points are identified as outliers if
they are larger than q3 þ w (q3 2 q1) or smaller than q1 2 w (q3 2 q1), where w ¼ 1.5 and q1 and q3 are the 25th and 75th percentiles, respectively.
(a) Frictional forces (Fx) for the different normal loads (Fz) applied to the dorsal and ventral skin surfaces of the manus of Ch. calyptratus. For each load, the
friction forces for the ventral skin are significantly higher than those for the dorsal skin. *p , 0.05; **p , 0.01. (b) Frictional coefficients (Fx/Fz) of the ventral
and dorsal manual skin surface of Ch. calyptratus. (c) Frictional coefficients (Fx/Fz) obtained for different normal loads imposed upon individual scales before and
after scraping the outer epidermal layer away. For the 1 and 2 mN load, the scales with the intact surface show trends towards higher frictional coefficients; for the
5 mN load the difference was significant (n.s., not significant; **p , 0.01).
greater forces for the scales prior to scraping. It is likely that
the higher noise-to-signal ratio of our force measurements
for the lower normal forces was responsible for the absence
of significant differences at these levels.
4. Discussion
Our results clearly indicate that for all species of chameleon
examined there are differences in the expression of the morphology of the outer epidermal generation and that these
differences are regionally specific, a finding that accords with
those of previous investigators [40–42]. Elongated seta-like
structures characterize the scales of the plantar surfaces of the
hands and feet, and the ventral and ventrolateral surfaces
of the distal part of the tail. Other areas of the body lack
these outgrowths. The skin regions bearing these filamentous
derivatives exhibit enhanced frictional capacity.
(a) The benefit of a friction-enhancing integument
to chameleons
Chameleons typically (although not exclusively) occupy
arboreal habitats, and all of the species examined by us are
arboreal. Exploitation of relatively narrow branches using
erect limbs and a narrow trackway [2,32,36] is associated
with gross morphological adaptations that permit this pattern of locomotion [2,45]. Chameleons typically grasp the
narrow and inclined perches upon which they move with
zygodactylous digits [45] and the tightly coiled tail tip [38].
Grasping the surface in this way, and in this posture, may
persist for prolonged periods while stationary and also
occurs when climbing. A tight grasp of the perch [33] and
the deployment of strong claws assists this, but there is
likely to be slippage between the feet/tail and the perch
that could be mitigated by enhancing the frictional interaction
between the two [45].
Proc. R. Soc. B 281: 20132334
0.20
Downloaded from http://rspb.royalsocietypublishing.org/ on June 15, 2017
The filamentous structures of the pedal and caudal scales
of chameleons are slender and bear globular, blunted or
attenuated blade-like distal tips. They differ from the
adhesion-inducing setae of gekkotans, anolines and scincids
[1,27,28,46] in that no clear triangular spatulate tips are present.
The spatulate structures described by Spinner et al. [42] are
quite different in form from those of gekkotans and anoles
and actually represent flattened, blade-like shafts rather than
expanded tips borne on narrowed regions of much more slender shafts. Among chameleons, there may be a phylogenetic
distinction between genera that exhibit (Furcifer, Calumma)
and lack (Chamaeleo) these flattened, blade-like epidermal
outgrowths, although Trioceros is apparently polymorphic in
this regard [42]. A more complete phylogenetic survey is
required to explore this question further, but it is evident that
the flattened, blade-like outgrowths are not common to all
chameleons. The integumentary, seta-like outgrowths of chameleons are not branched, are not directionally arranged and
are not carried on distinctive subdigital pads that are divided
into scansors or lamellae.
Chameleon ‘hairs’ are borne on tubercular scales that invest
the entire ventral surface of the manus and pes (and the ventral
and ventrolateral surfaces of the distal part of the tail) [42].
As such, they appear similar to the fields of elongated, attenuated spinules on the basal regions of the digits and adjacent
plantar regions of the manus and pes of the Tokay gecko
([23], figs. 4 and 5). Such tubercular scales, segregated by
hinge regions [44], provide the skin with greater local flexibility
and do not exhibit any directionality in terms of the orientation
of the filamentous shafts.
Applying Peterson’s [2] and Peterson & Williams’s [47]
system of terminology for filamentous integumentary outgrowths of squamates, the outgrowths of chameleons are
most similar to prongs and seta/prong intermediates (those
with flattened, blade-like shafts). Müller & Hildenhagen
[41] employed this terminology in their description of
chameleon integumentary filaments. The flattened, bladelike structures reported for Furcifer, Trioceros and Calumma
by Spinner et al. [42] appear to be structures unique to chameleons, and do not fulfil the criteria for recognition as setae as
defined earlier [2,47] and as routinely applied to structures
encountered in gekkotans and anolines.
Phylogenetic relationships of the major taxa of lizards
[48 –50] suggest that the pilose epidermis of chameleons is
derived independently from that of gekkotans, anolines and
scincids, and thus represents the second expression of this general morphology within the Iguania. Similar integumentary
(c) Functional attributes of the seta-like fields
of chameleons
Our investigation of the surface interactive properties of the
seta-like structures of chameleons showed no evidence of
generation of adhesive forces and exhibited only increased
frictional interaction with the probe. The form of the filaments, with attenuated, blunt or globose distal tips that are
arranged in a gradational pattern (from peripheral to central)
on the tubercular scales, appears to represent a favourable
configuration for frictional enhancement, as suggested by
Spinner et al. [42]. This pattern of distribution on tubercular
scales is reminiscent of that on tubercular palmar scales
of Tokay geckos [23]. These structures lack directionality,
which is more expected of friction-enhancing capabilities
than adhesion-inducing ones.
Our measurements demonstrate that the seta-like structures of chameleons impart a significant increase in the
frictional coefficient of the integument. Such structures might
be expected to decrease the surface area of contact, consistent
with the lotus effect [57]. This raises an interesting question
relating to the mechanism that underlies friction enhancement.
Because we carried out our experiments using a smooth sphere
as a probe, we can discount interlocking via a ‘Velcro-like’
mechanism. Geckos adhere using branched, spatulate-tipped
setae that act as an array of angled beams [9,58], each ending
in tiny strips of adhesive tape [22]. We propose that the friction-enhancing chameleon filaments depict what might be
expected of a convergent transitional stage in the evolution of
gekkotan and anoline adhesive setae, although they are also
an adaptive solution in their own right. This concept is supported by the friction-enhancing gecko-like materials that
have been engineered [30,31], which are remarkably similar
in form to chameleon ‘setae’. The contact mechanics of these
engineered materials have been explored in detail, and it has
been found that friction enhancement on smooth surfaces is
owing to a load-dependent increase in the contact area of the
side of each fibre. Under low loads, the fibres are nearly vertical
in orientation, contact the substrate only with their tips and
contact across the substrate is incomplete because of variation
in the lengths of the fibres. As the normal and shear loads
increase, the fibres bend and contact the substrate along their
sides [59]. This contact region grows as load increases, creating
a ‘smart’ material response: friction increases as load increases.
The broad, blade-like ‘setae’ of some chameleons reported
by Spinner et al. [42] would probably further enhance frictional
interactions by way of their bending and contacting the
substratum along their enlarged, flattened faces. We measured
frictional forces only in Ch. calyprtatus, a species also examined
by Spinner et al. [42], who, like us, found it to lack the
broadened, blade-like outgrowths.
Such contact mechanics depend strongly on a match
between the external load and the stiffness of the fibre
array. A rough estimate of the force Fb required to change
7
Proc. R. Soc. B 281: 20132334
(b) Comparison with the setae of other lizards
derivatives have arisen at least three times in insects [51 –54]
and three times in spiders [55]. This commonality of form
points to an optimized configuration for the enhancement of
surface contact and, ultimately, attachment [56]. The setalike structures of chameleons seemingly represent the hypertrophy of the spinules [42] located in the hinge regions
between tubercles and on less extensively modified scales
(figure 3e,f ).
rspb.royalsocietypublishing.org
Our observations indicate that such an enhancement of
frictional interaction is brought about by regional integumentary modifications, whereby the outer epidermal generation
is produced into densely packed fields of elongated, filamentous seta-like structures. The presence of these derivatives
on the plantar surface of the hands and feet, and also on
the ventral and ventrolateral surfaces of the distal, prehensile
[38] part of the tail, strongly suggests that the role of these is
directly related to friction enhancement. These structures are
similar to the setae of gekkotan and anoline lizards, but
differences indicate that they are friction-enhancing rather
than adhesion-inducing. Thus, the predictions of Spinner
et al. [42] are consistent with our experimental observations.
Downloaded from http://rspb.royalsocietypublishing.org/ on June 15, 2017
the configuration of an elastic beam from an upright contact
to a side contact is given by the formula of Euler:
p2 EI
;
ðKLÞ2
5. Conclusion
Our observations and experimental results indicate that regionally restricted filamentous integumentary arrays of chameleons
are located in areas where friction enhancement would be
of value in countering slippage when negotiating narrow
and inclined perches. Furthermore, we demonstrate that the
filamentous arrays exhibit enhanced frictional properties
when compared with regions of the integument lacking these
arrays. Our calculations of the properties of these arrays, and
our discovery that they are non-directional in their responses,
are consistent with friction enhancement of grasping structures. These findings also indicate that the adhesive setae of
gekkotan and anoline lizards may have been evolutionarily
preceded by friction-enhancing filaments—such structures
are present in the non-adhesive areas of the feet of both of
these taxa. Friction-enhancing filaments, in such a scenario,
would have become reconfigured and associated with more
active control mechanisms as transition to adhesion-inducing
properties occurred.
The experiments carried out at Glasgow University were conducted
in accordance with UK Legislation on Animal Experimentation.
Acknowledgements. We thank Jon Barnes for valuable discussions on the
experiments, Diana Samuel for revising the manuscript, Jonathan
Puthoff for help with predictions for fibre collapse and Erica Lai,
Margaret Mullin and Jörg Hardege for assistance with scanning electron microscopy. The comments of three anonymous referees helped
us to improve the substance of this paper.
References
1.
2.
3.
4.
5.
6.
Ruibal R, Ernst V. 1965 The structure of the digital
setae of lizards. J. Morphol. 117, 271–294. (doi:10.
1002/jmor.1051170302)
Peterson JA. 1983 The evolution of the subdigital
pad in Anolis. I. Comparisons among the anoline
genera. In Advances in herpetology and
evolutionary biology, Essays in Honour of
Ernest E. Williams (eds AGJ Rhodin, K Miyata),
pp. 245 –283. Cambridge, MA: The Museum of
Comparative Zoology, Harvard University.
Maderson PFA. 1964 Keratinized epidermal
derivates as an aid to climbing in gekkonid lizards.
Nature 203, 780 –781. (doi:10.1038/203780a0)
Hiller U. 1968 Untersuchung zum Feinbau und zur
Funktion der Haftborsten von Reptilien.
Zoomorphology 62, 307–362.
Autumn K, Majidi C, Groff RE, Dittmore A, Fearing R.
2006 Effective elastic modulus of isolated gecko
setal arrays. J. Exp. Biol. 209, 2558– 2568. (doi:10.
1242/jeb.02469)
Zhao B, Pesika N, Rosenberg K, Tian Y, Zeng H,
McGuiggan P, Autumn K, Israelachvili J. 2008
Adhesion and friction force coupling of gecko
setal arrays: implications for structured adhesive
surfaces. Langmuir 24, 1517 –1524. (doi:10.1021/
la702126k)
7.
8.
9.
10.
11.
12.
13.
Russell AP. 2002 Integrative functional morphology
of the gekkotan adhesive system (Reptilia: Gekkota).
Integr. Comp. Biol. 42, 1154–1163. (doi:10.1093/
icb/42.6.1154)
Gamble T, Greenbaum E, Jackman TR, Russell AP,
Bauer AM. 2012 Repeated origin and loss of
adhesive toepads in geckos. PLoS ONE 7, e39429.
(doi:10.1371/journal.pone.0039429)
Autumn K, Liang YA, Hsleh ST, Zesch W, Chan WP,
Kenny TW, Fearing R, Full RJ. 2000 Adhesive force
of a single gecko foot-hair. Nature 405, 681– 685.
(doi:10.1038/35015073)
Russell AP, Higham TE. 2009 A new angle on
clinging in geckos: incline, not substrate, triggers
the deployment of the adhesive system.
Proc. R. Soc. B 276, 3705– 3709. (doi:10.1098/
rspb.2009.0946)
Bauer AM. 1998 Morphology of the adhesive tail tips of
carphodactyline geckos (Reptilia: Diplodactylidae).
J. Morphol. 235, 41–58. (doi:10.1002/(SICI)10974687(199801)235:1,41::AID-JMOR4.3.0.CO;2-R)
Dellit WF. 1934 Zur Anatomie und Physiologie der
Geckozehe. Jena. Z. Naturwiss 68, 613 –656.
Maderson PFA. 1970 Lizard hands and lizard glands:
models for evolutionary study. Forma et Functio 3,
179 –204.
14. Mahendra BC. 1941 Contributions to the bionomics,
anatomy, reproduction and development of the
Indian house gecko Hemidactylus flaviviridis Rüppel.
Part II: the problem of locomotion. Proc. Indian.
Acad. Sci. B 13, 288 –306. (doi:10.1007/
BF03048487)
15. Russell AP. 1975 A contribution to the functional
analysis of the foot of the Tokay, Gekko gecko
(Reptilia: Gekkonidae). J. Zool. Lond. 176, 437–476.
(doi:10.1111/j.1469-7998.1975.tb03215.x)
16. Russell AP. 1976 Some comments concerning
interrelationships amongst gekkonine geckos. In
Morphology and biology of reptiles (eds
Ad’A Bellairs, CB Cox), pp. 437 –476. London, UK:
Academic Press.
17. Arzt E, Gorb S, Spolenak R. 2003 From micro to
nano contacts in biological attachment devices.
Proc. Natl Acad. Sci. USA 100, 10 603–10 606.
(doi:10.1073/pnas.1534701100)
18. Autumn K, Peattie AM. 2002 Mechanisms of
adhesion in geckos. Integr. Comp. Biol. 42,
1018– 1090. (doi:10.1093/icb/42.6.1081)
19. Autumn K et al. 2002 Evidence for Van der
Waals adhesion in gecko setae. Proc. Natl Acad.
Sci. USA 99, 12 252–12 256. (doi:10.1073/pnas.
192252799)
Proc. R. Soc. B 281: 20132334
where E is the elastic modulus of the material, I is the second
moment of area, L is the length of the beam and K is a factor
related to the constraints on the beam. For a fibre with uniform
circular cross section or radius r, I ¼ pr 4/4. For a fibre with one
end fixed and the other end free to move laterally, K 2.0, assuming a b-keratin modulus of 2.0 GPa [5,29,60,61]. Further studies
will be needed to determine the precise values in chameleons.
Taking r ¼ 1.0 mm and L ¼ 15 mm, we find Fb ¼ 17 mN.
For the taxa experimentally examined by us, there are
approximately 370 000 fibres mm22 on the gripping surface of
the palm. A grip pressure of 6.37 N mm22 would be required
to collapse the array fully and engage the fibres in a maximally
friction-enhancing configuration. In this configuration, the
fibres are flattened against the underlying surface, and the contact area increases relative to that of both an uncollapsed
configuration and, presumably, that of a stiff, inflexible scale.
Macroscopic friction, as described by Amonton’s Laws, is
typically taken to be unchanged by the apparent size of the contact between objects. At the microscale, however, this is not the
case, and the strength of individual contacts is proportional to
their interfacial area [62]. The dense arrangement of the fibres
on the feet and ventral caudal region of chameleons may
permit them access to this microscopic regime, in which areaenhancing mechanisms can provide additional friction force.
At grip pressures lower than the 6.37 N mm22 required for full
collapse, a lower but significant level of friction enhancement
will occur. It remains to be determined what pressure values
8
rspb.royalsocietypublishing.org
Fb ¼
are present during climbing. As well, rough substrates may
create stress concentrations that increase friction enhancement.
Downloaded from http://rspb.royalsocietypublishing.org/ on June 15, 2017
35.
36.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
Am. Mus. Nat. Hist. 310, 1–182. (doi:10.1206/
310.1)
Vidal N, Hedges SB. 2005 The phylogeny of
squamate reptiles (lizards, snakes and
amphisbenians) inferred from nine nuclear proteincoding genes. CR. Biol. 328, 1000– 1008. (doi:10.
1016/j.crvi.2005.10.001)
Wiens JJ, Kuczynski CA, Townsend T, Reeder TW,
Mulcahy DG, Sites JW. 2010 Combining
phylogenomics and fossils in higher-level squamate
reptile phylogeny: molecular data change the
placement of fossil taxa. Syst. Biol. 59, 674–688.
(doi:10.1093/sysbio/syq048)
Bullock JMR, Federle W. 2011 The effect of
surface roughness on claw and adhesive hair
performance in the dock beetle Gastrophysa
viridula. Insect Sci. 18, 298– 304. (doi:10.1111/j.
1744-7917.2010.01369.x)
Gorb SN. 1998 The design of the fly adhesive
pad: distal tenent setae are adapted to the delivery
of an adhesive secretion. Proc. R. Soc. Lond. B 265,
747–752. (doi:10.1098/rspb.1998.0356)
Gorb EV, Gorb SN. 2002 Attachment ability of the
beetle Chrysolina fastuosa on various plant surfaces.
Entomol. Exp. Appl. 105, 13 –28. (doi:10.1046/j.
1570-7458.2002.01028)
Niederegger S, Gorb S, Jiao Y. 2002 Contact
behaviour of tenent setae in attachment pads of the
blowfly Calliphora vicina (Diptera: Calliphoridae).
J. Comp. Physiol. A 187, 961 –970. (doi:10.1007/
s00359-001-0265-7)
Niederegger S, Gorb SN. 2006 Friction and adhesion
in the tarsal and metatarsal scopulae of spiders.
J. Comp. Physiol. A 192, 1223–1232. (doi:10.1007/
s00359-006-0157)
Federle W. 2006 Why are so many adhesive pads
hairy? J. Exp. Biol. 209, 2611–2621. (doi:10.1242/
jeb.02323)
Barthlott W, Neinhuis C. 1997 Purity of the sacred
lotus, or escape from contamination in biological
surfaces. Planta (Heidelberg) 202, 1–8. (doi:10.
1007/s004250050096)
Autumn K, Dittmore A, Santos D, Spenko M,
Cutkosky M. 2006b Frictional adhesion: a new angle
on gecko attachment. J. Exp. Biol. 209, 3569–3579.
(doi:10.1242/jeb.02486)
Majidi CS, Groff RE, Fearing RS. 2005 Attachment
of fiber array adhesive through side contact.
J. Appl. Phys. 98, 103521. (doi:10.1063/
1.2128697)
Peattie AM, Majidi C, Corder A, Full RJ. 2007
Ancestrally high elastic modulus of gecko setal
b-keratin. J. R. Soc. Interface 4, 1071–1076.
(doi:10.1098/rsif.2007.0226)
Prowse M, Puthoff JB, Wilkinson M, Autumn K.
2011 Effects of humidity on the mechanical
properties of gecko setae. Acta Biomater. 7,
733–738. (doi:10.1016/j.actbio.2010.09.036)
Bhushan B. 2002 Introduction to tribology.
New York, NY: John Wiley and Sons.
9
Proc. R. Soc. B 281: 20132334
37.
Acta. Zool-Stockholm 71, 189–192. (doi:10.1111/
j.1463-6395.1990.tb01195)
Fischer MS, Krause C, Lilje KE. 2010 Evolution of
chameleon locomotion, or how to become arboreal
as a reptile. Zoology 113, 67 –74. (doi:10.1016/j.
zool.2009.07.001)
Higham TE, Jayne BC. 2004 In vivo muscle activity
in the hind limb of the arboreal lizard, Chamaeleo
calyptratus: general patterns and the effects of
incline. J. Exp. Biol. 207, 249 –261. (doi:10.1242/
jeb.00745)
Hurle JM, Garcia-Martinez V, Gañan Y, Climent V,
Blasco M. 1987 Morphogenesis of the prehensile
autopodium in the common chameleon (Chamaeleo
chamaeleo). J. Morphol. 194, 187–194. (doi:10.
1002/jmor.1051940207)
Bergmann PJ, Lessard S, Russell AP. 2003 Tail
growth in Chamaeleo dilepis (Reptilia :
Chamaeleonidae): functional implications of
segmental patterns. J. Zool. 261, 417– 425.
(doi:10.1017/S095283690300428X)
Lange B. 1931 Integument der Sauropsiden. In
Hanbuch der vergeleichenden Anatomie der
Wirbeltiere 1 (II) (eds L Bolk, E Göppert, E Kallius,
W Lubosch), pp. 375 –448. Berlin and Vienna:
Urban and Schwarzenberg.
Schleich H-HW, Kastle W. 1979 Hautstrukturen als
Kletteranpassungen bei Chamaeleo und Cophotis
(Reptilia:Sauria:Chamaeleonidae, Agamidae).
Salamandra 15, 95 –100.
Muller R, Hildenhagen T. 2009 Untersuchungen zu
Subdigital- und Subkaudalstrukturen bei
Chamäleons (Sauria : Chamaeleonidae). Sauria 31,
41 –54.
Spinner M, Westhoff G, Gorb SN. 2013 Subdigital
and subcaudal microornamentation in
chamaeleonidae: a comparative study. J. Morphol.
274, 713 –723. (doi:10.1002/jmor.20137)
Humason GL. 1979 Animal tissue techniques, 4th
edn. San Francisco, CA: WH Freeman & Co.
Maderson PFA. 1985 Some developmental problems
of the reptilian integument. In Biology of the
reptilia (eds C Gans, F Billett, P Maderson),
pp. 523–598. New York, NY: Wiley.
Cartmill M. 1985 Climbing. In Functional vertebrate
morphology (eds M Hildebrand, DM Bramble,
KF Liem, DB Wake), pp. 73 –88. Cambridge, MA:
The Belknap Press of Harvard University Press.
Williams EE. 1963 Studies of South American
anoles. Description of Anolis mirus, new species fron
Rio San Juan, Colombia, with comment on digital
dilation and dewlap as generic and specific
characters in the anoles. Bull. Mus. Comp. Zool.
Harv. Uni. 129, 463 –480.
Peterson JA, Williams EE. 1981 A Case history in
retrograde evolution: the Onca lineage in anoline
lizards. II. Subdigital fine structure. Bull. Mus. Comp.
Zool. 149, 215– 268.
Conrad Jl. 2008 Phylogeny and systematics of
Squamata (Reptilia) based on morphology. Bull.
rspb.royalsocietypublishing.org
20. Autumn K. 2007 Gecko adhesion: structure,
function, and applications. MRS Bull. 32, 473–478.
(doi:10.1557/mrs2007.80)
21. Spolenak R, Gorb S, Arzt E. 2005 Adhesion design
maps for bio-inspired attachment systems. Acta
Biomater. 1, 5–13. (doi:10.1016/j.actbio.2004.
08.004)
22. Tian Y, Pesika N, Zeng H, Rosenberg K, Zhao B,
McGuiggan P, Autumn K, Israelachvili J. 2006
Adhesion and friction in gecko toe attachment and
detachment. Proc. Natl Acad. Sci. USA 103,
19 320–19 325. (doi:10.1073/pnas.0608841103)
23. Russell AP, Johnson MK, Delannoy SM. 2007
Insights from studies of gecko-inspired adhesion
and their impact on our understanding of the
evolution of the gekkotan adhesive system.
J. Adhes. Sci. Technol. 21, 1119 –1143.
(doi:10.1163/156856107782328371)
24. Autumn K, Gravish N. 2008 Gecko adhesion:
evolutionary nanotechnology. Phil. Trans. R. Soc. A
366, 1575 –1590. (doi:10.1098/rsta.2007.2173)
25. Bauer AM, Good DA. 1986 Scaling of scansorial
surface area in the genus Gekko. In Studies in
herpetology (ed. Z Rocek), pp. 363 –366. Prague:
Charles University.
26. Irschick DJ, Austin CC, Petren K, Fisher RN, Losos JB,
Ellers O. 1996 A comparative analysis of clinging
ability among pad-bearing lizards. Biol. J. Linnean
Soc. 59, 21 –35. (doi:10.1006/bijl.1996.0052)
27. Russell AP. 1979 Parallelsim and integrated design
in the foot structure of gekkonine and
diplodactyline geckos. Copeia 1, 1– 21.
(doi:10.2307/1443723)
28. Williams EE, Peterson JA. 1982 Convergent and
alternate designs in the digital adhesive pads of
scincid lizards. Science 215, 1509 –1511.
(doi:10.1126/science.215.4539.1509)
29. Puthoff J, Prowse M, Wilkinson M, Autumn K. 2010
Changes in materials properties explain the effects
of humidity on gecko adhesion. J. Exp. Biol. 213,
3699–3704. (doi:10.1242/jeb.047654)
30. Lee J, Majidi C, Schubert B, Fearing RS. 2008
Sliding-induced adhesion of stiff polymer microfiber
arrays. I. Macroscale behaviour. J. R. Soc. Interface 5,
835–844. (doi:10.1098/rsif.2007.1308)
31. Schubert B, Lee J, Majidi C, Fearing RS. 2008
Sliding-induced adhesion of stiff polymer microfiber
arrays. II. Microscale behaviour. J. R. Soc. Interface 5,
845–853. (doi:10.1098/rsif.2007.1309)
32. Gasc J-P. 1963 Adaptation à la marche arboricole
chez le cameleon. Arch. Anat. Histol. Embryol. 46,
81 –115.
33. Herrel A, Tolley KA, Measy JG, da Silva JM, Potgieter
DF, Booler E, Boistel R, Vanhooydonck B. 2013 Slow
but tenacious: an analysis of running and gripping
performance in chameleons. J. Exp. Biol. 216,
1025–1030. (doi:10.1242/jeb.078618)
34. Abu-Ghalyun Y. 1990 Histochemical and
ultrastructural features of the biceps brachii of
the African chameleon (Chamaeleo senegalensis).