I

Asiatic Herpetolo^ical Research

June 1995

DigitalVol. 6, pp. 85-96

Pad Morphology in Torrent-living Ranid Frogs

ANNEMARIE OHLER
Laboraloire des Reptiles el Amphibiens, Museum national d'llistoire nalurelle,
25 rue Cuvier, 75005 Paris, France
Abstract. -Digital pads of 24 species of ranoid frogs (Raninae, Dicroglossinae, Ranixalinae, Rhacophorinae,

Hyperoliinae) were studied by scanning electron microscopy. In many species of Raninae the cells of theadhesive pad are differentiated (elongated and wearing projections). Functional aspects of cell morphology anddigital pad expansion are discussed in relation with sticking condition in aquatic medium.

Key words: Amphibia, Anura, morphology

Introduction

could be distinguished by their digit
morphology. Ranines have digital pads

Digital pads occur in most of advanced
anuran families. This organ seems to be ofmultiple origin and of difficult use insystematics (Noble and Jaeckle, 1928;
McAllister and Channing, 1982). Digitalpads occur in arboreal anurans (Hyla), butthey can also be observed in torrent-livingwith a latero-ventral groove, often separated

frogs (Amolops), and in some fossorial
species (Kaloula). In Asian and African
frogs of the family Ranidae, several generaand species groups belonging to different
subfamilies have fingers and toes bearing
digital pads.

Jaeckle (1928) undertook a comparative
histological analysis of 47 species ofanurans. The fine structure of the epidermal
cells in the digital pad has been observed bytransmission electron microscope (Komnick

There exists no strong hypothesis ofphylogeny of ranids as a whole.
Phylogenetic analyses were undertaken only
for geographic and taxonomic limited

to describe morphology of digital pads,
often in view of taxonomic utilisation orfunctional interpretation (Welsch, Storchgroups (Liem, 1970; Clarke, 1981; Hillis,
1985; Emerson and Berigan, 1993). The
broadly accepted classification (Frost, 1985)
based on Boulenger's works dating fromthe beginning of this century (Boulenger,lines

terminally. Dicroglossines have digital pads
showing a dorso-terminal groove.

The histological structures of the digitalpads were first described by Schuberg(1895) and Siedlecki (1910). Noble and1969; Ernst, 1973 a-b).
electronmicroscopy has been used
Scanning

and Stockem,

and Fuchs, 1974; Green, 1979, 1980, 1981;
Emerson and Diehl, 1980; Mc Allister andChanning, 1983; Green and Simon, 1986;
Green and Carson, 1988).

is1882; 1920).
Recently Dubois (1986,
tried
toreview
the entire group and1992)
which heclassificationatentative
proposedIn thissees as a working hypothesis.
intoseveralranidsaresplithypothesisfamilies, subfamilies and tribes (Dubois,
1992). Species that are enclosed in thegenus "Rana" in Frost (1985) are in Dubois'
classification distributed in several

The epidermis of anurans

has a
of
or
hexagonal
pentagonal
superficial layersquamosal cells, which are disposed in a
regular way (Tyler and Miller, 1985).
Differentiation of the pad leads to prismatic

Their surface is usuallyorhexagonal
pentagonal, as is that ofbut their height is morecells,
generalized
inthe latter.
than
They areimportantin
theirdistalpart
forming
deep
separated

epithelial cells.

subfamilies (Table 1).

crypts.

Results from study of skeleton showed
several major lines in "Rana" (Deckert,
1938; Clarke, 1981).
Study of the

In the dermis of amphibians bothmucous and venomous glands are present.
Their aperture is situated between themorphology of the digital pads (Ohler andDubois, 1989) confirmed that two of theseepithelial cells of the epidermis.

1995 by Asiatic Herpetological Research

On the padVol. 6, p. 86

Asiatic Herpetological Research

June 1995

TABLE 1.

Classification of Dicroglossinae and Raninae as proposed by Dubois (1992) and numbers ofstudiedhere. D: digital pad or expanded digit tip present in some species at least; M: some species at
speciesleast in the genus Micrixalus in the classification given by Frost (1985); R: some species at least in thegenus Rana in the classification given by Frost (1985); the number indicates the number of species herestudied by morphometry, external morphology and/or scanning electron microscopy.

Dircoglossinae

June 1995

FIG. 1

Asiatic Herpetological ResearchVol. 6, p. 87

Generalized plan of digital pad. Left dorsal view; right ventral view, c: cover; df: dorsal fold; tk:
terminal knuckle; eg: circumferential groove; p: pad; bg: basal groove.
.

FIG. 2. Digital pad of Raninae with latero-ventral groove (Rana (Hylarana) erythraea, MNHN 1987.3343,
Thailand), a: dorsal view of finger III; b: ventral view of finger III; stippled area corresponds to the pad withprismatic cells.

only openings of mucous glands can beobserved.

The

first

authors (Schuberg,

lizards differ substantially from amphibians

having a dry or setal adhesive system(Green and Carson, 1988).

in

1895;

Siedlecki, 1910; Noble and Jaeckle, 1928)
supposed that the products of the mucous

Emerson and Diehl (1980) and Green(1981) independently showed that surface

glands were implicated in sticking function.

tension was mechanically responsible for theadhesive abilities of treefrog digital pads.
As the surfaces of plants have usually a low
surface tension, the structure of the pad cells
assures humidification responsible foradhesion. The grooves surrounding the pad

To complete sticking the epidermal cells
would allow attachement to natural surfacesthat are covered with irregularities (Welsch,

Storck and Fuchs, 1974), somehow close to
the mechanism of clinging in lizards. But

Vol. 6, p. 88

Asiatic Herpetological Research

June 1995

FIG 3. Digital pad of Dicroglossinae with dorso-tcrminal groove (Limnonectes (Bourretia) doriae, MNHN
1987.3130, Thailand), a: dorsal view of toe IV; b: ventral view of toe IV; stippled area corresponds to the padwith prismatic cells.

serve as a reservoir for the fluid wetting
agent (McAllister and Channing, 1983).

Numerous frog species with enlarged
digital tips have been studied (Hyperoliinae,

Hylidae, Telmatobiinae, Rhacophorinae,
and others), as well as the digital tips ofsome species without enlarged digital tips.
Only some species of the family Ranidae
have been studied in this respect, including
only species without digital pad. Here I will
present the structure of digital pads anddigital pad cells of subfamilies of ranoids
according to Dubois' (1992) classification,
Ranixalinae, Dicroglossinae, Raninae,

Rhacophorinae and Hyperoliinae. They
include arboreal ("Hylarana") and torrent-

formalin fixed and all had been stored in 70
alcohol. Finger II or IV or toe III were
cut on the terminal articulation. Cleaned
with ultrasonic sounds, they were
dehydrated in alcohol. After critical point%

drying, they were gold covered (2-4 A).
Specimens were observed with the Scanningelectron microscope (JSM-840) of the

MNHN SEM facilities. Photographs wereon
120 Ilford FP4 film.
Measurements were taken with a slide
caliper (SVL) or a binocular microscope
taken

SVL

FW

-

snout-vent length;
thirdfinger width (maximum width of tip of thirdTo eliminate size factor,
isfinger).
given as a ratio of SVL (per thousand).

(FW):

FW

living frogs (Amolops) that have digital pads
with grooves and modified cells. For theTerminology of digital padmorphology (Fig. 1, 2, 3)

torrent-living frogs a mechanism of sticking
proposed and the correlation of cell(1) The circumferential groove (Green andismorphology,

digit tip enlargement
biology of these frogs is outlined.

andSimon, 1986) (Fig.

1)

The groove may be
a distal zone ofor(withcomplete
opencontact between the dorsal and ventral part).
This is the generalized groove that is
modified in various manners according to
the group of frogs observed.
part from a ventral part.

Material and methods

Specimens representing 15 of 34 generaand subgenera, possessing digital pads, as
recognised by Dubois (1992) were chosen
in the collection of MNHN (see Table 1,
Appendix I). They had been generally

surrounds the digit

tip latero-terminally and separating a dorsal

June 1995

Asiatic Herpetological ResearchVol. 6, p. 89-»iFIG. 4. Squamosal cells with short spinulae,

FIG. 5. Squamosal cells with microridges, ventral

ventral view, proximal of pad of finger III

view, outside the circumferential groove of finger HI(Batrachylodes vertebralis,

MNHN 1970.1407,(Amolops marmoratus, MNHN 1988.2787, Nepal).

Salomon Islands).

r "V*'.J* tf"SS

FIG. 6. Squamosal cells with spongious
structures, dorsall view, on subunguis close to the

FIG. 7. Sqamosal cells with hallow tubercles,

dorso-tenninal fold of finger III (Ingerana tasanae,

tasanae,ventral viw, proximal of pad of finger III {Ingerana

MNHN 1987.2002, Thailand).

(a) The latero-ventral grooves (Ohler

and Dubois, 1989) (Fig. 2) close the pad,that is of triangular shape, laterally. In somespecies they join distally and close to aunique groove arround an oval or roundedpad.

MNHN 1987.2002, Thailand).

(2) The basal groove (Fig. 1) is the basal
limit of the digital pad. Fusion of this withthe circumferential

groove results in acircumplantar groove. The latter is notpresent in all digital pad types.
(3)

The

ventral

part

is

the

adhesive organ, the pad (Savage,

The dorso-terminal groove
and(OhlerDubois, 1989) folds on thedorsal part of the digit. The pad is of oval
or rounded form.
In species where theismoregroovepronounced its lateral parts(b)

can be observed ventrally (Fig. 3).

(Fig.

1).

Its

proper1987)

latero-terminal limits areusually distinct formed by the groove.

basal limit

is

Its

intergrading, and the basal

groove, if present, is not the limit of thefunctional part as indicated by presence ofmodified cells still beyond this limit distally.

Vol. 6, p. 90

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June 1995

TABLE 2. Distribution of prismatic cell types and relative width of tip of third finger in ranoid frogs.

-

Cell

differentiation: L: elongated prismatic cells; R: regularly outshaped prismatic cells; H: cells of heterogeneous

shape; P: projections on proximal border of prismatic cellls; S: small projections on proximal border of- noprismatic cells;
projections on prismatic cells; -: no prismatic cells in the digit tip. Relative width ofN

tip of third finger, measured by

FW/SVL: x: mean; s: standard deviation; n: number of specimens measured;

EV: extreme values of ratio FW/SVL in group.

Species studied

Asiatic Herpetological Research

June 1995

FIG. 8. Regular outshaped prismatic cells withmucous gland pore on pad of finger III (Hyperolius
viridiflavus karissimbiensis,

MNHN 1988.1055,

Vol.6, p. 91

FIG. 9. Elongated prismatic cells with disatal
projections on pad of finger III {Amolops sp. 1,

MNHN 1987.2163, Thailand).

Rwanda).

FIG. 10. Elongated prismatic cells with distal
projections on pad of finger III {Amolops sp. 3,

MNHN 1987.2140, Thailand).

FIG. 1 1. Elongated prismatic cells with small
distal projections on pad of finger III {Amolops
marmoratus, MNHN 1988.2787, Nepal).

FIG. 12. Orientation and distribution of prismatic
cells on distal part of the digital pad of finger III of
Rana (Odorrana) andersoni (MNHN 1938.57,

FIG. 1 3. Distribution of prismatic and sqamosalcells on the extreme distal part of the digital pad ofVietnam).

1987.3471, Thailand).

finger III of Rana (Sylvirana) sp.

(MNHN

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The epidermal cells
thisOn the tips of the digits one observes
two major cell types (squamosal cells andprismatic cells) with intermediary cells that

occur in high numbers in the proximal padzone.

June 1995

In the species of the genus Amolops,
kind of cells is present with welldeveloped projections (Fig. 9, 10). These
were also observed in different "subgenera"of the genus Rana (Odorrana, Amnirana,Hylarana, Chalcorana) and in Indirana
gundia (Ranixalinae). The prismatic cells ofthese species vary in their elongation, in the(1)

Squamosal

cells.

This

is

the

generalized cell type, covering the body ofamphibians (Tyler and Miller, 1985). The
cells often show short spinulae (Fig. 4) or
structures called microridges (Fig. 5). In
Ingerana tasanae the surface of thesquamosal cells is extremely rough and canshow spongious structures (Fig. 6). On
other parts of the epiderm the surface of thesquamosal cells of Ingerana tasanae shows
hallow tubercles (Fig. 7). The squamosalcells cover fingers and toes outside the pad.

The groove is generally the border, butsometimes the squamosal cells are present
on the border of the pad {Rhacophorus

leucomystax) or in the contrary they arepushed back by the prismatic cells evensize of the projection, and in the degree of

regularity.

They are often rather regularly

hexagonal, rounded proximally, with small
distal projections, as in Rana (Chalcorana)chalconota and in Ingerana tasanae. In somespecies the prismatic cells are elongated,rounded proximally without projections{Rana (Hylarana) erythraea). In otherspecies outlines are very variable among
neigbouring cells; the cells are elongatedforming a somehow triangular outshape
wearing a single or two distal projections(Fig. 11). In all species of Amolops of thisstudy, this kind of elongated cells withheterogeneous outlines was observed.

The prismatic cells are present outside
latero-ventral grooves in Rana
(Odorrana) andersoni and in Amolops sp. 3.
theoutside the groove (Amolops).

The prismatic cells
They are of regular

Observation of direction of the channels

outline in all the species already studied

generalized alignement in the direction of thespace between the pair of lateral grooves(Fig. 12). In other species the border of pad
is formed by squamosal cells, but a contact(2) Prismatic cells.

are present on the pad.

(Green, 1979; Green and Simon, 1986;
McAllister and Channing, 1982; Richards etal., 1977; Welsch, Strock, and Fuchs,
1974).

Among the species studied here,

Hyperolius vividiflavus karissimbiensis andRhacophorus leucomystax have prismatic
cells of regular outline (Fig. 8) like thosefound by previous authors. Also someother species of ranids (Limnonectes
doriae, Batrachylodes(Bourretia)
vertebralis) have this kind of prismatic cells.

most of the ranid species(Table2) in this study, theinvestigated
cells
are
not
of regular outline but
prismatic
Theirlong axis is oriented in theelongated.
However,inproximo-distal direction on the digital pad.
The ratio of the width to the length of thesecells is smaller than 60%, while in normal
prismatic cells this ratio is over 80%, often
close to 100%. On their narrow distal side,
the elongated cells

developed projections.

have more or

lessformed by the prismatic

cells

shows

a

between the ventral and dorsal part of digitaltip remains (ex. Rana (Sylvirana) sp., Fig.
13).

The development of the toe pad

The measurements of the digital width(Table 2) show an important variation thatcan be divided in several units. The species

formosus and Amolops
marmoratus show the most enlarged finger
pads (FW/SVL = 68 p.m.). Other species

Amolopsof Amolops, but also Rana (Chalcorana),

tasanae and Rhacophorus
leucomystax have very well developed= 47-57 p.m.). The
digital pads (FW/SVL

Ingerana

frogs of the subgenera Rana (Amnirana) andRana (Odorrana) show moderately enlarged= 35-43 p.m.). The
digital pads (FW/SVL

species of Rana

(Sylvirana) and Rana

(Hylarana), as the species of the subgenus

Vol. 6, p. 93

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June 1995

FIG. 14. Scheme of liquid fluid on a digital pad with regular outshaped cells (left) and with elongated cells(right).

Limnonectes (Bourretia) have very little(FW/SVL = 22-28

enlarged finger padsp.m.).

(Sylvirana), and Rana (Chalcorana); (5)arboreal frogs (Hyperolius, Rhacophorus).

The species studied that show no

digital pad formation have the lowest ratios(FW/SVL = 17-20 p.m.).
Discussion

Actually the Raninae, the Dicroglossinae
and the Ranixalinae do not include strictly
The closest group ofarboreal species.
treefrogs are the Rhacophorinae, an othersubfamily

The elongated cells here described insome species of Ranidae have not been
described in other anuran families.

In factthe species that have been studied until now

are "treefrogs", and no torrent-living frogs
have yet been investigated. Considering theecology of the studied species, five types
can be distinguished: (1) torrent-living frogsof the genus Amolops, Rana (Odorrana) (thepossible sister-group of Amolops) and Rana
(Amnirana); Ingerana tasanae should beplaced in this group; (2) aquatic frogs, like

Limnonectes (Limnonectes) kuhlii andPhrynoglossus laevis; (3) terrestrial frogs ofthe genus Limnonectes (Bourretia) and Rana
(Hydrophylax); (4) ground/vegetation living
frogs of the genera Rana (Hylarana), RanaofRanidae.

Hyperolius

viridiflavus karissimbiensis is anotherranoid treefrog studied.
Rhacophorus
leucomystax, Hyperolius viridiflavus
karissimbiensis and the species studies by
the previous authors (Green, 1979; Greenand Simon, 1986; Richards et al., 1977;
McAllister and Channing, 1982; Welsch,
Strock, and Fuchs, 1974) have prismaticThis kind ofcells of regular outshape.
regular cells was here also observed in
Limnonectes (Bourretia) doriae andLimnonectes (Bourretia) pileata, twoterrestial species. Elongation of digital pad
cells in Amolops, Rana (Odorrana), andRana (Amnirana) might be in relationshipwith their mode of life. The presence ofelongated cells in Rana (Hylarana) and in
Rana(Sylvirana) might indicate

Vol. 6, p. 94

Asiatic Herpetological Research

June 1995

phylogenetic relationship to Amolops. The
heterogeneous cells in some of these species

use of liquid (Fig. 14). The elongated cellsof Amolops guide the liquid in the disto-

might indicate a regression in comparison to
the elongated cells with projections in
Amolops. The functional analysis of cell

proximal direction. The digital pad is not
closed posteriorly and often also anteriorly

morphology underlines this interpretation.

restricted to the pad surface, but are alsopresent in the groove and outside to it.

The major sticking force of tree frogs is

Water can flow out of the pad and distancefrom pad to sticking surface is minimized,

surface tension (Emerson and Diehl, 1980;
Green, 1981). It is a kind of wet adhesion,

by a groove, and prismatic cells are notthus increasing the sticking force inversely.

where two surfaces are hold together by aninterlaying liquid. The prismatic cells, thechannels and the mucous glands are requiredin the humidification mechanism necessary

would be interesting to compare themorphology of torrent-living frogs ofother anuran families, like Ansonia
It

cell

for sticking. For torrent-living frogs thesurfaces to stick to are already humid or in a

(Bufonidae),medium. In liquid the force is no
more proportional to the surface, but to thesquared surface which reduces the sticking(Hylidae)liquidforce to its square root (Emerson and Diehl,
1980).

When sticking to glass at an angle of

90 to 180
it,a treefrog is inmerged in water,

Petropedetes(Phrynobatrachidae), some Litoria and Hylaand

Heleophryne(Heleophrynidae) to what is here describedin Raninae. A more detailed morphologicalanalysis of surface of digital pads should beundertaken to compare sticking surface intree and torrent-living frogs.

will separate almost immediately (Emerson

and Diehl, 1980). The force of attachment

Acknowledgments

in liquid medium is inversely proportional tothe distance of the two surfaces, separated
by the liquid.

To provide a good sticking in water, thesurface of the pads should be enlarged.
Some of the species of Amolops, as
Amolops formosus or Amolops marmoratus
have in fact very much enlarged digital pads
(Table 2). A correlation between the digitalpad development, as defined by the groups
A, B, C, D and E see Table 2), and theecology of the species may be found. The
species and the aquatic frogs
The group B
belong to the group A.
includes ground/vegetation-living frogs.
terrestial

The torrent-living frogs are distributed inthree groups: C {O dorr ana, Amnirana andHuia), D (Amolops, Ingerana), E (Amolops
formosus and Amolops marmoratus). The
treefrogs (Rhacophorus, Hyperolius) are allmembers of the group D, thus not the

species with the largest digital pads.

The photos of this work were realizedwith the precious help of Jean Menier in thefacilities of the "Service Commun deMicroscopie electronique" of the MuseumI
national d'Histoire naturelle of Paris.
express my gratitude to Alain Dubois foradvice and discussion.
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Catalogue of the

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June 1995

APPENDIX I
Specimens studied by scanning electronmicroscopy (origin and reference in the
catalogue of the Museum national d'Histoire naturelle of Paris).
Species studied

Amolops (Amolops) formosus
Amolops (Amolops) marmoratus
Amolops (Amolops) sp. 1
Amolops (Amolops) sp. 2
Amolops (Amolops) sp. 3
Amolops (Huia) kinabaluensis
Amolops (Huia) nasicus

OriginNamdu Khola, Nepal
Timal, Nepal

Khao Chong, Thailand
Doi Inthanon, Thailand
Phu Kradung, Thailand
Kina Balu, Borneo
Hanoi region, Vietnam

Batrachylodes vertebralis

Bougainville, Solomon

Rana (Amnirana) albolabris
Rana (Amnirana) lepus
Rana (Chalcorana) chalconota
Rana (Hydrophylax) galamensis
Rana (Hylarana) erythraea
Rana (Odorrana) andersoni
Rana (Sylvirana) sp.

Liberia

Collection Number

MNHN 1994.5559
MNHN 1988.2787
MNHN 1987.2163
MNHN 1987.2082
MNHN 1987.2140
MNHN 1889.240
MNHN 1938.70
MNHN 1970.1407

Islands

Ingerana tasanae

Limnonectes (Limnonectes) kuhlii
Limnonectes (Bourretia) doriae
Limnonectes (Bourretia) pileatus

Central African Republic

Khao Chong, Thailand"Afrique Orientale Francaise"

Chiangmai, Thailand
VietnamDoi Pui, Thailand
Khao Phra Tiu, Thailand

Phu Kradung, Thailand

Khao Chong, Thailand
Phu Kradung, Thailand

Phrynoglossus laevis

Khao Chong, Thailand

Platymantis corrugatus

New Guinea

Khao Chong, Thailand
Hyperolius viridiflavus karissimbiensis Gihirwa river, Rwanda
Indirana gundia
Gundia, India
Rhacophorus leucomystax

1.

Formerly Amolops afghanus: see Dubois (1992: 340).

MNHN 1989.3456
MNHN 1968.247
MNHN 1987.3490
MNHN 1920.145
MNHN 1987.3343
MNHN 1938.57
MNHN 1987.3471
MNHN 1987.2002
MNHN 1987.3332
MNHN 1987.3130
MNHN 1987.3140
MNHN 1987.2944
MNHN 1989.3461
MNHN 1987.3544
MNHN 1988.1055
MNHN 1985.607