CALIFORNIA STATE UNIVERSITY , NORTHRIDGE
Morphology of the P ineal Complex in
Hyla cadaverina
and
Hyla reg i lla, with Studie s
of Its Role i n Spatial Orientati on
A the s i s s ubmitted in partial sati s facti on of the
requirements for the degree o f Mas ter of Science in
B i ology
by
Eve lyn Marie Fergus on
June , 1978
The Thesis o f Evelyn Marie Ferguson is approved :
(Dr . George F . Fis�er)
(Dr
(Dr
•
\
Anjll ojiff J '; Gaudin )
Jim
w.
Dole , Chairman)
Cali fornia State University , Northridge
ACKNOWLEDGJ.VIENTS
I would like to express my appreciation to the members
o f my thesis committee , Drs . J im Dole , George Fisle r , and
Anthony Gaudin , for the i r help and interest in my research
and for reading this manuscript and offering helpful sug­
gesti ons for its i mprovement .
Dr . Anthony Gaudin also
provi ded the Hyla reg i lla larvae whi ch were used and
great ly assisted with the morphological analysis and the
photography; for this
I
am espec ially grate ful .
Finally,
I wish to extend my deepest grat i tude to Dr . Jim Dole for
his enthusiasm , encouragement and valuable ass istance
throughout this entire pro j ect .
I would also like to thank Dr. Robert Bezy , Associate
Curator of Herpetology at t he Los Angeles County Museum of
Natural History , for allowing me the use of the Hyla
cadaverina larvae for the studies of the p ineal complex .
I also appre ciate the ass istance Dr . Ri chard Potter
provided me in locat ing informati on on the amphibian brain .
I am also indebted to Mr. Charles Bufalino for his assis­
tance with the mi crotechnique .
I
would like to thank my husband , Mr . Ronald Koller ,
for his encouragement and assistance .
H owever ,
I
would
like to express special thanks to my parents , Mr . and Mrs .
W i lliam Ferguson , for their help in collecting , in building
iii
the apparatus used in the polarized light studies, in
proofreading , but most of all for their interest and
enthusiasm .
iv
TABLE OF C ONTENTS
PAGE
A CKN OWLEDGMENTS
iii
LIST O F FIGURES
vi
LIST O F TABLES
vii
ABSTRACT
viii
INTRODUCTION
1
M ORPHOL OGY OF THE PINEAL C OMPLEX
6
Y-AXIS ORIENTATION STUDIES
23
P OLARIZED LIG HT STUDIES
32
SUMMARY AND CONCLUSI ONS
48
LITERATURE CITED
51
v
LIST OF FIGURES
FIGURE
1.
PAGE
Photograph and diagram of midsagittal
secti on of the brain of Hyla cadaverina ,
stage 26.
11
2.
Transverse sections of t he brain of Hyla
cadaverina , stage 2 9 .
13
J.
Transverse and midsagittal sect i ons of
the brain o f Hyla cadaverina , adult .
15
4.
Transverse sect i ons of the brain of Hyla
regi lla , stage 29.
17
Directional choices of Hyla regi lla when
tested for Y-axis orientati on.
26
Arena for t esting ability o f Hyla regi lla
to detect the e-vector of polarized light .
35
5.
6.
vi
L IST OF TABLE S
TABLE
1.
2.
PAGE
D i s tribut i on o f individually marked Hyla
regi lla when tes ted for their ability to
detect the e-vector o f polarized light .
40
Dis tributi on o f groups of unmarked Hyla
regi lla when tes ted for their ability to
detect the e-vector of polarized light.
42
vii
ABSTRACT
Morphology of the P ineal Complex in
Hyla cadaverina and Hyla regilla , with Studi es
of Its Role in Spatial Ori entation
by
Evelyn Marie Ferguson
Master of Science in Biology
The morphology and development of the pineal complex
are s i mi lar in Hyla cadaverina and Hyla reg i lla .
The
larvae of both species develop all three s tructures of the
complex , end organ , nerve and body before stage 26.
Mature adults o f both species lack the pineal end organ
and nerve , retaining only the pineal body .
Growth of the
pineal body wa s ob s erved from stage 26 through 32, but the
structure was only s light ly longer in mature adult s than
in larval stage 32 .
I found no evidence that adult Hyla regil la used in
thi s s tudy are capable of using a sun-compas s for Y-axi s
orientation .
Sim i larly there was
no
evidence that Hyla
reg i l la i s able to learn t o different iate between parallel
and perpendi cular orientations of the e-vector of polarized
light .
Cons equently , it is not pos s ible yet t o determine
for this species i f the pineal complex , or any port ion of
it , plays a role in the s e aspects o f spatial orientation .
viii
INTRODUCTI ON
The exi s tence of a 11 third e ye 11 or p ineal end organ in
amphibians was first noted by Stieda in 1865 when he found
a small spheri cal s tructure beneath a clear patch of skin
in the center of the head of the frog Rana temporaria.
Following thi s d i s c overy , de Graaf (1886) identified the
same s t ructure in s evera l other European frogs , but notably
was unable to find i t in the treefrog , Hyla arborea .
Goette (1873) was t he first to not e that during deve lopment
thi s s t ru cture budded from brain t i s s ue.
It was not until
1890, howeve r , tha t a complete des cripti on of the anatomy
and innervati on of t he p ineal end organ in a frog was made
by Leydig .
Since that time other researchers have looked at
pineal complexes in surpri s ingly few species o f anurans .
The morphology o f the pineal complex i s well known in
� temporaria , which was used by mos t early workers , and
in� pipi ens ( Ke lly and Smi t h , 1 963), � e s culenta
and� catesbeiana (Ueck , et al . , 1971).
On ly one non­
ranid , Hyla regilla ( Eakin, 1961, 1973), has been care­
fully studied in this r egard .
Ralph (1975) superfica lly
investigated spec i e s from 13 fami lies of anurans and found
pineal end organs c ons i s tently in only t he Ranidae; he
s tated that while 11i t i s c ommonly implied that the frontal
1
2
organ {pineal end organ ) i s a constant s tructure in frogs
and toads, i t i s apparent in only a very few anurans . "
P ineal morphology i s s t i l l unknown i n many species of
anurans .
Shortly after the discovery o f the structure, the
functi ons of the pineal end organ and pineal body with
whi ch i t i s as s ociated, were debated and explored .
In
1918, Holmgren found three distinct cell types, sens ory,
supportive and ganglionic, in the p in eal end organ and
suggested a s en s ory function for the structure .
Although
s ev eral later workers ( Ri ech, 192 5 ; Kle ine, 1929 ;
Winterhalter, 1931 ) found no evi dence t o s upport Holmgren•s
ideas and disagreed with his find ings, Oks che ( 1952 ) ulti­
mately con firmed Holmgren's s ensory cell descripti on .
Actual s en s ory functi on remained unknown, however,
unti l Eakin ( 1961 ) provided the first good evidence that
the pineal complex of anurans i s involved in photoreception .
He showed that receptoral processes in the pineal end organ
of Htla regilla larvae were similar to the outer s egments
of rods and cones of the vertebrate eye .
Succeeding
s tudies quickly demonstrated that thi s i s also true for
other species, as well as for di fferent developmental
stages ( Ke lly and Smith, 1963 ; Ueck et al . , 1971 ; Eakin,
1973) .
The actual photoreceptive funct i on of the pineal
complex was demons trated by Eakin and Westfall ( 1961 ), who
showed that blanching behavior changed when the pineal
3
apparatus was removed .
Direct electri cal impuls es , which
change with the wavelength of light directed at the pineal
end organ , have been recorded by Dodt and Heerd ( 1962 ) in
a frog pineal end organ , further s ugge st ing its role as a
light receptor .
The d i s covery of the abi lity of amphibians to perce ive
light extraopt i cally via the pineal complex has quite
naturally led t o studies to determine what us es the animals
derive from this percept i on of light .
Recent studies have
indi cated that extraopti c photorecepti on in amphibians i s
important in pigment control ( Eakin , 1 96 1 ; Bagnara and
Hadley , 1970 ), the regulat i on of endogenous rhythms ( Adler ,
1970 ), spatial ori entat i on ( Landreth and Ferguson , 1 967 ;
Taylor and Ferguson , 1970 ) and percep t i on of polarized
light ( Adler and Taylor , 1 9 73 ) .
Both t he extracranial
pineal end organ and intracranial pineal body have been
implicated in the perception of light for the s e behavi oral
activities .
In the las t few years the role of the pineal complex
in spatial orientati on has been carefully explored .
Taylor
and Fergus on ( 1 970 ) demons trated extraopti c reception o f
cue s for celest ial ori entation in the s outhern cri cket fro&
Acris gryllus .
When they covered the pineal body with
opa�ue plast i c , blinded frogs were unable to orient to
their correct Y-axis .
They concluded t hat in this species ,
cues for celes ti al ori entati on may be perceived via ei ther
4
the eyes or an extraopt i c s ite which they presumed t o be
the pineal body .
More recently, Jus t i s and Taylor ( 19 76 )
have shown bul lfrog larvae, � catesbeiana, capable of
Y-axi s orientat i on and that both the pineal end organ and
the pineal body may be involv ed in perception of cues for
ce lestial orientat i on when the eyes are removed .
Although many other species o f anurans have also been
s hown t o pos s e s s t he abi l ity to orient to a Y-axi s us ing
celesti al cues perceived opti cally and extraopti cal ly
( Ferguson, 1963 ; Fergus on, et al., 1965, 1967, 1968 ;
Landreth and Fergus on, 1966, 1967 ; Dole, 1972 ) , little i s
known about thi s abi lity in two local hylid species, Hyla
regilla and Hyla cadaverina .
Alvey ( 19 7 7 ) was unable to
demonstrate clearly that Hyla reg illa can use celestial
cue s to orient to their Y-axi s .
No informat ion is avai l­
able to date on any aspect of celesti al orientat ion in Hyla
cadaverina .
Recently it has been demonstrated that at least one
amphibian species, Ambystoma tigrinum, can use polarized
light as a cue for spatial orientation and that it per­
ceives the plane o f polarization ( e-vector ) both via the
eyes and extraopti cally ( Adler and Taylor, 1973 ) .
abi lity o f animals to use the polarizat ion of
light
Thi s
as a
cue for spatial orientation, well known in bees ( von Fri s ch,
1967 ) and other invertebrates ( Shurcliff and Ballard, 1964 ;
Waterman and Horch, 1966 ), has not been well exp lored in
vertebrat e s .
It i s now clear , however , that s ome teleosts
( Forward and Waterman , 197J ) and pigeon s ( Kreithen and
Keeton , 1 9 74 ), in additi on to the salamanders used by
Taylor and Adler ( 1 97J ) , can uti lize such cues .
To date no
information is avai lable on the ability of anurans to
detect and use the plane of polarizati on of light .
In view o f the spar s e in formati on on the pineal
complex in local repre s entatives of the family Hylidae and
the role o f the vari ous structures within thi s complex in
celestial or spatial ori entation , a series of related
studies was undertaken in an attempt to shed s ome l ight on
the sub j ect .
threefold:
Thus , the purpose of the pres ent study was
( 1 ) to explore the general morphology o f the
pineal complex in both larval and adult forms of the
California treefrqg , Hyla cadaverina , and to compare thi s
to the morphology o f the pineal complex in s imi lar develop­
mental stages of a clos ely related , sympatri c speci e s , the
Pacific treefrog , Hyla regi lla ; ( 2 ) to determine i f stream
dwelling Hyla reg i lla pos s e s s the abi lity to use a sun­
compas s to ori ent t o the Y-axi s of the i r speci fic shore­
line and , i f s o , the role of the pineal complex in deter­
mining di rection ; and ( J ) to determine i f Hyla reg i lla
larvae and j uveniles are able to detect the plane of
polarizat i on of li ght , and i f s o , the role of the pineal
c omp lex in its detect i on .
MORPHOLOGY OF THE P INEAL COMPLEX
MATE RIALS AND METHODS
In September , 1 9 7 5, five subadult Hyla cadaverina were
collected along Big Tu j unga Creek , Los Angeles County ,
Cali fornia .
The s ex o f the frogs was not determined; the
average snout-vent length was 2 . 5 em .
When returned to the
laboratory , the animals were fixed in 10% formalin for
approximately two weeks .
The heads were removed and then
decalcified , embedded , and stained according to the Trueb
modi fication ( Trueb , 1 9 70 ) of Baldauf's ( 19 58 ) procedure
for staining and s ectioning anuran head s .
Four heads w ere
sectioned in the transverse plane , three at a thicknes s of
10 microns and one at 5 micron s .
One head was s ectioned in
the sagittal plane at 10 micron s .
Eighteen Hyla cadaverina larvae , three each of stages
26, 27, 29 , 30, 3 1 and 32 ( standard anuran developmental s tages,
Gosner , 1960) were �btained from the Los Angeles county
Museum of Natural His tory ; all had been fixed in formalin .
Thes e larvae , as well as six stages ( 26, 2 7, 28, 29,30, 32 ) of
Hyla regilla larvae obtained from Dr . Anthony Gaudin ,
Cali fornia State University , Northridge , were embedded and
stained , again using Trueb1s modi fication of Baldauf's
pro ce dure .
Two Hyla cadaverina larvae and one Hyla
regilla larvae for each stage were s ectioned in the
6
7
transvers e plane at 10 microns .
One individual o f each
stage of Hyla cadaverina larvae was als o s ectioned in the
sagi ttal plane at 1 0 micron s .
RESULTS
At stage 26 , the earli est deve lopmental s tage s tudi ed
in Hyla cadaverina , the pineal end organ , pineal nerve and
p ineal body were already pres ent
developed ( Figure 1).
and
apparent ly fully
In all later s tage s the appearances
and relative pos i t i on s of the pineal end organ , pineal
nerve and pineal body were s imi lar to those of stage 26 .
In tadpoles o f all s tages o f Hyla cadaverina examined ,
the pineal end organ was located approximate ly on the mid­
line beneath the derm i s and above the tectum synoticum ,
anteri or to both the eyes and the pineal body .
In both
sagittal ( Figure 1 ) and transverse ( Figure 2) sect i ons the
pineal end organ appeared as a small ovoid structure ,
slight ly longer ( anteri or-pos terior ) than wide .
The length
varied from 80 microns ( both specimens ) in stage 26 to 1 1 0
microns ( both speci mens ) in stage 32 ; the width in the same
s tages varied from 50 to 80 microns ( N
=
1 for each s tage ) .
The center o f the end organ was hollow and i t s ventral wall
was thicker than i t s dorsal and lateral walls .
The pineal
nerve ran pos teriorly from the pos tero-ventral regi on o f
the pineal end organ , through the tectum synot icum , and
connected to the anterior porti on of the pineal body
8
( Fi gure 1 ) .
In midsagi ttal s ections, the pineal body
i t s elf appeared as a long, hollow finger- like pro j ecti on
in the roof of the thi rd ventricle of the brain, just
anteri or to the thalamus and posterior t o the paraphys i s
( Fi gure 1 ) .
In transverse s ect i ons, t he pineal body was
located mid-dorsally in the brain and appeared as a hol low
ring of cells ( Figure 2 ).
It extended t hrough s ections
containing the choroid plexus and third ventricle and
terminated anterior to the l evel of the opti c chiasma .
Although there was no cons i s tent difference from one
s tage to t he next, the relative posi tion of the pineal end
organ varied s li ghtly from animal to animal wi thin each
deve lopmental category .
In s ome individuals i ts posi t i on
was ent irely anterior t o the eyes, whereas in others i t was
at the level of the anterior margins of the eyes .
Although
the end organ was centered approximately between the two
s ides of t he brain, its pos i t i on varied somewhat .
Usually
it was s hi fted s li ghtly to the right or left of the pineal
body, whi ch was con s i s t ently in the center of the brain ; in
a few individuals, the two structures were directly in l ine .
The anterior limit of the pineal body always fell
between the anterior and pos terior margins o f the eyes .
The di s tance between the pineal end organ and pineal body
increased from 1 50 microns ( both specimens ) in stage 26 to
a mean o f 275 microns ( range 270-280, N
=
2 ) in stage 32.
This di s tance also corresponded t o t he l ength of the pineal
9
nerve .
In both transversely sect i oned individuals of
stages 26 and 27 , the pineal body terminated anterior to
the opti c chiasma .
In stages 29- 3 1 , the pineal body ter­
minat ed at thi s same level in one individual, but extended
to the level of the opt i c chiasma in the other individual .
In both transvers e ly s e ctioned individuals of stage 32 , the
pineal body t erminated at the level of the optic chiasma.
Thus , i t appeared that the pineal body extended s omewhat
more pos teriorly from stage 26 to stage 3 2 as its length
increased from approximately 140 mi crons ( range 1 30- 1 50;
N
=
2) to 420 mi crons ( both specimens ) .
Sections of the five adult Hyla cadaverina showed no
traces of ei ther t he pineal end organ or the pineal nerve.
However , the pineal body was pres ent in all adults , and in
each cas e was simi lar in appearance to the structure in the
larval stages 26- 3 2 ( Figure 3 ) .
In sagittal section it was
a hollow finger- like pro j ection above the roof o f the
di encephalon extending the length of the third ventricle
from the paraphys i s to the thalamus ( Fi gure 3 ) .
In trans­
verse sect i ons it appeared as a hollow ring of cells in the
cent er of the most dorsal part of the brain .
Typi cally it
extended from a point s omewhere between the eyes to the
level of the opt i c chiasma.
( range 440-480; N
=
Its m ean l ength was 460 microns
4) .
The p ineal body was shorter in relati on t o the brain
in adults than in larval stages , although i t s abs olute
10
length i ncreas ed with age .
In the adult s the average
length of the p ineal body was 460 mi crons whereas that of
the brain from i t s most anterior point t o the level of the
opti c chiasma was 2750 microns , a ratio of 1 to 6.
For
larval stage 32 comparable figures were 420 and 1560
microns , a rat i o of 1 to 4.7.
The p ineal complex found in Hyla regilla larval stages
26- 32 was s imi lar in appearance and pos ition to that in
Hyla cadaverina larvae ( Figure 4).
In the former species
als o the pineal end organ , pineal nerve and pineal body were
pre s ent at the earliest developmental stage examined .
However , the pineal body in Hyla regilla larvae was pro­
j ected more dors ally in the brain and was not surrounded by
as much brain t i s s ue ( Figure 4 ) .
In the s ix specimens
s tudied , the pineal b ody increas ed in length from approxi­
mately 100 microns in s tage 26, to 260 mi crons in s tage 32 .
The s e lengths were s horter than those in Hyla cadaverina
larval s tages 26-3 2 .
As in Hyla cadaverina , the p ineal end
organ varied s omewhat in placement from one individual to
another regardless of developmental s tage ; in s ome i t was
located on a line connecting the anterior margin of the eyes
whereas in others , it was anterior to the eyes .
Again in
s tages 29-32 of Hyla regilla , just as in Hyla cadaverina ,
the p ineal body terminated at the level o f the optic
chiasma , in stages 26-28 it t erminated anterior to this
leve l .
11
FIGURE 1
Photograph ( A ) and diagram ( B ) of a midsagi ttal
section through the brain case of Hyla cadaverina ,
larval stage 2 6 showing the pineal end organ , pineal
nerve and pineal b ody .
c .p .
=
p.e.o.
sk
=
Abbreviat ions:
choro id plexus ; d
=
=
dermis ; p . b .
pineal end organ; p .n .
skull ; t . s .
=
=
brain;
b
=
=
pineal body ;
pineal nerve ;
te ctum synoticum .
12
A
d
p.e.o.
p.b.
t.s.
I
I
B
Figure
1
13
FIGURE
2
Transvers e s ections through ( A ) the cerebral
hemi sphere s at the level of the pineal end organ and ( B )
the di encephalon and third ventricl e at the level o f the
pineal body of Hyla cadaverina , larval stage 29 .
c. h.
Abbreviati ons:
di
=
=
diencephalon ; p. b.
end organ ; sk
=
cerebral hemispheres ; d
=
pineal body; p. e. o.
skul l ; t. s.
=
=
tectum synoti cum.
=
dermi s ;
pineal
14
1 p.e.o. tls. . r,. ...
·-�
� �::2?2' ·?�·
�
. ... .
·:-:.
· : ..
.
.
.·.···
·
.
A
sk
�h.
B
..., :·::.;� ::; ·
':t·: :.: .·.
.
,:1·!· �1 ;.-·· �z::::v
·
..:;
-- -----.�
- �· :;f�i;���::···
.
.
.
1'
•
•
2
. ..
. . .
-�
sk
di
Figure
•
•
•
.
.
•
15
FIGURE 3
Transverse ( A ) and midsagittal ( B ) s ections through
the brain cas e o f s exually mature Hyla cadaverina.
showing the pineal body .
Broken line in midsagi ttal
sec tion ( B ) indicate s the level of transverse s ection {A ) .
b
Abbreviat i ons :
d
=
dermi s ; di
=
o .n .
=
sk
skull; t . s.
=
=
brain; c . p .
=
choroid plexu s ;
d iencephalon; o . l .
optic nerve; pa
=
=
=
opt ic lobe;
paraphys i s ; p . b .
t ectum synot icum .
=
pineal body ;
16
. .·
.
. ·
. .· :
A
d
0.1.
o.n.
t.s.
B
�-T
-
-
sk
'J
�
� - - ----:--·
.
--·
�------ � .
c.p.
Figure
.� � - <·_
-
.
-
b
3
_-,---:
17
FIGURE 4
Transverse sections through ( A ) the cerebral
hemispheres at the level of the pineal end organ and
( B ) the diencephalon and third ventricle at the level of
the pineal body of Hyla regilla, larval stage 29 .
Abbreviations :
di
=
c . h.
=
diencephalon; p . b .
end organ; sk
=
cerebral hemisphere s; d
=
pineal body; p . e . o .
skull; t . s .
=
=
=
tectum synoticum .
dermis;
pineal
18
d
p.e.o.
A
c. h.
sk
di
sk
B
Figure
4
19
DIS CUSSION
In Hyla cadaverina the ent ire pineal complex ( end
organ, nerve, body ) develops fully before stage 26 and
remains through stage 32 .
The structures are similar in
appearance to those found in Hyla regilla larvae, stages
26- 3 2 by me and by Eakin and West fall (1961 ) .
These latter
authors have not ed that in Hyla regilla separati on of the
embryonic epiphys is into the two separate structures of
pineal end organ and pineal body begins at stage 24 and is
completed by stage 26 .
It is likely that this general
pattern is true for Hyla cadaverina as well, particularly
since other less closely related anurans, such as �
temporaria ( Riech, 192 5 , as c i t ed by Eakin and Westfall,
1961 ) , also follow this developmental s equence.
As development proceeds from stage 26 through 32, the
pattern of extension of the p ineal body posteri orly to the
level of the optic chiasma is similar in both species,
except for a s i z e di fferential .
At stage 26 in Hyla
cadaverina and Hyla regilla the lengths are similar, 140
microns and 100 microns respectively .
By stage 32 , the
last stage observed, there is a slight vairance, the
pineal body of Hyla cadaverina averaging 420 microns in
length whi le that of Hyla regilla has increased to only
260 microns, an increase of 3 and 2.7 t imes respect ively .
This di fference probably reflects the slower growth rate
( Gaudin, 1965 ) and somewhat smaller adult s i z e of Hyla
20
regilla ( Stebbins, 1966) although both have approximately
the same snout-vent length at stage 26 .
Eakin ( 1 973 ) noted that in Hyla regilla early
embryoni c development of the pineal end organ and pineal
body is characteri z ed by asymmetry between the two struc­
tures, the pineal end organ constri cting off at an oblique
angle to the left of the pineal body .
From my informati on,
there was no evidence that this asymmetry was maintained in
the fully developed complex of stage 26.
Similarly,
I
found that in Hyla cadaverina larvae, the pineal end organ
was usually not in l ine with, but off to either side of,
the p ineal body .
I cann o t be certain, h owever, whether
this is a real variation or an arti fact, s ince it is
possible that preserving, embedding and sect i oning tech­
niques may have caused a sli ght shi ft in the immediate
position of the pineal end organ .
Adult Hyla cadaverina lack the pineal end organ and
pineal nerve as Eakin ( 1973) has reported also to be true
in Hyla regilla .
Since the last larval stage that I
examined (32) in Hyla cadaverina had both structures, i t is
not possible from my data to determine when the end organ
and nerve are lest .
Eakin and Wes tfall ( 19 6 1 ) reported
both to be present in Hyla regilla through metamorphosis so
that a fully metamorphosed young Pacif i c treefrog still has
all three elements of the pineal complex in undiminished
size .
It is probable that the developmental pattern of the
21
pineal complex of Hyla cadaverina close ly follows that of
Hyla regi lla .
In adult Hyla cadaverina the pineal body is only
·sl ight ly larger than in larval stage 32 , 460 and 420
microns respectively .
Thus i t is probable that the pineal
body reaches i ts maximum length during late larval stages .
No simi lar measurements are available for Hyla regi lla .
Whether s i z e and complexity of a p ineal system in
vertebrates has any direct relation t o physiologi cal
importance and function is not known .
Ralph (1975),
however , suggested that those systems with clearly differ­
entiated pineal components , end organ, nerve and body,
probably have larger physiological roles than those whi ch
contain only the p ineal body .
This same reasoning could be
applied to developmental di fferences within a species .
Thus , it is likely that the p ineal system is more i mportant
in the larval than adult stages in Hyla regi l la and Hyla
cadaverina .
The habitats o f larval and adult stages are striking
contrasts in both species of tree frogs .
Larvae are
confined to water whereas adults are semi-aquati c , breeding
in water but spending the remaining time in moist areas
near pools and streams .
Therefore , the two l i fe forms
would receive d i fferent qualities and quantit ies of light
stimuli for various physiologi cal and behavioral processes
since light for larval forms is received through water,
22
whi le in adults light is perceived directly through air .
Bagnara and Hadley ( 1970) felt that the p ineal complex is
most important in regulating color in early developmental
stages .
S ince one of the functions of the pineal complex
in frogs is to regulate chromatophore change, and i f this
is most important in Hyla regilla and Hyla cadaverine
larvae, the pineal end organ could be important as a
receptor for this purpose during larval stages only, hence
in adults this imput would be unn e cessary and the end organ
superfluous .
However, Eakin ( 1 973 ) demonstrated that in
HYla regi lla the larval pineal end organ and pineal body
have simi lar cell types and the adult-pineal body maintains
these same cell types .
This and other similar work has led
Kelly and Smith ( 1964) to speculate that both the end organ
and body have i denti cal functions, but the location of the
pineal end organ, b eneath the dermis, is more favorable for
recept ion of light st imulus .
Again the habitat di fference
could explain the presence of the end organ in larvae and
its absence in adults .
Y-AXIS ORIENTATION STUDIES
MATERIALS AND METHODS
The arena for testing the Y-axis orientation of Hyla
regi l la was surrounded by a 1 m high wall of opaque black
p lastic arranged in the form of a 16-sided polygon , 8 m
between opposing walls .
The arena was locat ed on the roof
of the science bui lding at California State University ,
Northridge .
The gravel- over-tar roof of the bui lding
served as the floor of the arena .
From floor level in the
center of the arena , nothing was v isible above the walls
except the sky .
Frogs were re leased in the center of the arena by
p lacing them under a large opaque jar lid whi ch was
attached to a string stretched across the arena .
Once I
had placed a frog in the release container , had left the
arena , and was in position to observe , the frog was
released by pulling the string , thereby l i fting the lid .
Observati ons o f the activities of the frog were made
through a small hole in the plastic wal l .
The point at
whi ch a frog first touched t he arena wall was recorded as
its orientati onal choice .
In Apri l , 1977 , 23 breeding H�la regi lla were captured
during both night and day at two sites a long the north side
of Malibu Creek in Tapia Park , Los Angeles County,
23
24
California .
At the two sites the stream ran roughly west
to east, 290°
the second .
to
°
110
at the first, and 285
°
to 105° at
The expected Y-axis ( a line perpendicular to
the shoreline) o f the frogs when released on land for site
one was
200
0
, and for sit e two, 195
0
•
After capture the
frogs were housed, until tested, in glass containers which
were kept outdoors under natural lighting conditions .
Tests were run during daylight hours o f the day following
nighttime captures or the same day for daytime captures .
Each frog was marked by toe-clipping at the time o f capture
and then t ested individually .
After all tests were
comp le t ed the frogs were released again where originally
captured .
One individual was recaptured after release and
retested .
RESULTS
Adult Hyla regi lla were tested in the roof arena on
four different occasions during April and May, 1 977 .
The
tests were performed betwe en the hours of 10 AM and J PM
with the sun visible in a clear or partially cloudy sky .
As the position o f the sun changed the crescent-shaped
shaded p ortion o f the arena shifted from the s outheast in
the morning to the southwest in the afternoon.
Although the
arena was never without some shaded area, the width of
shade during testing was always less than 1 m .
Most frogs did not move immediate ly when the lid was
25
raised , but remained in the center of the arena for a few
moments during which time they pivoted around that point .
Some took s everal short hops in a circle as i f attempting
Once a frog 11chose11 a d irect i on ,
to get their bearings .
however , i t usually continued on that course , taking long
jumps unti l it reached the arena wall .
On
only a few
occasi ons was a frog seen to change course before reaching
the wall , first tak ing several long jumps and then stoppin�
pivot ing , and then leaping off on a new course .
Thus it
was clear that the final directi onal choice o f the frog was
not necessari ly the direction it was facing when released .
It was assumed that i f the frogs demonstrated Y-axis
orientat i on the maj ority of them would choose compass
directions within 90° to e ither side
direction .
of
the expected
However , no directional choice was obvious
s ince the distribut i on of di recti onal choi ces for frogs
taken from site one , site two , and the combined total
{ Figure 5) do not di ffer s ignificantly from that expected
for a random choice (X 2
with 1 d . f . ; P
=
o,
0.072 and 0 . 174 respectively
0.995, 0. 7 and 0 .5) .
Also it is clear that the dire cti onal choices of the
frogs did not favor the partially shaded southern half of
the arena .
From the two s ites , 1 0 animals chose the
southern half whereas 13 animals chose the northern half
( Figure 5, A and B ) .
This distribution does not di ffer
signi ficantly from a distribution expected for a random
26
FIGURE 5
Directional choi ces of adult Hyla regi lla released
in an arena between 1 0 AM and J PM with clear or partially
cloudy ski es ; sun visibl e ; no wind ; warm temperatures
prevai led .
Arrow indi cates expected direction .
dot equals directi onal choice of one frog .
0°
Each
=
north .
A.
Frogs from site one; expected Y-axis 2 00° .
B.
Frogs from site two; expected Y-axis 1 9 5° .
c.
Combined direct i onal choices for sites one and two .
Expected Y-ax is of site one has been rotated to
coincide with Y-axis of site two at 1 9 5° .
27
A
B
••
•
•
c
Figure
5
28
choice (X
2
=
0 . 1 74 with 1 d . f . ; P
0.5) .
DIS CUSSION
Since s everal hylids including Acri s crepitans
( Fergus on et al . , 1 96 7 ) , Acris gryllus ( Ferguson , 1 963;
Fergus on et al . , 1 96 5; Taylor and Fergu s on , 1970 ) and
Ps eudacri s tri seriata ( Ferguson , 1 963 ; Landreth and
Fergus on , 1 966 ) are clearly able to use celestial cues to
orient to their correct Y-axi s , the fai lure of individuals
of Hyla regi lla to s e le ct a compass bearing perpendicular
to their home s horeline i s rather surpri s ing .
Thi s lack of
clear directional s electi on does not , of c ourse , mean that
the animal s cannot orient under other circumstances .
Indee d , the stat i onary p ivoting behavior s een in all frogs
upon releas e suggests that the animals were "searching" for
meaningful cue s as to where they were and , pre sumably , al s o
the be st route of escape .
Unfortunately the preci s e
reas ons for the pivoting cannot b e determined from my data .
There are , however , habitat differences whi ch could
account for the di fferences in orientati onal abi l ities .
My Hyla regi lla were collected from the north s ide of a
fai rly narrow stream ( 2 . 5 m) whereas all other hylids
studi ed thus far have been collected from ponds .
Since the
range of movement of the individual Hyla regi lla i s unkno�
it i s pos s ible that each frog was familiar with both s hores
of the stream which may or may not be parallel to each
29
ot her and may be able to orient along a Y-axi s to e ither .
The pres ent testing procedure would probably not di s crim­
inate a bimodal di s tribution as there i s usually a
s ubstant ial amount o f s catt er , even among species which
are c learly capable of Y-axi s orientat i on .
With a small
sample sufficient s catter in a bimodal distributi on would
almost certainly produce a dispers i on whi ch fai led to
di ffer s ign i fi cant ly from that expected for a random
choi ce.
It was clear that the failure of the frogs to show
sun- compas s orientat i on was not because they followed
s cents left by previous ly tested animals .
Had this been
true , all would have presumably gone in the same general
direction , thus generating a non-random di s tributi on .
In
any cas e , after they 11 chose 11 a direction , frogs usually
reached the wa ll in only a few long j umps:
the s cented
areas probably would be widely spaced "points" rather than
11 line s 11 whi ch other frogs might eas i ly follow .
Most
important , though , was the posture of t he frogs during
te sting; a t no t ime did they have their nose s to t he ground
as i f 11 smelling , 11 but rather their heads were up as i f
11 look ing . 11
It wa s also c lear that auditory cues did not affect
the overall di s tribut i onal choices of t he frogs .
The
location of the tes ting s ite , on a bui lding roof top over
20 miles from the home s i t e s of the frogs , eliminated all
JO
fami liar s ounds .
Although other , presumably unfami liar
s ounds were present , mos t appeared to have no influence on
directional cho i ce s .
Sounds of humans on the campus below
the bui lding were minimal , as tests were performed during
summer sessions when s tudent attendance at the Univers ity
was low .
On
the other hand traffic no i s e was fairly s teady
and predominately t o the south o f the test s ite .
It is
doubtful that either of the s e attracted or repelled the
animals since the di stribut i on of directions does not
differ signifi cantly from that expected for a random
choice .
One sound did , however , influence the frogs .
On
all four occas i ons when one of the air condi t i oning vent
motors on the roof squeaked whi le a frog was being tested,
the frog approached the s ound .
Thi s account s for the
directional choice of four frogs to the northwest .
If
the s e four directional choi ces are eliminated the di stri­
but i on is s t i l l not signifi cantly di fferent from that
expected for a random choice .
It is doubt ful that the conditi ons under which the
frogs were housed for the 1 to 1 2 hours prior to t est ing
were respons ible for their fai lure to orient .
No s horeline
was avai lable in t he glass containers in whi ch they were
housed ( wet paper towels provided moisture ) so that they
c ould not have learned a new shoreline as Fergus on et al .
(1965) found in Acris gryllus .
Als o , s ince frogs were kept
outdoors under natural light ing during the interval between
31
capture and testing , their internal clock s hould not have
been altered , thus causing their Y-axis to shift ( Ferguson
et al . , 1967).
The random choice of direct ions , of course , may
reflect the fact that adult Hyla regilla , unlike other
hylids so far tested , do not have an ability to orient on
a Y-axis t o the shoreline of their breeding site using the
dis c of the sun as a visual cue .
During breeding season ,
Hyla regilla move t o the streams after
sundown
( personal
observat ion) as is typical of most amphibians ( Adler and
Taylor , 1 973 ) .
I have never seen these frogs moving about
during the daytime except when forced out of the vegetation
in which they are hidden .
Movement after sundown neces­
sarily would eliminate the disc of the sun as a s ource of
directional cues in orientation .
POLARIZED LIG HT STUDIES
MATERIALS AND METHODS
Juvenile and larval Hyla regilla were captured during
June and September, 1976, along Malibu Creek, in and near
Tapia Park, Los Angeles County, California .
These indi­
viduals were transported to the laboratory and h oused in
several small glass aquaria under artificial lighting .
At
periodic intervals they were placed in an experimental
chamber where an attempt was made to 11train11 them and to
test for their ability to detect the plane of polarization
( e-vector ) of light.
The apparatus used for training and testing consisted
of an arena in the form of a cross, with a central chamber
and four side chambers each measuring 3 0 . 5 em square
( Figure 6) .
The arena was constructed of black ABS plastic,
textured on the internal surface to prevent partial
polarization by re flection .
The interior of the arena was
uniform in all respects ; there were no distinguishing
-
features unique to any of the side chambers .
A release
apparatus, a 7.5 em high box of sanded, clear Plexiglas,
2 5 em square and with no top or bottom, was suspended in
the center chamber using lightweight mon ofilament nylon
line .
When in use the arena was filled with water to 1 em
below the top edge .
32
33
A wooden frame painted flat black rested on the rim
of the arena .
Attached to the frame were stainless steel
electrodes whi ch protruded downward int o the water at
8 . 3 em intervals along the interi or walls of the arena side
chambers ( Figure 6).
Each side chamber was equipped with
four negative and four positive electrodes .
The electrodes
for two opposite s ide chambers were connected to
an
electri cal s ource , a J v battery in an inductorium modi fied
to produce a constant , low voltage , strong enough to
produce an avoidance response in the frogs , but not strong
enough to harm the animals .
I
was able to feel the current
when I placed my hand in the water .
The electrodes of the
other two side chambers were not so connected , serving only
to present a uni form appearance within the arena .
Over each of the four side chambers of the arena was
placed a Bausch and Lomb light polari z ing film , )0 . 5 em
square.
The axes o f polarization were kept constant
throughout , so that a frog approaching e ither of t he two
electrified side chambers from the center chamber moved
parallel to the axis of polarizat i on , whereas a frog
approaching e ither of the two une lectri fied s ide chambers
moved perpendicular to the ax is o f polarizat ion .
made by sanding one side of a 9 5
em
A filter ,
square piece o f clear
Plexiglas until frosted , placed 75 em above the arena
served to depolarize the light provided by four 60 w light
bulbs , each centered 80 em above one of the four side
34
chambers and 5
em
above the depolariz ing filter .
The
entire arena , including polari z ing f ilms, was completely
surrounded by black cotton-polyester fabri c, attached to a
wooden frame, whic h s erved t o exclude all external light .
This arrangement thus assured that frogs in the s i de
chambers of the arena would be sub j ected t o polarized
light , while the c enter chamber received unpolarized light .
Each experiment consisted of two parts , a training
period lasting 3 0 minut es and a t esting period of equal
length .
To begin the training period a group of 4 to 6
animals was placed in the release container and allowed to
remain undisturbed for several minutes .
The e le ctri city
was then turned on in the electri fied chambers and the
animals were released from the center by raising the
release container .
For the next 30 minutes the animals
were allowed to move undisturbed by me throughout the
arena .
At the end o f this period the locat i ons of all
ind ividuals were recorded , and the animals were removed
from the arena .
The water was stirred , the frame was
turned 9 0° , and the animals were replaced in the release
container .
After the animals were quie t in the release
box it was again raised to allow them access to the rest
of the arena .
The electri cal current t o the e lectrodes
remained off .
The animals were again allowed to move
freely about the arena for 30 minutes , after which t he
locations of the individuals were again recorded .
In
35
FIGURE 6
Arena ( B )
and
frame ( A ) to whi ch e lectrodes were
attached and whi ch supported polariz ing films used in
test ing the ability of Hyla regi lla t o orient using the
plane o f polarizat i on o f l ight .
In use the wooden frame
res t ed on the rim of the water- fi lled arena with the
electrodes pro j ecting into the water along the interior
walls of the four side chambers .
36
Figure
6
37
s everal cases individual frogs were trained and tested
more than once ; however, no frog was used more than once
in a 24 hour peri od .
RESULTS
� regi lla larvae when placed in the arena showed
no evidence that they were able to feel the electri cal
current .
Even when the current was at maximum , a level at
which adult frog s responded and I was able to feel the
current with my hand , the larvae showed no pre ference for
the une le ctrified over e lectrified side chambers but swam
freely int o all chambers .
Being unable to evoke an
avoidance response in the larval Hyla regi lla , no test of
their abi lity to perceive polarized l ight was poss ible .
Juveni le frogs , on the other hand , clearly responded
to the current and avoided the e lectrif i ed side chambers
when the current was on .
At the end of JO minute s in the
arena wi th the current on , individually marked juveni le
Hyla regi lla were in the unelectri fied chambers , those with
the e-vect or perp endi cular to their line of travel , a
maj ority o f the t ime ( 2 5 out of 28 , Table 1 ) .
The
probab i lity that t hi s di stribution is the same as that
expected for a random choice is much less than 0 . 005
(X2
=
1 5 . 75 wit h 1 d . f . ) .
Simi lar results were obtained
with groups of unmarked Hyla regi l la when 12 of 1 5 times
the juveni les chose the unelectri fied s ide chambers
38
Thi s dis tributi on als o i s s igni ficantly di f­
( Table 2) .
ferent from that expected on the bas i s of a random choice
(X 2
=
4 . 27 with 1 d . f . ; P
0 . 05 ) .
Combining the marked
and unmarked di s t ribut i ons , the frogs chose the unele c­
tri fied s ide chambers during training far more frequently
( 37 of 4 3 ) than would be expected i f the choice were random
(X 2
=
20 . 93 with 1 d . f .; P
0.05).
At the end of the testing period , when the animals had
been in the arena for 30 minutes with the electri cal
current off , the dis tribut i on of marked individuals was not
s ign i ficantly different from that expected for a random
choice (X
2
=
0 . 8 9 with 1 d . f . ; P
0.1) .
The s e frogs chose
the s ide chambers with the e-vector perpendicular t o their
l ine of movement from the center chamber ( unelectri fied
during training ) only 1 7 o f 28 time s .
The same was true
for the unmarked animals s ince 9 out of 15 t imes the s ide
chambers with the e-vector perpendi cular were chosen; the
probability of thi s dis tributi on being di fferent from one
expected for a random choice i s greater than 0 . 5 ( X
with 1 d. f . ) .
2
=
0 . 27
Even when the sample i s increas ed by con­
s idering the marked and unmarked trial s together , t here i s
no s igni fi cant di f ference from that expected for a random
di stributi on at the 0 . 1 level (X
2
=
1 . 49 with 1 d . f . ) .
In
only 26 of 43 t imes did frogs choos e the s i de chambers with
the e-vector perpendicular to their l ine of travel from the
center chamber .
39
Even i f i t i s assumed that only certain frogs hav e
the abi lity t o learn us ing the present experimental
condi t i ons, no evidence of learning to u s e the e-vect or i s
evident.
If, for ins tance, only tho s e indiv iduals whi ch
con s i s tantly cho s e the une lectri fied s i de chambers with the
e-vector perpendicular to their line of travel during
training ( 2 , 3 , 4 , 6 , 8 ) are considered, there i s no evidence
of learning.
During the following testing period the frogs
entered the s ide chambers w i th the orientati on of the
e-vector perpendi cular only 1 5 out of 2 1 times, a dis tri­
buti on whi ch does not d i ffer s i gnificantly from that
expected for a random cho i ce (X2
p
=
3 .05 with 1 d . f . ;
0 . 05 ) .
A lthough the frogs s howed no evidence of learning to
avoid the prev i ou s ly e le ctri fied s ide chambers on the bas i s
o f the orientat i on of polariz ed l ight, the individually
marked frogs did s how a tendency to choose, during testing,
a s i de chamber w ith the s ame orientat i on of the e-vector as
the s ide chamber they had entered during training, regard­
less of the pres ence or absence of e le ctri city during
training .
Frogs entered s ide chambers o f the same light
orientati on in both the training period and the follow ing
test period, whether i t was perpendi cular or parallel, 2 0
out
of
28 t imes ( Table 1 ) .
This distribution d iffers
signi fi cantly from that expected for a
than the 5% level (X
2
=
random
4 . 32 with 1 d . f . ) .
choice at les s
40
TABLE 1
D i stribution o f e ight individually marked Hyla
regilla when tes ted for the i r abi lity to orient us ing
the e-vector of polariz ed l ight.
During training and
testing the e-vector was oriented perpend i cular and
parallel t o the l ine
of
travel o f the animals from the
center chamber to the unelectri fied and electri fied s ide
chambers respectively.
TABLE 1
Training Period
Testing Period
Number of times frogs
chose chambers with the
e-vector oriented:
Individual
Perpendicular
Parallel
( unelectrified ) ( electrified )
Number of times frogs
chose chambers with the
e-vector oriented:
Perpendicular
Parallel
1
0
1
0
1
2
5
0
5
0
3
4
5
0
4
1
5
0
3
2
5
4
1*
2
3
6
5
0
3
2
7
0
1
0
1
8
1
0
0
1
25
3
17
11
Total
*In the subsequent test period this frog entered an arm with parallel
e-vector orientation.
.{::"
.....
42
TABLE 2
Dis tribution of three groups of unmarked Hyla
reg il la when tes ted for their abi lity to orient us ing
the e-vector o f p olarized light .
During training and
testing the e-vect or was oriented perpendi cu lar and
parallel to the line of travel o f the animals from the
center chamber t o the unelectri fied and e lectri fied s i de
chambers r espe ctively.
TABLE 2
Testing Period
Training Period
Number of times frogs
chose chambers with the
e-vector oriented:
Group
Parallel
Perpendicular
(unelectrified) (electrified)
Number of times frogs
chose chambers with the
e-vector oriented:
Perpendicular
Parallel
1
4
0
3
1
2
4
2
4
2
3
4
1
2
3
Total
12
3
9
6
�
\....)
44
DIS CUSSION
The training di s tributi ons o f j uveni le Hyla regilla
clearly indi cate that they felt the e lectrical stimulus and
avoided the s ide chambers with the current .
Their di s tri­
buti on during the tests , however, provides no evidence that
they were able to learn to avoid the expected electri cal
s timulus us ing the e-vector of polariz ed light as a cue .
Even animals which cons i s t ently entered unelectri fied s ide
chambers ( perpendi cular orientati on of e-vector } during
training did not do s o cons i s t ently during testing .
More
than twi ce as many individuals ( 1 5 of 2 1 } entered the
une lectri fied s ide chambers .
This may indicate learning ,
however , t he s ample s i z e i s too small t o be s tatistically
s igni fi cant .
Thus , t he s e res ults allow no definitive
conclus ions on the ability of Hyla regi lla to perceive
differences in polariz ed light .
It i s ·pos s i ble that the lack o f pos i t ive �esults
during testing is because the experimental procedure was
not suited to the task at hand .
For instanc e , the animals
may have been able to detect the e-vector , but unable t o
learn t o respond t o t he e lectrical s timulus .
Other
anurans , s uch as � RiRiens ( Dole , pers . comm . ) , are
apparently unable to learn avoidance of electrical s timuli .
Als o , there may not have been s uffi cient t ime during
training for t he frogs t o learn the orientati on of t he
e-ve ctor .
4.5
Taylor and Adler ( 1973 ) noted that in aquati c s itua­
t i ons there may be bright spots v i s ible to animals due to
the reflecti on o f the polarized light s hining on the water ,
and that the s e bright spots would be at a maximum perpen­
di cular to the e-v e ctor .
I f the frogs had been attracted
to the bright spot s , the distributi on s hould have favored
side chambers wi th the perpendicular orientati on of the
e-vector of light in the t e s t ing p�riod just as it did in
the training period .
It i s clear this was not the cas e
s ince the dis tributi on after testing was not s ignificantly
different from that expected for a random choice .
There i s s ome indi cat i on , however , that the frogs may
be abl e to detect the e-vector o f polarized light and learn
to move e i ther parallel or perpendicular to i t .
The number
of t imes frogs moved to s ide chambers having the s ame
orientati on o f the e-vector in both the training
peried
and
following tes t ing period far exceeds the number of t imes
they chose different orientati ons .
The frogs felt the
current , but they may not have as s oci ated the presence or
abs ence of e lectri city with a particular orientat i on o f the
e-vector .
Ins tead , the frogs may have preferred the s ide
chamber with a parti cular orientati on of light int o whi ch
t hey first swam , a 11 familiar 11 s ide chamber , regardles s o f
the pres ence o r abs ence o f e lectri cal current .
Adler and Taylor ( 1973 ) have s ugges t ed that the
abi lity of amphibians to perceive the e-vector can be
46
us e ful for spatial orientati on ; even when the sun i ts e lf
i s not vis ible , as when i t i s obs cured by clouds or at dawn
and dusk , i t s pos i ti on can be determined by the patterns of
polarized light in t he sky .
They further sugges ted that
amphibians may use polarized light cues to maintain a
directional choice with respect to the sun after sundown .
Breeding HYla regi lla , individuals of which call from
e s s enti ally the same locat i on each night after moving t o
them at dusk ( Jame s on , 1 957 ) , may use polarized light cues
to locate them .
Depending on their locat i on with respect
to the s e tting sun and calling s i t e s , they would need t o
travel at di fferent angles to the s un and t o retain knowl­
edge of this direc t i on for at leas t a s hort t ime .
My data
are not contrary t o this type of me chanism , s ince frogs
s eemed to prefer a parti cular orientat i on of the e-vector
of polarized light , but the results were not conclus ive .
It i s pos s ible , o f course , that the negat ive results
were obtained becaus e the frogs lacked the s ensory appa­
ratus for detect ing polarized light , even though tadpoles
may have t he capacity .
The larvae pos s e s s all three
s tructures of t he pineal complex , but the juveniles in the
s tudy already may have lost the pineal end organ and
pineal nerve ; unfortunately no informat i on i s available on
the t ime o f loss of the s e two structures during develop­
ment .
In any case i t i s not known i f t he end organ and
nerve are neces s ary for the detection and us e of the
47
e-vector .
Thi s s e ems unlikely , however , s ince s alamanders ,
whi ch Adler and Taylor ( 1973 ) have demons trated are capable
of perceiving the e-vector and us ing it for spatial
orientation in laboratory s i tuati ons , do not have t he end
organ or nerve .
In the s e animals the polarized light i s .
perceived by the pineal body after penetrating skin and
skull .
The same may be true for Hyla regi lla after los s of
the end organ and nerve , e specially s ince the pineal body
is not covered by bone , but rather the t ectum synoticum ,
a layer o f connective t i s sue and the skin .
SUMMARY AND CONCLUSI ONS
The pineal complex was found to be s imi lar in
morphology and development in Hyla regi lla and Hyla
cadaverina .
The larvae o f both species have all three
s tructures of the pineal complex , end organ , nerve and
body , from stage 2 6 through stage 32 .
The s tru ctures are
s imi lar in pos it i on and appearance in both species during
all deve lopmental stages .
All are fully deve loped prior
to stag� 2 6 and , s ince adult forms of both species lack
them , the loss of the pineal end organ and nerve occurs
after stage 32 but be fore animals reach s exual maturity .
Probably the loss occurs after metamorpho s i s in both
species s ince Eakin ( 1973 ) found that t he pineal complex
in Hyla regi lla remains complete through metamorpho s i s .
Although there i s growth o f the pineal from stage 2 6
through stage 3 2 i n both species , there i s probably little
growth after metamorphos i s inasmuch as the pineal body in
adult Hyla cadaverina i s only s light ly longer than that of
the larval stage 3 2 .
Although all three stru ctures of the pineal complex
are funct i onal phot oreceptors in both larval and adult
forms ( Dodt and Heerd , 1 962 ; Adler , 1 9 7 0 ; Eakin , 1973 ) , the
reas on for the lack of the end organ and nerve in the
adults of the two species I studied i s not clear .
48
It i s
49
pos s ible that this structural di fference i s correlated with
a functional or habitat di fference between larval and adult
forms .
Breeding Hyla regi lla u s ed in this s tudy did not
demons trat e an abi l ity to use a sun-compas s to orient to
their correct Y-axi s .
The poss ibility that this speci es i s
incapable of thi s type of spat ial orientati on i s supported
by Alvey ' s ( 1977 ) failure , u s ing both s tream and pond
dwe ll ing Hyla regi lla , to demons trate any clear use of sun­
compass for Y-axis orientat i on .
I f , in fact , Hyla regi l la
do not have the ab ility to u s e celestial cue s for spat ial
ori entat i on , they di ffer in thi s respect from hylids
prev i ou s ly studied .
However , the possibil ity that my frogs
were familiar with and oriented to both shores of the
stream cannot be e liminated .
I was unable t o demonstrate that j uveni le Hyla regi lla
can learn to di fferent iate between parallel and perpendic­
u lar ori entat i ons of the e-vector of polariz ed light when
given an e lectri cal s hock in s ide chambers with parallel
e-vector orientat i on during training .
This lack of
response may have been due to the inability of the frogs to
learn to respond to the e lectrical s timulus rather than
the i r being unable to d i s t ingu i s h the e-vector of the
polarized light .
The fact that frogs preferred to enter
s ide chambers with the same light orientat i on in both
training and tes ting, regardle s s of the pres ence o f
50
e lectricit y , s ugge sts that they can di s tinguis h differences
in polarized light .
The di stribution obtained during
testing may be due to familiarity with and preference for
a parti cular orientation of light rather than to avoiding
the 11 charged 11 s ide chambers .
It remains unclear in Hyla regilla what role the
pineal complex plays in spat ial orientati on .
It i s
doubt ful , however , that the lack of the pineal end organ
in adult Hyla regi lla would render them incapable of using
e ither the sun or the e-vector of polarized light as cues
for directional choices .
Other hylids presumed to lack
the end organ ( Ralph , 1975 ) have been s hown to us e a sun­
compas s for Y-axi s orientati on ( Fergus on , 1967 ) .
Simi larly , Ambys toma tigrinum i s able to detect the
e-vector o f polari zed light and use i t s orientat ion for
directional cues even though it , as all salamanders , lacks
a pineal end organ ( Adler and Taylor , 1 9 73 ) .
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