A Field Study of the Headbob Displays of Male Green Iguana
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Transcript of A Field Study of the Headbob Displays of Male Green Iguana
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7/25/2019 A Field Study of the Headbob Displays of Male Green Iguana
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Anim . Behav., 1982, 30, 327-338
F I E L D S T U D Y O F T H E H E D B O B D I S P L Y S O F M L E G R E E N
I G U N S
IGUANA GUANA ) :
V R I T I O N I N F O R M N D C O N T E X T
BY BEVERLY DUGAN*
Department o f Psychology, The University o f Tennessee, Knoxville, T N
37916
Abstract. Headbob displays play an important role in the social behaviour of male green iguanas
Iguana iguana). Four types of headbobs were identified, and the variability in form and context were
determined from quantitative analyses of displays filmed in the field. The Roll is a conspicuous advertise-
ment display. The Shudder, the most variable in form, occurred during close contac t between males and
females. The Roll-shudder is intermediate between the Roll and Shudder in its form and use. The
Signature Bob, the most stereotyped display in form, punctuates much activity and is the most variable
in contextual use. This display encodes species identity, and possibly individual identity. The con-
spicuous nature of the headbob displays was enhanced by the intensified colour of territorial males, and
by the selection of highly visible display posts. This system of visual displays is viewed as the product of
related sets of constraints, including the iguanas' physical capacities, the forest environment, and the
social environment.
Central to the study of animal communication
is the idea that the structure of a display rep-
resents the optimum form for transmitting par-
ticular types of information in a particular
environment. One aspect of display form that
has received much at tention is variability. Many
investigators have sought correlations among
the variability of a display, its contextual use or
information content, and the species' social and
ecological environment in an attempt to under-
stand the functions of relative differences in
variability (see reviews in Barlow 1977; Marler
1977). The objective of the present investigation
was to apply this approach in a field study of
the headbob displays of the green iguana,
Iguana iguana
(Sauria, Iguanidae).
The most thoroughly studied displays among
lizards in the family Iguanidae are the headbob
displays, vertical head and/or body movements
that are usually accompanied by extension of
the dewlap, a flap of skin under the throat. Most
iguanids studied perform a display termed the
'signature bob' (Stamps Barlow 1973): it is
stereotyped in form at the species, population
and/or individual level but occurs in a variety
of contexts such as territory advertisement, threat
and courtship. At least one other display in
addition to the signature bob has been described
for the majority of species investigated (see
reviews in Jenssen 1977; Ferguson 1977).
Iguanas have been observed to perform head-
bob displays in both the field (Mtiller 1972;
Lazelle 1973; Hazlett 1980) and the laboratory
*Address correspondence to: Beverly Dugan, Route 15,
Box 367, Gray, TN 37615, USA.
(Distel Veazey, 1982), although data on
variability in form and context have not been
reported. The observations of iguana display
behaviour reported here are part of a more
general field study of the social behaviour
of adult iguanas (Dugan 1980; 1982). The
objectives in this report are (1) to describe the
(headbob) display repertoire of L iguana; (2) to
identify the situations in which each type of
display is performed in the field; (3) to quantify
the variability of each display type; and (4) to
relate the variation in form and context of each
display type to the iguana's natural history.
e t h o d s
The social behaviour of L iguana was studied
from October 1977 to January 1979 on Flamenco
Island, a 13.5 ha island (maximum elevation
75 m) at the Pacific entrance to the Panama
Canal. The annual climate is marked by distinct
wet (May through November) and dry (Decem-
ber through April) seasons. Observations were
confined to a 5.5 ha area on the northwest side
of the island. The study area contained 200 to
300 iguanas (Dugan 1980).
Direct observation and filming were employed.
The iguanas were habituated to an observer
sitting quietly within a minimum distance of
about 20 m. I did not detect any differences in
behaviour observed at this range and that
observed with a telescope from 200 m and more.
During 15-min observation sessions, verbal
accounts of the behaviour of a focal animal
(Altmann 1974) were recorded with a cassette
tape recorder. The iguanas were filmed with a
327
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328 A N I M A L B E H A V I O U R , 3 0 , 2
i z o
S - 80 S u p e r 8 m m m o v i e c a m e r a e q u i p p e d
w i t h a z o o m l e n s a n d s e t a t a n a d v a n c e s p e e d
o f 1 8 f r a m e s / s . O c c a s i o n a l l y i t w a s p o s s i b l e t o
f il m a n d c o n d u c t f o c a l a n i m a l s a m p l e s s i m u l -
t a n e o u s l y ; h o w e v e r , d u r i n g p e r i o d s o f in t e n s e
a c t i v i t y t h i s w a s n o t p o s s i b l e .
D i s p l a y s w e r e f i lm e d b e t w e e n O c t o b e r a n d
J a n u a r y , a p e r i o d d u r i n g w h i c h d i s p l a y r a t e s
w e r e h i g h . A l t h o u g h d i s p l a y s w e r e g i v e n b y a l l ,
l a r g e m a l e s d i s p l a y e d m u c h m o r e f r e q u e n t l y
t h a n o t h e r m e m b e r s o f t h e p o p u l a t i o n ( se e
b e l o w ) . T h u s , t h e d i s p l a y a n a l y s i s i s b a s e d o n
o b s e r v a t i o n s o f l a r ge m a l e s o n l y .
D i s p l a y - a c t i o n - p a t t e r n ( D A P ) g r a p h s ( C a r -
p e n t e r G r u b i t z 1 96 1) w e r e g e n e r a t e d f r o m
f r a m e - b y - f r a m e a n a ly s is o f f il m e d h e a d b o b s .
T h e l o c a t i o n o f t h e t ip o f t h e s n o u t i n e a c h
f r a m e w a s p l o t t e d o n g r a p h p a p e r . T i m e i n
s e c o n d s w a s p l o t t e d o n t h e X - a x i s ; t h e Y - a x i s
r e p r e s e n t e d t h e a m p l i t u d e o f e a c h v e r t ic a l h e a d
m o v e m e n t . S i n c e c a m e r a - t o - s u b j e c t d i s t a n c e
v a r i e d f r o m 2 5 t o 6 0 m , i t w a s n o t p o s s i b l e to
m e a s u r e a b s o l u t e a m p l i t u d e . H e n c e a m e a s u r e
o f r e la t i v e a m p l i t u d e w a s u s e d t o c o m p a r e a m p l i -
t u d e s w i t h i n a d i s p l a y p e r f o r m a n c e ; t h e h i g h
a n d l o w h e a d p o s i t i o n s w e r e e x p r e s s e d a s a
p r o p o r t i o n o f t h e h i gh e s t i n e a c h d i s p la y . O n l y
t h o s e d i s p l a y s i n w h i c h t h e a n i m a l w a s r o u g h l y
l a t e r a l t o t h e c a m e r a , a n d i n w h i c h s n o u t
p o s i t i o n c o u l d b e c l e a r l y s e e n i n e a c h f r a m e ,
w e r e g r a p h e d .
T w o m e a s u r e s o f c o n te x t , 'd i s t a n c e ' a n d
' s o c ia l ', w e r e r e c o r d e d f o r e a c h d i s p l a y . D i s t a n c e
c o n t e x t r e f e r s t o t h e d i s t a n c e b e t w e e n a d i s p l a y -
i n g m a l e a n d t h e n e a r e s t f e m a l e ( s ) . E s t i m a t e s
o f d i s t a n c e s w e r e c l a s s if i ed i n t o o n e o f f o u r
c a t e g o r i e s : ( a ) 0 . 0 t o 0 . 5 m ; ( b ) 0 . 5 t o 1 .5 m ;
( c ) 1 . 5 t o 3 . 0 m ; ( d ) g r e a t e r t h a n 3 m . S o c i a l
c o n t e x t c a t e g o r i e s w e r e a s f o l l o w s : ( 1 ) S o l o ,
t h e m a l e w a s t h e o n l y o c c u p a n t o f t h e t r e e ;
( 2 ) N o t D i r e c t e d , o t h e r i g u a n a s w e r e p r e s e n t i n
t h e s a m e t r e e b u t a t a d i s ta n c e o f g r e a t e r t h a n
1 m , a n d t h e d i s p l a y in g i g u a n a w a s n o t i n t e r -
a c t i n g w i t h a n o t h e r i n d i v i d u a l ; ( 3 ) G e n e r a l
S o c i a l , a m a l e a n d f e m a l e ( s ) w e r e w i t h i n 1 m o f
o n e a n o t h e r , b u t n o o b v i o u s i n t e r a c t i o n w a s
o c c u r r i n g ; (4 ) M a l e - F e m a l e I n t e r a c t io n , a n a c t -
r e s p o n s e - a c t s e q u e n c e o c c u r r e d b e t w e e n a n
i n i t ia t o r o f t h e i n t e r a c t i o n a n d t h e r e s p o n d e n t .
D i s t a n c e a n d s o c ia l c o n t e x t c a te g o r i e s a r e n o t
m u t u a l l y e x c lu s iv e . D i s ta n c e c a t e g o r y ' d ' a p p l i e d
w h e n t h e t r e e w a s o c c u p i e d b y f e m a l e s t h a t
w e r e m o r e t h a n 3 m f r o m t h e m a l e , a s w e ll a s
w h e n t h e m a l e w a s t h e o n l y o c c u p a n t o f t h e
t r e e. T h u s , t h is c a t e g o r y a t t i m e s o v e r l a p p e d t h e
s o c i a l c a t e g o r y S o l o . H o w e v e r , s o c i a l c o n t e x t
c a te g o r ie s c o n t a in i m p o r t a n t i n f o r m a t i o n a b o u t
t h e n a t u r e o f th e m o m e n t a r y r e l at i on s h i p
b e t w e e n t h e m a l e a n d f e m a l e t h a t i s l a c k i n g i n
t h e d i s t a n c e c a te g o r ie s . F o r e x a m p l e , m a l e s a n d
f e m a l e s b a s k i n g a t c l o s e d i s t a n c e s t y p i c a l l y d i d
n o t i n t e r a c t , e v e n t h o u g h p h y s i c a l c o n t a c t w a s
m a i n t a i n e d f o r l o n g e r t h a n 1 5 m i n . H o w e v e r ,
m o s t i n t e r a c t i o n s d i d o c c u r w i t h i n d i s t a n c e s o f
1 .5 m . A l t h o u g h i t w a s d e s i r a b l e t o d i s t i n g u i s h
a m o n g s u c h s i t u a t i o n s , s m a l l s a m p l e s i z e d i d
n o t p e r m i t t h e u s e o f o n e s y s t e m o f i n d e p e n d e n t
c a t e g o r i e s . T h u s , a l l d a t a w e r e i n i t i a l l y a n a l y s e d
a c c o r d i n g t o d i s t a n c e . I f t h i s a n a l y s i s s u g g e s t e d
t h a t c o n s i d e r a t i o n o f s o c ia l c o n t e x t w o u l d p r o -
v i d e m o r e i n f o r m a t i o n , t h e n t h e s e c a t e g o r i e s
w e r e e x p l o r e d .
I g u a n a s h e a d b o b b e d d u r i n g m a l e - m a l e i n t e r -
a c t i o n s . A g o n i s t i c e n c o u n t e r s u s u a l l y i n v o l v e d
m a l e s o f d i f f e r e n t s iz es . T h e s e e n c o u n t e r s w e r e
b r i e f , c o n s i s ti n g o f l u n g e s o r c h a s e s b y t h e
l a r g e r m a l e , a n d a r a p i d r e t r e a t b y t h e s m a l l e r .
T h e l a rg e r m a le t h e n m o v e d r a p i d ly t h r o u g h o u t
t h e t r e e , p a u s i n g f r e q u e n t l y t o d i s p l a y . I n i n t e r -
a c t i o n s b e t w e e n m a l e s o f e q u a l s iz e, a n e x c h a n g e
o f h e a d b o b d i s p la y s a c c o m p a n i e d b y la t e r a l
b o d y c o m p r e s s i o n , a s d e s c r ib e d in o t h e r i g u a n i d
l i z a r d s ( C a r p e n t e r 1 9 6 7 ) , m i g h t c o n t i n u e f o r a s
l o n g a s 1 . 5 h . T h e t e r m i n a l e v e n t in t h e s e e n -
c o u n t e r s , t h e r e t r e a t o f o n e l i za r d , w a s o c c a s i o n -
a l l y p r e c e d e d b y a s n o u t - t o - s n o u t p u s h i n g
b a t tl e . S u c h e n c o u n t e r s w e r e e x t r e m e l y r a r e ;
o n l y t h r e e w e r e o b s e r v e d i n m o r e t h a n 1 00 0 h
o f o b s e r v a t i o n . D u e t o i ts r a r i ty , t h i s c o n t e x t
w a s n o t i n d u d e d i n t h e a n a l y s i s . T h u s t h e
f u n c t i o n o f h e a d b o b d i s p la y s i n t h e c h a l le n g e
p h a s e o f a n a g o n i s t ic e n c o u n t e r is n o t a d d r e s s e d
i n t h i s r e p o r t .
F i v e t y p e s o f h e a d m o v e m e n t s w e r e i d e n t if i e d :
t h e H e a d J e r k , t h e R o l l , t h e S h u d d e r , t h e
R o l l - s h u d d e r , a n d t h e S i g n a t u r e B o b . F i l m e d
H e a d J e r k s w e r e t o o f e w in n u m b e r t o p e r m i t
q u a n t i t a t i v e a n a ly s is o f f o r m a n d c o n t e x t . B e -
c a u s e th e m o v e m e n t s o f t h e R o l l, S h u d d e r a n d
R o l l - s h u d d e r w e r e o f s u c h l o w a m p l i t u d e ,
s u b j e c t - t o - c a m e r a d i s t a n c e p r e c l u d e d d e t a i l e d
t r e a t m e n t o f t h e f o r m o f th e s e d is p l ay s . O n l y
o v e r a l l d u r a t i o n s a n d c o n t e x t u a l u s e o f t h e
t h r e e w i ll b e d i s cu s s ed . T e n c o m p o n e n t s o f th e
S i g n a t u r e B o b w e r e m e a s u r e d , a n d v a r i a t i o n s i n
d u r a t i o n a n d a m p l i t u d e m e a s u r e s , a s w e l l a s
c o n t e x t u a l u s e , w e r e a n a l y s e d .
S ince a l l d i sp l ays were f i lmed in t he f i e ld , i t
w a s n o t p o s s i b le t o h a v e e q u a l n u m b e r s o f o b -
s e r v a t i o n s i n e a c h c l a s s i f i c a t i o n c a t e g o r y . T h e r e -
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DUGAN: FIELD STUDY OF IGUANA HEADBOB DISPLAYS
329
fore, the General Linear Models Procedure for
unbalanced designs (Statistical Analysis System,
Barr et al. 1976) was used for analyses of
variance. Other statistical methods and refer-
ences are detailed in the following sections as
appropriate.
R e su l t s
Be h av i ou r a l Ob se r vat i on s
The following summary of the ecology and
social behaviour of the iguanas on Flamenco
provides a background within which to interpret
the use of the headbob displays. More detailed
accounts can be found in Dugan 1980, and
1982.
Iguanas are sexually dimorphic in body size,
shape, and coloration. Males have larger heads,
longer spines on the dorsal crest, larger femoral
pores, and a larger dewlap. Most females and
small male iguanas are green; larger males are
dull grey, gold or tan. The male's colour intensi-
fies during the breeding season, when the head,
forelegs, and/or body of most large males are
bright gold or red-orange. Secondary sex
characteristics are most well-developed in males
larger than 36 cm body length.
During the non-breeding months of February
to October, the home ranges of all sex and size
classes overlapped in both space and time. The
largest males confined their movements to an
average area o f 0.08 ha; the home ranges of
smaller males and females were roughly three
times that size.
In both 1977 and 1978 mating activity began
in the last half of October with increasing rates
of display by large males. Courtship began in
early November. Females visited and courted
several males, and less frequently, males left
their home tree to court nearby females. By late
November, most large males had established a
territory in a tall, conspicuous tree or group of
trees. Territorial males alternated periods of rest
with display bouts (non-directed displays given
while basking or resting), courtship bouts, and
territory patrols. A mating territory contained
from one to four resident females. Each female
was courted for at least two weeks before
becoming receptive. Copulat ion occurred during
the first six weeks of the dry season, from early
December through mid-January.
H e a d M o v e m e n t s
1. Head Jerk. The behaviour consists of a
variable number of abrupt up and down move-
ments of the head (Fig. 1A). This movement was
performed by individuals in exposed locations
(nesting females, animals basking on the
ground) and in response to the approach of a
human or (less frequently) another iguana. Rand
(1968) noted that this simple bob is performed
by nesting iguanas that seem to be nervous, and
argued that it functions as an aid to vision
rather than as a social signal. My observations
are consistent with Rand's, and as the Head Jerk
was not associated with any particular social
situation(s), it was not considered to be a
display.
2. Shudder. The Shudder corresponds to Distel
& Veazey's (1982) 'vibra tory head nodding' .
This bob type consists of very low amplitude
vertical head movements that are much more
rapid than those of the Head Jerk (Fig. 1B).
Head position is variable, but the head is most
often parallel to the substrate. Neck movement
is more reduced in this bob than in the others.
Occasionally, the animal raises the head or
swings it from side to side while performing the
Shudder. More commonly, the neck is motion-
less and only the head moves vertically, pro-
ducing a jiggling appearance. During the
Shudder, the dewlap is maximally extended, the
forelegs are one-half to fully extended. Two
hundred and twenty-nine filmed Shudders per-
formed by 16 individuals were suitable for
analysis. Of these 64, or 28 , were performed
while the iguana was walking.
3. Roll. This movement corresponds to
Miiller's (1972) 'high frequency nodding' and
to Distel & Veazey's (1982) 'rotatory head
nodding'. In performing the Roll, the head is
lifted at an approximately 45 ~ angle to the sub-
strate and rotated along the longitudinal axis of
the body. Vertical movements varying in dur-
ation and amplitude may occur simultaneously
with the head rotations (Fig. 1C, 1E). The dew-
lap is maximally extended, the forelegs are one-
half to fully extended. The Roll is much more
exaggerated than the Shudder. During the high
amplitude head rotations of the Roll, the long,
pendulous dewlap undulates as the head moves
from side to side. The iguanas typically per-
formed Rolls from exposed perches, thereby
enhancing the conspicuous nature of this head-
bob. Locomotion was never observed to occur
simultaneously with the Roll. Forty-nine Roils
performed by 12 individuals were analysed.
4. Rol l shudder . This bob type consists of
rapid alternations of Rolls and Shudders in
highly var i ab l e s e q u e n c e s (Fig. 1D). Body
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330 A N I M A L B E H A V I O U R , 3 0 , 2
c
[ _ . _ |
I s e c .
Fig. 1. Patterns of iguana head movem ents in w hich the horizontal ax is rep
resents time in seconds, the vertical axis represents amplitude: (A) H ead jerks
from three individuals; (B) Shudders from two individuals: (C) Two Rolls per-
formed b y the s am e individual; (D) Roll-shudders from two individuals;
(E) Rotatory head movements performed during the Roll . Top graph of D is a
Shudder-Roll-Shudder sequence; bottom graph is a Roll-Shudder sequence.
p o s i t i o n i s t h e s a m e a s d u r i n g t h e S h u d d e r a n d
R o l l . T h i r t y - f o u r R o l l - s h u d d e r s p e r f o r m e d b y
1 4 i n d i v i d u a l s w e r e a n a l y s e d . I n f o u r o f t h e s e ,
l o c o m o t i o n o c c u r r e d s i m u l t a n e o u s l y w i t h t h e
S h u d d e r c o m p o n e n t o f t hi s b o b t y p e .
5 . S i g n a t u r e B o b . T h i s b e h a v i o u r c o r r e s p o n d s
t o D i s t e l & V e a z e y ' s ( 1 9 8 2 ) ' s t e r e o t y p e d h e a d -
n o d d i n g ' , a n d t o M i iU e r 's (1 9 72 ) 'l o w f r e q u e n c y
n o d d i n g ' . H a z l e t t ( 1 98 0 ) r e f e r r e d t o t h is m o v e -
m e n t a s t h e a s s e r t io n d i s p la y . T h e S i g n a t u r e B o b
i s a h i g h a m p l i t u d e v e r t i c a l h e a d m o v e m e n t
d u r i n g w h i c h t h e h e a d i s e l e v a t e d a t a 6 0 t o 9 0 ~
a n g l e t o t h e s u b s t r a t e , f o l lo w e d b y a s e c o n d
h i g h a m p l i t u d e m o v e m e n t o f lo n g e r d u r a t i o n
( t h e p l a t e a u ) . A s e ri es o f s h o r t e r , l o w a m p l i t u d e
b o b s f o l l o w s ( F i g . 2 ) . D u r i n g t h e p l a t e a u , t h e
h e a d i s h e l d u p f o r a b o u t 1 s , a n d h e a d r o t a t i o n s
o c c u r . T h e s e h e a d r o t a t i o n s a r e m u c h r e d u c e d
c o m p a r e d t o t h o s e o f t h e R o l l. T h e S i g n a tu r e
B o b i s p e r f o r m e d f r o m b o t h s i t t i n g ( f o r e l e g s
e x t e n d e d ) a n d l y i n g ( v e n t r a l s u r f a c e i n c o n t a c t
w i t h s u b s t r a t e ) p o s i t i o n s . T h e d e w l a p i s m a x i -
m a l l y e x t e n d e d . T h e a n a l y s i s i n c l u d e d 2 5 8
f i l m e d d i sp l a y s p e r f o r m e d b y 1 3 i n d i v id u a l s .
ontextual orrelates
D i s p l a y r a t e s v a r i e d w i t h t h e s e x a n d s i z e o f
t h e i n d i v i d u a l . F e m a l e s d i s p l a y e d m u c h l e s s
t h a n m a l e s ; v i r t u a l ly a l l d is p l ay s g i v e n b y f e m a l e s
o c c u r r e d d u r i n g i n t e r a c t i o n s . L a r g e m a l e s
( l o n g e r t h a n 3 6 c m b o d y l e n gt h ) d i s p l a y e d m o r e
o f t e n t h a n s m a l l e r m a l e s t h r o u g h o u t t h e y e a r .
L a r g e m a l e s w e r e m o r e l ik e ly t h a n m e d i u m
m a l e s ( 3 2 t o 3 6 c m ) t o p e r f o r m a t l e a s t o n e d is -
p l a y d u r i n g a n o b s e r v a t i o n p e r i o d ( 2 = 2 6 . 9,
f = 1 , P < 0 .001).
O v e r a l l d i s p l a y r a t e s a l s o v a r i e d s e a s o n a l l y ,
b e i n g g r e a t e s t w h e n m a t i n g a c t i v i t y w a s m o s t
I G U A N A F ~ O . 1 9
i I r z ~ I i I
I I E I I I 1
0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 ~ . 0
A NO. I0
t I P v I l
t t i t
I 1 I [ I I I I I I I ]
0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 ~ . 0
" I /~ . /~ /~ 9 I G U A N A N O 2
t t I | t I r I I I L . ? q I I t I [ I f f I
I ] ] 1 ~ I I I I I ]
0 0 . 5 1 . 0 1 . 5 2 . 0 2 : 5 3 . 0
TIME IN SE ONDS
Fig. 2. Samples of two D AP graphs of Signature Bobs
from each of three different male iguanas. Displays for
each iguana w ere filmed on the sa me day.
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D U G A N : F IELD S TU D Y O F IG U A N A H EA D BO B D ISP LA Y S 331
i n t e n s e ( F ig . 3 ) . T h e p r o p o r t i o n o f d i s p l a y s o b -
s e r v e d t h a t w e r e R o l l - s h u d d e r s a n d S i g n a t u r e
B o b s , h o w e v e r , r e m a i n e d r e l a t i v e l y c o n s t a n t
t h r o u g h o u t t h e y e a r , w i t h t h e R o l l - s h u d d e r
a c c o u n t i n g f o r 3 . 2 o v e r a l l, a n d t h e S i g n a t u r e
B o b f o r 6 5 . 0 , o f t h e to t a l. T h e R o l l a n d
S h u d d e r , o n t h e o t h e r h a n d , s h o w e d m o r e d r a s -
t i c f l u c t u a t i o n s ( F i g . 3 ). T h e r e l a t i v e f r e q u e n c y
o f S h u d d e r s i n c r e a s e d d u r i n g a l l b r e e d i n g s e a s o n
m o n t h s . T h e R o l l s h o w e d t h e g r e a t e s t i n c r e a s e
i n O c t o b e r , w h e n t h e n u m b e r o f R o ll s e x ce e d e d
t h e n u m b e r o f S h ud d e r s. T h e f l u c t u a ti o n i n
r e l a t i v e f r e q u e n c y s i g n i f i c a n t l y d e v i a t e d f r o m
t h e e x p e c t e d f o r t h e R o l l (; (2 --_ 2 3 . 6, d r = 9
P < 0 .01 ) an d the S hu dd er ( 2 = 34 .6 , d r = 9
P < 0 .001) .
T h e S h u d d e r w a s m o s t li k e l y t o o c c u r a t c lo s e
d i s t a n c e s ( 0 .0 t o 1 .5 m ) . D i s p l a y p e r f o r m a n c e
w a s m u c h m o r e v a r i a b l e a t i n t e r m e d i a t e d i s -
t a n c e s . W h e n t h e d i s t a n c e b e t w e e n a m a l e a n d
f e m a l e w a s g r e a t e r th a n 3 m , t h e R o l l t e n d e d t o
b e m o r e f r e q u e n t t h a n e i t h e r t h e S h u d d e r o r t h e
R o l l - s h u d d e r ( F i g. 4) . T h e r e l a t i v e p r o p o r t i o n s
o f t h e t h r e e h e a d b o b s d i f f e r e d s i g n i f i c a n t l y f r o m
t h e o v e r al l p r o p o r t i o n s b e t w e e n N o v e m b e r a n d
J a n u a r y a t a l l d i s t a n c e s e x c e p t 1 .5 t o 3 . 0 m ( 0 .0
to 0 .5 m , ; (2 = 37 .2 , d f = 2 , P < 0 .001 ; 0 .5 to
1 .5 m , 2 = 20 .3 , d f = 2 , P < 0 .001 ; g r ea te r
t h a n 3 m , K o l m o g o r o v - S m i r n o v t es t, P < 0 .0 1) .
B o b p e r f o r m a n c e a l s o v a r i e d a c c o r d i n g t o t h e
s o c i a l c o n t e x t . S o l i t a r y m a l e s u s u a l l y p e r f o r m e d
R o l l s ; o v e r 9 5 o f d i s p l a y s w e r e S h u d d e r s
d u r i n g M a l e - F e m a l e I n t e r a c t i o n ( F ig . 5 ). S o l i-
t a r y m a l e s d id p e r f o r m t h e S h u d d e r o r R o l l -
s h u d d e r ; h o w e v e r , t h e s e w e r e r a r e e v e n t s t h a t
n e v e r o c c u r r e d d u r i n g f i l m i n g s e s s i o n s . T h e f r e -
q u e n c y o f t h e t h r e e b o b s d i f f e r e d si g n i fi c a n tl y
f r o m t h e o v e r a l l p r o p o r t i o n s i n a ll c a s e s ( S o lo ,
K o l m o g o r o v - S m i r n o v t es t, P < 0 . 0 1 ; N o t D i r -
ec te d, ;(2 _= 10.2, d f = 2 , P < 0 . 0 1 ; G e n e r a l
So cia l, ;(2 = 11.9, d f = 2 , P < 0 . 0 1 ; M a l e -
F e m a l e I n t e r a c t i o n , ;(9 = 4 6 . 5 , d f = 2 , P < 0 . 0 0 1 ) .
I n t h e N o t D i r e c t e d c a t e g o r y , d i ff e r e n ce s w e r e
p r i m a r i l y d u e t o a g r e a te r t h a n e x p e c t e d n u m b e r
o f R o l l - s h u d d e r s , w h i l e th e l o w f r e q u e n c y o f
R o l l s c o n t r i b u t e d m o s t t o t h e d i f fe r e n c e s i n t h e
G e n e r a l S o c i a l c a t e g o r y .
I n s u m m a r y , t h e R o l l is m o r e l i k e ly t o o c c u r
i n ' n o n - s o c i a l ' s i t u a t i o n s ; t h e S h u d d e r i s m o r e
l i k e ly in s o c i a l c o n t e x t s. A l t h o u g h l o w i n f re -
q u e n c y i n a l l c o n t e x t s , t h e R o l l - s h u d d e r t e n d s
t o o c c u r m o s t o f t e n w h e n a r e c i p i e n t i s j u s t o u t -
s i d e t h e d i s p l a y e r ' s i n d i v i d u a l d i s t a n c e ( I o b -
s e r v e d it a t b e t w e e n 1 a n d 1 .5 m , t h e d i s t a n c e a t
O
[ ] Roll [ ] Roll Shudder
r~ Shudder T [ ] SignatureBob
I BREEDINGACTIVITY
M o n t h
N o . 1 5 - r a i n . i n t e r v a l s )
Fig. 3. Mean num ber o f headbob displays observed per 15-min observation
interval, and proportion of total displays observed that were Rolls, Shudders,
Roll-shudders, and Signature Bobs. Large males only, 1977-1979. Vertical lines
represen t :E SD a/2/N where N = num ber of intervals. Non-overlapping lines
indicate a significant difference (P = 0.05) between an y two pairs (see Burghardt
1969).
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332
A N I M A L B E H A V I O U R , 3 0 , 2
w h i c h e s c a p e o r a v o i d a n c e r e s p o n s e s o c c u r r e d ) ,
a n d i n t h e i n t e r m e d i a t e s o c i a l c o n t e x t s ( N o t
D i r e c t e d a n d G e n e r a l S o c i a l ) . I t i s a t t h e s e d i s -
t a n c e s a n d i n t h e s e c o n t e x t s t h a t t h e a p p a r e n t
s o c ia l n a t u r e o f th e l a r ge m a l e s b e h a v i o u r m a y
c h a n g e ( b e t w e e n N o t D i r e c t e d a n d G e n e r a l
S o c ia l , o r G e n e r a l S o c ia l a n d M a l e - F e m a l e
I n t e r a c t i o n ) . I n o r d e r t o d e t e r m i n e w h e t h e r t h e
R o l l - s h u d d e r i s r e s t r i c t e d t o t r a n s i t i o n s b e t w e e n
s o c i a l c o n t e x t s , t h e s i t u a t i o n s i n w h i c h t h e R o l l -
s h u d d e r o c c u r r e d w e r e f u r t h e r s u b - d i v i d e d i n t o
( 1 ) t r a n s i t i o n a l c a t e g o r i e s , i n c l u d i n g R o l l -
s h u d d e r s p e r f o r m e d d u r i n g t r a n s i t i o n s b e t w e e n
1.0
0 . 8
F -
0
I--
ta_
0 . 6
0
Z
0
F- o.a
2
O
~ 0.2
0
0 ROLL
H
ROLL-SHUDDSR
O - 0 . Sr n 0 . 5 - I .S m
03,120) {9,88)
i
.5 - 3.Ore
(6,461
D I S T A N C E F R O M F E M A L E
(NO, INDIVIDUALS, NO. DISPLAYS)
> &Ore
(6,41)
Fig. 4. Median proportion of total Roll, Shudder, and
Roll-shudder displays given by males at va rying distances
fro m females. Line denotes interquartile range; numbers
in parentheses refer to the number of individuals and
number of displays, respectively.
I . O
- . 1 0 . 8
o
I . l . 0 .6
0
0 . 4
o
~ 0 0 . 2
fl..
Solo
3,14)
0 ROLL
A SHUDDER
t D ROLL-SHUDDER
o __ I~ ~ L o
Not Directed General Social M/F Interact ion
9,91) 12~78) 10,112)
S O C I A L C O N T E X T
(NO. INDIVIDUALS, NO. DISPLAYS )
Fig. 5. Median propo rtion of total Roll, Shudder, and
Roll-shudder displays given by m ales in d ifferent social
co n t ex t s . Line denotes interquartile range; numbers in
p a r e n t h e s e s r e f e r t o t h e
number of individuals and num-
ber o f d i s p l a y s , r e s p e c t i v e l y .
s e q u e n c e s o f R o l l s a n d s e q u e n c e s o f S h u d d e r s ,
b e t w e e n s o c i al c o n t e x t s , a n d b e t w e e n p a t r o l s
a n d s o c i a l c o n t e x t s , a n d ( 2) n o n - t r a n s i t i o n a l
c a t e g o r i e s , i n c l u d i n g R o l l - s h u d d e r s i n t e r s p e r s e d
a m o n g s e q ue n c e s o f S h u d d e r s o r a m o n g s e -
q u e n c e s o f R o l l s , a n d ( 3 ) i n d e t e r m i n a t e c o n -
t e x ts , i n w h i c h t h e a c t i v i t y p r e c e d i n g a n d / o r
f o l l o w i n g t h e d is p l a y w a s n o t o b s e r v e d . T w e n t y -
t w o R o l l - s h u d d e r s o c c u r r e d i n t r a n s i t i o n a l s i t u -
a t i o n s , a n d o n l y n i n e i n n o n - t r a n s i t i o n a l s i t u -
a t i o n s ( Z 2 = 5.45,
df
1 , P < 0 .02 ). Th ree
w e r e in d e t e r m i n a t e . R o l l - s h u d d e r s w e r e m o r e
l i k e ly to o c c u r d u r i n g s e q u e n c e s o f S h u d d e r s
t h a n s e q u e n c e s o f R o l l s , b u t w e r e n e v e r m i x e d
w i t h S h u d d e r s i n t h e c o u r s e o f a s o c i al in t e r -
a c t i o n .
T h e S i g n a t u r e B o b p u n c t u a t e d a c t i v i t y i n a l l
c o n t e x t s . F r o m t h e f il m e d r e c o r d , I n o t e d t h e
e v e n t i m m e d i a t e l y p r e c e d i n g 7 2 3 d is p l a y s . N o
b e h a v i o u r p r e c e d e d 5 4 o f t h e 7 23 d i s p la y s . S o m e
o f t h e d i sp l a y s m a y h a v e f o l l o w e d a n e v e n t t h a t
w a s n o t r e c o r d e d o n f i lm . T w e n t y - e i g h t o f th e
d i s pl a y s o c c u rr e d a t t h e a p p r o a c h o f a f e m a l e ,
a n d t h e r e m a i n i n g 6 4 1 f o l l o w e d s o m e o t h e r
b e h a v i o u r b y t h e d i s p l a y e r ( T a b l e I ) . T h e s e
S i g n a tu r e B o b s w e r e n o t p r o p o r t i o n a t e l y a s s o c i-
a t e d w i t h a l l p r e c e d i n g a c t s , b u t r a t h e r t e n d e d
t o f o l l o w m o r e p e r c h c h a n g e s a n d f e w e r S h u d -
d e r s a n d R o l l - s h u d d e r s t h a n e x p e c t e d ( Z z = 4 8 .4 ,
df
5 , P < 0. 0 0 1) . S e q u e n c e s o f t h e s e a c t s
o f t e n p r e c e d e d S i g n a t u r e B o b s . T h e e x t e n t t o
w h i c h t h e s e q u e n t i a l o r d e r i n g o f b e h a v i o u r i n fl u -
e n c e d t h e a s s o c i a ti o n b e t w e e n t h e S i g n a t u r e B o b
a n d o t h e r a c t i v it i es i s n o t k n o w n . N e v e r t h e l e s s ,
i t i s c l e a r t h a t t h e S i g n a t u r e B o b o c c u r r e d d u r i n g
p a u s e s in , a n d a t t h e t e r m i n a t i o n o f , a h n o s t a n y
a c t i v i t y i n w h i c h a l a r g e m a l e e n g a g e d . A f t e r
p e r f o r m i n g a S i g n a tu r e B o b , a n i n d i v i d u a l m i g h t
c o n t i n u e t h e s a m e a c t i v i t y , c h a n g e t o a n o t h e r
a c t i v i t y , o r c e a s e a c t i v i t y .
T a b l e I . F r e q u e n c y w i t h w h i c h V a r i o n s B e h a v i o u r a l P a t -
t e r n s b y t h e D [ s p la y e r P r e c e d e d S i g n a t u r e B o b s , a n d t h e
O v e r a l l F r e q u e n c y o f t h e B e h a v i o u r . P r o p o r t i o n a l F r e -
q u e n c i e s a r e i n P a r e n t h e s e s
No. that preceded Overall
Behaviour Signature Bobs frequency
Ro ll 60 (0.094) 80 (0.089)
Shud der 173 (0.270) 308 (0.342)
Roll-shu dder 31 (0.048) 46 (0.128)
Perch chan ge 233 (0.363) 283 (0.314)
Position chan ge 114 (0.178) 141 (0.157)
Ton gue-tou ch 30 (0.047) 42 (0.047)
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DUGAN: F IELD STUDY OF IGUANA HEADBOB DISPLAYS 333
Variation in Form
Th e Ro ll Shudder and Roll-shudder. T h e
Rol l , a ve r a g ing 1 .26 s i n d u ra t ion (SD = 0 .41 ) ,
t e n d e d t o b e s h o r t e r a n d l e s s v a r i a b l e t h a n t h e
S h u d d e r ( ~ = 2 . 79 , SO = 1 .2 4) a n d R o l l - s h u d d e r
( -~ = 2 .68 , so = 1 .09) . I nd iv idu a l d i f f e r ences in
d i s p la y p e r f o r m a n c e f o l l o w e d t h is o v e r a l l t r e n d ,
w i t h t h e r e l a ti v e d u r a t i o n o f th e R o l l b e i n g
s h o r t e r a n d l e ss v a r ia b l e t h a n t h a t o f t h e
S h u d d e r ( F i g . 6 ) . I n t e r - i n d i v i d u a l c o m p a r i s o n s
i n th e p e r f o r m a n c e o f t h e R o l l - s h u d d e r a r e
d i f fi c u lt t o i n t e r p r e t d u e t o i ts l o w f r e q u e n c y .
W i t h a n o v e r a l l c oe f fi c ie n t o f v a r i a t i o n ( C V )
o f 3 2 . 3 8 , t h e R o l l f a l l s w i t h i n t h e t y p i c a l r a n g e
o f v a ri a b i li t y o f m o d a l a c t i o n p a t t e r n s ( M A P s )
t h a t h a v e b e e n q u a n t i f i e d f o r a l a r g e n u m b e r o f
s p e ci es r e p r e s e n t i n g d i f f e r en t t a x o n o m i c g r o u p s :
C V s o f d u r a t io n s b a s e d o n i n t e r - in d i v i d u a l
v a r i a b i l i t y r a n g e f r o m 1 5 t o 3 5 ( s u m m a r i z e d i n
B a r l o w 1 97 7 ). T h e S h u d d e r ( C V = 4 4 .4 6 ) a n d
R o l l - s h u d d e r ( C V = 4 0 . 8 4 ) e x c e e d t h e s e v a l u e s .
A l t h o u g h t h e d i f fe r e n ce s a m o n g t h e t h r e e a r e
n o t l a r g e , t h e S h u d d e r i s s i g n i f i c a n t l y m o r e
v a r i a b l e t h a n t h e R o l l (c s t a ti s ti c , D a w k i n s &
Dawkins 1973) .
T h e r e l a t i o n s h i p b e t w e e n d i s p l a y d u r a t i o n
a n d d i s t a n c e b e t w e e n , t h e d i s p l a y i n g m a l e a n d
f e m a l e a c c o u n t s f o r s o m e o f t h e d i ff e re n c e s
a m o n g d i s p l a y s i n t e m p o r a l v a r i a b i l i t y ( F i g . 7 ) .
T h e S h u d d e r b e c a m e s h o r t e r a n d l e s s v a r i a b l e
a s d i s t a n c e f r o m t h e f e m a l e i n c r e a s e d . S h u d d e r
d u r a t i o n w a s i n f l u e n c e d b y b o t h i n d i v i d u a l
i d e n t i t y ( F = 2 . 7 0 , d f 15, 193, P < 0 .001)
a n d d i s t an c e f r o m t h e f e m a l e ( F = 5 .1 1 , d f
3 , 1 5, P < 0 . 0 25 ) . T h e d u r a t i o n o f t h e R o l l w a s
l i tt l e a f f e c t e d b y d i s t a n c e f r o m t h e f e m a l e , b u t
w a s s i g n i f i c a n t l y r e l a t e d t o i n d i v i d u a l i d e n t i t y
(F = 2 .91 , d f 10 , 32 , P < 0 .025) . The Ro l l -
s h u d d e r i n c r e a s e d s l ig h t ly in d u r a t i o n a t 0 . 5 -
1 .5 m , b u t o t h e r w i s e f o l lo w e d t h e s a m e g e n e r a l
t r e n d a s t h e S h u d d e r . N o s i g n i f ic a n t e f fe c t s w e r e
f o u n d f o r th e R o l l - s h u d d e r , a l t h o u g h a g a i n
s m a l l s a m p l e s i z e m a k e s i n t e r p r e t a t i o n d i f f i c u l t .
S i n c e t h e S h u d d e r t e n d e d t o d e c r e a s e i n d u r -
a t i o n a s d i s t a n c e f r o m t h e f e m a l e i n c r e a s e d , i t
w a s o f i n te r e s t to a s k w h e t h e r o r n o t s i g n if i ca n t
d i f f e re n c e s i n t h e d u r a t i o n o f t h is d i s p l a y m i g h t
b e f o u n d a m o n g t h e t h r e e s o c i a l c o n t e x t c a t -
e g o r ie s (r e c al l t h a t S h u d d e r s d i d n o t o c c u r w h e n
t h e m a l e w a s s o li ta r y ). D i f f e re n c e s a m o n g t h e
s o c i a l c o n t e x t s d i d o c c u r ( F = 1 0 .0 2 , d f = 2, 17,
P < 0 . 0 01 ). T h e d u r a t i o n o f t h e S h u d d e r w a s
s i g n i fi c a n t ly g r e a t e r d u r i n g i n t e r a c t i o n s ( , ~ =
3 . 19 s ) t h a n i n t h e G e n e r a l S o c i a l ( ) ? = 2 . 5 6 s )
a n d N o t D i r e c t e d ( J? = 2 .2 3 s) c a t e g o r ie s ( D u n -
c a r t ' s t e s t ) . T h i s p a t t e r n w a s t h e s a m e i n a l l
i n d i v i d u a l s . T h e G e n e r a l S o c i a l a n d N o t D i r -
e c t e d c a t e g o r i e s i n c l u d e i n s t a n c e s w h e r e m a l e s
a n d f e m a l e s a r e c l o s e t o g e t h e r b u t n o t i n t e r -
a c t i n g . T h u s i t a p p e a r s t h a t S h u d d e r d u r a t i o n i s
m o r e i n f l u e n c e d b y t h e o c c u r r e n c e / n o n - o c c u r -
r e n c e o f a n i n t e r a c t i o n t h a n b y d i s t a nc e .
Th e Signature B o b . T h e p a t t e r n o f h e a d m o v e -
m e n t s c h a r a c t e r is t i c o f t h e S i g n a t u r e B o b w a s
d i v i d e d i n t o t w o u n i t s ( F i g . 8 ) . U n i t 1 ,
c o n s i s ti n g o f t h e p r e - p l a t e a u b o b , t h e p l a t e a u ,
a n d t h e p o s t - p l a t e a u b o b , o c c u r r e d i n a ll S ig -
n a t u r e B o b s a n d w a s c o n s i d e r e d t o b e t h e
s p e c ie s - ty p i c al c o m p o n e n t o f t h is d i sp l a y. U n i t 2
w a s m o r e v a r i a b le ; a n y n u m b e r o f U n i t 2 b o b s
m i g h t f o l l o w U n i t 1 . O n l y o n e o f 2 5 9 d is p l a y s
a n a l y s e d s t o p p e d a f t e r U n i t 1 .
8 . 0
6 . 0
Z 4 .0 5
2 0
[ ] ROLL
[ ] s H u ~ R
[ ] ROLL- HUODER
o
2o Toc SPMF H CR SPST
I N D I V I D U L S
Fig. 6. Mean duration 4- SD of R olls, S~ -dders, and Roll-shudders
performed by six individuals. N of each bob type is above the
vertical line.
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334
A N I M A L B E H A V I O U R , 3 0 , 2
Th e i n i t i a l an a ly s i s c o n s id e r e d 1 0 c o m p o n e n t s
o f t he S i g n a t u re B o b . D u r a t i o n m e a s u r e s m -
c t u d e d th e d u r a t i o n o f t h e p l a t e a u ( P L T ) , t h e
s u m m e d d u r a t i o n s o f t h e p r e- a n d p o s t - p l a t e au
b o b s ( U 1 - P ) , a n d U n i t 2 ( U 2 ) . A m p l i t u d e
m e a s u r e s i n c lu d e d t h e p r e - p l a t e a u p e a k ( A - l )
a n d d ip ( A - 2 ) , t h e p l a t e a u p e a k ( A - 3 ) , t h e p o s t -
p l a t e a u d ip ( A - 4 ) a n d p e a k ( A - 5 ) , a n d t h e m e a n
a m p l i t u d e o f U n i t 2 b o b s (A - 6) . T h e n u m b e r o f
U n i t 2 b o b s ( U 2 b ) w a s a l so c o u n t e d . T h e m e a n ,
s t a n d a r d d e v i a t i o n , a n d c o e f fi c ie n t o f v a r i a t i o n
o f e a ch c o m p o n e n t a n d o f t o t al d u r a t i o n a r e
p r e s e n t e d i n Ta b l e I I .
N e i t h e r d i s t a n c e f r o m f e m a l e n o r p r e c e d i n g
b e h a v io u r ( Ro l l , S h u d d e r , Ro l l - s h u d d e r , l o c o -
m o t io n , o th e r , o r u n id e n t i f i e d ) i n f l u e n c e d t h e
v a r i a b i l i ty o f t h e I 0 c o m p o n e n t s . O n ly i n d i-
v idua l iden t i ty (F = 8 .39 , d f = 120, 1334, P .~.O m OVERAt,
DISTANCE FROM FEMALE
F i g . 7 . M e a n d u r a t i o n s SD o f d i s pl a y s p e r f o r m e d b y
m a l e i g u a n a s a t v a r y i n g d i s t a n c e s f r o m f e m a l e s .
d i s p l a y c o m p o n e n t s . N o r w a s t h e s t r e n g t h o f
e a c h c o m p o n e n t a s a u n i q u e d i s c r i m i n a t o r
a m o n g i n d i v i d u a ls k n o w n . I f t h e S i g n a t u re B o b
d o e s c o m m u n i c a t e i n d i v i d u a l i d e n t it y , m u s t t h e
ig u a n a s w i tn e s s t h e o v e r a l l p a t t e r n o f m o v e m e n t ,
o r m i g h t a c e r t a in s e g m e n t b e t h e i m p o r t a n t c u e ?
A s t e p w i s e d i s c r im in a n t a n a ly s i s w a s u s e d t o
i d e n t i f y t h e c o m p o n e n t s t h a t b e s t s e p a r at e d
ind iv idua ls .
I n d i s c r im in a n t a n a ly s i s , t h e v a lu e s o f d i s -
c r im in a t i n g v a r i a b l e s a re w e ig h t e d a n d c o m -
b i n e d t o f o r m o n e o r m o r e l i n e a r f u n c t i o n s t h a t
m a x im iz e t h e v a r i a n c e b e tw e e n g r o u p s w h i l e
m i n i m i z i n g t h a t w i t h i n g r o u p s :
Di
d izZ1 d~2Z2 . . . . . . d ipZp
w h e r e D f i s t h e s c o r e o n t h e d i s c r im in a n t f u n c -
t io n i , the d i ' s a re we igh t ing coe f f ic ien ts , and
th e Z ' s a r e t h e s t a n d a r d i z e d v a lu e s o f t h e p d i s-
c r im in a t i n g v a r i a b l e s u s e d i n t h e a n a ly s i s . Th e
coef f ic ien t re f lec t s the re la t ive co n t r i bu t ion o f i t s
a s s o c i a t e d v a r i a b l e t o t h a t f u n c t i o n .
A s tepwise p roc edu re f i r s t se lects the s t ronges t
p r e d i c to r v a r i a b l e , a n d t h e n e n t e r s t h e ' n e x t b e s t '
d i s c r im in a to r a t e a c h s t e p ( N i e e t a l . 19 75 ).
D e p e n d e n c i e s a m o n g v a r i a b l e s a r e w r i t t e n i n to
th e m o d e l ; t h e r e l a t iv e s t r e n g th o f e a c h v a r i a b l e
a s a n i n d e p e n d e n t p r e d i c to r i s i n d i c a t e d b y i t s
c o n d i t i o n a l F - v al u e . T h e c o n d i t i o n a l F ' s a r e
d i f f ic u l t t o i n t e r p r e t , h o w e v e r , i f e x t r e m e d e -
p e n d e n c i e s e x i st b e tw e e n tw o o r m o r e v a r ia b l e s.
P e a r s o n p r o d u c t - m o m e n t c o r r e l a t i o n c o e f fi ci e nt s
r e v e a l e d t h a t a l t h o u g h 2 8 o f 45 p a i r s w e r e s ig -
n i f i c a n t l y c o r r e l a t e d , t h e c o r r e l a t i o n b e tw e e n
o n l y o n e p a i r, U 2 a n d U 2 b ( r = 0 .8 9) w a s
grea te r than 0 .60 , an a rb i t ra r i ly se lec ted cu t -o f f
T O T A L ~
U N I T : - - , - , - - U N I T 2 - - - ~ -[
i I
I ^ i P L T E U I ~ ~
~ J r ~ A 6 l
V v v v v v \
i A 4 I
R E L A T I V E A M P L I T U D E S
A I : P r e p l o t e o u P e a k A 4 : P o s t p l a t e a u D i p
A E : P r e p l a te a u D i p A 5 : P o s t p l a t e a u P e a k
A 3 = P l a t e a u P e a k A 6 : X a m p l i t u d e o f U n i t ?
b o b s
F i g . 8 . C o m p o n e n t s o f t h e S i g n a t u r e B o b u s e d i n a n a ly s i s.
S e e t e x t f o r e x p l a n a t i o n .
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7/25/2019 A Field Study of the Headbob Displays of Male Green Iguana
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D U G A N : F IEL D S TU D Y O F IG U A N A H EA l)BO B D ISP LA Y S 335
v a l u e . T h e u n i v a r i a t e F - v a l u e s i n d i c a t e d t h a t
U 2 b ( F = 1 2. 84 ) m i g h t b e a s t r o n g e r p r e d i c t o r
t h a n U 2 ( F = 4 .5 5) , a n d t h u s U 2 w a s o m i t t e d
f r o m t h e a n a l y s i s .
T h e s t e p w is e p r o c e d u r e u s e d a l l o f th e r e m a i n -
i n g n i n e v a r i a b l e s i n f o r m i n g t h e d i s c r i m i n a n t
f u n c t i o n s . T h e f u n c t i o n s s i g n if i c an t l y d is c r i m i -
n a t e d a m o n g i n d i v i d u a l s ( F = 2 5 .1 ,
d f =
108,
1 7 38 , P < 0 . 0 0 01 ) . N i n e d i s c r i m i n a n t f u n c t i o n s
w e r e fo r m e d . T h e d u r a t i o n o f t h e p l a t e a u a n d
o f t h e r e m a i n d e r o f U n i t 1 c o n t r i b u t e m o s t t o
t h e s e p a r a t i o n a m o n g i n d i v i d u a l s o n t h e f i r s t
f o u r f u n c t i o n s , w h i c h a c c o u n t f o r 9 0 .6 % o f t h e
v a r i a b i l i t y a m o n g i n d i v i d u a l s ( T a b l e I I I ) . T h u s ,
a l t ho u g h U 2 b a n d t h e a m p l i t ud e m e a s u r e s
c o n t r i b u t e t o t h e s e p a r a t i o n o f i n d i v i d u a ls , w h e n
a l l v a r i a b l e s a r e c o n s i d e r e d t o g e t h e r i n d i v i d u a l
d i f f e r e n c e s a r e p r i m a r i l y d u e t o d u r a t i o n
m e a s u r e m e n t s m a d e o n U n i t 1 .
T h e c o n d i t i o n a l F - v a l u e s ( T a b l e I V ) i n d i c a t e
t h a t p l a t e a u d u r a t i o n i s b y f a r th e b e s t p r e d i c t o r
o f a n i n d i v i d u al 's b o b p e r f o r m a n c e , f o l l o w e d b y
t h e a m p l i t u d e o f t h e p r e - p l a t e a u d i p , p o s t -
p l a t e a u d i p , a n d U 2 b , r e s p e c ti v e l y . T h u s , w h e n
o v e r l a p a m o n g t h e v a r i a b l e s i s t a k e n i n t o c o n -
s i d e r a t io n , t h e s t r o n g e s t i n d e p e n d e n t d i s c r i m i -
n a t o r s a r e t o b e . f o u nd w i t h i n U n i t 1 .
T h e d i s c r i m i n a n t f u n c t i o n s c o r r e c t l y c la s si fi e d
8 8 .4 % o f t h e S i g n a t u r e B o b s . I n a m o r e e x a c t i n g
t e s t, t h e ja c k - k n i f e c l a s s i fi c a ti o n , 8 3 . 7 % o f t h e
d i s p l a y s w e r e c l a s s i f i e d c o r r e c t l y . T h e e x p e c t e d
p e r c e n t a g e o f c o r r e c t c la s s if i c a ti o n s b a s e d o n
c h a n c e ( d e t e r m i n e d f r o m t h e p r o p o r t i o n a l d i s -
t r i b u t io n o f s a m p l e d d i sp l a y s a m o n g i n d iv i d u a ls )
w as 9 .1 .
D i s c u s s i o n
E a c h i g u a n a h e a d b o b t y p e r ep r e s en t s a u n i q u e
c o m b i n a t i o n o f a p a r ti c u l a r f o r m , d e g re e o f
T a b l e H . M e a n , S t a n d a r d D e v i a t io n , a n d C o e f f i c i e n t o f
V a r i a t i o n ( C V ) o f S i g n a t u r e B o b C o m p o n e n t s . D u r a t i o n
M e a s u r e s ( D ) a r e S e c o n d s ; A m p l it u d e M e a s u r e s ( A ) a r e
P e r c e n t a g e o f H i g h e s t
Component Mean so CV
(D) PL T 1.061 0.107 10.068
(D) U1-P 0.722 0.076 10.588
(D) U2 1.235 0.322 26.095
To tal duration 3.0 12 0.419 13.900
A -I 98.043 3.346 3.413
A-2 56.310 9.203 16.344
A-3 92.752 5.245 5.655
A-4 28.911 11.909 41.192
A-5 69.857 11.289 16.160
A-6 62.120 13.006 20.937
~ U 2 b 4.725 0.993 21.025
s t e r eo t y p y , a n d p a t t e r n o f o c c u r re n c e . I n o r d e r
t o u n d e r s t a n d t h e d i f f e r e n t a s s o c i a t i o n s a m o n g
t h e s e d i s p l a y f e a t u r e s , i t i s h e l p f u l t o f u r t h e r
c o n s i d e r t h e u s e o f t h e d i s p l a y s , t h e p r o b l e m s
t h a t a n i g u a n a d i s p l a y i n g i n a f o r e s t h a b i t a t
m u s t s o l v e , a n d g e n e r a l s e l e c t i v e p r e s s u r e s
t h o u g h t t o i n f l u e n c e s t e r e o t y p y .
T h e
R o l l . B e c a u s e o f it s h i g h r e l a t iv e f r e q u e n c y
i n t h e e a r l y s t a g e o f t h e b r e e d i n g s e a s o n ,
i t s
e x a g g e r a t e d m o v e m e n t p a t te r n s , a n d i t s p e r -
f o r m a n c e f r o m h i g h l y c o n s p i c u o u s d i s p l a y p o s t s,
I c o n s i d e r t h e R o l l t o b e a n a d v e r t i s e m e n t d i s -
p l a y , a l e r t in g n e i g h b o u r s t o t h e p r e s e n c e o f a
r e s id e n t m a l e . I n t h e e a r l y p a r t o f th e b r e e d i n g
s e a s o n , a m a l e s u c c e s s f u l i n e s t a b l i s h i n g a t e r r i -
t o r y a n d a t t r a c ti n g f e m a l e s m u s t m a k e h i s
l o c a t i o n a n d s t a t u s k n o w n . A s t h i s p e r i o d
c o i n c i d e s w i t h t h e e n d o f t h e r a i n y s e a s o n , t h e
f o l i a g e i s m u c h t h i c k e r t h a n a t o t h e r t i m e s o f t h e
y e a r . T h e R o l l a p p e a r s t o b e u n i q u e l y s u i t e d t o
o v e r c o m e o b s t a c l e s t o v i s i o n . T h e c o n s t a n t
r h y t h m o f t h e h e a d a n d d e w l a p s t a n d s o u t
a g a i n s t t h e b a c k g r o u n d o f o t h e r e n v i r o n m e n t a l
' n o i s e ' , s u c h a s l e a v e s s w a y i n g i n t h e b r e e z e . A s
i g u a n a s a r e d i s p e r s e d a t t h i s t im e , a n a d v e r t i s e -
m e n t d i s p l a y m u s t t r a n s m i t o v e r so m e d i s ta n c e .
V a r i a n c e i n s i g n a l p e r f o r m a n c e s h o u l d b e l o w
i n a l o n g d i s t a n c e s i g n a l , t h e r e b y f a c i l i t a t i n g
a c c u r a t e d e t e c t i o n o f t h e s i g n a l f r o m i t s b a c k -
g r o u n d ( M a r l e r 1 97 7). I n k e e p i n g w i t h t h i s p r e -
d i c t i o n , t h e R o l l i s m o r e s t e r e o t y p e d t h a n t h e
S h u d d e r , a d i s p l a y u s e d p r i m a r i l y i n i n te r -
a c t i o n s .
T h e c l os e t e m p o r a l r e l a ti o n s h ip b e t w e e n t h e
R o l l a n d t h e S i g n a t u r e B o b s u g g e s t s a s e c o n d
T a b l e H I . S t a n d a r d i z e d D i s c r i m i n a n t F u n c t i o n C o e f -
f i c ie n t s ( d ~ s ) S h o w i n g t h e R e l a t i v e C o n t r i b u t io n o f t h e
V a r i a b l e s t o E a c h D i s c r i m i n a n t F u n c t i o n ( d r s )
d~
Components d f d f 2 d f 3 d f 4
U1 -P
P L T
A -I
A-2
A-3
A-4
A-5
A-6
~ U 2 b
Percentage of
variance among
individuals 55.5
Cumulative p e r c e n t a g e
--5.349 --3.431 --2.017 0.1 41
--7.936 3. 94 5 --1.934 0.981
--0.027 --0.018 --0.070 0.1 33
0 . 0 0 5 0 . 0 7 7 0 . 0 3 3 -- 0. 03 7
--0 .033 - -0 .023 0 .0 8 2 0 .013
--0 .036 0 .0 15 - -0 .087 0 .006
0.002 --0.028 0. 0 27 --0.052
0.036 --0.005 --0.020 --0.052
0.416 0 . 31 4 - -0 .269 - -0 .069
16.7 11.6 6.8
72.2 83.8 90.6
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336 A N I M A L B E H A V I O U R , 3 0 , 2
f u n c t i o n f o r t h e R o l l : i t m a y i n t r o d u c e t h e
S i g n a t u r e B o b t h a t i n v a r i a b l y f o l l o w s . S t a m p s
Ba r lo w ( 1 97 3) r e p o r t e d t h a t i n
Anolis aeneus
i n t r o d u c t o r y h e a d m o v e m e n t s w e r e m o r e l i k e l y
to p r e c e d e a S ig n a tu r e Bo b w i th i n c r e a s in g d i s -
t a n c e b e tw e e n i n t e r a c t a n t s . Th e y s u g g e s t e d t h a t
i n t r o d u c t o r y m o v e m e n t s f u n c t i o n t o c a l l a t t e n -
t i o n t o t h e S i g n a t u r e B o b . T h e R o l l m a y s e r v e
a s im i l a r f u n c t i o n i n
L iguana
s a y in g ' h e r e I a m ,
S ig n a tu r e Bo b t o f o l l o w ' . Th e a p p r o p r i a t e r e -
s p o n s e w o u ld d e p e n d o n t h e s e x o f th e r e c ip i e n t
o f t h e s i g n a l . M a le s m ig h t a v o id t h e d i s p l a y in g
m a l e o r a p p r o a c h t o c h a l l en g e h i m ; f e m a l e s
m ig h t a v o id , o r a p p r o a c h t o i n s p e c t , a p o t e n t i a l
m a t e . Th i s i n t e r p r e t a t i o n i s c o n s i s t e n t w i th t h e
c h a n g e i n r e l a t i v e p r o p o r t i o n s o f e a c h d i s p l a y
ty p e a c r o s s s e a s o n s . O n ly i n O c to b e r , i n t h e
i n i ti a l s ta g e s o f t h e b r e e d in g s e a s o n w h e n m a n y
la r g e m a le s w e r e s o l i t a r y , d id t h e Ro l l e x c e e d
t h e S h u d d e r i n p r o p o r t i o n o f t o t a l d i s p la y s g iv e n.
T h e S h u d d e r . S h a ll o w , ra p i d h e a d b o b b i n g h a s
b e e n r e p o r t e d i n a v a r i e t y o f i g u a n id l i z a rd s ( se e
Jenssen 1977) . In
L iguana
t h e S h u d d e r o c c u r s
i n
m a l e - f e m a l e in t e r a c ti o n s b o t h d u r i n g a n d o u t -
s i de o f t h e m a t i n g s e a s o n, a n d t h u s i s n o t c o n -
s id e r e d t o b e a c o u r t s h ip b o b . O u t s id e o f th e
m a t i n g s e a s o n , S h u d d e r s w e r e m o s t c o m m o n l y
o b s e r v e d w h e n a m a l e a n d f e m a l e m o v e d c l o s e
t o o n e a n o t h e r w h i l e b a s k in g . T h i s d i s p l a y m a y
s ig n a l t h a t t h r e a t o r a t t a c k i s u n l i k e ly , a n
e x p l a n a t i o n t h a t i s i n k e e p in g w i th s u g g e s t i o n s
t h a t t h e S h u d d e r e v o l v e d t o a l l o w c lo s e c o n t a c t
b e t w e e n m a l e s a n d f e m a le s ( S t a m p s B a r l o w
1973; Barlow 1977).
A s o b s e r v e d i n o th e r i g u a n id s ( s e e r e v i e w in
J e n s s e n 1 9 7 7 ) , l o c o m o t io n m a y o c c u r s im u l -
t a n e o u s l y w i t h S h u d d e r m o v e m e n t s , b u t n e v e r
w i th o th e r h e a d b o b d i s p l a y s . I n 6 1 o f 68 f ilm e d
c a s e s i n w h i c h l o c o m o t i o n a n d S h u d d e r s o c -
c u r r e d t o g e th e r , t h e d i s p l a y in g i g u a n a w a s
a p p r o a c h in g a f e m a le . I f t h e S h u d d e r i n d i c a te s a
m a le ' s l a c k o f a g g r e s s iv e n e s s , t h e r e b y r e d u c in g
th e p r o b a b i l i t y t h a t t h e f e m a le w i ll r e t r e a t a t h i s
a p p r o a c h , t h e a b i l it y t o d i s p l ay a n d a p p r o a c h
s i m u l t a n e o u s l y w o u l d h a v e a n o b v i o u s a d v a n -
tage .
T h e S h u d d e r i s th e l e a s t s te r e o t y p e d o f ig u a n a
h e a d b o b d i s p l a y s. M a r l e r ( 19 77 ) s u g g e s t e d t h a t
g r e a t e r v a r i a b i l it y , a s w i th g r a d e d d i s p l a y s , p r o -
v i d es t h e p o t e n t ia l t o c o m m u n i c a t e m o r e r e fi n e d
i n f o r m a t i o n o v e r s h o r t d i s t a n c e s a n d t o a d a p t
t o f e e d b a c k f r o m a r e s p o n d e n t . S in c e t h e i n t e r -
a c t i o n s b e t w e e n m a l e a n d f e m a l e i g u a n a s w e r e
n o t a n a l y s e d , M a r l e r ' s h y p o t h e s i s c a n n o t b e
e v a lu a t e d f u l l y . H o w e v e r , u n l i k e s o c i a l s i g n a l s
t h a t e x h ib i t a ' t y p i c a l i n t e n s i t y ' ( M o r r i s 1 95 7) ,
t h e d u r a t i o n o f t h e S h u d d e r is n o t c o n s t a n t
a c r o s s s o c i a l c o n t e x t s . I t s d u r a t i o n i s i n v e r s e ly
r e l a t e d t o d i s t a n c e a n d i s s i g n i f ic a n t l y g r e a t e r
d u r i n g M a l e - F e m a l e I n t e r a c t i o n s t h a n i n n o n -
i n t e r a c ti o n a l c o n te x t s. A s s u m i n g t h a t t h e m a l e ' s
l e v e l o f a r o u s a l i s g r e a t e r d u r in g c o u r t s h ip t h a n
w h i l e r e s t i n g n e a r a f e m a le , t h e n t h e d u r a t i o n o f
t h e S h u d d e r m a y p r ov i d e i n f o r m a t i o n a b o u t t h e
m a le ' s l e v e l o f a r o u s a l . S h u d d e r in g s e e m s t o
r e f l e c t a r o u s a l l e v e l s i n s c e lo p o r in e i g u a n id s a s
w e l l ( Ru b y 1 97 7; R o th b lu m J e n s s e n 1 97 8) .
S h u d d e r d u r a t i o n m a y a l s o r e d u c e a m b i g u i t y
r e g a r d i n g t h e m a l e ' s i n t e n t i o n . T h e m o s t d i s -
t inc t ive d i f fe rence obse rved in a la rge ma le ' s be -
h a v i o u r i n t h e G e n e r a l S o c ia l a n d M a l e - F e m a l e
I n t e r a c t i o n c o n t e x t s w a s t h e d u r a t i o n o f t h e
S h u d d e r . O th e r w i s e t h e m a le b e h a v e d s im i l a r l y
i n b o t h c o n t e x ts : t h e S h u d d e r w a s t h e m o s t
f r e q u e n t l y o c c u r r i n g d i s p l a y , t h e m a le w a s o f t e n
c lo s e to m o r e t h a n o n e fe m a l e , a n d m o v e m e n t
to w a r d f e m a le s o c c u r r e d i n a v a r i e t y o f c o n t e x t s .
I n e f f ec t, t h e g r e at e r d u r a t i o n o f t h e S h u d d e r m a y
b e e q u i v a l e n t t o t h e m a l e ' s s a y i n g: ' I a m t a l k i n g
t o y o u ' . T h i s m e s s a g e , a l o n g w i t h o t h e r c o n -
t e x tu a l c u e s ( e . g . d i s t a n c e f r o m m a le , m a l e
a p p r o a c h e s f e m a le ) m a y f a c i l i t a t e t h e f e m a le ' s
r e c o g n i t i o n t h a t s h e i s t h e r e c ip i e n t o f t h e m a le ' s
a t t e n t i o n .
T h e R o l l - s h u d d e r . A g r o ss e x a m i n a t i o n o f t h e
c o n t e x tu a l c o r re l a t e s o f t h e R o l l - s h u d d e r i n d i -
c a te s t h a t i t d o e s c o n v e y i n f o r m a t i o n a b o u t t h e
r e s p o n s e t e n d e n c i e s o f th e d i s p l a y in g i g u a n a : a
c h a n g e f r o m o n e s e t o f a ct i v it i es t o a n o th e r s e t
o f a c t i v it i e s is h ig h ly l ik e ly . M y im p r e s s io n w a s
t h a t o n e c o u l d p r e d i c t t h e d i r e c t i o n o f t h e
b e h a v i o u r a l t r a n s i t i o n t o b e m a d e b y t h e d i s -
p l a y e r b a s e d o n t h e b o b s e q u e n c e . F o r e x a m p le ,
a R o l l t o S h u d d e r s e q u e n c e i s l i k e ly t o p r e c e d e
m o v e m e n t i n t o a m o r e s o c ia l c o n te x t , w h e r ea s a
S h u d d e r t o Ro l l s e q u e n c e i s m o r e l i k e ly t o p r e -
Table IV. C onditional F-values o f Signature Bob Components Ran ked in Descending
Order. Al l Values are Significant at P < 0.001
d f
12 237
PLT A-2 A-4 7~U 2b A-3 A-5 U 1 -P A-6 A-I
71 .38 28 .11 26 .4 6 22 .4 7 13 .6 7 13 .4 2 10 .27 7 .29 5 .87
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DUGAN: FIELD STUDY OF IGUANA HEADBOB DISPLAYS 337
cede leaving a social situation. However, more
complex sequences occurred, such as the
Shudder-Roll-Shudder sequence shown in Fig.
1D. The performance of such a sequence while
close to, but not interacting with, a female may
reflect ambivalence on the part of the male
regarding approach versus withdrawal.
Since the Roll-shudder is an apparently vari-
able intergradation between the Roll and the
Shudder in its form, variability and use, one
might ask whether it should be classified as a
separate display. Due to the small sample of
filmed Roll-shudders, it could no t be determined
whether the behaviour consists of a very few
fixed sequences or represents a range o f variable
combinations. It seemed that the form(s) of the
Roll-shudder would most likely fit Smith s (1977)
category of variable display sequencing, in which
patterns of behaviour have not been formalized,
and that the head movements should not be
classified as a separate display.
The Signature Bob. Signature Bobs in L iguana
as in other iguanids, occur in a variety of cir-
cumstances such as aggressive and courtship
encounters, territory advertisement or assertion,
and after locomotion. Based on the display s
pattern of occurrence, it fits the message cat-
egory that Smith (1977) termed general set
alternatives . These displays are always associ-
ated with some definable activity, but give no
information about the activities that are being
changed or the alternatives replacing them. They
occur in a wide variety of circumstances, and
information concerning the probability of future
behaviour is derived from the context.
The Signature Bob is more stereotyped than
most MAPs (Barlow 1977) in duration measures
made on Unit 1, total duration, and two of the
amplitude measures (Table II). Stereotypy and
species uniqneness are common characteristics
of iguanid Signature Bobs, and a species recog-
nition function is usually ascribed to these dis-
plays (see review in Ferguson 1977).
Although experimental support is lacking,
Signature Bobs may communicate individual as
well as species identity. Individual recognition
should evolve if advantages are derived from
discrimination between comparably detected
signals. Iguana iguana is territorial only during
the breeding season, when nearest neighbours
are 25 to 100 m distant. Although movements of
large males overlap spatially at other times of the
year, temporal overlap is rare. Individual recog-
nition might allow a male to avoid a larger,
dominant animal, and thereby avoid a potentially
costly encounter. Individual recognition might
also facilitate mate acquisition. Where males
are spread apart and females pass through the
territories of several males before mating, the
female must presumably be able to remember
and compare displays from several suitors
(Brander 1967; Shepard 1975). Since much early
courtship between iguanas occurs in locations
other than those selected for territories in late
November, remembering the male s identity,
rather than the locations of early courtship
interactions, would aid the female in her final
choice of mate.
The Signature Bob resembles the Roll in tha t
it is an exaggerated headbob that is conspicuous
in its patterned rhythm, high amplitude head
movements and dewlap motion. Given this
similarity, one might ask why the Roll has
evolved, and why it prefaces a similarly con-
spicuous display in territorial advertisement.
However, although each Signature Bob compo-
nent reliably discriminated among individuals,
the most distinctive part of the Signature Bob,
Unit 1, occurs at the beginning of the display.
If individual recognition is important, then the
introduction of the Signature Bob with a Roll
should ensure that Unit I is noticed by con-
specific observers from a distance. The question
remains, why were not all Signature Bobs pre-
ceded by a Roll? Presumably, activities per-
formed in other contexts serve to introduce the
Signature Bob. In contrast, the Roll appears to
be specialized to introduce the Signature Bob
in one context only: advertisement.
Conclusion. Signal transmission in L iguana is
accomplished with a highly redundant system
of visual signals, including selection of con-
spicuous trees and conspicuous perches in those
trees, frequent performance of conspicuous dis-
plays, and colour intensification. The redundancy
in the system may be viewed as an adapta tion to
a very complex habitat , where both distance and
visual obstacles must be overcome. Among forest
birds and monkeys that require signals to main-
tain intergroup spacing, audi tory exchange is
common since it is less hindered than vision by
obstacles. The importance of visual signals and
the variability of the signals tend to increase in
close-range, intragroup communication (Marler
1977). Iguanid lizards do not have the option of
using auditory signals. They must depend for
the most pa rt on visual communica tion, particu-
larly for long distance signaling. However, it
appears that patterns of variation in visually
communicating iguanas parallel such patterns
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338
A N I M A L B E H A V I O U R , 3 0 , 2
in species living in similar environments but
employing other sensory modalities for com-
munication
cknowledgments
Based in part upon a dissertation submitted in
partial fulfillment of the requirements for the
Ph.D. degree at The University of Tennessee.
This research was supported by NSF Grant
BNS-77-11344 by grants from the Smithsonian
Tropical Research Institute Sigma Xi The Uni-
versity of Tennessee Department of Psychology
and by NSF grants BNS-75-02333 and BNS-78-
14196 awarded to G. M. Burghardt. I owe many
of the ideas in this paper to discussions with
A. S. Rand. 1 thank H. Molina and J. Philpot
for statistical advice and G. M. Burghardt A. S.
Rand and T. A. Jenssen for helpful comments
on various drafts of the manuscript.
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