a field study of the headbob displays of male green iguana

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


4/12


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.


5/12

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).


6/12

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)


7/12

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.


8/12

334


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 .


9/12

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


10/12


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


12/12

338


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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(Received 25 February 1981 ; revised 15Ju l y 1981 ;

MS. number: A2614)

a field study of the headbob displays of male green iguana

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