Sociality, stress, and the corpus striatum of the green anolis lizard
Department of Ecology and Evolutionary Biology, University of Tennessee, Walters Life Science Building, Room F-241, Knoxville, TN 37996, USA
Received 4 April 2003; accepted 17 April 2003
The green anolis lizard, Anolis carolinensis, is a uniquely convenient species with great potential for providing insights about the causes
and consequences of social behavior from an evolutionary perspective. In this species, social interactions are mediated by visual displays in
which specific units of behavior are combined in various ways to communicate several more-or-less specific messages. Two related research
programs that utilize this species converge in provocative ways to provide insight into this phenomenon. The first program is centered on the
basal ganglia, now known to be crucial to the expression of aggressive territoriality in this species, and the second research program examines
the way the physiological stress response is involved in aggression and its subsequent adaptive outcomes. Both the neural and the
neuroendocrine systems affect the progress of social interactions as well as the subsequent social dominance relationships when combatants
subsequently live together. Further, because body color depends almost exclusively on the stress response, skin color provides a unique in
situ bioassay of otherwise inaccessible information about the animal’s internal state. The fullest understanding of the physiological ethology
of this model species will depend on an interdisciplinary approach that considers both proximate (physiological) and ultimate (evolutionary)
causes of displays. Questions thus arising include how the nervous system controls and assembles the specific units of behavior—motor
patterns and autonomic reflexes—into displays that are adaptive in specific contexts.
D2003 Elsevier Inc. All rights reserved.
Keywords: Sociality; Stress; Corpus striatum; Basal ganglia; Behavior; Social behavior; Display; Anolis
Between stimulus and action, the intervening neurobiol-
ogy of display behavior is poorly understood. Displays of
more-or-less complexity are manifest in all taxa and are
often presumed to represent or have evolved from chains of
reflexes or fixed action patterns (FAPs). The adaptive value
of modulating and coordinating such behavioral patterns to
help organisms deal with vagaries, exigencies, and emerging
challenges of their environments is a major force in the
evolution of the brain. Among the best studied of these
behavioral patterns are social displays, and among the most
interesting of these displays are those of lizards. Among
lizards, the green anole, Anolis carolinensis, may be the
Two ideas converge in this brief account of the neuro-
ethology of display behavior in the green anole: the role of
the basal ganglia in the coordination and expression of
social displays and the influence of the physiological stress
response on displays during and subsequent to aggressive
encounters. First, I will review the social behavior of the
green anolis lizard with an emphasis on units of behavior.
Then I will review and discuss brain research on the social
displays of the green anole inspired by and first done in
collaboration with Paul D. MacLean , and more recently
extended by Cliff Summers (e.g. Refs. [81,82]) and Lewis
Baxter (e.g. Ref. ). Next, research on the interplay of
stress endocrinology, brain, and behavior will be outlined.
This work followed the brain research but was inspired by
Daniel Lehrman and David Crews, and often researched in
collaboration with Crews (e.g. Ref. ). Finally, I’ll
comment on implications of these projects for understanding
the evolution of brain and behavior.
2. The green anole—a model reptile
The small, diurnal, arboreal lizard, the green anole
(Anolis carolinensis) is one of the most scrutinized lizards
in science, and a valuable model for several biomedical
research programs . Specific elements of social behavior
0031-9384/$ – see front matter D2003 Elsevier Inc. All rights reserved.
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E-mail address: firstname.lastname@example.org (N. Greenberg).
Physiology & Behavior 79 (2003) 429 – 440
Inventory of social behavior in A. carolinensis
Dewlap extension of gular flap produced by the erection of the retrobasal process of the hyoid apparatus upon the fulcrum of the basi-hyal
component (TCM) [fan]
Push-up a raising and lowering of the forebody by rhythmic flexion and extension of the forelimbs (TCM) [bobbing]
Four-leg push-up push-up performed with all four limbs (T)
Head nod vertical movements of the head, (submission, subordination), often coordinated with pushups [bobbing, assertion, signature] (TCM)
Rapid nod an oscillating vertical movement of the head, often following an arrhythmic nod+ pushups, occasionally appearing without
preceding display (C) [jiggling]
Sagittal expansion enlargement of the sagittal profile of the animal by lateral compression of the body (T) [lateral flattening, lateral compression;
with ‘‘arrhythmic’’ nod = challenge]
Extended throat enlarged profile of throat produced by erection of the basi-hyal component of the hyoid apparatus (T) [engorged throat]
Nuchal crest elevated ridge of tissue along the back of the neck (T)
Dorsal crest elevated ridge of tissue, slightly narrower than the nuchal crest, extending along the spine from the posterior margin of the nuchal
crest to the base of the tail. Occurs shortly after nuchal crest in prolonged interactions (T)
Gape wide sustained opening of jaws, often accompanied by tongue-gorge (TD)
Tongue-gorge tongue apparently enlarged and pushed forward along the floor of the mouth creating a ridge near the front of the mouth (TDM)
Tongue-out tip of tongue appears between loosely closed jaws (TM)
Tongue-touch apparent touching of substrate or specific target with tongue  (TM)
Air-lick tongue extruded but never contacts surface (TM)
Tail-writhe slow sinuous lashing movements of the distal tail (T) [tail waggling]
Tail-lash wide side-to-side sweeping movements of the tail from the base (TCDM)
Head-up-high head tipped upward from the neck at a right angle to the body axis; suggestive of arousal and active surveillance (T)
Head-down [chin-down] head pressed to the substrate; effected even if the movement is against gravity (T)
Brown body color, sometimes combined or blending into symmetrical areas of green (TCMD)
Green body color, sometimes combined or blending into symmetrical areas of brown (TCMD)
Dark brown body color (TD)
Blotchy green and brown coloration simultaneously but in asymmetrical patches; generally includes eyespot (TD)
Eyespot darkening of postorbital patch of temporal scales (TD)
Defecate extrusion of fecal material (TDM)
Cloacal discharge contents of cloaca discharged; may be fluid or feces (TD)
Lateral orientation sagittal plane of lizard is made to face (‘‘aimed’’ at) stimulus point, generally an adversary, by postural adjustment (T)
Face-off two lizards in mutual lateral orientation, generally facing opposite directions with their heads at right angles to their body axes (T)
[often with mutual circling = parallel advance and retreat]
Stalk slow cautious approach to stimulus (TM)
Limp – stalk slow cautious approach to stimulus, rear legs appear limp or stiff and are often dragged (T)
Lunge rapid short range movement of body towards stimulus; typically combined with bite (TDM)
Bite sustained gripping with teeth, frequently follows lunge (TCDM)
Circling mutual stalking during a face-off (T)
Jaw spar mutual attempts to orient gaping jaws in order to bite the jaw of the antagonist (T)
Jaw-lock mutual sustained bite of two antagonists’ jaws; accompanied by twisting (T) [interlocking bite]
Strut forward movement with stiff front legs creating a unique gait (C)
Neck-bend raising neck while nose tipped down; only seen in females (CM)
Neck-grip gripping the skin around the neck or shoulders of another lizard (C)
Straddle while maintaining neck-grip, one lizard (usually male) rests parallel next to and partly upon another lizard (usually female) (C)
Tail-tuck the base of the tail of a straddling lizard is tucked under the base of the tail of an adjacent lizard bringing cloacae into apposition (C)
Insertion insertion of hemipenis into the vent of tail-tucked lizard during apposition of cloacae
body axis perpendicular to stimulus point, head facing away
body axis perpendicular to stimulus point, head facing point (CM)
Rear legs-back rear legs extended back alongside tail (TDM) (contributes to crypsis)
Squirrel abrupt lateral movement to side of perch away from stimulus (TDM)
Posture change adjustments in body posture not associated with locomotion, predominantly head movement [visual surveillance, scanning] (TCDM)
Site change displacement of the body’s center of gravity; slow, deliberate movements of entire animal in habitat; may be positive, negative,
or indifferent [exploration, foraging] (TCDM)
Charge rapid approach towards stimulus (TD)
Escape rapid movement away from stimulus (TCD)
Allogroom bite and pull at loose slough on another lizard; slough usually ingested (M)
Autogroom bite and pull at loose slough which is usually ingested (M)
Food-steal lunge and bite at object held in the jaws of another lizard; object or part of object pulled or broken off ingested if possible (M)
Adapted and updated from Ref. . Behavioral units delineated from observations of lizard interactions. Letters in parentheses indicate the context(s) in which
a behavioral unit has been observed: T=territorial defense and fighting; C=courtship and mating; D=nonspecific defensive behavior; M=maintenance behavior.
[Terms in brackets are synonyms in the literature].
An updated annotated version of this table is maintained at http://notes.utk.edu/bio/greenberg.nsf.
N. Greenberg / Physiology & Behavior 79 (2003) 429–440430
have been studied and reported since the 1930s [21,25,68].
More recent detailed ethological accounts were prepared in
support of neuroethological studies of social behavior .
These and subsequent reports detailing the display behavior
of the species  and its behavioral ecology [47,49] as
well as behavioral endocrinology of reproductive patterns
[17,18] have contributed to a detailed inventory of behav-
ioral patterns characteristic of the species (Table 1).
2.1. Units of behavior
Many social displays are found to consist of multiple
units of behavior, the forms and coordination of which are
valuable sources of clues about the evolutionary background
to the behavioral pattern. An inventory of 50 units of
behavior (‘‘ethogram’’) associated with sociality has been
developed for the green anole (Table 1). It is important to
note that units of behavior in such lists must be identified
with as little reference as possible to function because it is a
common observation in comparative behavior studies that
similar behavioral patterns can serve very different functions
in different individuals or species (or in the same individual
at different times). The units in Table 1 are also identified
with respect to the life-history contexts in which they
appear—maintenance behavior (such as foraging, feeding,
defecating, grooming), aggression (territorial defense and
conspecific fighting), reproductive behavior (courtship, mat-
ing, egg-laying), and nonspecific defensive behavior. The
occurrence of specific units of behavior in either multiple
categories or in very restricted contexts suggests more-or-
less conservatism in their stimulus control as well as hy-
potheses about their proximate (physiological) causes and
consequences. For example, some units of display behavior
are commonly seen expressed in the absence of any specific
stimulus (dewlap) and suggest nothing more than elevated
arousal, while others (such as rapid nodding) are restricted to
very specific contexts that may require endocrine priming
and a specific stimulus (receptive female).
Most units of behavior fit the criterion of reflexes or
fixed action pattern (FAP). A reflex is often regarded as the
simplest of behavioral units. They are highly stereotyped
and can be chained together in cascades of highly complex
motor patterns. An FAP, on the other hand, refers to a more
complex ensemble of motor acts orchestrated into a perfor-
mance involving an unlearned stereotyped temporal and
spatial pattern. The term ‘‘fixed action pattern’’ is a mis-
leading translation of the original German, Erbkoordination,
which is more correctly rendered as ‘‘inherited movement
coordination’’ . When found to be species-typical, FAPs
are regarded much like a morphological trait as a distinctive
attribute of a particular species.
2.2. The social behavior of the green anole
What follows is an account of social behavior seen in
naturalistic laboratory vivaria. In many specific details,
particularly those involving FAPs and social displays, they
are much like those seen in the field. Still, there are
important differences (see Ref. ), apparently attributable
to the larger diversity of alternative actions available in the
field, but also as a result of the opportunities for closer
scrutiny in the laboratory. Ideally, findings in both kinds
of studies would inform each other in reciprocal fashion
In nature, male green anoles emerge from seasonal
inactivity and establish territories by aggressively compet-
ing with other males of the same species (see Ref.  but
also Ref.  for key differences between laboratory and
field). Species recognition and subsequent competition
usually involves exchanges of distinctive displays. When
aggressive, animals will face-off and begin circling each
other, displaying occasionally, possibly jaw-sparring (see
Table 1) and manifesting signs of acute stress indicated by
their body color changes (see below); only rarely is physi-
cally dangerous combat observed—the contest appears to
be one of stamina. Losers typically flee, but there is some
evidence that in nature they may remain in a winners
territory as a social subordinate. A typical first display
has been termed ‘‘assertion,’’ and includes a distinctive
pattern of vertical movements of the head performed with
more-or-less amplitude (head-nods) that provides the defin-
itive species-typical ‘‘signature’’ display for many lizards
, the green anole included. The assertion display con-
sists of such head-nods emphasized with coordinated push-
ups and accompanied by a brief dewlap extension (Tables 1
In the lab as in the field, males often ‘‘spontaneously’’
manifest assertion displays often as they move about
‘‘patroling’’ their territories. At least there is no external
stimulus the human observer can detect that might evoke
such displays. Such displays suggest elevated nonspecific
arousal rather than a response to any specific evocative
stimulus. The display also serves as an ‘‘advertisement’’. If a
male’s assertion display is observed by another male not
previously observed, that second male may call attention to
himself by reacting with his own sequence of head-nods
coordinated with push-ups and extension of the dewlap. The
resident, observing this, may then rapidly escalate its
display into ‘‘threat’’ (with extended throat only) or ‘‘chal-
lenge.’’ In this display, the elements of assertion are
complemented by extended throat and sagittal expansion
(of the body profile) and (in interactions of sufficient
duration) erection of nuchal and dorsal crests along the
back, all effectively enlarging the animal’s apparent size. If a
male’s display is observed by a female, on the other hand,
her head-nod response will cause the aggressive male to
switch to ‘‘courtship’’: he will approach the female with a
unique ‘‘strutting’’ gait punctuated by one or more series of
rapid nods. Interestingly, this head-nod display (no push-ups
or dewlap) is also occasionally performed by males defeated
in combat and are taken by some observers to express
‘‘subordination’’ (Table 2).
N. Greenberg / Physiology & Behavior 79 (2003) 429–440 431
Displays exchanged between two lizards are easily stud-
ied in the laboratory by carefully removing an opaque
divider between two vivaria in which the animals appear
acclimated. When two males, each the exclusive occupant of
adjacent vivarium, have their divider removed, they each act
as though the other is an intruder in their territory (details in
Ref. ). This procedure minimizes the stress of handling
or an observer effect. Reproductively competent males that
see each other in this way almost always respond to each
other with an assertion or challenge display. When territorial
males escalate their competition, a full ‘‘challenge’’ display
is seen. This is an assertion display (the species-typical
component) complemented by postural changes (‘‘modi-
fiers’’). An early response might also be ‘‘extended throat’’
but an experienced aggressor might rapidly effect the chal-
lenge display, in which an enlarged sagittal profile of his
body complements the assertion display to the intruder. As
aggressive encounters proceed, the male’s behavior is ac-
companied by autonomic responses: Body color may at first
darken rapidly and then revert back to green—but with a
critical difference, a dark eyespot will appear just rostral to
the eye. In some cases, the initial darkening does not occur
and the animal’s color changes quickly to green. In as little as
30 s, a crest of erectile tissue will appear along its neck and
back. The antagonist typically responds in kind and they
stalk each other with slow, deliberate, apparently tense
movements; the tips of their tails may twitch. Prolonged
encounters by evenly matched males may result in jaw-
sparring or (more rarely) jaw-locking and biting (Table 1).
Most commonly however, fights conclude with no trauma to
In the course of such extended interactions, the animals
appear to assess their position relative to each other. This
may be reflected in multiple changes between green and
brown body color, although once present, the eyespot will
remain. There is evidence that the eyespot can serve as a
signal that evokes sympathetic activation and inhibits ag-
gression in conspecifics . Body color ‘‘reversals’’ of
aggressively engaged lizards is attributable to highly ele-
vated epinephrine (EPI) and sometimes they go directly
from green to green with an eyespot and apparently skip the
intervening brown phase. Colors may then darken consid-
erably, and in (rare) extreme situations, colors may become
blotchy (Table 1). As aggressive posturing and displaying
subsides, the animal that is brown is probably the one
subdued, typically lowering their chins to the substrate
(head down). Even if clinging to the underside of a limb,
the ‘‘loser’’ will press his chin to the perch surface. The
winner climbs to the top of his perch and may perform a few
assertion displays with his head raised (head-up-high). The
head-lowering of losers and raising of winners (seen in
many reptile taxa) may be a potential evolutionary origin of
the bobbing display, corresponding to Desmond Morris’s
category of ‘‘alternating ambivalent movements’’ in his
analysis of ritualization .
After territorial confrontations, winners, apparently little
affected, return to their routine, while losers, if forced by the
vivarium or environmental circumstances to remain in sight
of the winner, change in obvious ways: They behave as
social subordinates, selecting lower perches, and do not
court females; they also manifest a brown body color most
of the time . There may be brief aggressive skirmishes
for another day or two, but by Day 3, the relationship seems
stabilized and such pairs can, in the laboratory, cohabit for
extended periods. The preference for lower perches and the
disinterest in females appear to be more an altered motiva-
tional state rather than a response learned in the presence of
the winning male—even when the dominant is removed,
subordinates may take up to 2 days to recover their former
The several types of social displays identified in the
green anole represent a more-or-less specificity in form and
of stimulus control. There is an apparently highly conser-
vative central species-typical element (the head nod), the
meaning of which is modified by the coordinated expression
of hormone-dependent or context-dependent display com-
ponents (Table 2). The display modifiers have been charac-
terized as ‘‘static’’ (such as crests or eyespot) or ‘‘dynamic’’
(such as a pulse of dewlap erection) by Jenssen .
2.3. Assembling the units of behavior
Taken together, the information in Tables 1 and 2 indicate
that the species-typical bobbing pattern, while stereotyped,
is evoked by a broad spectrum of situations but specific
units of behavior are added or deleted in more limited
contexts, presumably to modify the meaning of the display
[47,48].Head nods are commonly emphasized with fore-
limb movements and a brief dewlap extension, which as an
ensemble constitute the assertion (‘‘signature’’) display.
Complementing the bobbing pattern and observed in more
specific contexts are slight variations such as rapid nods
(‘‘courtship’’), a hindlimb contribution to the head-nodding
movement (four-leg push-up), erection of a fleshy nuchal
crest (‘‘challenge’’), or erection of parts of the hyoid
apparatus (‘‘assertion’’ or ‘‘threat’’). The hyoid can be
erected in two stages: extension of the long retrobasal
Shared elements of social displays in the green anole
Display Display components
‘‘Assertion’’ T T T
‘‘Threat’’ T T T T
‘‘Challenge’’ T T T T T
‘‘Courtship’’ T T T T
The ‘‘core’’ species-typical element of head-nod may be deleted from a
sequence of displays after an initial performance; the display component
dewlap is frequently deleted from displays of combative males in close
proximity to each other.
N. Greenberg / Physiology & Behavior 79 (2003) 429–440432
process will extend the dramatic red dewlap (an element of
the ‘‘assertion’’ display) and extension of the basihyal
element (the fulcrum upon which the retrobasal element
rests) will simply enlarge the apparent size of the throat
(extended throat, ‘‘threat’’). The expression of these and
related units of behavior is presumed to have become
progressively more precise and stereotyped because of an
advantage that precision confers, such as the correct iden-
tification of the species or gender doing the display or the
motivational state of the performer (but see Ref. ).
Autonomic responses include color changes (of which only
the eyespot appears to have signal value—see below).
3. Basal ganglia influences social displays of anolis
Social displays are of great intrinsic interest, but their
relative stereotypy and well-understood stimulus control
provides powerful models for structuring investigations of
neural mechanisms. The basal ganglia is of particular
interest because of their long historical association with
control of motor sequences. ‘‘Basal ganglia’’ is an alternate
term for an array of related structures called the striatal
complex, nicknamed the R-complex by Paul D. MacLean
(‘‘R’’ for ‘‘reptilian’’) because of its remarkable prominence
in reptiles, seemingly corresponding to evolutionary inno-
vations in behavior first seen in reptiles (see Ref. ). The
basal ganglia includes the corpus striatum (caudate and
putamen) and is sometimes termed ‘‘non-limbic’’ or dorsal’’
striatum. The putamen is so intermeshed with an afferent
projection (the globus pallidus,pallidum) that the two
structures are occasionally regarded together as the lentic-
ular nucleus. The nucleus accumbens is, along with the
olfactory tubercle, sometimes called the ‘‘ventral’’ or ‘‘lim-
bic’’ striatum. The caudate, putamen, and globus pallidus
are sometimes referred to collectively as the ‘‘neostriatum’’
while the nucleus accumbens, olfactory tubercle, and ventral
pallidum are called the ‘‘paleostriatum’’ (PS) . Closely
associated structures are the substantia nigra (possessing
reciprocal connections with the dorsal striatum) and the
ventral tegmental area (possessing reciprocal connections
with the ventral striatum) .
The reptilian PS was the focus of an investigation of
forebrain control of display behavior in the green anole.
Paul D. MacLean’s finding that lesions of the globus
pallidus in squirrel monkeys disrupts species-typical dis-
plays  and studies that indicated that species-typical
displays could be reliably evoked in a reptile [26,28]
converged at MacLean’s Laboratory of Brain Evolution
and Behavior at the National Institute of Mental Health on
an investigation of the function of the PS in green anolis
display behavior. Preceding investigations of forebrain
influences on reptilian behavior [20,77,83] used stimulation
techniques and provided clues about our candidate sites for
the neural control of displays but were not conclusive. With
the help of a forebrain atlas developed for A. carolinensis
, the paleostriatal complex was probed with micro-
The lesion studies took advantage of the absence of a
corpus callosum in this taxon, providing us with a natural
split-brain preparation. By making only unilateral lesions
and directing visual input to the lesioned or the intact
hemisphere, each individual served as its own progressive
matched control (; animal care and experimental proto-
cols in Ref. ). There was little concern about lateralized
brain function, but hemispheres to be lesioned were selected
at random. Recent findings of right versus left dominance in
control of lizard aggression  were not observed in these
Lesioned lizards recovered very rapidly and appeared
amazingly unaffected. Most animals remained alert, for-
aged and fed as normal, often expressing the assertion
display. Only when an intruding conspecific was provided
was a profound deficit observed . When vision was
restricted to the lesioned hemisphere, the subject remained
responsive to the presence of an intruder but was unre-
sponsive to the releasers of territorial aggression it provided
, a behavioral deficit that might be characterized as
More recent investigations of basal ganglia in social
displays of green anoles have been conducted by Lewis
Baxter , who initiated a series of experiments based on
his insight that the control of the stereotyped displays of
anoles shared some features with the control of obsessive–
compulsive disorder (OCD). Using the Anolis lizard model,
he showed that beyond a sharp increase in forebrain
serotonin during dominant displays and a decrease during
subordinate displays, there was an activation of dorsolateral
basal ganglia and deactivation of the ventromedial area.
Related experiments went further to analyze the subtypes of
serotonin receptors and their distribution in A. carolinensis,
confirming important commonalities with other taxa .In
a series of analyses on dopamine receptors, the occurrence,
distribution, and pharmacological specificity of dopamine
receptor subtypes were also seen to be similar to
those of mammals. One interesting difference, however, is
that neural tissue in the parts of basal ganglia outside the
ventral striatum characterized by D
types is largely separated, rather than co-mingled as in
mammalian basal ganglia .
4. Neurotransmitters in the behavior of anoles
The relatively stable changes in body color that accom-
pany reduced social status in green anoles indicate impor-
tant changes in endocrine tone. Might the differences in
the display behavior of social dominants and subordinates
be attributable to comparable changes in the brain? Cliff
Summers led a series of investigations that analyzed
specific neural structures isolated by micropunches through
slabs of brain tissue. The use of Coulochem electrode array
N. Greenberg / Physiology & Behavior 79 (2003) 429–440 433
high-pressure liquid chromatography allowed analysis of
specific sites in the brains of dominant and subordinate
males for indolamines, catecholamines, and their metabo-
lites (summarized in Refs. [78,79]). We learned that central
serotonin production and turnover is more rapidly activated
in losers of fights (destined to behave in a subordinate
way) than in winners. A closer analysis revealed that
serotonergic activity in dominants and subordinates had a
distinctive time course as well as a regional distribution in
the brain .
There is abundant evidence that changes in serotonergic
activity in the brain is associated with stress and subordinate
social behavior in many taxa including Anolis (see Refs.
[4,58]). In green anoles, the greatest serotonergic changes
were detected in the telencephalon of subordinate males.
One hour after a fight, the hippocampal cortex and nucleus
accumbens showed increased ratios of 5-hydroxyindole-
acetic acid/serotonin. Just as in earlier studies of the brain-
stems of these animals , the ratio gradually decreased as
the animal’s social status became consolidated, and within
one month, ratios had returned to normal. Measured in the
brains of lizards sacrificed at an hour, day, week, and month
following a fight, changes were seen to be more rapid in
dominant males. The patterns of serotonergic activation are
so similar in the hippocampus, nucleus accumbens, and
brainstem that a coordinated response may be involved in
mediating short-term social stress and aggression. Similarly,
medial and lateral amygdala exhibit corresponding but
delayed patterns in subordinate males, suggesting a coordi-
nated response in these regions mediating a longer-term
stress response (; summarized in Refs. [78,79]). Work
on free-ranging spiny lizards (Sceloporus jarrovi) provided
consilient findings: brain serotonin activity and turnover
was greater in nonterritorial males than in those holding
territories . Comparable findings in fish  and mam-
mals (, in primates) suggest a phylogenetically con-
served mechanism of monoamine behavioral modulation of
5. Stress in anoles
5.1. Body color in anolis is uniquely sensitive to stress
The body color changes seen in green anoles occasion-
ally during their maintenance and often when subjected to
any of a wide array of disturbances has given rise to their
popular nickname, ‘‘American chameleon.’’ The appear-
ance of a potential predator in the field or a careless
observation protocol in the lab will evoke a brown body
color. During aggressive interactions (as mentioned above),
color shifts also occur as interactions proceed. Both males
will likely develop eyespots early in their interaction, but
near their conclusion, probable losers are usually brown,
and winners green.
Unlike chameleons and other lizards investigated, body
color changes are attributable only to circulating hormones
(Fig. 1). A role for direct neural control was excluded in a
series of studies by Kleinholz [55,56]. This allows body
color in green anoles to serve as a partial in situ assay of the
endocrine tone of the chromoactive hormones: EPI, norepi-
nephrine (NE), and melanotropin (melanocyte-stimulating
hormone [MSH]). Several patterns of body color can be
distinguished (Table 1) that suggest the underlying acute flux
of circulating hormones. Body color can also be affected by
nonsocial activities such as predator avoidance, but generally
in contexts reasonably construed as stressful. A shift from
green to brown, or darkening involving speckling, and the
appearance of a small ‘‘eyespot’’ just behind the eye indicate
specific patterns of activation of a
- and h
Fig. 1. The effects of stress on a dermal chromatophore of A. carolinensis.
Acute and chronic stressors are integrated to cause the release of hormones
that interact with each other and converge in affecting the darkness of a
chromatophore. ACTH, adrenocorticotropic hormone; CS, corticosterone;
E, epinephrine; MSH, melanocyte stimulating hormone; NE, norepinephr-
ine. CS can elevate the ratio of E to NE by facilitating a key enzyme within
the adrenal gland. E stimulates receptors preferentially and then receptors
resulting in opposing effects (adapted from Ref. ).
N. Greenberg / Physiology & Behavior 79 (2003) 429–440434
(sympathetic elements of the acute stress response in Fig. 1),
5.2. Stress responses are both causes and consequences of
social dominance relationships
Stress is inevitably evoked in agonistic or competitive
behavior. Our research has shown that in the dominant –
subordinate relationships that are established by green
anoles in laboratory vivaria, the typically brown subordi-
nates have elevated circulating corticosterone levels 
and lowered androgen . Subordinates also manifest
changes in activity of central neurotransmitters consistent
with elevated stress and lowered aggression (see Refs.
[79,82] and references therein). It is significant that elevat-
ed circulating corticosterone levels can potentially result in
a shift in the ratio of EPI to NE released from the adrenal
chromaffin tissue because of its facilitation of a key en-
zyme in the conversion of NE to EPI. An increase in EPI
relative to NE is associated with behavioral patterns cha-
racteristic of fearful or subordinate animals. Further, rela-
tive autonomic tone of the two combatants may be im-
portant: the male that first manifests the eyespot invariably
wins the contest .
6. Prospects for insights about the evolution of brain and
My approach to understanding the evolution of brain and
behavior was to look at the influences of the basal ganglia
and the physiological stress response on the expression of
stereotyped social displays in the green anole. The display
repertoire of the green anole is much like other species that
involve a conservative ‘‘base’’ display evoked in many
contexts that serves as the core element in other displays
that are more precisely controlled and convey more specific
information. Such adaptive variations on a theme, to the
extent that their substrate is understood, can suggest specific
hypotheses about the mutual influences of brain and behav-
ior in evolution.
For example, how do the psychoactive properties of the
hormones involved in the physiological stress response
affect the pathway to the basal ganglia or one of its parallel
loops with the thalamus or cortex? Most displays are ‘‘motor
programs’’ and often involve both highly stereotyped as
well as more flexible elements, depending on the context
and stimulus. Motor programs range in complexity from
strings of reflexes through automatized learned behavior. An
additional influence, that of reproductive status and the
presence of relevant hormones, has been relatively neglec-
ted but is almost certainly deeply involved. For example,
sex steroid hormones, like those of stress, can affect
virtually every major component of the path from input to
action. How does control of a specific unit of behavior
‘‘shift’’ from internal control of a fragment of a motor
pattern or an autonomic phenomenon to external control by
a specific stimulus and or a specific environmental context.
More specifically, how do behavioral responses to indis-
tinct but arousing stimuli become progressively more
specific in their control? Here is where an understanding
of the stress response may be of value, since responses to
potentially challenging perturbations are often hierarchical-
ly arranged. In such a scheme, a minor disturbance evokes
a modest response, and progressively more challenging
disturbances evoke responses at progressively higher levels
6.1. Fixity and flexibility: how are the functions of units of
A perspective that can illuminate some of the most
compelling questions—those related to how units of behav-
ior come to have their communicative function—was en-
gaged by the ethologist Desmond Morris  almost 50
years ago. In his review of ‘‘ritualization,’’ the evolutionary
changes that result in communicative displays, Morris
identifies and describes somatic and autonomic units of
behavior. Somatic units such as fragments of motor pro-
grams and autonomic responses such as the green anole’s
body color changes, either individually or as a coordinated
ensemble, were initially associated with relatively nonspe-
cific phenomena. Among the autonomic responses, Morris
identified alimentary (changes in salivation, sphincter con-
trol, urination, defecation), circulatory (pallor, flushing,
vasodilation of organs, fainting), respiratory (changes in
rate or amplitude, sighing, panting, vocalizing), and ther-
moregulatory (panting, sweating, pilomotor) responses.
Morris also iterated the most common kinds of changes
that could occur to isolate or emphasize a unit of behavior,
including changes in thresholds, rhythmic repetition, exag-
geration of certain components of the movement, omission
of components, ‘‘freezing’’ of movements, changes in
sequence or in coordination of components, and change in
speed or vigor of performance. The known specific and
nonspecific effects of stress-related hormones on the ner-
vous system can contribute substantially to hypotheses
about how such changes are effected. For a recently dis-
cussed example, an acute stress episode can impair the
ability of ‘‘higher’’ neural centers to inhibit ‘‘conservative’’
patterns of behavior controlled by lower centers (see Ref.
). Basic information about how specific aspects of the
stress response affect specific neural areas may provide the
key to understanding the control and evolution of core
theme, variation, and how modifiers act in display reper-
toires. Although it is reasonable that adaptive variations in
the regional distribution of neurotransmitter and hormone
receptors play a large role in evolutionary change, there is as
yet little comparative data. Although of great intrinsic
interest for understanding the neuromodulatory influence
of experience on brain function, the basic information being
provided for the brain of the green anole by researchers such
N. Greenberg / Physiology & Behavior 79 (2003) 429–440 435
as Cliff Summers (see Ref. [78,79]) may prove to be of
comparably great value for comparative studies and insight
into evolutionary processes.
6.2. The stress response
Broadly construed, stressors are any of a large array of
real or perceived challenges to an organism’s ability to meet
its real or perceived needs. These challenges activate an
ensemble of coordinated physiological coping mechanisms
collectively called the stress response. Traditional defini-
tions of stress have historically been rooted in a medical
model and typically focus on coping with challenges to
homeostasis (for example, Ref. ). While arguably the
most compelling of needs, homeostasis is, in terms of an
animal’s Darwinian fitness, only the most urgent of several
needs. The broader definition used here avoids the limita-
tions of traditional models and more fully accommodates
Hans Selye’s original vision , as well recent views such
as that of McEwen’s , who sees stress as ‘‘a threat, real
or implied, to the psychological or physiological integrity of
an individual.’’ Similarly, Mac Hadley , in his popular
textbook, wrote, ‘‘Discrepancies between perceptions of
internal or external circumstances and innate or acquired
expectations lead to patterned stress responses....’’ Such
definitions (see also Goldstein  and Levine ) allow
the extension of insights from medically oriented research to
the growing interest in subclinical expression of stress and
its subtle if relentless influence on the evolution of life
histories (see Refs. [34,36]).
The stress response involves fairly well-understood
phases that provide both rapid response and long-term
accommodation. The rapid system involves an ensemble
of responses centered on the sympatho-adrenomedullary
system (SAMS), involving release of EPI and NE from
specialized extensions of the sympathetic nervous system,
adrenal chromaffin tissue (adrenal medulla in mammals).
Continued (or frequently repeated) stressors then activate
the hypothalamic– pituitary –adrenal (HPA) axis (Fig. 1).
Although the stress response is prominently associated with
coping with significant threats to survival, it is important to
note that many coping responses are ‘‘subclinical’’ and are
manifest mainly in modest, sometimes difficult to detect,
adjustments of tone in an endocrine or neurophysiological
system. Further, most hormones are ‘‘pleiotropic’’ in that
they have multiple effects some of which may be unrelated
to the phenomenon that evoked them (below).
6.3. Hormonal pleiotropy
It is significant that most hormones are pleiotropic—they
manifest multiple effects. Hormone release may have been
evoked in a specific adaptive context, but their other
(‘‘collateral’’) effects may or may not complement or
support the primary effect. In any event, they are available
to be transformed or incorporated into other adaptive traits,
including life-history habits or social displays (see Ref. ,
and references therein). This diversity of hormone effects, in
concert with variations in the distribution of receptors on
neurons in different parts of the brains of closely related
species (see, for example, Ref. ) suggests an important
emerging perspective on the evolution of species-specific
Relevant examples of the multiple—pleiotropic—effects
of hormones are provided by adrenal corticosterone and the
pituitary hormone that causes its release, corticotropin (ad-
renocorticotrophic hormone, ACTH); they each have inde-
pendent psychoactive effects that include amelioration of
aggressive responses, at least in rodents . In our lizard,
the stress of social subordination may be responsible for
reduced androgen  and reduced motivation to court.
When a dominant male is removed from a laboratory vivar-
ium that he has cohabited with a subordinate, the recovery of
interest in courtship may take many hours or even days .
Interestingly, if testosterone in subordinates is artificially
increased by means of an implant placed before the domi-
nant– subordinate relationship is established, the subordinate
will court as soon as the dominant is removed (unpublished
observations)—a situation that may be much more like that in
nature, and consistent with observations of the effects of
testosterone on arousal and attention (see Ref. ).
Stress results in elevated circulating corticosterone. One
potential consequence of the release of pituitary corticotro-
pin (ACTH) needed to stimulate release of this adrenal
steroid is a collateral release of melanotropin (MSH) 
and this is, in fact, detectable in the blood of subordinate
animals . Melanotropin has positive effects that aid in
growth and recovery from trauma and psychoactive prop-
erties that reduce aggression. In addition, subordinate ani-
mals select different perch sites than dominates where the
effect of the darkening effect of melanotropin on dermal
chromatophores may provide a significant survival advan-
tage (Ref.  and references therein).
In summary, stressors that challenge homeostasis, the
most urgent of needs, are the best known but by no means
the only experiences that can activate the stress response.
Further, the direct effects of coping mechanisms frequently
have collateral effects that may or may not reinforce each
other. Indeed, a collateral effect of a specific hormone might
well serve other needs. The evolutionary process is intel-
lectually fascinating in part because of its capacity for
making the most of available resources to serve adaptive
needs, a process sometimes nicknamed ‘‘bricolage,’’ after
the French term (bricoloeur) for a handyman able to make a
virtue of necessity.
6.4. Stereotyped behavior, stereotypies, stress, and the basal
Species-typical displays and clinical stereotypies are
related not only by the fixity of expression but by their
responsiveness to stress. All contexts in which green
N. Greenberg / Physiology & Behavior 79 (2003) 429–440436
anoles display reasonably involve elevated arousal and at
least a mild stress response. In other words, this ‘‘core’’
display (‘‘assertion’’) can be performed even in the ab-
sence of specific stimuli, but always in situations of ele-
Dysfunctional behavior such as stereotypies, addictions,
neuroses, and psychoses are all known to be affected by the
stress response. This is reasonable given the well-known
psychoactive effects of stress-sensitive hormones on alert-
ness and arousal as the organism under stress adjusts to
enhance its assessment of potential environmental stressors.
The physiological stress response, in its fullest expression,
can also affect integrative and efferent components of
behavior. Altogether, we can expect enhanced arousal and
vigilance, lowered sensory thresholds, increased attention
width and capacity for sustained attention, and conservatism
in the perceived salience of stimuli. These are all stress-
sensitive aspects of behavior (,Tab le 1), so it is
unsurprising that energized or aroused lizards may repeat
specific patterns frequently. But there is as yet no clarity as
to where in the circuit from input to output the stress
hormones are most active. Some clues are likely to emerge
from examinations of regional neurotransmitter changes
correlated with behavior [78,79] and regional changes in
metabolism detected by various imaging technologies (,
Clues will also emerge from fuller understanding of the
causes of clinical stereotypies in which repetition is clearly
inappropriate or dysfunctional. Most dysfunctional stereo-
typies are manifest in abnormal contexts such as zoos or
laboratories or as a result of stress where they are often
viewed as evoked by stress or an errant attempt at stress
reduction ([7,14,15] but see Ref.  for a critical review).
Such dysfunctional stereotypies may be unlike only in
degree from the adaptive expressions of stereotyped behav-
ior observed to be spontaneously expressed in natural
habitats. The form of such ethological stereotypies, often
correspond to the ‘‘fixed action patterns’’ of early etholo-
gists , which were presumed to be genetically deter-
mined responses to specific stimuli (a ‘‘sign stimulus’’ or
‘‘releaser’’). FAPs also resemble clinical stereotypies in that
although they may be shaped by external influences (and to
that extent ‘‘modified by experience’’ and therefore
‘‘learned’’), they complete themselves with relative inde-
pendence of external feedback—They will continue until
their pattern is concluded even though their functional ends
have been accomplished.
Why should we suspect that the performance of a
stereotyped display or even a dysfunctional stereotypy is
stress reducing? Real or perceived familiarity and a sense of
control are additional variables in the stress response that
must color an interpretation of anxiety. Recalling Seligman’s
views of the modulation of the stress response by perceived
helplessness (e.g., Refs. [73,74]), the apparent ‘‘controlla-
bility’’ of a stress-evoking situation is at the heart of Geralt
Huether’s  concept of a ‘‘central adaptation syndrome.’’
In Huether’s view, different coping strategies are effected
depending on the animal’s perception of the controllability
of the stressor. Controllable situations refine existing strat-
egies while uncontrollable situations can cause changes in
behavioral responsiveness and a reorganization of neural
circuits affecting learning—an ‘‘adaptive reorganization of
the associative brain.’’
Perceived controllability of a stressor was specifically
identified as an influence on the basal ganglia system’s
mesoaccumbens dopaminergic system , a phenomenon
that might be linked with emerging understanding of the
basal ganglia’s role in expectations [54,72].
The strategies of the ‘‘central adaptation syndrome’’ are
likely related to those of passive versus active coping
strategies evoked to cope with unescapable versus escapable
stressors discussed by Bandler et al. . In their work,
alternative autonomic strategies (sympathoexcitatory or
sympathoinhibitory) were correlated with activity in discrete
columns of the midbrain periaqueductal gray .
6.5. Basal ganglia connection: clues from dysfunction
Stereotyped displays have been compared to obsessive –
compulsive behavior , possibly associated with the im-
pairment of one of the several parallel thalamocortical loops
in which the basal ganglia participates. The architecture of
the motor loop, involves a direct pathway (ultimately
facilitatory) and an indirect (inhibitory) pathway (see Ref.
 for a brief review). In that respect, it is interesting that
OCD, like many other neuropsychiatric disorders, is exac-
erbated by the stress response. Alternatively, at least in some
cases, trauma to the striatum rather than reconfiguration may
be implicated in the pathophysiology of OCD. For example,
striatal neurons might be destroyed by prolonged immuno-
logic stress triggering a cross-reaction between antistrepto-
coccal antibodies and striatal neurons .
The several social displays of green anoles are stereo-
typed and more or less context-dependent. In their form the
recall motor plans, in which specific simple acts are
performed in set sequences . Sequential triggering can
be visual or proprioceptive feedback, but failing that,
internal cues can be generated by the motor system .
Interestingly, in Parkinson’s disease, the most prominent of
the degenerative disorders involving basal ganglia and
responsible for profound problems in motor control, the
deficits in sequencing attributable to faulty basal ganglia can
sometimes be overridden by external stimuli that demand
heightened arousal. This phenomenon, known as paradox-
ical kinesia led Brown and Marsden  to hypothesize that
the basal ganglia is integral to nonconscious attention.
Anne Graybiel’s  work, extending our understanding
of basal ganglia and its adaptive possibilities, has led her to
hypothesize that the sequences of units organized by central
pattern generators of the motor system are complemented by
‘‘cognitive pattern generators’’. She suggested that ‘‘by
analogy with the central pattern generators of the motor
N. Greenberg / Physiology & Behavior 79 (2003) 429–440 437
system ...these pattern generators operate to organize neural
activity underlying aspects of action-oriented cognition.
Disorders of the basal ganglia may thereby contribute to
neural circuit dysfunctions that are expressed as positive and
negative symptoms of schizophrenia.’’ A specific mode of
basal ganglia influence is indicated by the observation that an
apparent imbalance of activation between the neurochemical
zones of the striatum—the striosomes and the matrix in
which they are embedded—can result in stereotypies. When
psychomotor stimulants were applied in concert with dopa-
mine receptor agonists, the degree of motor stereotypy
manifest by rats could be predicted by the imbalance created
between activity of striosomes and their matrix .
6.6. Proximate and ultimate causation of behavior
It is an ethological truism that questions about ‘‘how’’
behavior is caused and regulated involve proximate factors
such as physiological mechanisms. ‘‘Why’’ questions, on
the other hand, emphasize the adaptive and therefore evo-
lutionary significance of behavior, the so-called ultimate
factors. Between these extremes, developmental and eco-
logical factors abound and must also be considered if
behavior is to be most fully understood. Ethology is a
profoundly interdisciplinary enterprise.
The proximate expression of behavior is frequently
viewed as the outcome of a hierarchical organization. The
most proximate causes of overt behavior are the activities at
a neuromuscular junction. Working backwards, then, from a
manifest action, we are often led to more central neural
structures and pathways. For example, beginning with the
ceratohyoid muscle that controls the anolis dewlap. A path
could be traced by a retrograde neuronal tracer back to the
motoneurons in the nucleus ambiguus, an element in the
brainstem motor system associated with pharyngeal and
laryngeal muscles  and with vocalization and swallow-
ing in higher vertebrates.
The specific paths that information takes from the afferent
stimulus to the efferent action are all more or less responsive
to modulating agents such as the powerfully psychoactive
hormones associated with the stress response. Motor systems
are often viewed as hierarchical, such that activation at a
relatively centrally located level of limited activation
diverges to affect progressively more peripheral levels until
they get to final expression. Unlike a simple ‘‘military’’
hierarchy, however, there is also converging information
from other sources of information as well as information
flow in the opposite direction to effect a feedback consoli-
dation or reconfiguring of the activity along the path.
Although the behavioral system in which causes and
consequences are envisioned is immensely rich and there
are numerous sites at which alternative actions and inter-
actions can be brought into play, their ultimate causation is
constrained by history and thereby limits the possible
evolutionary mechanisms we can propose. The ultimate
consequences of this richness, on the other hand, can only
be imagined. The manifest adaptive functions of displays,
with their expressions of fixity and flexibility tightly corre-
lated with specific environmental contexts, are among the
most likely of social phenomena to yield significant insights
into the past processes and future possibilities of the
coevolution of brain and behavior.
I am grateful to an anonymous reviewer and three recent
meetings that provided opportunities to explore the impli-
cations of the work and ideas reviewed here with colleagues
in allied disciplines. The catalytic effects of such interdisci-
plinary meetings cannot be underestimated. The Across
Species Comparisons and Psychopathology group met in
Boston, July 16 and 17, 1999 (proceedings published );
The Society for Integrative and Comparative Biology
(formerly the American Society of Zoologists) hosted a 3-
day symposium devoted to stress during their annual meeting
in Chicago, January 3 –7, 2001 (proceedings published ),
and the satellite meeting devoted to the implications of the
work of Paul D. MacLean, at the International Behavioral
Neuroscience Society in Capri, Italy, June 19–23, 2002 (this
issue of Physiology and Behavior). And above all, I must
acknowledge my gratitude for the empowering and inspira-
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