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6 6 BULLETIN CARNEGIE MUSEUM OF NATURAL HISTORY NO. 32
An additional series of evolutionary modifica-
tions that is presumably associated with male dis-
play behavior are associated with lateral tail com-
pression in the common ancestor of C. dickersonae,
C. grismeri. C. bicinctores, C. vestigiurn, and C. in-
sularis (Fig. 3 18, 32A-D). This character complex
includes the derived acquisition of dorsal and ven-
tral caudal fat bodies as well as modifications of the
neural and haemal arches and transverse processes
of the caudal vertebrae. Lateral tail compression
presumably increases the apparent size of adult males
in lateral view. The evolution of sexual dichroma-
tism, the acquisition and modification of black color
pattern components that are restricted to males, and
the development oflateral tail compression in males
each suggest an increase in the importance of male
display within Crotaphytus.
Bipedalism. -The form of bipedalism present in
Crotaphytus appears to be unique among iguanians
(see below). Several morphological modifications
within the genus appear to be related to this behav-
ior, including the loss of autotomic fracture planes
of the caudal vertebrae (character 39), the modifi-
cation of the skin of the distal portion of the tail
such that the skin may easily slip free (character 52;
Fig. 34), the acquisition of lateral tail coiling be-
havior (character 87; Fig. 36), and the contact of the
medial and lateral plantar tubercles of the fifth meta-
tarsal such that they form an arch (character 45; Fig.
17). The reference to the last character requires some
explanation. Snyder (1 952, 1954, 1962) observed
that M. gastrocnemius is usually slightly larger in
bipedal lizards than in quadrupedal species. Al-
though he emphasized that the differences in muscle
mass between quadrupedal and bipedal lizards are
not usually great, he noted that M. gastrocnemius
was conspicuously larger in Crotaphytus than in any
other quadrupedal or bipedal lizard that he exam-
ined (Snyder, 1962). Because M. gastrocnemius in-
serts on the medial and lateral plantar tubercles, it
is possible that the arch structure found in Crota-
phytus increases the surface area for insertion of this
muscle.
Crotaphytus utilizes a unique form of bipedal lo-
comotion, wherein individuals jump bipedally from
rock to rock on the boulder-strewn hillsides that
they inhabit. This saltatory form of bipedalism al-
lows them to move rapidly over a complex substrate
and, presumably, an individual would be at a dis-
advantage if it were not able to maintain a bipedal
gait. Snyder (1 949, 1954, 1962) found that the tail
of Croraphytus acts as a counterbalance during bi-
pedal locomotion and that the removal of between
25 and 33 percent ofthe tail prohibits a bipedaI gait
for more than three to five strides, while the removal
of more than 50 percent prevents bipedal locomo-
tion for more than one step. This may have been
the selective factor that lead to the loss ofautotomic
fracture planes in the common ancestor of Crota-
phytus. However, the tail of Crotaphytus is very long
and it seems likely that there would be strong se-
lective pressure to prevent predators from capturing
them by this appendage, especially given that the
tail cannot be broken easily (tail breakage can still
occur, but requires an intervertebral separation or
a fracture of the caudal vertebra itselc Etheridge,
1967). At least two evolutionary modifications have
occurred in Crotaphytus that appear to play a role
in minimizing predation by "tail capture." First, the
lateral tail-coiling behavior utilized by Crotaphytus
when taking refuge from predators beneath rocks or
surface debris, during hibernation, and when resting
beneath stones appears to function as a means of
keeping the tail out of the reach of potential pred-
ators. Second, the presence of loosely adherent skin
over the distal approximately 20 percent of the tail
allows the skin of the caudal terminus to slip off
when grasped, thus providing an alternative to cau-
dal autotomy over the portion of the tail the lizard
can lose without hindering its ability to run bipe-
dally. Once the skin is removed, the underlying ver-
tebrae and soft tissues wither and are lost. This hy-
pothesis for the function of the loosely adherent
caudal skin is based on three separate instances in
which 1 experienced this phenomenon while at-
tempting to capture lizards, as well as on the ob-
servation of numerous museum specimens that lack
the skin of the distal portion of the tail.
Gravid Coloration. -Gravid coloration occurs in
all crotaphytid taxa and a similar color pattern de-
velops in subadult male Crotaphytus (character 59).
Although gravid coloration itself may be a plesiom-
orphic retention of Crotaphytidae, the subadult male
coloration is almost certainly derived (see above).
The coloration of the subadult males, which devel-
ops soon after hatching and fades just before ma-
turity (Fitch, 1956; Rand, 1986) is virtually indis-
tinguishable both in terms of its chromatic char-
acteristics and in its anatomical placement and,
therefore, suggests that the young males may be
mimicking females in order to incur some selective
benefit. In fact, the presence of this coloration in
young males may provide a clue to its function both
in females and in the subadult males themselves.