Abstract
A hybrid zone involving Cnemidophorus tigris punctilinealis (formerly
gracilis) and C. tigris marmoratus in southwestern New Mexico
and adjacent Arizona is narrow and characterized by abrupt and concordant
change in both morphological characters and allele frequencies studied
by protein electrophoresis. We compared adult C. tigris sampled
from three locations that span the hybrid zone. Body mass was positively
associated with both treadmill endurance at 1.0 km/h and maximal sprint
running speed on a high-speed treadmill, although the largest individuals
were not the fastest sprinters. Males and females differed significantly
for maximal sprint running speed, liver mass, and kidney mass (ANCOVA with
body mass as covariate). We found no statistically significant population
differences for body mass, maximal sprint running speed, standard metabolic
rate at 40C, blood hematocrit levels, or heart mass. Hybrids tended to
have lower treadmill endurance running capacities as compared with the
pure forms, but the difference was not statistically significant. Cnemidophorus
tigris punctilinealis and the hybrids both had significantly heavier
kidneys, relative to body mass, than did C. tigris marmoratus. Hybrid
individuals also had significantly heavier livers as compared with either
pure population. However, the present data cannot rule out the possibility
that the observed differences in organ masses were related to reproductive
status as opposed to being genetically based population differences. Thus,
our results do not suggest that hybrid individuals differ from nonhybrids
with respect to Darwinian fitness.