Summary: LOCOMOTOR PERFORMANCE AT LOW TEMPERATURE AND THE EVOLUTION OF NOCTURNALITY
IN LIZARDS. K. Autumn. Univ. of California, Berkeley.
Nocturnal lizards provide an ideal model system in which to study the effects of a substantial evolutionary shift
in environment. Lizards are ancestrally diurnal and the majority of lizard species, genera, and families have
remained diurnal. Nocturnality requires activity at body temperatures 10-30_C lower than does diurnality. Be-
cause ectotherms are profoundly affected by changes in body temperature, this is potentially a major obstacle
to nocturnal activity. Optimality theory predicts the coadaptation of thermal optima and activity temperatures.
However, I have shown that nocturnality imposes a thermal handicap which constrains growth rate and en-
durance to submaximal levels. Physiologically, activity at low temperature reduces the maximal rate of oxy-
gen consumption, and therefore the maximum aerobic speed, which in turn reduces endurance capacity.
Data from several gecko species shows that these nocturnal lizards have excellent fuel economy (Cmin 1/2
to 1/3 that of phylogenetically comparable diurnal lizards of similar mass) which partially offsets the thermal
handicap. If a low Cmin is an adaptation to nocturnality, the equilibrium approach to the study of adaptation
predicts that a low Cmin will be correlated with nocturnality and a normal (or high) Cmin will be correlated with
diurnality in various taxa. The phylogenetic approach predicts that a low Cmin and nocturnality will be shared,
derived characters. The historical pattern of nocturnality in lizards favors the phylogenetic approach. Concor-
dance not convergence will provide the test of adaptive hypotheses concerning the evolution of nocturnality