How often do lizards "run on empty"?

Energy balance is relevant to diverse issues in ecology, physiology, and ev olution. To determine whether lizards are generally in positive energy bala nce, we synthesized a massive data set on the proportion of individual liza rds (N = 18 223) with empty stomachs (127 species), representing nine famil ies distributed on four continents, primarily in temperate zone deserts but also in the neotropics. The average percentage of individuals with empty s tomachs is low (13.2%) across all species, even among desert lizards, sugge sting that most lizards are in positive energy balance. Nevertheless, speci es vary substantially in this regard (among all species, 0% to 66% of indiv iduals have empty stomachs). Several patterns are detectable among species with unusually high frequencies of empty stomachs. In particular, nocturnal lizards "run on empty" more often on average than do diurnal species (24.1 % vs. 10.5%); and this pattern holds even for nocturnal vs. diurnal geckos (21.2% vs. 7.2%, respectively). Several (but not all) top predators have a higher frequency of empty stomachs than do species that feed at lower troph ic levels. Diet breadth and body size appear unrelated to frequency of empt y stomachs. Widely foraging species sometimes have a high frequency of empt y stomachs relative to sit-and-wait species, but patterns vary among contin ents and appear to be confounded by phylogeny and trophic level. Ant-eating specialists have uniformly low frequencies of empty stomachs. Diurnal term ite specialists also have low frequencies of empty stomachs, but nocturnal ones have high frequencies. Lizards from certain families (Gekkonidae [incl uding Pygopodidae],Gymnophthlamidae, and Varanidae) are more likely to have empty stomachs than an those of other families (Agamidae, Iguanidae, Lacer tidae, Scincidae, and Teiidae).