Diet of the Mexican Fringe-Toed Lizard (Uma exsul)

Abstract

Seasonal differences in environmental conditions, such as precipitation, appear to regulate food diversity and abundance, and thereby indirectly influence changes in the diet of some lizards (Ballinger and Ballinger, 1979). Whitford and Creusere (1977) showed that the availability of insect prey varied according to the distribution and amount of rain throughout the year, and Maury (1995) found that variability in prey diversity and abundance was correlated with the effect of precipitation on primary productivity. The diet of the genus Uma remains somewhat unclear (D. S. Turner and C. R. Schwalbe, unpbul.). Commonly, it has been considered a carnivore, or perhaps opportunistic omnivore, but the data are contradictory. In Sonora (Mexico), Ortega-Rubio et al. (1995) found no plant matter in the diet of Uma notata. They reported a purely insect diet. In sharp contrast, a study of the more distantly related Mexican species, Uma paraphygas, found plant parts in nearly 56% of the 153 lizards examined, providing more than 40% of the total prey volume for adult males in two of four seasons (Gadsden and PalaciosOrona, 1997). The purpose of our paper is to describe seasonal variation in the diet of an endemic Mexican lizard species, Uma exsul, and to investigate sexual differences in diet. The study was conducted in the sand dune systems within the Mapimian subprovince of the Chihuahuan Desert (25023'N, 103?30'W), in the dry Laguna de Viesca in southwestern Coahuila, Mexico (1100 m elevation; INEGI, 1988). The vegetation is dominated by Larrea tridentata and Suaeda nigrescens (Instituto Nacional de Estadistica, Georgrafia e Informatica, 1988). Average monthly temperature ranges from 13?C in January to 28?C during the summer. Average annual rainfall is 200 mm but varies greatly among years (Schmidt, 1979). In 1992, adult U. exsul were noosed or shot with BB rifles each month (8.3 + 4.0 individuals each month), immediately placed on ice, and subsequently preserved in 10% formalin. Lizards were collected between 1000 and 1600 h. Monthly samples for each season were combined (winter = January-March, spring = April-June, summer = July-September, and autumn = October-December). Stomach contents were removed, and food items