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Diet of juvenile lizards, Uma exsul, from Coahuila, Mexico.

The diets of juvenile lizards can differ from those of adults, either in breadth or composition (Znari and El Mouden, 1997; Duffield and Bull, 1998; Azocar and Acosta, 2011; Raya-Garcia et al., 2015), although this is not always the case (Teixeira-Filho et al., 2003). In addition, the foraging ecology and profitability of neonate or juvenile lizards may differ from that of adults (Watters, 2010). However, few studies have examined large series of juveniles, nor are juvenile diets available for many species of lizards.

We examined the stomach contents of 43 juvenile Uma exsul from the Herpetological collection of the unidad de Biologia, Tecnologia y Prototipos, Laboratorio de Ecologia, Facultad de Estudios Superiores, Universidad Nacional Autonoma de Mexico, Iztacala, Mexico, Mexico. These specimens were collected from Dunas de Bilbao, municipio Viesca, Coahuila (25[degrees]25'26.7"N, 102[degrees]53,40.2"W; 1,115 m) on 13-14 July 2004 (Smith et al., 2005). The Dunas de Bilbao is an extensive sand dune area of approximately 10 [km.sup.2] located approximately 65 km east-southeast of the city of Torreon. The area is composed of active, semi-stabilized, and stabilized sand dunes. Individual U. exsul are typically found in the semistabilized sand dunes, characterized by sparse vegetation that includes mesquite (Prosopis glandule> sa) and a variety of herbaceous plants that often grow in dense clusters which serve as refugia for U. exsul (J. A. Lemos-Espinal, pers. observ.). Uma exsul is the most abundant and conspicuous reptile in this area (J. A. Lemos-Espinal, pers. observ.). Other reptile species in the study area include Gambelia wislizenii, Coleonyx brevis, Phrynosoma cornutum, Apsidoscelis marmorata, Uta stansburiana, Masticophis flagellum, Rhynochelius lecontei, Pituophis catenifer, Thamnophis marcianus, and Crotalus scutulatus.

Adult U. exsul have been described as generalists in terms of their diet (Gadsden et al., 2001). Gadsden et al. (2001) found that formicids were the most common item in the diet of adult U. exsul across the year, with other hymenopterans and hemipterans also being important seasonally. Uma exsul also respond to chemical cues of both plants and animals, suggesting a potentially broad diet (Cooper et al., 2006). Our goal was to compare the diets from the juvenile U. exsul that we studied to the previous study of diet in adult U. exsul to see if there are any indications they might differ.

We measured snout-vent length (to nearest 0.1 cm using a plastic ruler), head width (to nearest 0.01 cm using a digital caliper), and head length (to nearest 0.01 cm using a digital caliper) of each specimen (one specimen was inadvertently not measured, thus n for measurements is 42). We removed the stomach contents of each individual via dissection. We identified each prey item to the lowest taxonomic level possible (generally order) and measured the maximum length and width using a digital caliper (to nearest 0.1 mm). We used BugRun to estimate prey volume (using the equation of a prolate spheroid) as well as niche breadth on prey number and volume (Vitt and Zani, 2005). We calculated an importance value (IV) for each prey taxon (i) using the sum of the proportions of total prey items ([p.sub.Ni]), total prey volume ([p.sub.Vi]), and total number of stomachs ([p.sub.Si]) represented by prey item i (Powell et al., 1990):

[IV.sub.i] = [p.sub.Ni] + [p.sub.Vi] + [p.sub.Si]

Mean snout-vent length of the juvenile U. exsul was 3.77 [+ o -] 0.05 cm (n = 42; range = 3-4.5 cm). Mean head width was 0.708 [+ o -] 0.007 cm (n = 42; range = 0.58-0.8 cm) and mean head length was 0.897 [+ o -] 0.010 cm (n = 42; range = 0.79-1.02 cm).

For the juvenile U. exsul that we examined, the diet was composed primarily of adult and larval beetles (Table 1). Indeed, adult beetles represented 72.4% of the total number of prey items, 68.3% of the total volume of prey, and were found in 97.7% of all stomachs. Larval beetles were less prevalent but much more important than other prey items.

We also examined the stomach contents of two adult female U. exsul (snout-vent length = 6.8 and 6.5 cm) collected at the same time and place as the series of juveniles. The diets of these two females included beetles as the most important item, but hemipterans were also important as were ants and hymenopterans (Table 2).

Our observations on the diets of juvenile U. exsul differ from previous reports of adult diets in U. exsul. While our study and that of Gadsden et al. (2001) both identified insects as the main food items, these two studies identified different main food items (beetles, our study; ants, Gadsden et al., 2001). However, beetles and hemipterans were present in summer (July-September) samples from adults of U. exsul in Gadsden et al. (2001) but not at the same proportions as in the juveniles in our study. Possible explanations for these differences include differences in the seasonal timing of collection and differences in the local availability of prey. However, the diets of the juveniles we collected in July were still different from the adult females we collected as well as from the summer (July-September) sample examined by Gadsden et al. (2001). Thus, it seems unlikely that season could explain much of the differences that we observed. Unfortunately, data on prey availability at the study site are not available. In addition, differences between our observations and Gadsden et al. (2001) could reflect differences in ontogenetic stage. The limited data on adult diet from our study suggest that the differences observed above likely reflect both local abundances of prey (i.e., beetles still important in the two adults in our study but not in the Gadsden et al., 2001) and potential ontogenetic changes (i.e., beetles important for both adults and juveniles, but with other diet items such as ants also important for adults, even when only two adults were examined). Differences between adults and juveniles could also reflect differences in gape size. However, in the two adult female U. exsul we examined, the maximum prey width of the prey items observed was 0.53 cm, which is less than the mean head width for the juveniles we examined. Also, only one of the prey items (out of 19) found in the two adult females we examined had a length (0.92 cm) greater than the mean head length of the juveniles we examined. In addition, one of the biggest differences between adults and juveniles in our study is the higher abundance of ants, a small prey item, in the adult individuals as compared to the juveniles. Thus, while gape size may contribute to some of the differences that we observed, we do not believe it is a major factor driving prey differences between adults and juveniles. Clearly, additional work is needed to address these possibilities, but our observations do suggest that juvenile diets in U. exsul may differ from those of adults.

Submitted 20 April 2016. Accepted 12 December 2016.

Associate Editor was Charles Matthew Watson.

We thank the late Hobart Smith for facilitating the loan of these specimens. Two anonymous reviewers helped improve the manuscript.


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Department of Biology, Denison University, Granville, OH 43023 (JJS, GRS)

Laboratorio de Ecologia, Unidad de Biologia, Tecnologia y Prototipos, FES Iztacala, Universidad Nacional Autonoma de Mexico, Av.

De los Barrios # 1, Col. Los Reyes Iztacala, Tlalnepantla, Estado de Mexico, Mexico (JLE)

* Correspondent:
TABLE 1--Diet of 43 juvenile Uma exsul from Coahuila, Mexico,
collected on 13-14 July 2004. Proportion of totals given in
parentheses. IV = importance value.

Taxon                Number of        Volume        Stomachs     IV

Araneae              1 (0.002)       0.01 (0)      1 (0.023)    0.025
Coleoptera-Adults   425 (0.724)   295.4 (0.683)    42 (0.977)   2.284
Coleoptera-Larvae   119 (0.203)   136.41 (0.316)   16 (0.372)   0.891
Diptera              6 (0.010)    0.03 (0.0001)    5 (0.116)    0.126
Hemiptera           21 (0.036)     0.1 (0.0002)    12 (0.279)   0.315
Homoptera            1 (0.002)       0.01 (0)      1 (0.023)    0.025
Hymenoptera         10 (0.017)    0.06 (0.0001)    9 (0.209)    0.226
Hymenoptera (ant)    1 (0.002)        0 (0)        1 (0.023)    0.025
Unidentified         3 (0.005)    0.33 (0.0008)    2 (0.046)    0.052
Total                   587           432.35           43        --
Niche breadth          1.77            1.77            --        --

TABLE 2--Diet of two adult Uma exsul from Coahuila, Mexico,
collected on 13-14 July 2004. Proportion of totals
given in parentheses. IV = importance value.

Taxon          of items     Volume    Stomachs    IV

Coleoptera    10 (0.526)    0.166 (0.42 (1.0)    1.99
Hemiptera      4 (0.210)    0.146 (0.41 (0.5)    1.12
Hymenoptera    1 (0.053)    0.025 (0.01 (0.5)    0.623
Formicidae     4 (0.210)    0.021 (0.01 (0.5)    0.769
Total             19         0.358       2
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Article Details
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Title Annotation:NOTES
Author:Schulte, Joshua J.; Smith, Geoffrey R.; Lemos-Espinal, Julio A.
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1MEX
Date:Mar 1, 2017
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