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Dietary composition of the Mexican spadefoot toad (Spea multiplicata) from a sand dune habitat in southwestern Coahuila, Mexico.

Abstract. -- Diet composition of the Mexican spadefoot toad has been poorly studied, especially in the southern part of this species' geographic range. This study reports the diet composition of a population inhabiting a sand dune system in southwestern Coahuila, Mexico. The stomach contents of 43 specimens revealed 24 items, 22 were arthropods and two were vegetal material. Numerically, ants comprized the greater percentage of the stomach contents (49.7%) followed by homopterans (16.7%) and hemipterans (12.8%). Volumetrically, beetles were the most important item (42.4%). There was a linear relationship between SVL and stomach volume, SVL and snout length, and snout length and length of one prey group (Hemiptera). In comparison with northern populations which feed primarily on termites, it is possible that toads in this population will need more than seven feedings to survive for one year.

Resumen. -- La composition de la dieta del sapo Spea multiplicata a sido escasamente estudiada, especialmente en el rango de su distribucion mas sureno. Este estudio muestra la dieta de una poblacion de S. multiplicata en un sistema de dunas en el suroeste de Coahuila, Mexico. Se encontraron relaciones significativas entre la LHC y el contenido volumetrico estomacal, la LHC y la longitud del hocico y la longitud del hocico y la longitud de los hemipteros. El contenido estomacal de 43 sapos mostro 24 categorias, de las cuales, 22 fueron de artropodos y dos de vegetales. Numericamente, las hormigas mostraron el mayor porcentaje del contenido estomacal (49.7%) seguido por los homopteros (16.7%) y los hemipteros (12.8%). Volumetricamente, los escarabajos fueron los mas importantes (42.4%). En comparacion con las poblaciones nortenas, las cuales se alimentan basicamente de termitas, es posible que la poblacion estudiada requiera de mas de siete alimentaciones para sobrevivir un ano.

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Although moisture conditions of North American deserts are not favorable for a wide diversity of amphibians as are tropical regions, there are many species particularly adapted to tolerate droughts and the high temperatures of the arid environments (Bentley 1966; Mayhew 1968; Rubial et al. 1969). Due to the high permeability of the skin and to the susceptibility to desiccation under arid conditions, the annual activity period of the amphibians like Spea multiplicata (formerly Scaphiopus multiplicatus) depends upon the availability of moisture; consequently, the daily activity tends to be concentrated at night (Degenhardt et al. 1996). This toad exhibits a generalist diet, however the composition varies geographically (Dimmit & Rubial 1980; Punzo 1991; Degenhardt et al. 1996). The groups most consumed by this amphibian are termites, beetles, orthopterans, ants, and spiders (Dimmitt & Rubial 1980; Punzo 1991; Degenhardt et al. 1996). Likewise, Dimmitt & Rubial (1980) suggest that S. multiplicata require at least seven feedings to accumulate sufficient amounts of fat to survive for 12 months. However, because the diet composition of this toad varies among localities and because this species inhabits many habitat types, the number of feedings required to survive one year could change geographically depending on the availability of food resources and the major groups consumed (Degenhardt et al. 1996).

There are few studies concerning food patterns on desert toads; particularly for S. multiplicata. Populations in Texas represent the southernmost location where diet composition of this toad has been studied (Anderson et al. 1999; Whitaker et al., 1977; Punzo 1991), and dietary patterns of Mexican populations of S. multiplicata are unknown. Furthermore, it is important to know how the sand dune vegetation can influence diet composition of this toad in comparison with other populations. This study provides the diet composition of a population of the Mexican spadefoot toad in a sand dune habitat in southwestern Coahuila, Mexico.

STUDY SITE

The climate of the sand dunes in southwestern Coahuila is dry and very warm. Mean annual precipitation vary between 200 to 300 mm, occurring primarily during July to September. Mean annual temperature is about 22[degrees]C. December and January are the coolest months, and July and August the warmest (Garcia 1981). The collecting site is near the lowest part of the Aguanaval River which has water only in rainy years (INEGI 1988).

Mean altitude of the area is 1100 m. Vegetation is xerophitic (Redowsky 1978) with a greater abundance of creosote bush (Larrea tridentata) and desert seepweed (Suaeda nigrescens) and lesser abundance of mesquite (Prosopis glandulosa) and Christmas cactus (Cylindropuntia leptocaulis). The soil is basically Si[O.sub.2] sand forming dunes of varying elevation.

METHODS AND MATERIALS

Stomach contents of 43 spadefoot toads (Spea multiplicata) from the herpetological collection of the Facultad de Ciencias Biologicas of the Universidad Autonoma de Nuevo Leon, Mexico were analyzed. The specimens were collected in November, 2003 in the sand dunes region of Viesca, Coahuila.

During November 2003 a large number of spadefoot toads were observed in the sand dune system of Viesca, Coahuila. Although this toad is basically nocturnal (Degenhardt et al. 1996), the toads at Viesca were active during the morning before 09:00 hrs (at an air temperature lower than 23.8[degrees]C and a relative humidity of 43.3%). However, by afternoon activity had ceased. By sunset (after 19:00 hrs) a few isolated active toads were seen, when the air temperature had dropped to 22.0[degrees]C and the humidity increased to 36.5%. Active toads were seen under different kind of plants, however, conglomerated groups (by the morning) were frequently observed in depressions shaded partially by grasses with damp substrates.

Toads were preserved in alcohol (70%) three hours after been collected and sacrificed by cooling. Snout-vent length (SVL) and snout length (SL) of each toad were measured with a caliper to the nearest 0.01 mm. A Pearson's regression analysis was applied to determine any relationship between these two variables and between SVL and the stomach volume. Stomach contents (S = number of stomachs containing item i, where i = prey species) were examined under a stereomicroscope. Individual prey items (n = number of prey items) were identified and percentage of stomachs with that item i (S%) was calculated. The volume of food items (V) of each taxonomic category in a single stomach was estimated by using the length and width (0.01 mm) of intact preys with caliper. Prey volume was calculated as an ellipsoid (V = 4/3 [pi] (w/2)[.sup.2] (l/2)), where w is prey width and l is prey length (Punzo 1991; Oliveira-Mesquita & Rinaldi-Colli 2003).

The sum of the above three relative abundance measures (in percentage) for item i was used to estimate the importance value (Acosta 1982; Oliveira-Mesquita & Rinaldi-Colli 2003).

Niche breadth for pooled stomachs was determined from numeric and volumetric percentages of preys using the Shannon Index (H' = -[SIGMA] [p.sub.i] [log.sup.2][p.sub.i]) (Pianka 1973; Oliveira-Mesquita & Rinaldi-Colli 2003). The Shannon Index was preferred instead of the Simpson index (D) commonly used in lizard studies (Pianka 1973) because the former is less sensitive to the frequency of dominant prey items, and unlike D, the casual ingestion of a prey item by an opportunistic predator does not disturb H' (May 1975).

For the main prey items identified in the stomach contents, the relationship between toad SVL and the largest body length of the prey were examined using Pearson's regression analysis (Gadsden & Palacios-Orona 1997; Ramirez-Bautista & Lemos-Espinal 2004). Statistical analyses were performed with SPSS Ver. 10 considering a P < 0.05 to assess statistical significance. Values are showed as mean [+ or -] 1SE.

Specimens examined are deposited in the Herpetological Collection of the Universidad Autonoma de Nuevo Leon, Facultad de Ciencias Biologicas. UANL-6696-6738.

[FIGURE 1 OMITTED]

RESULTS

The mean SVL of the toads was 23.22 [+ or -] 0.29 mm, and the mean snout length was 7.82 [+ or -] 0.11 mm, both variables were related significantly (R = 0.76; [F.sub.1,41] = 56.51; P = 0.000). The mean stomach volume was 102.67 [+ or -] 113.46 [mm.sup.3] and there was a significant linear relationship between SVL and the prey volume (R = 0.58; [F.sub.1,41] = 21.42; P = 0.000) (Fig. 1).

Snout length was related only with hemipteran length (R = 0.30; [F.sub.1,73]; = 7.47; P = 0.008), and not with ant (R = 0.041; [F.sub.1,284] = 0.47; P = 0.492), beetle (R = 0.136; [F.sub.1,76] = 1.43; P = 0.235), homopteran (R = 0.058; [F.sub.1,171] = 0.57; P = 0.451) or larval lepidopteran lengths (R = 0.85; [F.sub.1,44] = 0.32; P = 0.572). Twenty-four items consumed by S. multiplicata were identified; of which 22 were arthropods (four in larval stages) and two were vegetal material (Table 1).

Numerically, the family Formicidae (separate from Hymenopteran per se) represents the greatest percentage of the stomach contents (49.7%) followed by the order Homoptera (16.7 %) and Hemyptera (12.8%). Volumetrically, coleopterans were the most important item (42.4%) followed by lepidoptera larvae (26.8%) and homopterans (11.1%). On the other hand, the greater importance values associated with the five orders stated above (Table 1), indicate that these are the insect groups widely consumed by S. multiplicata in the southwestern region of the sand dune habitat in Coahuila.

The low amount of vegetal material found in the stomachs suggests that plants are not an important component in the diet of this toad (Whitaker et al. 1977), and may have been consumed inadvertently while capturing animal pray items (Table 1). The Shannon-Index for the numeric data was H' = 2.48 with an evenness of [E.sub.H] = 0.78. The index for the volumetric data was H' = 2.35 with an evenness of [E.sub.H] = 0.73. The SVL has a significant relationship with the prey length for prey of the order Hemiptera, but SVL was not correlated with prey length in the other groups of insects (Table 2).

DISCUSSION

Dimmitt & Rubial (1980) indicate that S. multiplicata in San Simon Valley Arizona, feeds basically on termites and beetles. Conversely, Punzo (1991) showed that S. multiplicata fed on beetles, orthopterans, ants, spiders, and termites (comprising 93.8% of its diet) in western Texas. This current study shows that S. multiplicata in southwestern of Coahuila feeds on ants, homopterans, and hemipterans as the numerically most common food items, but that volumetrically, beetles represented more than 40% of the stomach content followed by larvae of lepidopterans and homopterans. These results coincided with the observed by Anderson et al. (1999), where a population of S. multiplicata (from southern high plains of Texas) feeds on beetles of the Carabidae family. These diet preferences probably reflect that in sand dune systems, homopterans, lepidopterans and coleopterans are uncommon among the perennial plants during drought periods, but these kinds of arthropods are particularly abundant in shaded areas with grass after the rains and when water ponds begin to dry up.

Dimmitt & Rubial (1980) suggest that the Arizona population requires seven feedings to accumulate the necessary fat to survive 12 months considering a diet made up mainly of beetles and alate termites. However, termites are more digestible and have higher lipid and caloric contents than most of the insects consumed by this amphibian (Fast 1964). If it is considered that the population of S. multiplicata studied here feeds on less nourishing insect species (mainly ants, homopterans and hemipterans) than termites, the number of feedings they require to survive one year could be greater than the San Simon Valley population. However, body size of this population was less than that estimated by Dimmit & Rubial (1980) and K. Pfennig (pers. comm.) for populations of this toad in Arizona. This could also compensate and influence the number of total feedings to survive one year.

Brown (1976) found that the western population of S. multiplicata has a mean SVL of 46.9 mm and the Arizona's population reaches 40 mm (K. Pfennig, pers. comm.). The studied population has the smallest size (23 mm) which probably was represented by young adults. However, although this population was represented by adults, it is possible that size and collection date of toads may have an influence on diet composition. Likewise, the insects more commonly eaten by this population could reflect environmental arthropod availability and not a preference for the more nutritious orders. Dietary breadth of the Mexican population (24 items consumed) was similar to the population of Texas (20 items) studied by Anderson et al. (1999). These also suggest that food availability in dune systems could be similar to the southern high plains of Texas, or that diet preferences of this toad can be comparable in a wide geographic range. Differences in diet composition of spadefoot toads among the populations studied could be related to dissimilarity in habitat composition, habitat use, size and collection season.

ACKNOWLEDGMENTS

We thank Isabel Vazquez Velez, Alfonso Garza and Fernando Gonzales for their help in the identification and quantification of arthropods. Our thanks and appreciation to Karin Pfennig for her support and the revision of a previous version of this paper. To Robert Edwards, Ned E. Strenth and two anonymous reviewers for providing helpful comments and suggestions for the manuscript. We also thank to Consejo Nacional de Ciencia y Tecnologia (CONACyT) for support to G.C.G y C.G.P during doctoral programs. This manuscript is especially dedicated to Elsa Kai, the young daughter of K. Pfennig.

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Bentley, P. J. 1966. Adaptations of amphibians to arid environments. Science, 152:619-623.

Brown, H. A. 1976. The status of California and Arizona populations of the western spadefoot toads (genus Scaphiopus). Contrib. Sci. Nat. Hist. Mus. Los Angeles Co., 286:1-15.

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Gadsden, H., & L. E. Palacios-Orona. 1997. Seasonal dietary patterns of the Mexican fringe-toed lizard (Uma paraphygas). J. of Herpetology, 31(1):1-9.

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GC at: gamaliel.cg@gmail.com

Gamaliel Castaneda-Gaytan, Cristina Garcia-De La Pena, David Lazcano and Armando J. Contreras-Balderas

Laboratorio de Herpetologia, Facultad de Ciencias Biologicas

Universidad Autonoma de Nuevo Leon, A.P. 513. C.P. 66450

San Nicolas de los Garza, Nuevo Leon, Mexico
Table 1. Contents of the stomachs of Spea multiplicata (n = 43). S =
number of stomachs containing item i; S% = percentage of stomachs with
item i; n = number of prey items; N% percentage of items in total
sample; V = volume in [mm.sup.3]; V% = percentage of total volume
represented; VI = importance value and VI/3 = standardized importance
value. Formicidae is excluded from the order Hymenoptera.

Item S S % N N % V V % VI

Acari 12 4.13 18 0.78 3.81 0.08 4.99
Araneae 11 3.79 22 0.96 42.28 0.97 5.72
Chilopoda 1 0.34 1 0.04 1.7 0.03 0.41
Coleoptera 33 11.37 113 4.94 1834.75 42.49 58.8
Coleoptera larvae 4 1.37 6 0.26 6.75 0.15 1.78
Collembola 11 3.79 28 1.22 10.09 0.23 5.24
Diptera 9 3.1 14 0.62 17.18 0.37 4.09
Diptera larvae 12 4.13 21 0.91 11.36 0.26 5.3
Formicidae 43 14.82 1137 49.75 256.22 5.93 70.5
Hemyptera 35 12.06 294 12.86 346.63 8.21 33.13
Homoptera 37 12.75 382 16.71 483.42 11.19 40.65
Hymenoptera 6 2.06 6 0.26 29.57 0.68 3
Isoptera 9 3.1 72 3.15 14.93 0.34 6.59
Lepidoptera 10 3.44 12 0.52 29.03 0.67 4.63
Lepidoptera larvae 23 7.93 47 2.05 1157.56 26.81 36.79
Neuroptera larvae 4 1.37 6 0.26 7.82 0.18 1.81
Plants 3 1.03 3 0.13 19.1 0.44 1.6
Pseudoscorpionida 3 1.03 5 0.21 2.77 0.06 1.3
Psocoptera 1 0.34 6 0.26 1.8 0.04 0.64
Scorpionida 2 0.68 2 0.08 7.08 0.16 0.92
Seeds 15 5.17 84 3.67 20.82 0.48 9.32
Solifuga 2 0.68 2 0.08 8.47 0.19 0.95
Thysanoptera 3 1.03 3 0.13 0.65 0.01 1.17
Tysanura 1 0.34 1 0.04 1.01 0.02 0.4
Total 290 99.85 2285 99.89 4314.8 99.99 299.88

Item VI/3

Acari 1.66
Araneae 1.9
Chilopoda 0.13
Coleoptera 19.6
Coleoptera larvae 0.59
Collembola 1.74
Diptera 1.36
Diptera larvae 1.76
Formicidae 23.5
Hemyptera 11.04
Homoptera 13.55
Hymenoptera 1
Isoptera 2.19
Lepidoptera 1.54
Lepidoptera larvae 12.26
Neuroptera larvae 0.6
Plants 0.53
Pseudoscorpionida 0.43
Psocoptera 0.21
Scorpionida 0.3
Seeds 3.1
Solifuga 0.31
Thysanoptera 0.39
Tysanura 0.13
Total 99.82

Table 2. Relationship between the snout-vent length of Spea multiplicata
and the length of the main insect prey items.

 R [r.sup.2] F df P

SVL vs Hemiptera 0.32 0.1 8.81 1,73 0.004
SVL vs Formicidae 0.05 0.003 0.84 1,284 0.35
SVL vs Coleoptera 0.20 0.04 3.19 1,76 0.69
SVL vs Homoptera 0.03 0.001 0.15 1,171 0.69
SVL vs Lepidoptera larvae 0.17 0.02 1.30 1,44 0.26
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Author:Castaneda-Gaytan, Gamaliel; Garcia-De La Pena, Cristina; Lazcano, David; Contreras-Balderas, Armando
Publication:The Texas Journal of Science
Geographic Code:1MEX
Date:Feb 1, 2006
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