Dibble, C. J.

Lemos-Espinal, J. A.

Smith, Geoffrey R.

English

Denison University

Denison University Denison Digital Commons Denison Faculty Publications 2008 Diet and Sexual Dimorphism of the Desert Iguana, Dipsosaurus Dorsalis, From Sonora, Mexico C. J. Dibble Geoffrey R. Smith J. A. Lemos-Espinal Follow this and additional works at: https://digitalcommons.denison.edu/facultypubs  Part of the Biology Commons Recommended Citation Dibble, C. J., Smith, G. R., & Lemos-Espinal, J. A. (2008a). DIET AND SEXUAL DIMORPHISM OF THE DESERT IGUANA, DIPSOSAURUS DORSALIS, FROM SONORA, MEXICO. Western North American Naturalist, 68(4), 521-523. doi:10.3398/1527-0904-68.4.521 This Article is brought to you for free and open access by Denison Digital Commons. It has been accepted for inclusion in Denison Faculty Publications by an authorized administrator of Denison Digital Commons. The desert- and arid-scrubland-dwellingdesert iguana, Dipsosaurus dorsalis, is mainlyherbivorous but will occasionally consumeinsects, fecal matter, and carrion (Norris 1953,Asplund 1967, Minnich and Shoemaker 1970,Pianka 1971). However, previous studies on thediet of D. dorsalis have found variation amongpop ulations in the extent of omnivory. Forexample, in some populations adult diets arecomposed of >80% plant material (Norris 1953,Pianka 1971, Mautz and Nagy 1987), but in atleast one population a much more omnivorousdiet, composed of 54% plant material (Asplund1967), has been observed. Thus the diet compo-sition of D. dorsalis appears to show somedegree of intraspecific variation. Indeed, thepopulation with the greatest degree of omnivoryis the southernmost population examined (As -plund 1967), suggesting that there may be alatitudinal gradient in herbivory in D. dorsalis.However, we are unaware of any reports on thediet of D. dorsalis from northern Mexico. Wethere fore examined the diet of D. dorsalis fromSonora, Mexico, to help determine the extentof intraspecific variation in diet composition.We also examined sexual dimorphism in D.dorsalis from this population. Carothers (1984)found no sexual dimorphism in body or headsize in D. dorsalis, but intraspecific variationin sexual dimorphism has been observed insome other lizard species (McCoy et al. 1994,Molina-Borja et al. 1997). Therefore, it is use-ful to examine other populations of D. dorsalisto determine if there is any intraspecific varia-tion in sexual dimorphism in this species.Dipsosaurus dorsalis individuals (n = 56)were collected near Ortiz, Sonora, Mexico(28°1723.0N, 119°430.8W) during summer2005 as part of a geographic survey of the her-petofauna of the Sonoran and Chihuahuandeserts and nearby regions (Smith et al. 2005).Specimens were humanely euthanized, fixedin formalin, and preserved in 90% ethanol.Specimens were deposited in the ColecciónHerpetológica of the Laboratorio de Ecologíaof the Unidad de Biotecnología y Prototipos ofthe Facultad de Estudios Superiores Iztacala-UNAM. We measured snout–vent length (SVL),head length (HL), and head width (HW) to thenearest 0.1 mm using digital calipers. StomachsWestern North American Naturalist 68(4), © 2008, pp. 521–523DIET AND SEXUAL DIMORPHISM OF THE DESERT IGUANA, DIPSOSAURUS DORSALIS, FROM SONORA, MEXICOChristopher J. Dibble1, Geoffrey R. Smith1,3, and Julio A. Lemos-Espinal2ABSTRACT.—We examined the diet and sexual dimorphism of the desert iguana, Dipsosaurus dorsalis, from Sonora,Mexico. The diet consisted primarily of vegetation, with insect material contributing little to the overall volume of thediet. Plant matter was restricted to leaves and seeds. Animal prey consisted mostly of ants, termites, and beetles. Malesand females did not differ significantly in snout–vent length, head length, or head width. These results confirm that ingeneral D. dorsalis is herbivorous and not sexually dimorphic through much of its range.Key words: diet, sexual dimorphism, desert iguana, Dipsosaurus dorsalis, herbivory, insectivory.RESUMEN.—Examinamos la dieta y dimorfismo sexual de la Iguana del Desierto, Dipsosaurus dorsalis, en Sonora,México. La dieta consistió principalmente de vegetales, con insectos contribuyendo poco al volumen total de ésta. Lamateria vegetal estuvo restringida a hojas y semillas. Las presas animales consistieron principalmente de hormigas, ter-mitas, y escarabajos. Machos y hembras no difirieron significativamente en longitud hocico cloaca, longitud de lacabeza, o ancho de la cabeza. Estos resultados confirman que en general D. dorsalis es una especie herbívora que nopresenta dimorfismo sexual en la mayor parte de su intervalo de distribución.Palabras clave: dieta, dimorfismo sexual, iguana del desierto, herbívoro, insectívoro.1Department of Biology, Denison University, Granville, OH 43023.2Laboratorio de Ecología, Tecnología y Prototipos, Facultad de Estudios Superiores Iztacala, UNAM, Apartado Postal 314, Avenida de los Barrios No. 1,Los Reyes Iztacala, Tlalnepantla, Estado de México, 54090 México.3Corresponding author. E-mail: smithg@denison.edu521were removed and prey identified to the lowestpossible taxonomic level, usually order. Wedivided plant material into leaves, seeds, andunknown vegetation and used volumetricdisplacement to determine the volume of veg-etation in the stomach. We measured the lengthand width of invertebrate prey items to thenearest 0.1 mm using digital calipers and cal-culated insect volumes using the expressionfor a prolate spheroid (Vitt et al. 2005). We cal-culated niche breadth based on volume (withseeds, leaves, and unidentified plant materialrepresented as a prey category) for each indi-vidual, using the reciprocal of Simpson’s Index(Krebs 1989). ANOVA was used to compareSVL and the proportion of plants in the dietbetween males and females, and ANCOVAwith SVL as a covariate was used to compareHW, HL, prey size, and stomach volume be -tween males and females. In all cases theinteraction between sex and SVL was not sig-nificant and was not included in the finalANCOVA model. Proportion data were arcsinetransformed prior to analyses. In the results,means are accompanied by one standard error.Diets of 56 D. dorsalis consisted of bothplant and animal material (Table 1). Plant mat-ter composed 94.6% of the total volume of D.dorsalis diets, with leaves being the predomi-nant category (43.4%). Less than half (25 of 55;45%) of the adults had animal prey in theirstomachs. Numerically, ants were the mostimportant animal prey item (154 items in 24stomachs) but contributed little to the totalvolume of the stomach contents (1.60%). Wefound no statistically significant relationshipbetween adult SVL and proportion of vegeta-tion eaten (n = 55, r2 = 0.026, P = 0.24).Mean reciprocal Simpson’s index was 1.87+– 0.09 (n = 56). Males and females had simi-lar mean reciprocal Simpson’s indices (males1.91 +– 0.12, females 1.85 +– 0.13; F1, 53 = 0.11,P = 0.74). The reciprocal Simpson’s indexincreased with SVL (n = 56, r2 = 0.18, P =0.0012; 1/D = 0.46 + 0.015SVL).For individuals that consumed animal prey,mean prey length increased with SVL (n = 26,r2 = 0.24, P = 0.0103; mean prey length =–0.11 + 0.010SVL). Similar trends were seenfor mean prey width and SVL (n = 26, r2 =0.21, P = 0.017; prey width = –0.0081 +0.0036SVL), and for total stomach volume andSVL (n = 57, r2 = 0.102, P = 0.015; volume= –0.12 + 0.015SVL).Males consumed longer insect prey thanfemales did (Table 2; F1,22 = 9.1, P = 0.006).Mean prey width did not differ between malesand females (Table 2; F1,22 = 3.0, P = 0.10).The proportion of vegetation eaten by maleswas not significantly different from that eatenby females (Table 2; F1,53 < 0.01, P = 1.0).Snout–vent length did not differ significantlybetween males and females (Table 2; F1,54 =2.79, P = 0.10). Male and female head size didnot significantly differ (Table 2; HL: F1,53 =0.76, P = 0.40; HW: F1,53 = 0.87, P = 0.35).Our results indicate that D. dorsalis fromSonora is primarily herbivorous, with almost95% of the diet consisting of plant material.The relatively low reciprocal Simpson’s indexvalues are consistent with a relatively narrowdiet. Pianka (1971) found a similar diet compo-sition for D. dorsalis from the southwestern522 WESTERN NORTH AMERICAN NATURALIST [Volume 68TABLE 1. Diet of 57 desert iguanas, Dipsosaurus dorsalis,from Sonora, Mexico. Because one stomach was empty, fre-quencies were created using n = 56.Stomachs Items Volume_________ ___________ __________Prey taxon n % Freq. % cm3 %INVERTEBRATESInsectaColeoptera 12 21.43 13 6.02 1.46 2.05Larvae 2 3.57 3 1.39 0.07 0.11Diptera 2 3.57 2 0.93 0.38 0.53HymenopteraAnts 24 42.86 154 71.29 1.14 1.60Wasps 1 1.78 1 0.46 0.04 0.06Isoptera 5 8.93 42 19.44 0.60 0.85Orthoptera 1 1.78 1 0.46 0.13 0.18VEGETATIONLeaves 51 91.07 — — 43.40 60.96Seeds 36 64.28 — — 17.75 24.93Other 17 30.36 — — 6.22 8.74TOTAL 56 216 71.19TABLE 2. Mean snout–vent length (SVL), head length(HL), head width (HW), prey length (PL), prey width(PW), and proportion vegetation consumed (prop. veg.) ofmale and female desert iguanas, Dipsosaurus dorsalis,from Sonora, Mexico. Means are given with one standarderror; sample sizes are in parentheses.Males FemalesSVL (mm) 98.8 –+ 3.3 (29) 90.8 –+ 3.4 (27)HL (mm) 11.1 –+ 0.3 (29) 10.1 –+ 0.3 (27)HW (mm) 15.9 –+ 0.2 (29) 14.5 –+ 0.2 (27)PL (mm) 10.8 –+ 1.1 (13) 6.4 –+ 1.2 (12)PW (mm) 3.86 –+ 0.44 (13) 2.76 –+ 0.46 (12)Prop. veg. 0.932 –+ 0.024 (29) 0.932 –+ 0.026 (26)2008] 523United States, recording 94.2% vegetativematerial by volume. Norris (1953) observedvolumetric percentages of vegetation from80%–95% for D. dorsalis from California. Mautzand Nagy (1987) found that D. dorsalis fromCalifornia ate only 1% invertebrate prey. Incontrast, Asplund (1967) found that D. dorsalisfrom southern Baja California had only 59%plant material in their diet. Taken together,these results from the literature and our ob -servations on the diet of D. dorsalis fromSonora suggest that if there is a latitudinaltrend in herbivory in D. dorsalis, it has a ratherrapid shift. It seems more likely that there issomething unique about the southern BajaCalifornia population of D. dorsalis studied byAsplund (1967), and further study of that pop-ulation may elucidate why it is more omnivo-rous than other populations of D. dorsalis.Smaller or juvenile D. dorsalis may con-sume insects and then switch to herbivorousdiets as they grow (Asplund 1967, Sokol 1967,Pianka 1971). Asplund (1967) found that 68%of stomach contents from 12 juvenile (SVL <65 mm) D. dorsalis was animal material, whileonly 41% was animal material in adults. Ourspecimens contained only one individual clas-sified as a juvenile based on Asplund’s criteria,and its stomach contained only insects. Whileour research is consistent with an ontogeneticshift in diet, the small sample size for juve-niles makes conclusions tenuous. However,the increase in the reciprocal Simpson’s indexvalues with increases in SVL suggests thatlarger individuals do consume broader dietsthan smaller individuals, but SVL explains rel-atively little of the variation in the lizard diets.Male and female D. dorsalis did not differin body or head size in our specimens fromSonora. This is consistent with the one otherstudy on sexual dimorphism in D. dorsalis(Carothers 1984). Thus it appears a lack of sex-ual dimorphism may be characteristic of D.dorsalis. Carothers (1984) offered an explana-tion for this lack of differences, proposing thatD. dorsalis, an herbivorous species with lowlevels of male aggression, would have littleselective pressure for dimorphism in head size.However, there does appear to be some male–male aggressive behavior that includes directconfrontation and fighting (Norris 1953).ACKNOWLEDGMENTSFunding for this research was provided bythe Denison University Research Foundation.All appropriate permits and approvals forspecimen collection were obtained by J.A.Lemos-Espinal. We thank H.M. Smith for hissupport and encouragement. We also thank J.Zúñiga-Vega and 2 anonymous reviewers fortheir very helpful comments on an earlier ver-sion of this note.LITERATURE CITEDASPLUND, K.K. 1967. Ecology of lizards in the relictualCape Flora, Baja California. American Midland Nat-uralist 77:462–475.CAROTHERS, J.H. 1984. Sexual selection and sexual dimor-phism in some herbivorous lizards. American Natu-ralist 124:244–254.KREBS, C.J. 1989. Ecological methodology. HarperCollins,New York.MAUTZ, W.J., AND K.A. NAGY. 1987. Ontogenetic changesin diet, field metabolic rate, and water flux in theherbivorous lizard Dipsosaurus dorsalis. Physiologi-cal Zoology 60:640–658.MCCOY, J.K., S.F. FOX, AND T.A. BAIRD. 1994. Geographicvariation in sexual dimorphism in the collared lizard,Crotaphytus collaris (Sauria: Crotaphytidae). South-western Naturalist 39:328–335.MINNICH, J.E., AND V.H. SHOEMAKER. 1970. Diet, behaviorand water turnover in the desert iguana, Dipso -saurus dorsalis. American Midland Naturalist 84:496–509.MOLINA-BORJA, M., M. PADRÓN-FUMERO, AND M.T.ALFONSO-MARTÍN. 1997. Intrapopulation variabilityin morphology, coloration, and body size in tworaces of the lacertid lizard, Gallotia galloti. Journalof Herpetology 31:499–507.NORRIS, K.S. 1953. The ecology of the desert iguana Dip-sosaurus dorsalis. Ecology 34:265–287.PIANKA, E.R. 1971. Comparative ecology of two lizards.Copeia 1971:129–138.SMITH, H.M., J.A. LEMOS-ESPINAL, AND P. HEIMES. 2005.2005 amphibians and reptiles from northwesternMexico. Bulletin of the Chicago Herpetological Soci-ety 40: 206–212.SOKOL, O.M. 1967. Herbivory in lizards. Evolution 21:192–194.VITT, L.J., S.S. SARTORIUS, T.C.S. AVILA-PIRES, P.A. ZANI,AND M.C. ESPÓSITO. 2005. Small in a big world:ecology of leaf-litter geckos in New World tropicalforests. Herpetological Monographs 19:137–152.Received 10 August 2007Accepted 21 March 2008NOTES

Diet and Sexual Dimorphism of the Desert Iguana, Dipsosaurus Dorsalis, From Sonora, Mexico

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Diet and Sexual Dimorphism of the Desert Iguana, Dipsosaurus Dorsalis, From Sonora, Mexico

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