The Sonoran Desert Toad


Bufo alvarius

Breeding Activity, Estimated Age-Structure, and Growth in Sonoran Desert Anurans
Brian K. Sullivan and Philip J. Fernandez
Herpetologica Sept. 1999; 55 (3): 334-343.
We investigated breeding activity of a community of desert anurans at two sites in north-central Phoenix, Maricopa County, Arizona, during 1990-1995. Four species used these sites for breeding during the summer monsoon season: Colorado River toad, Bufo alvarius, Great Plains toad, B. cognatus, red-spotted toad, B. punctatus, and Couch's spadefoot toad, Scaphiopus couchii. Breeding activity was restricted primarily to 1990 and 1992 when significant rainfall events (>25 mm of rainfall within 24 h) occurred repeatedly. No breeding (i.e., oviposition) occurred during 1991 or 1993, and only S. couchii bred on a single night in 1994 and B. alvarius on a single night in 1995. We undertook a skeletochronologic analysis of lines of arrested growth in digits and femurs with all four study species. Conservative estimates of age based on lines of arrested growth (LAG's) suggest that populations of all four taxa were relatively young (average age between two and four years) at both sites in 1994 and 1995. Growth, as indicated by the amount of bone deposited between LAG's, was greatest during the second year in B. alvarius and B. cognatus, but not B. punctatus or S. couchii. These results indicate rapid growth to maturity but do not support the notion that these anuran amphibians are long-lived organisms that exhibit an extended reproductive lifespan as adults in response to a highly variable, harsh desert environment.

Natural Hybridization Among Distantly Related Toads (Bufo alvarius, Bufo cognatus, Bufo woodhousii) in Central Arizona
Erik W.A. Gergus, Keith B. Malmos, and Brian K. Sullivan
Copeia May, 1999; 199 (2): 281-286.
Putative hybrid toads of the genus Bufo were collected in central Arizona and identified using allozymes, qualitative and quantitative morphological characters, and release call characteristics. Data suggest one hybrid resulted from mating between Bufo cognatus and Bufo woodhousii, and the other three resulted from matings between Bufo alvarius and B. woodhousii. Natural hybridization between these taxa has not been previously reported. To date, B. woodhousii has been found to hybridize with four species of the Bufo americanus species group, of which B. woodhousii is a member, and five other species which represent three outgroups to the B. americanus group. Fossil evidence suggests some genomic compatability has been retained between B. alvarius and B. woodhousii despite at least 6 million years of independent evolution. Hybridization among members of Bufo may be due to alternative mating tactics of males, such as active searching, and by alteration of historical habitats in central Arizona.

Call Variation in the Colorado River Toad (Bufo alvarius): Behavioral and phylogenetic Implications
Brian K. Sullivan and Keith B. Malmos
Herpetologica 1994; 50 (2) 146-156.
We studied variation in advertisement calls, release calls, and calling behavior in three populations of Bufo alvarius in central Arizona over a 3-yr period. Of advertisement call variables, pulse rate was the only variable significantly (positively) related to temperature, and no variables were correlated with male snout-vent length. For release calls only pulse rate was significantly (negatively) related to temperature, and no variables were related to male size. Mean advertisement call pulse rate was approximately 30% of average release call pulse rate; such dramatic differences in temporal structure of advertisement and release calls are previously unreported in the genus Bufo. These results support the hypothesis that B. alvarius generates advertisement calls without passive vibrations of the arytenoid cartilages. Overall patterns of call variation in B. alvarius more closely parallel the valliceps rather than the boreas species group. Preliminary discrimination trials indicate that females are attracted to male, advertisement calls. Within relatively low density choruses, some large males produced advertisement calls consistently, and active-searching behavior was size-related. Male advertisement calls may play an important role in mate selection by females under some conditions, but an adequate test of this hypothesis awaits additional study.

Cell & Tissue Research

The Spinal Nerves Innervate Putative Chemosensory Cells in the Ventral Skin of Desert Toads, Bufo alvarius
Hiromichi Koyama (1), Takatoshi Nagai (2), Hiro-aki Takeuchi (3), Stanley D. Hillyard (4)
(1) College of Nursing, Yokohama City University, Yokohama 236-0004, Japan
(2) Department of Physiology, Teikyo University School of Medicine, Kaga 2-11-1, Itabashi-ku, Tokyo 173-8605, Japan E-mail: Phone: +81-45-566 1328 Fax: +81-45-566 1328
(3) Department of Biology and Geoscience, Faculty of Science, Shizuoka University, Shizuoka 422-8529, Japan
(4) Department of Biological Sciences, University of Nevada, Las Vegas, Nevada 89154, USA
Received: 30 August 2000 / Accepted: 30 January 2001 / Published online: 27 March 2001
Toads normally obtain water by absorption across their skin from osmotically dilute sources. When hyperosmotic salt solutions are presented as a hydration source to dehydrated desert toads, they place the ventral skin onto the source but soon afterwards escape to avoid dehydration. The escape behavior coincides with neural excitation of the spinal nerves that innervate putative chemosensory cells in the ventral skin. In the present study, fluorescent dye translocated through the spinal nerves to those receptor cells in the epidermis was photoconverted in the presence of 3, 3'-diaminobenzidine tetrahydrochloride for electron-microscopic observation of the cells and associated nerve terminals. Most of the photoconverted cells were located in the deepest layer of the epidermis, with some being in more intermediate layers. No labeled cell was seen in the outermost layer of living cells. In desert toads, flask cells and Merkel cells are occasionally seen in the epidermis. An association of nerve fibers with these epidermal cells has been reported in some species of the anurans. In the present study, however, the cytological features of the photoconverted cells are neither reminiscent of flask cells nor Merkel cells, but are similar to those of surrounding epithelial cells in each layer of the epidermis. We hypothesize a sensory function for these cells, because they have a close association with nerve fibers and participate in the transepithelial transport of salts that must pass through all cell layers of the skin.