Mexican Burrowing Toad, Burrowing Toad
© 1962 Paul Licht (1 of 22)
Rhinophrynus dorsalis Duméril and Bibron, 1841
M.J. Fouquette Jr.1
1. Historical versus Current Distribution. Burrowing toads (Rhinophrynus dorsalis) have been a well-known, if infrequently encountered, burrowing anuran of lowland coastal areas in Mesoamerica for some 150 yr (Fouquette, 1969), but were not discovered as a component of the U.S. herpetofauna until 1964, when James (1966) found breeding populations in southern Texas. These toads surface only to breed during heavy rains, perhaps only once per year; thus, they are encountered infrequently by humans. Few additional reports of this species in Texas have been published since the first one.
Burrowing toads have been documented in a limited portion of Starr and Zapata counties in extreme southwestern Texas (James, 1966; Garrett and Barker, 1987; Dixon, 1987, 2000). Across their range, burrowing toads occur from sea level to 600 m, from southern Texas and northern Tamaulipas, Mexico, to Guanacaste Province in Costa Rica. In Texas, they occur in the Tamaulipan Biotic Province (Edwards et al., 1989), in the arid subtropical zone (Matamoran District; Blair, 1952). Duellman (1971) and Villa (1984) extended the southward range in Honduras and Nicaragua, beyond that indicated by the map of Fouquette (1969), and Foster and McDiarmid (1983) noted additional Costa Rican localities. Dixon (1987) commented that burrowing toads have not been found in Texas since about 1972; however, Dixon (2000) indicated specimens had been taken in 1984 and calls heard in 1998, presumably in the same limited area (although no specific literature records were reported).
2. Historical versus Current Abundance. No basis exists for evaluating any change in abundance over the 35 yr they have been known from the Texas localities. These animals are ephemerally active in limited arid habitats following heavy rains with flooding. James (1966) noted one moderate chorus that was active one night and had no activity on subsequent nights, but a few nights later (following another heavy rain), new choruses were found. There are no reports evaluating changes in abundance throughout the rest of their range.
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. Burrowing toads will breed any time of the year after rains create sufficiently deep breeding pools. Males will call from burrows, then emerge to breed in groups that can be large (Garrett and Barker, 1987). Calling and breeding may occur at the surface of the water or on soil along the flooded sites (Fouquette and Rossman, 1963; James, 1966). The distinctive advertisement call has been described by several authors (Fouquette and Rossman, 1963; James, 1966; Fouquette, 1969), and brief analysis with audiospectrogram was made by Fouquette (1969). Individuals occasionally are encountered on roads (Fouquette and Rossman, 1963; James, 1966) when there is breeding activity nearby, but there is no information available on dispersal or migratory movements.
ii. Breeding habitat. Seasonal pools and flooded areas created by heavy rainfall (James, 1966; Garrett and Barker, 1987).
i. Egg deposition sites. In ephemeral wetlands, females lay eggs in small clumps that soon separate and float to the water surface (Stuart, 1935).
ii. Clutch size. No information is available.
i. Length of larval stage. At least 2 mo, likely longer for most animals (Stuart, 1961). Growth rates, and thus times to metamorphosis, are dependent on food availability (Stuart, 1961). Larvae were not observed by James (1966) in the Texas localities, although she searched for them, and those of other species were abundant.
ii. Larval requirements.
a. Food. Tadpoles are phytoplankton feeders and their guts are lined with large caecum-like areas (Altig and Kelly, 1974). Occasionally, tadpoles may feed on conspecifics (Starrett, 1960), although cannibalism was not observed by Stuart (1961).
b. Cover. Tadpoles school (Stuart, 1961; Foster and McDiarmid, 1982). When schooling, tadpoles form massed aggregates of varying sizes (Stuart, 1961). Schools may be mediated by either lateral line mechanoreceptive cues or olfaction, but apparently not by vision; they also may be influenced by socialization (Foster and McDiarmid, 1982). Generally, these masses are disc-like, with one estimated to be 25 cm in diameter and 10 cm deep that could have contained thousands of individuals (Stuart, 1961). Smaller schools may join to form larger schools, larger schools may break up (Stuart, 1961). Occasionally, larvae in schools will suddenly form a column and swim swiftly towards the surface (surfacing behavior; Stuart, 1961).
iii. Larval polymorphisms. Unknown and unlikely, although Starrett (1960) reported occasional cannibalism.
iv. Features of metamorphosis. No information available.
v. Post-metamorphic migrations. No information available.
D. Juvenile Habitat. No information available, but juveniles presumably burrow in the soil, similar to adults.
E. Adult Habitat. Friable soils that permit burrowing, including cultivated fields and gardens, are required (Garrett and Barker, 1987). Duellman (1971) pointed out that habitats for this species are typically arid or savanna, tropical or subtropical, non-forested areas. Villa (1984) also emphasized that habitats are in dry rather than moist environments, and activity is opportunistic with heavy rainfall. Fenolio and Ready (1995) indicated a strong correlation between breeding and rainfall. James (1966) described in detail the habitat in Texas.
F. Home Range Size. No information available.
G. Territories. No information available.
H. Aestivation/Avoiding Dessication. Fouquette and Rossman (1963) reported a captive individual that did not feed for ≥ 18 mo, and during at least the last 5 mo, it apparently was aestivating in dried soil; upon rehydrating, the toad recovered and was quite active. No information is available for natural populations, but the soils where they burrow are often dry for weeks or months, so aestivation is presumed during such periods.
I. Seasonal Migrations. No information available.
J. Torpor (Hibernation). See "Aestivation/Avoiding Dessication" above.
K. Interspecific Associations/Exclusions. James (1966) noted western narrow-mouthed toads (Gastrophryne olivacea), Texas toads (Bufo speciosus), and Gulf Coast toads (B. nebulifer) calling abundantly in the choruses with burrowing toads in 1964, and Couch's spadefoot toads (Scaphiopus couchii) calling along with the Mexican burrowing toads in 1965.
L. Age/Size at Reproductive Maturity. Adults reach sexual maturity at 60–65 mm (Fouquette, 1969) with larger animals to 88 mm (Nelson and Nickerson, 1966). Females tend to be larger than males.
M. Longevity. No information available.
N. Feeding Behavior. Burrowing toads specialize in feeding on ants and termites in their underground nests. Hindlimbs have keratinized tubercles used in burrowing, and front limbs and snout are specially adapted for entering termite and ant tunnels to access prey (Trueb and Gans, 1983; Garrett and Barker, 1987). The tongue protrusion mechanism is uniquely designed for feeding on these insects underground (Trueb and Gans, 1983).
O. Predators. No information available.
P. Anti-Predator Mechanisms. Burrowing toads will grossly inflate their bodies, nearly concealing their head and limbs (Garrett and Barker, 1987). This is useful for subterranean burrowing (Trueb and Gans, 1983) and may also discourage predation on the surface. Other burrowing anurans (e.g., pelobatids and microhylids) may have noxious skin secretions that repel ants and termites from biting or stinging and are effective in discouraging many predators; no information is available on skin secretions of burrowing toads.
Q. Diseases. No information available.
R. Parasites. Santos-Barrera (1994) reported opalinid protozoans (Zelleriela sp.) from the gut of burrowing toads.
4. Conservation. Burrowing toads are considered Threatened by Texas, the only U.S. state where they occur, but are not listed by the U.S. Federal Government. Because these are secretive animals, determining their distribution and abundance, and therefore their conservation status, is a problem. There are no data that indicate changes in abundance or distribution over the 35 yr they have been known from the Texas localities.
1M.J. Fouquette Jr.
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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Citation: AmphibiaWeb. 2017. <http://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 29 May 2017.
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