© 2007 Twan Leenders (1 of 18)
Country distribution from AmphibiaWeb's database: Belize, Costa Rica, Guatemala, Honduras, Mexico, Nicaragua, United States
Hypopachus variolosus (Cope, 1866[b])
Frank W. Judd1
1. Historical versus Current Distribution. Sheep frogs (Hypopachus variolosus) range from Costa Rica northward through Mexico and reach the northern limit of their distribution in southern Texas (Smith and Barlowe, 1978; Conant and Collins, 1991). In Texas, B.C. Brown (1950) gave the range as extending from Cameron and Hidalgo counties along the Rio Grande, and north to Duval County; subsequent maps have extended the range farther north. The distribution as mapped by Raun and Gelbach (1972) showed sheep frog records from 11 counties. Dixon (2000) mapped additional records filling in the gap along those counties bordering the coast, but provided no additional records to the north or west. Based on county records, Dixon (2000) provides the most current distribution in Texas. Sheep frogs are currently known from 15 counties ranging from Cameron, Hidalgo, and Starr along the Rio Grande, north to Aransas and Refugio in the Coastal Bend. It is not known if these range extensions reflect more extensive collecting or a northward movement of the species. Sheep frogs are found where the relative humidity is high (typically 80% or greater) and therefore do not extend inland from the Gulf of Mexico > 160 km. Conant and Collins (1991) show a hiatus in the geographic range from central Tamaulipas, Mexico, to the Rio Grande, thus making the Texas population disjunct.
Sheep frogs are currently listed as Threatened by the Texas Parks and Wildlife Department and are protected from collection.
2. Historical versus Current Abundance. Sheep frogs have never been considered abundant in the United States. Publications from early in the nineteenth century regard them as uncommon and rarely seen (Wright and Wright, 1933; Mulaik and Sollberger, 1938). However, in areas with intact native brushland, after sufficient rainfall, I have found them to be common in Hidalgo, Starr, and Willacy counties, Texas (K.J.I., personal observations).
3. Life History Features.
A. Breeding. Reproduction is aquatic. Wright and Wright (1949) state that breeding takes place from March–September following heavy rains; Mulaik and Sollberger (1938) report that eggs are laid from April–October. Breeding also may be stimulated by irrigation of fields (Wright and Wright, 1949; Conant and Collins, 1991). Wiewandt et al. (1972) found a breeding chorus in southern Sonora, Mexico, at an air temperature of 24.5 ˚C and a water temperature of 27.7 ˚C. Nelson (1973) recorded choruses from 26 locations in Texas, Mexico, and Central America at temperatures ranging from 18–30 ˚C. The combined information from these reports suggests that breeding may occur at any time from March–October. This is in keeping with the temperature and rainfall regimes in southern Texas. Mean air temperature for March at Brownsville, Cameron County, Texas, is 20.2 ˚C (Lonard et al., 1991) and 69% of the annual rainfall of 68.2 cm occurs from May–October (Lonard et al., 1991). One of us (K.J.I., personal observations) recorded breeding from mid July to mid September in Hidalgo, Starr, and Willacy counties in southern Texas. Recently metamorphosed juveniles were observed on 15 July in Willacy County, Texas, indicating that breeding had occurred earlier in the season (K.J.I., personal observations).
i. Breeding migrations. Migrations occur from underground sites to ephemeral wetlands formed during and after rains (Wright and Wright, 1933). In Willacy County, Texas, on 18 July, I found adults active on the surface at 26 ˚C from 2100–2400 h after rainfall earlier in the day. Although calling males were not heard, available breeding sites were observed nearby (K.J.I., personal observations).
The call of sheep frogs is a clear, resonant bleat, resembling a sheep, from which the species derives its common name. Wiewandt et al. (1972) characterize the call as sounding like a deep, droning, nasal "aaaaaa" (as in bat). The call duration is 1.8–2.7 s and is emitted at 10–20 s intervals. The dominant frequency is in the range of 960–1,272 Hz (Wiewandt et al., 1972). Nelson (1973) reports the harmonic interval increases at about 5 Hz/˚C. His analyses of calls from 26 localities throughout the range of the species showed no evidence of geographic variation.
ii. Breeding habitat. Mulaik and Sollberger (1938) report that breeding typically occurs in temporary pools following heavy rains. Wiewandt et al. (1972) found a breeding chorus in a temporary pond in a deciduous subtropical thorn forest in Sonora, Mexico, and McDiarmid and Foster (1975) found a tadpole in the hollow of a tree 87 cm above the ground in Costa Rica. One of us (K.J.I., personal observations) found breeding choruses in areas of intact Tamaulipan brushland of Hidalgo, Starr, and Willacy counties in southern Texas. Sheep frogs have been observed calling in temporary wetlands ranging from roadside ditches, railroad right-of-way ditches, and natural pothole basins. Thus, it appears that breeding may occur in a variety of temporary to permanent aquatic habitats.
Mulaik and Sollberger (1938) report males call while floating free at the water's surface. Conant and Collins (1991) add that floating males at times hold on to stems as they call. In Sonora, Mexico, calling males were at the water's edge, by sticks, or in shallow depressions in mud (Wiewandt et al., 1972).
B. Eggs. Laid within 24 hr following heavy rains.
i. Egg deposition sites. Eggs are laid in rafts loosely held together (Mulaik and Sollberger, 1938) and usually in temporary pools, but see "Breeding habitat" (above) for departures.
ii. Clutch size. Mulaik and Sollberger (1938) report collecting ~700 eggs from a breeding pair. Ova with one envelope range from 1.5–2.0 mm in diameter (Mulaik and Sollberger, 1938).
i. Length of larval stage. Eggs hatch in ~12 hr and larvae transform in 30 d (Mulaik and Sollberger, 1938).
ii. Larval requirements.
a. Food. Unknown, but the mouth parts suggest tadpoles are generalized feeders that take a variety of organic and inorganic material.
b. Cover. Unknown.
iii. Larval polymorphisms. Unknown and unlikely.
iv. Features of metamorphosis. Tadpoles are small (27–30 mm TL). Newly metamorphosed animals measure from 11.0–16.0 mm (Wright and Wright, 1949). Gills are resorbed about 30 hr after hatching (Wright and Wright, 1949).
v. Post-metamorphic migrations. Newly metamorphosed sheep frogs migrate from wetlands to underground upland sites. During these migrations they seek cover under surface objects, including cow dung, old shoes, and litter (H.C. Blanchard, in Wright and Wright, 1933). Recently transformed individuals were found in wet sedges of a water lily pond in San Benito, Cameron County, Texas (Wright and Wright, 1949). On 15 July, five recently metamorphosed juveniles were found active on the surface at 1200 h around the margins of a temporary pond in Willacy County, Texas (K.J.I., personal observations). The weather was overcast, with light rain and 27–29 ˚C. Although the sample size is small, this observation indicates that newly metamorphosed animals make dispersal movements from breeding sites when humidity or moisture conditions are favorable.
D. Juvenile Habitat. Unknown, presumably similar to adults.
E. Adult Habitat. Sheep frogs occur in warm temperate and tropical thorn scrub and savanna habitats (Nelson, 1974). Throughout their geographic range, sheep frogs are most frequently encountered in open woodlands or pasturelands with abundant short-grass cover (Nelson, 1974). In these habitats, they are found in moist subterranean burrows. Mulaik and Sollberger (1938) state that they do not come out into the open except when driven out by excessive rains, and that they can be found in burrows among the trash (perhaps they mean dens) of packrats and in hollows under trees (presumably fallen trees). Sheep frogs are also commonly found in vegetative debris near ponds and irrigation ditches (Garrett and Barker, 1987). Nelson (1974) suggests that unsuitability of mesic areas may partly account for the northern limit of the species in Texas as “adjacent regions to the north and east (but not the west) are predominantly tall grass or forest.” One of us (K.J.I., personal observations) once found four adults under the same log, adjacent to a breeding pond on 15 July in Willacy County, Texas. The surrounding habitat was mature coastal brushland (see Jahrsdoerfer and Leslie, 1988).
F. Home Range Size. Unknown.
G. Territories. Unknown.
H. Aestivation/Avoiding Dessication. The moist, underground or covered sites used by adults prevent desiccation. Mulaik and Sollberger (1938) note that as humidity decreases, sheep frogs seek out deeper and moister situations or they burrow backwards into the soil where they are not subjected to rapid drying. Brown (B.C., 1950) reported that specimens were found at a depth of 76 cm at the bottom of postholes during the dry season.
I. Seasonal Migrations. Unknown, other than breeding migrations.
J. Torpor (Hibernation). Unknown.
K. Interspecific Associations/Exclusions. Wright and Wright (1949) note co-occurrences with Coastal-Plain toads (Bufo nebulifer) and chorus frogs (Pseudacris sp.). One of us (K.J.I., personal observations) found sheep frogs calling syntopically with Great Plains narrow-mouthed toads (Gastrophryne olivacea) and Couch’s spadefoot toads (Scaphiopus couchii) in Starr County, Texas. Sympatric amphibians observed in or near sheep frog breeding sites in Willacy County, Texas, include Texas toads (Bufo speciosus), spotted chorus frogs (Pseudacris clarkii), Great Plains narrow-mouthed toads (Gastrophryne olivacea), Rio Grande leopard frogs (Rana berlandieri), plains spadefoot toads (Spea bombifrons), Coastal-Plain toads, and Couch’s spadefoot toads. Reptiles observed include yellow mud turtles (Kinosternon flavescens), Texas tortoises (Gopherus berlandieri), Texas spiny lizards (Sceloporus olivaceus), Texas patch-nosed snakes (Salvadora grahamiae), western ribbon snakes (Thamnophis proximus), checkered garter snakes (Thamnophis marcianus), Mexican milksnakes (Lampropeltis triangulum), coachwhips (Masticophis flagellum), Schott's whipsnakes (Masticophis schotti), and Texas indigo snakes (Drymarchon corais; K.J.I., personal observations).
L. Age/Size at Reproductive Maturity. Males 25.0–37.5 mm SVL, females 29.0–41.0 mm (Wright and Wright, 1949).
M. Longevity. Unknown.
N. Feeding Behavior. Sheep frogs have a specialized diet, taking predominantly ants and termites (Hymenoptera). Mulaik and Sollberger (1938) comment: "Adults driven from under mesquite trees during irrigation were found to have been feeding on termites and minute dipterous insects. Specimens in captivity likewise fed readily upon termites. Even though buried several inches underground, while in captivity, the placing of termites upon the surface readily brought them into the open." Dundee and Liner (1985) recorded the presence of ants in the gut of a sheep frog taken as a prey item of the hylid Phrynohyas venulosa in Yucatán, Mexico. Two adults were found in drift fence funnel traps in Hidalgo County, Texas, on the morning of 20 August 1992 (K.J.I., personal observations). This surface activity was prompted by rainfall the previous night and considered to be foraging-induced, as no breeding sites were found nearby nor were choruses heard.
O. Predators. Single ribbon snakes (Thamnophis proximus) may take up to nine or ten newly metamorphosed sheep frogs (Wright and Wright, 1949). These authorities also suggest that cooters (Pseudemys sp.) might prey on newly metamorphosed sheep frogs.
P. Anti-Predator Mechanisms. Nocturnal, secretive.
Q. Diseases. Unknown.
R. Parasites. Mulaik (1945) described a new species of mite Caeculus hypopachus (Caeculidae) taken from the venter of a specimen from Hidalgo County, Texas.
4. Conservation. Sheep frogs are currently listed as Threatened by the Texas Parks and Wildlife Department and are therefore protected from collection. They are not listed by the U.S. Fish and Wildlife Service, nor are they proposed for listing. There are no data on population density anywhere within the species' geographic range, thus statements about their conservation status are based on the general impressions of scientists and resource managers familiar with the species. Certainly, there are no quantitative data to determine if numbers are increasing or decreasing. These data are sorely needed, but are unlikely to be produced unless the Texas Parks and Wildlife Department or the U.S. Fish and Wildlife Service initiate a comprehensive study of amphibian abundance in southernmost Texas. A 3–5 yr mark-recapture study of the population density of all the amphibians at ≥ 5 sites in the Lower Rio Grande Valley of Texas (LRGV) would yield valuable information of the abundance of 23 amphibian species, 5 of which are listed as Threatened in Texas.
Proposed efforts by the U.S. Fish and Wildlife Service, Texas Parks and Wildlife Department, and Texas Nature Conservancy to re-vegetate the LRGV with late successional species promise to restore habitat for sheep frogs and other species. However, if we are to know how effective these re-vegetation programs are, we need baseline data on species richness and abundance at the time of the plantings.
1Frank W. Judd
2Kelly J. Irwin
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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