Comprehensive DescriptionRead full entry
A rotund, egg-shaped amphibian, the Northern Sheep Frog (Hypopachus variolosus) inhabits dry and moist forests of lowland and premontane zones from southeastern Texas, USA, to central Costa Rica (Savage 2002), mainly below 1000 m in elevation but up to 2200 m on the Mexican plateau (Nelson 1974). A thick skin fold on the neck and a triangular snout readily identify this species as a member of the Microhylidae family; another distinctive feature includes the pair of metatarsal tubercles, or spades, on each hind leg (Leenders 2001). Typically, a smattering of dark patches and a single yellow streak extend down the gray, olive, and brown dorsum, with a similar light stripe standing out amongst the dark mottling of the white or grey belly (Conant & Collins 1991; Leenders 2001). A white diagonal stripe runs from each golden eye the towards the front limbs, usually with a dark eyemask also present (Leenders 2001). Adults measure roughly 30 to 50 mm in length, with females being the larger sex and lacking the dark throat coloration of males (Savage 2002). Stocky limbs, unwebbed digits, and nostrils near the very tip of the snout also characterize the body of the Sheep Frog (Savage 2002).
The species prefers areas dominated by short grasses, leading it to often be found in disturbed areas, particularly pastures and open woodlands (Nelson 1974). Secretive and strictly nocturnal, H. variolosus lives underneath rocks or logs, or underground (Leenders 2001). Locomotion primarily involves hopping as well as backward 'sliding' to burrow; the frog's sharp heel tubercles aid in the creation of a burrow which may stretch up to 1 m below the surface (but is less deep during the wet season) (Savage 2002).
When threatened, Sheep Frogs tend to freeze and rely on camouflage, though their stubby legs may power surprisingly lengthy jumps to allow escape (Leenders 2001). They also produce heavy mucous secretions which are irritating to the skin and eyes of humans—and can often stimulate sneezing spells in one who handles a Sheep Frog (Savage 2002). The main predators of the species, namely frog-eating snakes of the genera Leptodeira (cat-eyed snakes) and Coniophanes (black-striped snakes), however, do not seem deterred by these mucous secretions; instead, these secretions appear to better serve as a means to discourage stings from insects (Savage 2002). As the diet of these frogs consists primarily of ants and termites, they will actively forage for high concentrations of these prey underground (Leenders 2001).
H. variolosus employs a reproductive strategy known as explosive breeding: Though the frogs spend the majority of the year in a subterranean existence, following the first heavy rains of the rainy season (or flooding by irrigation), they surface (Conant and Collins 1991). Breeding commences in temporary pools, marshy sites, or standing water in pastures and agricultural fields (Savage 2002). While floating, sexually active males produce a nasal advertisement call resembling a sheep's bleat, spanning roughly two seconds, and rarely being repeated at less than fifteen-second intervals (Conant & Collins 1991). They display a shift from stationary to actively swimming behavior with increasing chorus size and density that may be an adaptation for both predator avoidance and competition for mates (Nelson 1973). Females lay 600 to 800 eggs in a loose mass on the water's surface (Savage 2002). Within twelve hours hatching occurs, and, facing transient ponds, tadpoles develop rapidly, metamorphosing into froglets in about thirty days (Leenders 2001; Savage 2002). The tadpoles lack tooth rows and beaks but possess oral flaps that are papillate or scalloped (Lips & Savage 1996). Though their diet has not been studied, their oral anatomy suggests they are generalized feeders, consuming a variety of organic and inorganic material (Judd & Irwin 2005).
The IUCN Red List has categorized the Sheep Frog as a Species of Least Concern, owing to its wide distribution, generalist habitat tolerance, and presumed large population (Santos-Barrera et al. 2013); the Texas Parks and Wildlife Department, however, currently lists the frog as Threatened and prohibits its collection (Judd & Irwin 2005).