Venezuela Pebble Toad

Males measure 16.5-23.5 mm in SVL, while females reach 20.4-30.0 mm in SVL. The head is slightly wider than long. The eye-snout length measures less than the eye diameter. The snout is truncate in profile, with a short and steep canthal ridge and a smooth, non-tuberculate loreal region. Nostrils are rounded, directed anterolaterally, and lack tubercles; the internarial region is concave. The interorbital distance is slightly larger than the width of the upper eyelid, which is itself covered in dense, sharp tubercles. No head crests are present. The tympanum is not visible. No teeth are present. The tongue is oval and attached anteriorly (Señaris et al. 1994).Arms and legs are thin. Hands are flat and palmate and are covered withtubercles. Fingers have expanded tips. Relative finger lengths are 3>4>2>1. Axillary membranes are lacking. When the leg is pressed against the body, the ankle reaches slightly above the shoulder. Toes are short and flattened, with three connected at a common base; relative lengths are 4>5>3=2>1. Skin is soft, sometimes wrinkled, and tuberculate, with tubercules on the dorsal and lateral sides of the body being larger and more dense than those on the ventral side. The vent has no ornamentation (Señaris et al. 1994).The body color is completely black, ventrally and dorsally, and lacks patterning (Señaris et al. 1994). In preservative, the coloration is a uniform dark brown or black, with the ventral side being slightly lighter (Señaris et al. 1994; Lathrop and MacCulloch 2007).Oreophrynella nigra was first described by Señaris et al. (1994). The species name, nigra, is Latin for "black," referring to its dark coloration.Video footage of O. nigra from the BBC documentary, Life (2009):

Endemic to the Guyana Highlands in Venezuela (McDiarmid and Gorzula 1989), a place high in biodiversity with many unique amphibian species (Lathrop and MacCulloch 2007; MacCulloch et al. 2007). This species has been found only on the tepuis Kukenan and Yuruaní in Bolívar State, Venezuela, from 2300-2700 m above sea level (Señaris and MacCulloch 2005). Oreophrynella nigra appears to be one of the few species of the genus Oreophrynella that is not restricted to solely one tepui (MacCulloch et al. 2007).The toads live around patches of vegetation scattered amid the bare sandstone rock of the tepuis (Señaris et al. 1994). Vegetation mainly consists of shrubs, flowering plants, and bromeliads such as Connellia spp. (Bromeliacea), Everardia spp. (Cyperaceae), Cyrilla recemiflora (Cyrillaceae), and others plants from the families Eriocaulaceae and Xyridaceae (McDiarmid and Gorzula 1989; Señaris et al. 1994).

Oreophrynella nigra is a diurnal, rock-dwelling species (McDiarmid and Gorzula 1989), inhabiting open areas of bare limestone (McDiarmid and Gorzula 1989). Its movements are slow and it travels by walking rather than by hopping (McDiarmid and Gorzula 1989).In the wild, O. nigra only eats insects from the groups Acari, Myrmicidae, and Coleoptera, but in the laboratory, it has been shown to eat any insect species it is presented with, apparently limited only by the frog's body size (Señaris et al. 1994). For example, O. nigra will eat Acheta domesticus, Collembola sp., Drosophila hydei, and Galleria mellonella in the laboratory (Señaris et al. 1994).The call consists of 6-16 pulsed notes lasting 0.5-0.11 seconds (McDiarmid and Gorzula 1989). The notes begin at a low frequency of about 2650-3200 Hz and then rise to a higher frequency of around 3000-3650 Hz, ending with a decrease of 300-350 Hz (McDiarmid and Gorzula 1989). There is considerable modulation within notes and between calls (McDiarmid and Gorzula 1989).Oreophrynella nigra is particularly known for its interesting defensive tactic: when faced with a predator, the frog will curl into a ball and roll away, tumbling over rocks to escape (McDiarmid and Gorzula 1989). (See video footage of O. nigra below.) This rolling behavior is thought to work in combination with the toad's cryptic skin coloration to help it avoid predators (McDiarmid and Gorzula 1989). The main predators in the area are large spiders in the family Theraphosidae (McDiarmid and Gorzula 1989). Interestingly, the rolling defensive technique is also used by the North American salamander Hydromantes platycephalus, demonstrating a case of convergent evolution (García-París and Deban 1995).The breeding behavior of these frogs has been contested (Señaris et al. 1994). On the one hand, communal nesting has been observed in populations from Kukenan tepui, where clutches of 8-35 eggs were attended by multiple toads (McDiarmid and Gorzula 1989). The eggs and toads were found within elaborate systems of tunnels, which McDiarmid and Gorzula (1989) suggested were built by male frogs in an elaborate social behavior (Señaris et al. 1994). On the other hand, others have asserted that the tunnels belong to arthropods and that the frogs do not have such an elaborate and communal breeding system, instead preferring to lay their eggs under moss and rocks in groups of 8-13 eggs (Señaris et al. 1994). In either case, the eggs are about 5-6 mm in diameter (Señaris et al. 1994). Embryos undergo direct development (McDiarmid and Gorzula 1989) and the timing of the development of the eggs is staggered (Señaris et al. 1994).

Although it is not directly threatened right now, its highly restricted range makes it vulnerable to future challenges (La Marca and Señaris 2004).

Oreophrynella nigra, or pebble toad, is a species of toad in the family Bufonidae. It is endemic to the Guiana Highlands in Bolívar State, Venezuela, and known from two tepuis, Kukenan-tepui and Yuruani-tepui, both belonging to the Eastern Tepuis.