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Duttaphrynus melanostictus, known as the Asian common toad, is widely distributed in South and Southeast Asia. It goes by a multitude of other common names including Asian toad, black-spectacled toad, black spined toad, common Sunda toad and Javanese toad. Ecological, geographical and genetic analyses have distinguished three distinct lineages (Asian mainland, coastal Myanmar, and Sundaic islands). It is probable that D. melanostictus is actually a complex of multiple species, but more research is needed to confirm species delineations (Wogan et al. 2016).
Asian common toads live in a wide variety of tropical Southeast Asian habitats, especially disturbed lowland areas. They are recorded from sea level up to 1,800 m (5,900 ft) in altitude and are considered a “human commensal” as they thrive in human-dominated agricultural and urban areas. Scientists estimate the original range of this species around the Myanmar-China border. These toads easily disperse as a result of human activities, leading Wogan et al. (2016) to describe them as tropical Asia’s “weediest” amphibian. Common Asian toads have invaded Borneo, Sulawesi, Seram, Indonesian New Guinea and multiple other Indonesian islands. Most recently this species was brought to Madagascar where it has spread rapidly. Its presence poses dire threats to native amphibians and other fauna as a predator, competitor, toxic threat to potential mammalian, avian and reptilian predators, and disease bearer (Kolby 2014). Wogan et al. compare its potential impact in Asia (and beyond) to that of the Cane toad (Rhinella marina) across Australia and Oceania.
Asian toads are a large species; females can grow to 20 cm (8 in) snout-vent length. Males are considerably smaller in size. Their dorsum can be yellow, red, brown or grey and is covered with black spiny warts and ridges. These wart patterns are unique and used in studies to identify individuals. Their underside is off-white in color, unmarked or spotted. They have a distinct tympanum (ear drum). Males have a subgular vocal sac and black pads on the inner fingers that help in holding the female during copulation (Khan and Koo 2016; van Dijk et al. 2004)
Adult toads are terrestrial and nocturnal. During the day they return to a selected hiding place with several other individuals. These daytime hiding spots may be holes or crevices or cover of rocks, leaf-litter and logs. Adults are active at night and feed upon a wide variety of invertebrates. They are slow and shy. In urban areas they frequently hunt insects attracted to streetlights, especially in times when winged termites swarm (Khan and Koo 2016; van Dijk et al. 2004).
The monsoon season often triggers breeding, although in some areas these toads breed year round. Males congregate around slow moving rivers, ponds, and pools calling with a “curr, curr, curr” to attract females. (Call recordings are available at http://amphibiaweb.org/cgi/amphib_query?where-genus=Duttaphrynus&where-species=melanostictus&account=amphibiaweb). Far outnumbered by males, females are quickly pounced upon by aggressive suitors and held in a nuptial clasp. Females lay a double jelly string containing the eggs, which gets entangled in vegetation in the waterbodies. The uniformly black tadpoles school in large groups, eating algae. Studies show that the tadpoles can recognize their kin. They grow to 26-27 mm in length before they metamorphose, then leave the water in swarms of froglets, many of which succumb to predators (Khan and Koo 2016; van Dijk et al. 2004).
Although the Asian toad is common, widespread and listed as “of least concern” by the IUCN, further research into the hidden diversity of D. melanostictus (in the form of distinct lineages and cryptic species included in this species name) may have some conservation implications as this becomes better understood (Wogan et al. 2016; Rowley et al. 2010). Asian toads are sometimes found in the pet trade, and eaten in northern Thailand (Khan and Koo 2016; van Dijk et al. 2004).
Kolby J.E. 2014 Ecology: stop Madagascar's toad invasion now. Nature 509, 563. (doi:10.1038/509563a)