Sunday, July 9, 2017

Small headed Treefrog, Dendropsophus microcephalus

A small yellow frog: 12-17 mm SVL; with the canthus forming a distinct ridge, and a light line from the anterior of eye to nostril. Dorsal skin smooth, ventral skin and skin under are thighs granular. Fingers with some webbing mostly reduced to bases of digits and lateral fringe; toes moderately webbed. H.goughi has a rounded canthus and lacks a lateral cream or enamel stripe. To distinguish it from H. m. misera: both species have a canthal ridge, but H. minuscula is much smaller, calling males tan and red, with side stripe that makes this species unique, and readily identified in the field.

Size. A small tree frog: males are typically 18-25 mm SVL (mean: 22 mm) and females 21-28 mm SVL (mean: 25 mm).

Identification. Dorsal skin smooth and variable in color, ranging from pale yellow and orange-brown. Venter is white, with distinct tubercular skin on the belly and thighs, which are used as complementary adhesive surfaces. Tubercles on the ventral thigh are yellow.  Reproductively active males have a yellow throat pouch and, when calling, are solid yellow in color. Dorsal markings are variable, often consisting of a network of darker brown speckled lines giving a ‘marbled’ appearance or creating an H-shaped marking between the shoulders. A dark interorbital line or blotch is common. A narrow brown line extends laterally from the nostrils, along with the canthal ridge, over the eye to midway down the body. This gives the frog’s head a distinctive triangular appearance. Dark brown flecks are scattered across the dorsum, the forelimbs and the lower hind limbs, with these forming two to three indistinct transverse bars on the shin in many individuals. Thighs are translucent and yellow with no pigmentation. Eyes are prominent, with a bronze iris and horizontally elliptical pupils. Eye diameter is generally greater than or equal to the eye-nostril distance. The tympanum is distinct and speckled with gold flecks. Webbing on the fingers is reduced, toes have moderate webbing. Fingers and toe tips are yellow, with expanded discs to aid with climbing at the ends of the digits. The feet have prominent subarticular tubercles, which are yellowish in color. 

Vocalization. Males call from marginal vegetation, emergent grasses and shrubs, usually at heights of 20-50 cm above the water. Dendropsophus microcephalus call in large assemblages, often with multiple species present (including D. goughi, D. minusculus, L. validus, S. ruber, H. punctatus, P. trinitatis and S. lacteus).  The call is a high-pitched “skrreeet, riti-titi-titi-titi” with the last phase of the call having an almost sawing rhythm.

Similar species. Small-headed Treefrog is one of three closely related frogs found in Trinidad (see also D. goughi and D. minusculus) all of which are small and yellowish in color. Dendropsophus microcephalus has a distinct pale line that runs from the back of each eye to just over the nostril, which makes the head appear almost pointed in shape. Dendropsophus minusculus is smaller as an adult, and has an enamel colored lateral stripe. The belly of Dendropsophus goughi is yellow rather than white, it has white markings on the rump and heels, and a rounded canthus all are absent in the other two dendropsophids. See the other species descriptions for comparison. 

Biology. Diet includes ants, spiders, ticks and mites. Arachnids are a particularly important element of their diet and can comprise over 80% of prey. Soil organisms, such as collembolans are a significant prey item for the wet season. Adults and tadpoles are vulnerable to predation by a wide range of both vertebrate and invertebrate predators, including snakes, (Leptophis), large spiders, and water bugs. Crabs are thought to be significant predators of both the adult frogs and their eggs in Trinidad. Dendropsophus microcephala is an opportunist breeder, with reproductive activity triggered by significantly increased rainfall. In Trinidad, males have been recorded calling throughout the rainy season with peaks in activity in June-July. Frogs can be found calling in mixed-species choruses, in the north of Trinidad, particularly alongside D. microcephalus which often outnumbers them significantly. In Trinidad, calling males are often attended by silent satellite males which attempt to intercept females attracted to the breeding ponds. 
Distribution. Widely distributed in Middle and South America and has been recorded from Mexico, Guatemala, Honduras, Nicaragua, Costa Rica, Panama, Colombia and Venezuela as well as from Trinidad and Tobago. The island subspecies is D. microcephalus misera, and is also known from the llanos of Colombia and Venezuela. In Trinidad, D. microcephalus is found island wide, at low elevation sites ranging from the southwestern peninsula to Chaguaramas. On Tobago, it occurs at scattered localities but widespread.

Habitat. The Small-headed Treefrog is highly adaptable and is often found in disturbed habitats, forest edges, and savanna habitats. Breeding sites include temporary ponds, shallow roadside ditches, and flooded pasture land as well as swamps and marshes. They are rarely recorded from more enclosed, forest habitat.    

Biology.  Nocturnal, but can be found during the day close to their calling sites, concealed in vegetation. Diet consists of small flies, spiders, and cockroaches. Dendropsophids fall prey to a wide range of predators including bats, snakes, other frogs and large invertebrates such as water bugs, fishing spiders, and centipedes. 

Calling can be heard throughout the year as long as water is available. Studies in Colombia and Venezuela suggest that D. microcephalus is an opportunistic breeder, with activity increasing during and after rainfall. Eggs are laid on or near the surface of the water, usually attached to emergent vegetation. Spawn is present as a small cohesive mass, with each dark-colored egg within its own jelly capsule. Reported clutch sizes range from 150-400 eggs, though clutches of around 200 eggs are more typical for Trinidad. 

Tadpoles are tiny and black-brown when they first hatch. Later stage tadpoles are distinctive, and colorful in appearance with a silver-white belly flecked with black, a yellow-brown dorsum and tail, which are mottled and marbled with black. The tail is deep and leaf-shaped and tapers off to a distinct filament, which is barred with black bands. A dark-brown stripe runs from the nose through the eye and along the side of the tadpole’s body. Tadpoles are filter-feeders and are often observed in open water, hanging vertically and maintaining their position by rotational movements of the filamentous tail tip. Development is rapid, with metamorphosis occurring at about six weeks. Maximum lengths are around 35 mm. Newly metamorphosed froglets are about 10 mm SVL.

Conservation status. Least Concern. Data suggests it is both abundant and increasing in numbers. Taxonomic status of subspecies needs clarification. It is less susceptible to chytridiomycosis than many other species. There is some suggestion, therefore, that it might act as a ‘reservoir taxa’ and it may be important to consider the nature of interactions that might occur in mixed species assemblages at breeding sites.

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