Silverstone’s Poison Frog (Ameerega silverstonei)

The Silverstone’s poison frog is a large Dendrobatid frog and there is little external sexual dimorphism except in size. Male adult Silverstone’s poison frogs are large and have a maximum snout-vent length of 38.3 millimeters (1.5 inches). Females are typically slightly larger than males with a maximum snout-vent length of 42.8 millimeters (1.7 inches).

The Silverstone’s poison frog’s head is characterized with a sloping and rounded snout that is lateral in profile and truncates to bluntly rounded in dorsal or ventral aspect. The head is generally as wide as the body, except gravid females tend to be more rotund. When described, head width averaged 99% of the greatest body width in 12 males (range 87-115%) and 88% in six females (78-118%). The widest part of the head is between the jaw articulations.

The nares are situated toward the tip of the snout and are directed posterolaterally. Both nares are visible from the front and from below but not visible from above, although nareal bulges cause the truncated aspect of the snout in dorsal (and ventral) view.

The canthus rostralis is rounded and the loreal region is vertical and very slightly concave. The interorbital area is wider than the upper eyelid.

The Silverstone’s poison frog’s fingers are appressed and flattened and have small, expanded discs at their tips. The relative length of the appressed fingers is 3>4≥1≥2, each terminating in slightly expanded disc. If measured from the base, finger 1 is distinctly longer than fingers 2 and 4, but when appressed their discs overlap and the three are nearly equal. The disc of third finger is 1.20-1.56 times wider than distal end of the adjacent phalanx, with no appreciable sexual dimorphism. A large, circular to elliptical tubercle is located on the median base of palm, and a small elliptical one (=inner metacarpal tubercle, but sometimes indistinct) is on the base of the first finger. These tubercles are low, with rounded surfaces. Some individuals have a suggestion of small, low, and poorly defined tubercles on the palm distad from the large one. One or two subarticular tubercles are on the fingers (one each on fingers 1 and 2, two each on fingers 3 and 4). The subarticular tubercles are somewhat protuberant in lateral profile. A few specimens have a keel-like ridge extending from the outer side of the large palmar tubercle to the proximal subarticular tubercle on finger 4.

There is no webbing between the digits of either the hands or feet. They also lack distinct interarticular tubercles or lateral fringe.

The Silverstone’s poison frog’s hind limbs are moderately long, with the heel of the appressed limb reaching the eye or beyond. The tibia/snout-vent length is 0.45-0.50.

The relative lengths of the appressed toes is 4>3>5>2>1, each terminating in a slightly expanded disc. The toe discs tend to be slightly wider than the finger discs.

The appressed first toe reaches the subarticular tubercle of the second. There are small inner and outer rounded metatarsal tubercles, and often a smaller or equivalent-sized median metatarsal tubercle in between. Some individuals have the suggestion of additional, smaller tubercles distad from the metatarsal tubercles. There are one to three subarticular tubercles (one each on toes 1 and 2, two each on toes 3 and 5, three on toe 4). The distal half (or more) of the tarsus with a keel-like ridge extending to the inner metatarsal tubercle. The slightly elevated, oblique proximal end of the keel is interpreted as a tarsal tubercle. The foot tubercles, particularly the subarticular tubercles, are protuberant in lateral profile.

On the thigh, the deep m. semitendinosus is independent of the superficial m. sartorius, and the distal tendon of the semitendinosus pierces that of the mm. gracilis is complex prior to insertion. This peculiar condition of the distal thigh tendons has been documented in a few species of dendrobatids and other frogs, and it was found no exceptions in a sample of 41 species of Dendrobatidae.

The skin of the Silverstone’s poison frog’s lower body and hind limbs is coarsely granular, especially on the back. It gradually smooths out in the skin atop the head, and on the forelimbs and ventral surfaces.

The flesh of freshly skinned specimens of Silverstone’s poison frog is pinkish white, more like the usual condition in Colostethus than the gray or blackish-colored flesh of most Dendrobates and Phyllobates. Melanophores are present, albeit sparse, on blood vessels and muscle fascia at all levels.

Teeth are present on the Silverstone’s poison frog’s maxillary arch.

The Silverstone’s poison frog’s tympanum, or eardrum, is concealed post-dorsally where it subcutaneously dips under the anterior edge of the m. depressor mandibulae.

The m. depressor mandibulae is comprised of three slips. A large, superficial slip originates from the dorsal fascia and conceals a deeper slip originating on the otic ramus of the squamosal bone. A shorter, poorly defined slip originates on the posterior part of the tympanic ring. There is no m. adductor mandibulae externus superficialis. Thus, in the aforesaid characters, the jaw musculature is of the normal dendrobatid pattern.

Generally in dendrobatids, the large superficial slip of the depressor mandibulae muscle tends to slightly overlap the tympanic ring and, in any case, holds the skin away from the rear part of the tympanum, thus accounting for the fact that the tfympanum is only partially indicated externally.

The tympanum is seen by dissection to be circular or slightly vertically elliptical with an area greater than 50% of the eye.

The upper third of the Silverstone’s poison frog’s iris is pale bronze with some black suffusion, turning black on the lower two-thirds, with possible minute bronze flecking. Eye length is greater than the distance from the anterior edge of the naris to the eye (latter distance 60-88% of eye length). Eye/snout length is 0.70-0.90.

Despite lacking great potency, the Silverstone’s poison frog has bright coloration which deters predators. The color pattern is a convergent autapomorphy.

The head, body, and forelimbs of an adult Silverstone’s poison frog is orange or red, sometimes with heavy black spots, marbling, or mottling, especially towards the rear. Sometimes a black reticulum extends forward on the back, at times so extensively as to demarcate bright spots on a black ground. Individuals having extensive black on the dorsum usually have the tympanum concealed in a black postocular spot, and, with addition of a black preocular stripe, some have a complete face mask. Such individuals also may have black markings on the forelimbs. The hind limbs are largely or partially black and may have spots of body color on the thighs or with bright body color extending dorsally over the thighs and onto the shanks. Some specimens (e.g., holotype) have a large calf spot of body color concealed below the knee on the anteroventral face of the tibia. This mark is reduced or absent in others, or present but fused with bright color from atop the thighs in individuals having brightly pigmented limbs. The forelimbs and undersides of the head and body are variably pigmented, ranging from black to uniformly pale, faded orange. Any light ventral color is much paler and not as bright as on the dorsum and often suffused with gray, except the undersides of the arms. The palms of the hands and feet and the underside of the digits also vary from grey to immaculate, bright orange. The gular area is often pigmented gray or black in females as well as in males.

This pattern is developed within a month or so after birth, and expansion of the orange-reddish areas occurs after twelve months.

Bright orange or reddish parts of the body turn pale yellowish or grayish in preservative.

There is interpopulational variation in the color pattern of the Silverstone’s poison frog, and perhaps also in the ontogenetic development of the pattern. Geographic variation is likely to be extensive if the species proves to occupy a large range in the montane forest of Cordillera Azul.

SEXUAL DIMORPHISM

Smaller Males with Vocal Sacs

BODY LENGTH

38.3-42.8 mm. / 1.5-1.7 in.

RELATIVE FINGER LENGTH

3>4≥1≥2

RELATIVE TOE LENGTH

4>3>5>2>1

TIBIA/SNOUT-VENT LENGTH

0.45-0.5

EYE/SNOUT LENGTH

0.7-0.9

Silverstone’s poison frog was described by Myers & Daly in 1979 under the original scientific name of Epipedobates silverstonei.

Prior to the official description in 1979, this species was known to many people but under incorrect names, such as Phyllobates bicolor. The frog is now recognized under the name Ameerega silverstonei.

The Silverstone’s poison frog is a species of frog in the family Dedrobatidae. This family consists of poison dart frogs, also known as dart-poison frogs, poison frogs, or formerly poison arrow frogs.

These amphibians are often called dart frogs due to the Native Americans’ use of their toxic secretions to poison the tips of blowdarts. However, of over 170 species, only four have been documented as being used for this purpose as curare plants are more commonly used. All of the species known to be used for weaponizing purposes come from the genus Phyllobates, which is characterized by the relatively large size and high levels of toxicity of its members.

The Silverstone’s poison frog can be distinguished from other members of the Dendrobatidae family by its large size and the lack of stripes in its orange-and-black or red-and-black coloration. Dendrobatids with uniformly colored bodies that superficially resemble the black-legged poison dart frog (Phyllobates bicolor), can be readily differentiated by observing the coarsely granular skin on the lower back that is smooth in P. bicolor, more prominent foot tubercles, and by various other differences.

KINGDOM

Animalia

PHYLUM

Chordata

CLASS

Amphibia

ORDER

Anura

FAMILY

Dendrobatidae

GENUS

Ameerega

SPECIES

Silverstonei

SUBSPECIES

None

Prior to its official description in 1979 as Ameerega silverstonei, this species was known to many people but under incorrect names, such as Phyllobates bicolor or Epipedobates silverstonei.

The Silverstone’s poison frog was discovered in the 1940’s during road construction across the Cordillera Azul on the Amazonian flank of the Peruvian Andes. John C. Pallister collected the first specimen in 1946 while on an entomological collecting trip for the American Museum of Natural History. The specimen was duly recognized as a new species of dendrobatid frog and set aside for description by Emmett Reid Dunn, then a research associate in the Museum’s Department of Amphibians and Reptiles, but the description never materialized as Dunn died in 1956 and the specimen languished in the unidentified collections. In 1954, a male frog with tadpoles on its back was caught and photographed by Edward S. Ross, an entomologist from the California Academy of Sciences. Unfortunately, Cochran misapplied the old name Phyllobates bicolor to the new frog.

In 1976, Philip A. Silverstone observed that the black-legged poison dart frog (Phyllobates bicolor) was restricted to the northern Andes of Colombia and it was therefore highly unlikely that the Peruvian Silverstone’s poison frog was related to the Colombian P. bicolor. Silverstone refrained from naming the species in deference to ongoing work.

In 1979, Charles Myers and John Daly proposed that the frog be named after Silverstone for making this distinction. The newly named silverstonei belonged to a group containing the type species of Dendrobates rather than to the demonstrably monophyletic group containing the type species (bicolor) of Phyllobates. The frog was first put in the genus Epipedobates and was known for some time as E. silverstonei. Presently, it is recognized as a member of the genus Ameerega and is now known as Ameerega silverstonei.

ALTERNATE

Rana Venenosa

GROUP

Army, Colony

YOUNG

Froglet, Larvae, Tadpole

The Silverstone’s poison frog is endemic to the Cordillera Azul, a small mountain range that flanks the eastern side of the Andes, Huánuco Region in Peru. This stands between the northward-flowing Huallaga and Ucayali rivers, two major tributaries of the upper Amazon, and between the towns of Tingo Marfa and Pucallpa on the rivers. The frog can be found at elevations ranging from 1,200–1,800 meters above sea level. Its range is not precisely known and might be wider than is thought.

It has also been introduced to the Tarapoto area of San Martín Region, although very little is known about this biological population.

It is considered to occur in two threat-defined locations (based on the currently known disjunct geographical areas and trade pressure exerted in Cordillera Azul). Its extent of occurrence (EOO) is 2,874 square kilometers.

EXTANT

Peru

The Silverstone’s poison frog is a terrestrial and freshwater species that is found among the leaf litter of the forest floor. Its natural habitats are subtropical or tropical moist montane forests and wetlands. It is a montane tropical rainforest specialist.

Reports from locals state that this species is abundant in both pristine forested and agricultural areas. While mainly found under logs and large piles of leaves, the frogs have also been found in open areas in the forest and in cleared areas near a tea plantation, and other agricultural areas.

It has been observed that these frogs have a preference for living in undisturbed forests.

FOREST

Subtropical/Tropical Moist Montane

WETLANDS

Permanent Rivers/Streams/Creeks/Waterfalls, Seasonal/Intermittent/Irregular Rivers/Streams/Creeks, Permanent Freshwater Marshes/Pools, Seasonal/Intermittent Freshwater Marshes/Pools

The Silverstone’s poison frog is a diurnal, terrestrial and freshwater species. It is wary and usually quick to hide, and seems to have some preference for edge situations.

To defend against predators, the Silverstone’s poison frog’s piperidine skin secretions contain small amounts of alkaloids, mainly of the pumiliotoxin-A class, a toxin found in the skin of poison frogs. However, the secretions lack the more potent batrachotoxin alkaloids of Phyllobates.

CIRCADIAN RHYTHM

Diurnal

MOVEMENT PATTERN

Non-Migrant

Silverstone’s poison frogs eat many kinds of small insects, including fruit flies, ants, termites, young crickets, and tiny beetles, which are the ones scientists think may be responsible for the frogs’ toxicity.

Silverstone’s poison frog tadpoles feed on invertebrates in their nursery.

PREFERENCE

Generalist

STYLE

Forager

There have been no observations or records of Silverstone’s poison frog courtship.

Silverstone’s poison frog males are territorial during the breeding season. When they reach sexual maturity, males begin mating trills to attract females.

There have been recordings of Silverstone’s poison frog calls, which have been described as retarded trill calls, the fourth class of dendrobatid vocalizations to be defined. Their call is a series of short, similar abrupt whistles, distinctly spaced and produced at 4-6 notes per second.

Only adult males possess the small, shallow, subgular vocal sac in the throat rarely indicated externally by weak longitudinal crease (not a definite fold) on each side of throat and well-developed vocal slits on the floor of the mouth.

The Silverstone’s poison frog probably breeds throughout the year. It is assumed that female Silverstone’s poison frogs lay eggs more than once a year, and possibly throughout the entire year due to the constantly wet climate in which they live.

The female lays her eggs terrestrially in a closely packed single layer underneath leaves and leaf-litter. Clutches are small, usually made up of about 30 eggs, and have been observed on leaves. The male holotype was found in November guarding a clutch of 30 eggs in a dry leaf.

The male Silverstone’s poison nurse frog guards the eggs until they hatch, then carries the hatched larvae on his back to a suitable terrestrial body of water, such as an ephemeral pool, stream, puddle, or water-filled crevice where the tadpoles develop until undergoing metamorphosis.

Although clutches have been found on dead leaves on the forest floor with male frogs in attendance, males have not been observed to defend eggs when clutches were approached.

Silverstone’s poison frog tadpoles are grayish or blackish-brown, with a few smatterings of darker spotting. The beak of the tadpole is keratinized, but does not appear of considerable size. The mouth has delicately toothed edges, and the lower beak has a broad V-shape. They have a single row of pointed papillae.

As they age, late tadpoles acquire dull yellow pigmentation on their forelimbs, snout, and upper eyelids. The dark brown of the larvae darkens to black as they become froglets and the yellow changes to light orange, and sometimes into red thereafter. The orange area expands over the head and torso. This pattern is developed within a month or so, and expansion of the orange-reddish areas occurs after twelve months.

BREEDING SEASON

Year-Round

BREEDING INTERVAL

Less than 1 Year

PARENTAL INVESTMENT

Paternal

CLUTCH

30

The Silverstone’s poison frog is known to be distasteful to snakes. In one observation, a frog-eating snake (Chironius sp.) of the Cordillera Azul area, seized and then immediately released a member of the species. Afterwards, the snake seemed disoriented and tried to rub out its mouth with branches, indicative of the toxic deterrent in the frog’s skin. However, the poison was not fatal and the snake seemed to recover fully.

Not much more is known of the Silverstone’s poison frog predators.

The Silverstone’s poison frog is a target for illegal collection and is consistently smuggled for the international pet trade, via Pucallpa in the Ucayali Region.

In the late 1980s, over 60 individuals were released at kilometer 20 on the Tarapoto-Yurimaguas road by authorities in charge of animal confiscations.

This species continues to be collected for illegal exportation. One farmer from the mountains east of Tingo Maria stated that he continues to sell “40–60 adults a month” and has done so for the last several years. Despite this, relatively few individuals have been found on the black markets, suggesting that most of them probably died during transit through the cities of Iquitos and Pucallpa. Conversations with locals in December 2016 indicate that the collection and selling by the farmer continues.

PETS/DISPLAY ANIMALS, HORTICULTURE

International

Due to harvesting of the Silverstone’s poison frog for the pet trade, the population is thought to be decreasing. Current subpopulations appear to be a small fraction of what they once were, although there is not enough data to determine population size.

In 1979, 26 individuals were recorded with a survey effort of 6 person/days, while in 2006 only eight individuals were observed in 15 person/days. In December 2016 near the town of Divisora, surveys of 9 person-hours encountered one adult, one sub-adult, and several dozen tadpoles in water pools on a trail. This survey was conducted during heavy rainfall so no adult frogs would have been calling, therefore more individuals could have been present but could not be detected.

Surveys between 2004-2010 at the release site for the introduced subpopulation near Tarapoto have never recorded any individuals of this species. The survey effort varied between two to three people with two to five hours of searching each.

The Silverstone’s poison frog is threatened by habitat destruction, accommodating the agricultural expansion of humans. Much of the frog’s habitat has been cleared for agricultural activities, including cattle pastures and tea farms.

For the past 15 years, this species has also been under threat due to heavy smuggling activity and illegal harvesting for the international pet trade by local people for dealers.

Although there is not enough data to determine population size, it has been observed that these frogs have a preference for living in undisturbed forests. This makes them vulnerable to agriculture, logging operations, and road constructions that border their habitat.

AGRICULTURE & AQUACULTURE

Annual & Perennial Non-Timber Crops

BIOLOGICAL RESOURCE USE

Hunting & Trapping Terrestrial Animals, Logging & Wood Harvesting

In the past, the habitat area where the Silverstone’s poison frog occurs had been somewhat protected by the presence of terrorist groups (excepting drug crops used in drug trafficking). More recently, coca eradication campaigns and alternative development programs have diminished the presence of drug crops in the region.

Conservation of other areas where this species occurs is required, as is enforcement on smuggling activities.

Further research is needed into the distribution, population status, ecology, and threats affecting the Silverstone’s poison frog, especially with regards to the impact of illegal trade. The population status and trends of this species are not fully known, as there is insufficient data for the species.

There is also a need for population monitoring of the status of this species given the threats of habitat loss and harvesting.