Description
Allobates chalcopis is a small, slender-bodied frog with a snout-vent-length of 17.4 mm for males, and 16.1-18.4 mm for females. The head is a little bit wider than it is long. The snout is short and bluntly round in dorsal view and truncate in profile. The eye diameter is 2.9 mm. The diameter of the tympanum is 40% that of the eye. The diffused, weakly defined supratympanic fold obscures the posterodorsal part of the tympanum. The loreal region is slightly concave and the nostrils weakly project laterally. The dorsum, flanks, and ventral skin are all smooth, with some poorly defined tubercles located anterior to the anal opening, which is directed posteroventrally. Dorsolateral stripe, oblique lateral stripe, and ventrolateral stripe are absent. The forelimbs are slender and relatively long with unwebbed fingers lacking fringes. The first finger is barely shorter than the second, or equal in length. The terminal discs are somewhat expanded, with the disc on finger 3 being 1.4 times larger than the discs on the adjacent phalanges. The subarticular tubercles are oval-shaped and low. The thenar tubercles are large and round but the palmar tubercles are twice the size and are somewhat rounded. The hind limbs are moderately slender. The proximal half of the tarsus has a low, small tubercle. Inner and outer tarsal folds are absent. The outer metatarsal tubercle is rounded, elliptical, and three-fourths the size of the round, inner metatarsal tubercle. The toes lack lateral fringes. Some remnant, skin-like webbing is barely visible. The discs on the toes are slightly expanded. Toes 1-5 have 1-1-2-3-2 subarticular tubercles, respectively. The tubercles on the toes are small and rounded. Vocal slits are present. The dentigerous processes of the vomers are absent (Kaiser et al. 1994).

At the type locality, Allobates chalcopis can be potentially confused with Eleutherodactylus johnstonei and Eleutherodactylus martinicesis. The juveniles of these species are very similar in coloration to the juvenilles of Allobates chalcopis, which have not yet developed their ventral coloration. The presence of digital scutes quickly allows for differentiation. Also, the snout of Eleutherodactylus is more elongate. Other diagnostic characteristics of Allobates chalcopis include: disc of Finger 3 is expanded, Finger 1 same length or barely shorter than Finger 2, vestigial toe webbing that is barely visible, absence of dorsolateral, oblique lateral, and ventrolateral stripes, absence of outer tarsal fold, disc on toe 3 is expanded, some individuals have markings on the chest and throat, light orange belly, males have dark pigmentation on throat that cover the entire hyoid region, endotrophic, nidicolous larvae, and the finger III is not swollen (Kaiser et al. 1994).

In life, the dorsum is a pale brown with some darker brown markings. There is a triangle mark in between the eyes, but this character is not well defined in many of the paratopotypes. There is also a “u”-shaped marking on the snout, which was also indistinctive in some of the paratopotypes. Males have a dark, black, throat coloration, which fades into a dark gray anteriorly, and a black collar which covers the hyoid region. Females do not have such coloration of throats or hyoid regions and instead, have a homogenous pale orange throat and venter. Males also have a light orange venter. The eye is a brown with the upper portion of the iris being a copper hue. When preserved, the brown and dark brown markings on the dorsal side become gray and dark gray, respectively. The “u”-shaped marking between the nostrils is dark, the bottom on this marking being on the upper lip. The triangle in between the eyes has its apex region pointing posteriorly. The head has a strong strip along the canthus rostralis from eyes to nostril. There is an additional line, not as defined, which goes along the upper lip, parallel to the canthus rostralis. There is a dark supratympanic stipe that extends from the eye just past the tympanum that connects to a dark wedge. At the scapular level, there are two small, dark bilateral marks with pale centers. There are two more bilateral black spots at the posterior lateral sacral regions. On the anterior sacral region, there is an expansive dark mark. A diffuse pale band that stretches transversally across the thighs surrounds the dark brown anal region. The gray flanks have two dark brown bands; one of these bands is posterior to the forelimbs while the other band is anterior, obscuring the upper portion of the tympanic region. The forelimbs are light brown on the dorsal surface; the upper arm has some gray shading, diffused dark longitudinal stripes both anteriorly and posteriorly. There are also two dark stripes across the lower arm and lighter ones across the fingers. The digital discs of Fingers 1 and 2 are white while those of Fingers 3 and 4 are a little darker. The hind limbs are light brown on the dorsal side with dark bars across the entire length. Each limb has four narrow bars: one on the thighs, one on the shanks, one of the tarsus, and on across the base of toes. The toe pads and scutes have color to them while the throat is a homogenous gray with a black collar decorating the hyoid region. The chest and the abdomen have lost their pale orange coloration and are instead a pale gray with a light reticulated pattern on the abdomen. The limbs are grayish white ventrally (Kaiser et al. 1994).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Martinique

View distribution map in BerkeleyMapper.

Allobates chalcopis is found on the island of Martinique, which is part of the Lesser Antilles, a series of oceanic islands that make up the eastern part of the Caribbean. The first specimens that were formally described in 1990 were found on the ground close to a deep ravine in a southeastern slope of Montagne Pelee, approximately 500 m in altitude. The area is montane rainforest, with vegetation mainly composed of scattered tall trees (approximately 30 m), ferns, bamboos, and palm brakes. Some individuals were also heard calling outside of this range in smaller ravines in elfin woodland. These forests are abundant with liverworts and moss mats (Kaiser et al., 1994). A study done 20 years after the frog’s initial description and last observation found that the maximal distribution of the species has changed. The recent study failed to locate the species in the type locality despite intensive searching, leading to the conclusion that Allobates chalcopis has gone locally extinct form its type locality. Instead, the species was found between 800 and 1400 m a.s.l. The density of calling males increased with altitude, with a maximum at 1200 m. Taking the lowest and the highest altitudinal occurrences into account, the total range was estimated to be 3.7 km², a dramatic decrease from the previously estimated 33 km² 30 years ago (Fouquet et al. 2013).

Life History, Abundance, Activity, and Special Behaviors
Allobates chalcopis exhibits crepuscular daily activity; this is based on peak calling behavior being at dawn and dusk, with the latter being more intense, and the difficulty of finding active specimens during mid-day and night. Some calling males were heard throughout the day. The frogs were secretive and cryptic: when approached they would retreat under rocks and dry leaves, blending in with their surroundings. Most of the individuals from the 1994 study that were found during the day were under large leaves and rocks in the ravine. Some specimens however were found in the small streams. When approached, these frogs would quickly flee; a succession of rapid jumps and sudden changes in direction makes them hard to catch. Some individuals would try to escape by jumping into the water, which actually made them easier to catch because they are relatively weak and slow swimmers (Kaiser et al. 1994).

Allobates chalcopis is the only known dendrobatid frog that is endemic to an oceanic island. It was first formally described in 1990 by Kaiser et al. Twenty years after this last observation, Fouquet et al. found an isolated population on the same island but at higher altitudes. The rediscovery of this species allowed for a confirmation of its molecular-based phylogenetic position: it is nested within a clade of lowland Amazonian Allobates (trilineatus clade) but only distantly related to any species within this group. All of the species within this clade occur in lowland Amazonia and are morphologically distinct from Allobates chalcopis. Molecular dating shows that it actually diverged from the rest of these species 11.3 Ma, which is in line with the emergence of the island of Martinique (9 Ma) and the establishment of the South Pole ice caps and consequently, a low eustatic sea level. This is also the period in which the Andean chains closed, the Pebas system of South America ended, and the Amazon and Orinoco modern drainages were established. This changing topography and low sea level could have been the cause of a large fresh water discharge into the sea of the Guiana Shield which faces the Caribbean. According to Measey et al. (2007), these fresh water connections between the Amazon and the Caribbean would have facilitated migration on rafts, such as those documented in the “congo plume” (as cited in Fouquet et al. 2013). This scenario seems like a likely explanation for the arrival of Leptodactylus fallax in the Lesser Antilles, which is similar in age to Allobates chalcopis. While salt-water dispersal is possible, there are some groups that are more prone to it; dendrobatids, given their small size and forestrial life history, are not among them, making the distribution of Allobates chalcopis unique. It's presence on an oceanic island might be the result of direct raft dispersal from South America to Martinique or a result of stepping stone dispersal by the island of the Grenadines, St Vincent, and Ste Lucia (Fouquet et al. 2013).

Trends and Threats
Given that there have not been any major modifications to the habitats of the Montagne Pelee in the past decade, one possible explanation for the change in range and elevation is climate change. Because Kaiser et al. did not note if they looked beyond the altitudes at which the original specimens were found, there are two possible explanations for the change in distribution of Allobates chalcopis: (1) there was a range reduction: the species originally was found between 500 and 1400 m, but now the lower limit has risen to 800 m, or (2) there was a range shift from the original 500 m to above 800 m. This upward shift in elevation has left the species without any higher ground. Currently, the population of Allobates chalcopis, 800-1400 m, is found within Réserve biologique intégrale de la Montagne Pelée, which is protected by the Office National des Forêts - Direction régionale de la Martinique. There is a possibility of finding other populations outside of this area. As of today, Allobates chalcopis is not protected by any kind of national conservation law and only holds the IUCN status of “Vulnerable.” Given the status as the only dendrobatid frog endemic to the Martinique Island, the only frog from an oceanic island, sharp decline in range and upward shift in altitude, and a frog originating from ancient oceanic dispersal, it has been highly recommended that the IUCN elevate the status to “Critically Endangered” and that a national program be developed to fully protect this unique species (Fouquet et al. 2013).

Possible reasons for amphibian decline

Subtle changes to necessary specialized habitat
Climate change, increased UVB or increased sensitivity to it, etc.

Comments
Allobates chalcopis is the only known oceanic dendrobatid. Most of the native amphibians known from the Lesser Antilles are from the genus Eleutherodactylus but others include Leptodactylus fallax, Prisimantis euphronides, Pristimantis shrevei, and Allobates chalcopis (Fouquet et al., 2013).

References

Fouquet, A., Pineau, K., Rodrigues, M.T., Mailles, J., Schneider, J.B., Ernst, R., and Dewynter, M. (2013). ''Endemic or exotic: the phylogenetic position of the Martinique Volcano Frog Allobates chalcopis (Anura: Dendrobatidae) sheds light on its origin and challenges current conservation strategies.'' Systematics and Biodiversity, 11(1), 87-101.

Kaiser, H., Coloma, L.A., and Gray, H.M. (1994). ''A new species of Colostethus (Anura: Dendrobatidae) from Martinique, French Antilles.'' Herpetologica, 50(1), 23-32.

Measey, G.J., Vences, M., Drewes, R.C., Chiari, Y., Melo, M., and Bourles, B. (2007). ''Freshwater paths across the ocean: molecular phylogeny of the frog Ptychadena newtoni gives insights into amphibian colonization of oceanic islands.'' Journal of Biogeography , 34, 7-20.

Species Account Citation: AmphibiaWeb 2013 Allobates chalcopis: Martinique Volcano Frog <https://amphibiaweb.org/species/1551> University of California, Berkeley, CA, USA. Accessed Nov 25, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 25 Nov 2024.

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