Description
Brachycephalus darkside is a robust, bufoniform frog with adults having a snout vent length range of 14.8 - 18.5 mm, juveniles have snout vent length ranges between 11.3 – 15.2 mm. The head is shorter than wide and characterised by its large size and the presence of plates on the skull and post-cranial area. In both the dorsal and lateral view, the snout is rounded. The lips are cresent-shaped. The slightly protuberant nostrils have slit-shaped openings that are directed anterolaterally. The canthus rostralis is indistinct and the loreal region is sligtly concave. The slightly protruding eyes are directed anterolaterally. The tympanum isn’t obvious and there are no external vocal sacs. The forelimbs are slender with the fore- and upper arm being approximately equal in length. The forearm is hypertrophied and longer than the hand. The relative finger lenghts are IV < I < II < III, with fingers I and IV being almost vestigial and fingers II and III being robust. The tip of finger I is rounded but the tips of the other fingers are pointed. There are no tubercles on the hands. The upper and lower legs are about equal in length to each other. The foot is longer than the tarsus, but shorter than the shank. The relative toe lengths are I < V < II < III < IV with toes II – IV having distinctly pointed tips and toes I and V being vestigial. Like the hands, there are no tubercles. The head and the dorusm have rough skin due to skull and post-cranial plate ornamentation. Scattered warts can appear on both the ventrum and dorsum, however, wart density can vary among individuals. This species is characterised by lack of ossified warts (Guimarães et al. 2017).

Brachycephalus darkside can be distinguished from all other species of Brachycephalus by the characteristic dark region on the underside of the body, which covers the dorsal muscles on all sides. In life, the body of B. darkside is bright orange-yellow, which is distinct from the pigmentation patterns of B. auroguttatus, B. boticario, B. crispus, B. ferruginus, B. fuscolineatus, B. guarani, B. hermogenesi, B. izecksohni, B. leopardus, B. mariaeterezae, B. pernix, B. pitanga, B. pombali, B. albolineatus, B. olivaceus, B. toby, B. verrucosus, B. tridactylus, B. brunneus, B. didactylus, B. nodoterga, B. pulex, B. quiririensis, and B. sulfuratus. In preservative, the dark pigmentation of B. darkside’s dorsum can distinguish it from the pale cream coloured dorsum of B. alipioi, B. atelopoide, B. bufonoides, B. ephippium, B. garbeanus, and B. margaritatus (Guimarães et al. 2017).

Post-cranial and skull plates can distinguish B. darkside from B. albolineatus, B. auroguttatus, B. boticario, B. brunneus, B. didactylus, B. ferruginus, B. fuscolineatus, B. hermogenesi, B. izecksohni, B. leopardus, B. mariaeterezae, B. olivaceus, B. pernix, B. pombali, B. pulex, B. quiririensis, B. sulfuratus, B. tridactylus, and B. verrucosus. Paravertebral plates can distinguish B. darkside from all other members of the genus Brachycephalus excepting B. ephippium, B. garbeanus, and B. margaritatus (Guimarães et al. 2017).

The lack of osteoderms can distinguish B. darkside from B. atelopoide, B. crispus, and B. margaritatus. The presence of laterally projected parotid plates and convex paravertebral plates distinguishes B. darkside from B. ephippium. The presence of a dorsal shield that isn’t projected over the vertebral spine, and the presence of paravertebral plates with rounded edges and convex shape distinguishes B. darkside from B. garbeanus. The trapezoidal shape of the first spinal plate, and the rectangular shape of the second distinguishes B. darkside from B marganitatus, as does the presence of rounded edges on the dorsal plate. The absence of metatarsal and metacarpal tubercles in B. darskside can distinguish specimens from, B. auroguttatus, B. boticario, B. brunneus, B. fuscolineatus, B. hermogenesi, B. izecksohni, B. leopardus, B. mariaeterezae, B. olivaceus, B. pitanga, B. pulex, B. quiririensis, B. toby, and B. verrucosus. The lack of maxillae teeth distinguishes B. darkside from B. brunneus, B. bufonoides, B. ferruginus, B. izecksohni, and B. pombali (Guimarães et al. 2017).

The head of B. darkside, which is wider than it is long, is distinct from the heads of B. ephippium and B. garbeanus. Larger snout-vent length can differentiate B. darkside from B. auroguttatus, B. boticario, B. brunneus, B. albolineatus, B. didactylus, B. fuscolineatus, B. guarani, B. hermogenesi, B. izecksohni, B. leopardus, B. mariaeterezae, B. nodoterga, B. olivaceus, B. pitanga, B. pulex, B. quiririensis, and B. verrucosus. In dorsal view, a bufoniform body and rounded snout can also be used to distinguish B. darkside from B. didactylus, B. hermogenesi, and B. pulex (Guimarães et al. 2017).

The presence of harmonic structures in the advertisement calls in B. darkside are distinct from all other members of Brachycephalus. Pulsed notes in advertisement calls distinguishes B. darkside from B. tridactylus. The characteristic 5 - 8 pulsed notes per vocalisation by B. darkside is distinguishable from B. pernix. The proportion of few pulses per note in advertisement calls of B. darkside distinguishes vocalisations from those of B. pitanga. Shorter notes distinguish B. darkside from B. crispus, B hermogenesi, and B. pitanga. Higher note rate distinguishes B. darkside from B. crispus and B. hermogenesi. Compared to all other Brachycephalus, B. darkside calls have the lowest peak frequency and lowest dominant frequency in calls (Guimarães et al. 2017).

In life, B. darkside has a bright yellow-orange dorsum with a dark region starting from the posterior edge of the skull and extending to the inguinal region. Brachycephalus darkside has mildly dark dorsal plates. The venter is yellow-orange but paler than the rest of the body. The eyes are totally black. When in preservative, the body takes on a cream colour and the dark area from centre of dorsum to the posterior skull region remains darkened. The skull, dorsal plates, and other ossified regions take on a grey colour. The eyes remain black (Guimarães et al. 2017).

Between individuals, the density of warts on the posterior and granular regions varies. In some cases, there are no warts and the body is smooth. Juveniles may lack bone development that is observed in adults, in addition, they may not present highly ornamented skulls or ornamented post-cranial skeleton. In adults, paravertebral plates can vary with size or can vary among individuals of the same size. The second spinal plate may touch adjacent spinal plates which are fused with paravertebral plates. The presacral VIII spinal plate and the corresponding sacral vertebra can be separated from dorsal shield. The top of the squamosal can be dermally ossified. One third of tongue can be fused to the floor of the mouth, and in the mouths of some there are oval choanae (Guimarães et al. 2017).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Brazil

View distribution map using BerkeleyMapper.

Brachycephalus darkside is endemic to the Atlantic semidecidous seasonal montane forests (Floresta Estacional Semidecidual Montana) of Southeastern Brazil, and are found in ‘Trilha do Ceuseiro' leaf litter. With an altitudinal range of 1266 - 1498 m, B. darkside is known to inhabit the Serra do Brigadeiro mountains, and has been observed within the towns of Ervália and Miradouro in the region. The species has also been reported in southeastern Brazil, north of the Mantiqueira mountain region, and in the state of Minas Gerais. Brachycephalus darkside is well distributed in the areas it has been found (Guimarães et al. 2017).

Life History, Abundance, Activity, and Special Behaviors
Individuals are active during the wet season from October to December between 12:00 – 19:00. Males vocalize from the leaf litter, exposed or under, as females walked through the leaf litter. They were found in deeper layers of the leaf litter and in tree roots in the dry period from July to September (Guimarães et al. 2017).

Brachycephalus darkside is known to have unique ’advertisement’ and ‘aggressive’ calls. Of all calls recorded, 76% are classified as advertisement calls, which are a long sequence of pulsed notes. Intervals between calls ranged from 6.3 - 11.2 seconds. There are 9 – 253 notes per call. Typically, the first note is the strongest followed by a second with a decreased volume, and an increase in the third or fourth pulses, and then a decrease of volume in the last pulses. The duration for each note and the interval between is 83 – 163 ms and 122 – 215 ms, respectively, for a total of 186.4 – 243.4 notes per minute. Each note had 5 – 8 pulses that were vocalized at a rate of 36.8 – 78.4 pulses per second. The dominant frequency of calls is between 2484.4 - 5765.6 Hz and peaks in energy range from 2856.4 – 3796.9 kHz. A maximum of 3 harmonics can be observed in advertisement calls (Guimarães et al. 2017).

The aggressive call of B. darkside has been observed in conspecific males within close proximity (20 cm). This call represents 23% of all observed vocalizations, lasting 2.4 – 6.9 seconds, and emitted roughly 8 times per minute. Aggressive calls are vocalized in shorter pulsed notes, compared to the advertisement call, with 15 – 41 notes per call that last 18 – 44 ms and have short intervals of 96 – 334 ms for a total of 343.7 – 364.2 notes per minute. There are 2 – 3 pules per note at a rate of 57.1 – 111.1 pulses per second. The volume of notes increases from the first pulse to the second, and declines from the second to third. The second and third pulses have been considered juxtaposed. The dominant frequency is between 2906.2 - 4406.2 Hz and the peak energy range is 3046.9 – 3984.4 kHz. The first pulse has the most dominant frequency and highest peak energy, and thus the aggressive call has downward frequency modulation (Guimarães et al. 2017).

Brachycephalus darkside reproduces in the same space and time as members of the Ischnocnema genus (Guimarães et al. 2017).

Comments
The species authority is: Guimarães CS, Luz S, Rocha PC, Feio RN (2017). “The dark side of pumpkin toadlet: a new species of Brachycephalus (Anura: Brachycephalidae) from Serra do Brigadeiro, southeastern Brazil.” Zootaxa 4258(4):327–344.

At the time of the species description, the genus Brachycephalus had three known species groups: B. pernix, B. didactylus, and B. ephippium. Based on the morphology and distribution of B. darkside, it was assigned to the B. ephippium group (Guimarães et al. 2017).

The species epithet, “darkside”, is in reference to the dark region around the black dorsal muscles, which is contrasted with the otherwise yellow-orange color of the body. The epithet “darkside” is also used in reference to Pink Floyd’s “Dark Side of the Moon” (Guimarães et al. 2017).

Owing to B. darkside’s small size, sex determination from the observation of vocal slits is often inconclusive without damaging specimens. Additionally, vocal slits have been observed in females in other members of the genus Brachycephalus potentially leading to misidentification (Guimarães et al. 2017).

References

Guimarães CS, Luz S, Rocha PC, Feio RN (2017). ''The dark side of pumpkin toadlet: a new species of Brachycephalus (Anura: Brachycephalidae) from Serra do Brigadeiro, southeastern Brazil.'' Zootaxa, 4258(4), 327-344. [link]

Species Account Citation: AmphibiaWeb 2019 Brachycephalus darkside <http://amphibiaweb.org/species/8618> University of California, Berkeley, CA, USA. Accessed Jul 7, 2020.

Citation: AmphibiaWeb. 2020. <http://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 7 Jul 2020.

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