Bioacoustics of Leucostethus bilsa

Etymology. The specific epithet is used as a noun in apposition and refers to the type locality, the Reserva Biológica Bilsa; Bilsa is the name of a nearby river. RBB protects the headwaters of several rivers and belongs to the Chocó Darien-Tumbesino region, one of 25 global “hot spots” known for their high biodiversity and endemism of flora and fauna. This name is also intended to honor the conservation efforts of Fundación Jatun Sacha, which owns and administers the RBB and several other reserves in Ecuador . Definition . Leucostethus bilsa is characterized by the following combination of characters: (1) large sized, snout-vent length (SVL) in females 27.4–28.2 mm (n = 3, wild caught), 22. 5–28.0 (n = 3, lab-raised); SVL in males 21.7–24.2 mm (mean = 22.7; n = 4); (2) skin of dorsum posteriorly tubercular, smooth anteriorly, but with scattered low tubercles on the lateral sides of the dorsum anteriorly; (3) Finger I slightly longer than Finger II when adpressed; (4) length of Finger IV reaching distal half of distal subarticular tubercle of Finger III; (5) Finger III not swollen in adult males; (6) lateral fringes absent on fingers; (7) discs on fingers weakly to moderately expanded; (8) webbing absent between fingers; (9) outer tarsal fold absent, inner tarsal fold well-developed; (10) lateral fringe absent on Toe IV, but keels are present in toes II–IV; (11) discs on toes weakly to moderately expanded; (12) basal webbing between toes III–IV–IV, only trace webbing between toes II–III; (13) ventrolateral stripe absent, but junction of dark lateral with pale ventral surfaces abrupt, forming a sharply distinct linear border; (14) cream oblique lateral stripe present, usually incomplete; (15) in life, mustard-yellow paracloacal marks present; (16) in life, bright mustard-yellow flash marks in axillae, groin, and antero-ventral surfaces of hindlimbs; (17) in life, ventral surfaces of body white with yellow in posterior portion of belly, with gray mottling on chin; throat, and chest in adult males; (18) testis color polymorphic, white or brown-gray; (19) thin distinct dark stripe or flecks present along lower lip; (20) advertisement call composed of a series of peep notes emitted at a mean rate of 81 notes/minute, with dominant frequency of 3310–3722 Hz.

Comparisons with other species. Leucostethus bilsa differs from all Leucostethus (except L. argyrogaster ) by lacking the Finger III swollen in adult males. Leucostethus bilsa is compared with all other Leucostethus described ( L. argyrogaster , L. brachistriatus ( Rivero & Serna 1986 ) , L. fraterdanieli ( Silverstone 1971 ) , L. fugax , L. jota Marin et al. 2018 , and L. ramirezi ( Rivero & Serna 2000 ) , and similar species of Dendrobatidae occurring in the lowlands of northwestern Ecuador ( Hyloxalus toachi ( Coloma 1995 ) , H. awa ( Coloma 1995 ) , H. infraguttatus ( Boulenger 1898 )) . Leucostethus bilsa is distinguished from L. argyrogaster by having bright mustard-yellow flash marks (absent in L. argyrogaster ). Leucostethus bilsa differs from L. fraterdanieli (characters in parenthesis) in that the flash marks have diffuse borders (more defined), and form crescent-shaped arcs around arm insertions, extending to ventral surfaces (do not consistently form crescent-shaped arcs). Also, L. bilsa differs in having a complete to incomplete oblique lateral stripe (prominent and fully complete oblique lateral stripe), a dark line below oblique lateral stripe is absent (the oblique lateral stripe is bordered below by a distinct dark line). Ventrolateral patterns also differ; while both species lack a pale ventrolateral stripe, the border between the dark flanks and pale ventral surfaces is abrupt and linear in L. bilsa , forming a sharply defined straight or slightly sinuous line (fragmented). Dorsally (in life) both species are cryptically colored, with L. bilsa varying from brown to charcoal (dorsum may also have green, orange or golden tinges). Leucostethus bilsa further differs from L. brachistriatus , L. fugax , L. jota , and L. ramirezi by having an oblique lateral stripe that varies from complete to anteriorly faded or broken along its posterior length (pale oblique lateral stripe complete in L. brachistriatus , L. fugax , L. jota , and L. ramirezi ). Additionally, it differs from L. jota by having mustard-yellow paracloacal marks (paracloacal marks absent in L. jota ) and by lacking a ventrolateral stripe (ventrolateral stripe present in L. jota ). Leucostethus bilsa is larger than L. ramirezi (maximum female SVL = 28.0 mm vs 22.8 mm in L. ramirezi ). Leucostethus bilsa and Oophaga sylvatica ( Funkhouser 1956 ) are the largest species of dendrobatid frogs yet reported from western Ecuador ; only in Hyloxalus toachi does the size of adults of either sex approach or exceed the sizes of adults in the type-series of L. bilsa (maximum sizes of adult males and females respectively for H. toachi are 23.1 and 28.2 mm SVL). Leucostethus bilsa may readily be distinguished from H. toachi , H. awa , and H. infraguttatus by having bright mustard-yellow axillary and groin flash marks, and presence of mustard-yellow paracloacal marks (absent in H. toachi , H. awa , and H. infraguttatus ). Acoustic comparisons. The call of Leucostethus bilsa (see description later under Vocalization) is compared to the calls of L. fraterdanieli and L. jota described by Marin et al. (2018) . The calls of L. brachistriatus and L. ramirezi are unknown. The calls of all three species consist of a long string of roughly equally spaced notes, but the calls mostly differ in the call repetition rate of 98 notes per minute for L. fraterdanieli , 87 for L. bilsa , and 22 for L. jota . Leucostethus bilsa and L. jota produce much longer notes, note duration being 139 ms in L. bilsa , 140 ms in L. jota vs. 90 ms in L. fraterdanieli . The dominant frequency in L. fraterdanieli is 3.1–3.5 Khz, whereas the L. bilsa (3.6–3.7 KHz) and of L. jota (3.7–4.9 Khz) are higher.

Vocalization. A chorus of three or four individuals was heard calling from the forest floor in a relatively flat area of forest at the base of a steep slope beside the Green trail, ~ 1.2 km SW of the RBB main station complex on the morning of 5 January 2004 . These individuals were calling within a relatively small area, spaced about ten meters apart. Typically, when one frog commenced calling, the other frogs would then start calling; between periods of calling continuing for only a few minutes, there would be long periods of silence. The individual recorded was perched in a prominent position on a dead leaf, raised a few centimeters above the rest of the leaf litter on the forest floor. Only a few individuals were heard calling elsewhere at Bilsa from 3–10 January 2004 , and so L. bilsa was presumed to be uncommon at that time, but during the rainy season in February of 2007, calling males were more commonly heard in many areas, suggesting that L. bilsa may be fairly abundant at the RBB.