Eleutherodactylus sentinelus, Grünwald & Reyes-Velasco & Franz-Chávez & Morales-Flores & Ahumada-Carrillo & Rodriguez & Jones, 2021

Grünwald, Christoph I., Reyes-Velasco, Jacobo, Franz-Chávez, Héctor, Morales-Flores, Karen I., Ahumada-Carrillo, Ivan T., Rodriguez, Christopher M. & Jones, Jason M., 2021, Two new species of Eleutherodactylus (Anura: Eleutherodactylidae) from Southern Mexico, with comments on the taxonomy of related species and their advertisement calls, Amphibian & Reptile Conservation (e 272) 15 (1), pp. 1-35 : 9-14

Eleutherodactylus sentinelus sp. nov.

El Balsamo Peeping Frog, Rana piadora del Puerto El Balsamo.

Figs. 4–5, 7B.

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Holotype. MZFC 33306 ( CIG 00913 ). Adult male ( Fig. 4), 8.9 km SW of Puerto El Balsamo, Municipality of José Azueta (17.9549, -101.2253, 1,354 m asl; datum = WGS84 ), Guerrero, Mexico ( Fig. 8B), collected on 14 July 2016 by Christoph I. Grünwald and Héctor Franz-Chávez.

Paratypes (n = 7; Fig. 5). MZFC 33302–33305 ( CIG 00907–910 ), four adult males, collected at Puerto El Balsamo , Municipality of José Azueta (17.9813, -101.2291, 1,900 m asl; datum = WGS84 ), Guerrero, Mexico on 14 July 2016 by Christoph I. Grünwald and Héctor Franz-Chávez ; MZFC 33031–33033 ( CIG 00333–335 ), three adult males, collected at Puerto El Balsamo , Municipality of José Azueta (17.9812, -101.2292, 1,900 m asl; datum = WGS84 ), Guerrero, Mexico on 5 June 2015 by Christoph I. Grünwald, Nadia Pérez-Rivera, and Héctor Franz-Chávez .

Diagnosis. Based on the phylogenetic analysis, this species is a member of the genus Eleutherodactylus , subgenus Syrrhophus , as defined by Hedges et al. (2008); and in the Eleutherodactylus (Syrrhophus) nitidus species series and the Eleutherodactylus (Syrrhophus) nitidus species group as defined by Grünwald et al. (2018) based on the condition of the tympanic annuli, ventral epidermis, and visceral peritoneum. A small frog, adult males measure 23.3–25.3 mm SVL; vocal slits are present in males; digital tips are expanded, 1.4–2.3 times the width of the narrowest part the finger on the third and fourth fingers; finger lengths are I-II-IV-III, fingers moderately long, with third finger length ranging from 13–21% of SVL; compact lumbar gland in the inguinal region present but indistinct, visible in live specimen; ventral epidermis semi-translucent and the visceral peritoneum is clear, not white, abdominal vein barely visible on the venter of live specimens against the background of the viscera; limbs moderate, TL/SVL ratio is 0.41–0.56, FeL/SVL ratio is 0.38–0.46 and TotFL/ SVL ratio is 0.61–0.74; snout short, END/ SVL ratio is 0.10–0.11; tympanum small, indistinct and round, tympanic annuli not visible in live specimen; TW/EW ratio is 0.26–0.28. The dorsal skin is smooth to slightly pustulate. Dorsal coloration is reddish-tan or brown. A pale brown or reddish interorbital bar always present, and a pale mid-dorsal blotch of the same color as the interorbital bar is present. Upper arms pale and unmarked, dark transverse bands present on forearms and legs, and inguinal flash coloration orange or yellow present on groin and sometimes on posterior portion of thighs. Ventral coloration whitish or gray with some darker gray spots or indistinct marbling. The mating call of adult males is a quick chirp (“peep,” see below; Fig. 3).

Comparisons. Eleutherodactylus sentinelus can be distinguished from all species in the Eleutherodactylus (Syrrhophus) longipes species series by: possessing a small, indistinct tympanum with no tympanic annulus visible and with a diameter less than 30% of diameter of the eye; by possessing a visceral peritoneum which is not white, so that the abdominal vein on the venter is not clearly evident against a white background in life; and by possessing a distinct, raised lumbar gland above the inguinal region.

Eleutherodactylus sentinelus can be distinguished from species of the Eleutherodactylus (Syrrhophus) modestus species group by the combination of possessing a compact, protruding lumbar gland above the inguinal region, digital tips which are expanded more than 1.4 times the width of the narrowest part the finger on the third and fourth fingers, and the presence of an interorbital bar.

Within its own species group, E. sentinelus can be distinguished from most species by possessing a compact inguinal gland which is indistinct but visible in live specimens. This differs from E. pipilans , E. erythrochomus , and E. nebulosus , which lack a distinct visible inguinal gland altogether. All other known species in the E. (Syrrhophus) nitidus species group have readily visible compact lumbar glands above the inguinal region. Eleutherodactylus sentinelus can be further distinguished from E. pipilans , E. erythrochomus , and E. nebulosus by the presence of distinct pale interorbital bar and pale-yellow inguinal flash coloration. It may be distinguished from E. nitidus , E. petersi , and E. orarius by the combination of smoother skin, longer limbs, and tips of digits which are expanded more than 1.4 times the narrowest part of the finger on the third and fourth fingers. It further differs from these three species by call, which is a short chirp rather than a whistle. It is distinguished from E. albolabris by the following characters (characters of E. albolabris in parentheses): lip dark, never white, with some flecking (lip white, immaculate), inguinal flash coloration always yellow (always fiery orange), ventral coloration translucent and white, with some black markings (ventral coloration completely white with bold black markings), tympanum small with TW/ED 0.25–0.28 (tympanum slightly larger with TW/ ED 0.27–0.32). Eleutherodactylus sentinelus may be distinguished from E. maurus by the following (characters of E. maurus in parentheses): by the presence of pale interorbital bar same color as snout ( E. maurus presents no pale interorbital bar), smooth dorsal and ventral skin (rugose or slightly rugose dorsal and ventral skin), bright yellow or orange flash colors on thighs (flash colors absent, or barely discernible), pale interorbital stripe distinct between darker head coloration (pale interorbital stripe absent or same color as dorsal surface of snout). Eleutherodactylus sentinelus may be distinguished from E. syristes and E. maculabialis by its mating call, which is a rapid chirp (“peep”) as opposed to a trill. Eleutherodactylus sentinelus can be further distinguished from the closely related E. syristes by having more expanded finger tips, 1.4–2.3 times the narrowest part of the digit on fingers three and four, and usually more than 1.6 (vs. 1.1–1.5 times the narrowest part of the digit on fingers three and four), and by having yellow or yellowishorange flash colors (vs. orange or reddish flash colors). Eleutherodactylus sentinelus is most similar to E. dilatus , from which it may be distinguished by the by much smoother skin on both the dorsum and venter ( E. dilatus has rugose dorsal skin and pustulate venter), smaller, less distinct inguinal glands ( E. dilatus has large, distinct inguinal glands), and a paler dorsal ground coloration ( E. dilatus is dark brown).

Description of the holotype. Small frog (24.1 mm SVL); male; head slightly longer (7.1 mm) than wide (6.4 mm), head slightly wider than body; snout subovoid from a dorsal view and rounded from a lateral profile; tympanum indistinct, rounded with no supra-tympanic fold present; tympanum small, circular, greatest width of tympanum 0.8 mm; greatest diameter of eye 2.9 mm; tympanum width to eye diameter ratio 0.28; eyelid width 1.6 mm, a third of the IOD; first finger shorter than second finger; finger lengths from shortest to longest I-II-IV-III; digital pads on fingers two, three, and four slightly expanded, 1.6 times the narrowest point of the digit on second finger and 2.3 times the narrowest point of the digit on fingers three and four; expanded finger pads truncate; three palmar tubercles; inner palmar tubercle about 75% as large as middle palmar tubercle, outer palmar tubercle about half the size of middle palmar tubercle ( Fig. 6B); toe lengths from shortest to longest I-II-V-III-IV; outer metatarsal conical with a round base, small, approximately 50% of inner metatarsal tubercle; inner metatarsal tubercle spherical shape with oval base, large, approximately 0.9 mm in length; dorsal skin smooth, lateral skin slightly shagreened, ventral skin smooth. Vocal slits present.

In life, the holotype had a dark reddish-brown dorsal ground coloration, with pale reddish interorbital bar, mid-dorsal blotch, and upper arm coloration, with the upper arm the palest. Lateral portions of the head were dark brown coloration, with small white flecks on the labial and loreal regions. Lateral coloration was brown and white. Legs and arms were ochre with dark brown transverse bars. Yellow flash colors present on the groin. Ventral coloration flesh colored with white spots and black melanophores. See Fig. 4A–C for photographs of the holotype in life.

Coloration in preservative is brown dorsum, with some paler brown areas on the lower parts of the back. Indistinct cream-colored interorbital bar, upper arms and groin pale tan. Inguinal gland black. White marbling on lateral surfaces. Legs and arms pale tan with darker brown transverse bands. Ventral surfaces white, unmarked, throat yellowish ( Fig. 4D–E).

Measurements of the holotype (in mm). IND 2.4, IOD 4.8, END 2.5, ETD 1.1, UpL 6.2, FoL 7.9, HaL 6.3, F1L 2.3, F1 PW 0.5, F1 W 0.3, F2L 2.7, F2 PW 0.6, F2 W 0.4, F3L 4.0, F3 PW 1.0, F3 W 0.4, F4L 3.1, F4 PW 1.0, F4 W 0.4, IPTL 0.6, MPTL 0.9, OPTL 0.4, FeL 11.3, TL 12.3, TaL 7.2; FL 10.9, T 2L 3.1, T 2 PW 0.7, T 2 W 0.4, T 3L 4.9, T 3 PW 0.7, T 3 W 0.4, T 4L 7.9, T 4 PW 0.8, T 4 W 0.4,

described above, and the related species in the Eleutherodactylus nitidus species group, important morphological, mensural, and call differences of all of the species in this group are collated into a single table ( Table 3), and photographs of related species in the E. nitidus species group are provided ( Figs. 9–10). The distributions of the species in the E. nitidus group are mapped in Figs. 7 and 13.

T 5L 3.5, T 5 PW 0.5, T5 W 0.4, IMTL 1.0, OMTL 0.6, FeL/SVL 46%, TL/SVL 50%, Ha/SVL 25%, FL/SVL 44%, HL/SVL 33%, HW/SVL 29%.

Variation. SVL from 23.3–25.3 mm (24.2 + 0.81). Expanded finger pads on third and fourth fingers vary from 1.4–2.3 times the narrowest part of the digit, with average 1.85 + 0.26 on the third finger and average 1.78 + 0.36 on the fourth finger. Dorsal ground coloration ranges across different shades of reddish brown ( MZFC 33304), tan ( MZFC 33031, 33306), and brown ( MZFC 33032–3, 33304–5). Venter typically gray, with white and black markings, although these markings range from sparse to almost complete reticulation. Morphological variation is presented in Table 2.

Differentiating the Advertisement Calls of Closely Related Eleutherodactylus nitidus Species Group Members

Recordings were made from 120 calling males of all 24 species of Eleutherodactylus from western Mexico. The calls of the species analyzed were found to fall into five different categories of a rapid burst whistle (trill), a drawn-out whistle (whistle), a strong high-pitched chirp (peep), a soft high-pitched chirp (chirp), and a drawn-out chirp (pipe). Of the species analyzed herein, four ( E. dilatus , E. nebulosus , E. pipilans , E. sentinelus ) produce a strongly high-pitched chirp (peep), four ( E. albolabris , E. nitidus , E. orarius , Distribution and ecology. This species is known only from the vicinity of the type locality in the western-most extension of the Sierra Madre del Sur of Guerrero ( Fig. 7A). It has been collected at elevations ranging from 1,300 –1,900 m asl, on steep mountain sides in humid pine-oak forest, oak woodland and pine-oak woodland, and tropical deciduous forest ecotone. At the type locality, this species is sympatric with E. petersi . All individuals of E. sentinelus have been observed after the onset of the rainy season in the months of June and July. Individuals were found calling on small bushes or rocks. All were found active at night.

Etymology. This species is named after latin sentinel, meaning guard or outpost, in reference to its type locality, a mountain which stands out from the north and west as the first outpost of the Sierra Madre del Sur of Guerrero.