Mutinondo Grass Frog
Species Description: Channing A, and Willems F. 2018. A new grass frog with rupicolous tadpoles from northern Zambia (Anura: Ptychadenidae). Zootaxa 4462: 349–366.
Ptychadena mutinondoensis is a slender-bodied Grass frog with a snout-vent length ranging from 35.0 - 38.8 mm for males and 41.6 - 43.5mm for females. The head is short with a length that is approximately equal to the head width, both of which are about a third of the snout-vent length. The long, sharply rounded snout has a length of a little over half the head length. It has a rounded canthus rostralis and a slightly concave loreal region, with subtly projected nostrils that are nearer to the eyes than the snout. Its small, slightly protruding eyes are directed anterolaterally and are less than half of the snout length in diameter. The interorbital and internarial distance are nearly equal in length; they are less than the upper eyelid and just a little less than the eye diameter. Near its eyes, P. mutinondoensis has a visible tympanum that is smaller than the eye’s diameter. It has vomerine teeth that are hidden by its pale upper lip, which turns into a thickened gland under the tympanum that extends all the way to the top of its arm. In addition, the upper jaw has a small, needle-like dentition. The choanae is small and rounded, and the tongue is slightly notched towards the back with no median lingual process. There is a paired vocal sac that starts below the arm insertion. On its dorsum, there is a complete median skin fold that runs from the level of the eye to the level of the thigh, a complete post eyelid fold, and interrupted dorsolateral and lateral folds. The dorsolateral fold is interrupted both anteriorly and posteriorly. Laterally and between folds, the skin is rough, while on the ventral surface of the limbs, gular, and abdomen, the skin is smooth (Channing and Willems 2018).
Ptychadena mutinondoensis has slender arms with moderately large hands. The metacarpals lack supernumerary tubercles. The tips of its fingers are not enlarged into discs, and the relative length of the fingers is II < IV < I < III. There is one subarticular tubercle on fingers I, II, and IV, and two on finger III. The fingers are unwebbed with a distinct thenar tubercle. In breeding males, there are pale nuptial pads on the upper surface of fingers I, II, and III (Channing and Willems 2018).
Ptychadena mutinondoensis has two gracile hind limbs with small, pale, and distinct tarsal tubercles. The foot is just a bit shorter than its long tibia, and when the knees are flexed and thighs are at a right angle to the body, its heels overlap. The relative length of the toes, which do not expand into discs, is I < II < V < III < IV. There is one subarticular tubercle on toes I and II, two on toes III and V, and three on toe IV. The toes have thin webbing in the following formula: I2 - 2.2 II2 - 3III2 - 3.2IV2 -1V that extends to the tips. The inner metatarsal tubercle is prominent with an oval smaller than its eye. There is a thin tarsal ridge, (Channing and Willems 2018).
One tadpole, in Gosner stage 40, had a total length of 32.9 mm. It had a bluntly rounded head from both the dorsal and lateral views, and large, dorsolateral eyes. The oral disc was located anteroventral and had extra paella laterally as well as a single row of rounded marginal papillae below it. The rostral gap in the papillae extended across the whole labium, and the labial tooth row formula was 2/1. The upper and lower jaw sheaths were finely serrated. The width of the oral disc was 1.7. The external nares were closer to the eye than the tip of the snout, and the proportion of the rostronasal distance to the orbitonasal distance was 1.5. The circular narial aperture did not have a distinct margin, and the internarial ratio was 0.15. There was no narial valve, orbitonasal line, or pineal spot. The distance from the snout tip to the nostril was 1.6 mm . Furthermore, the distance from the snout tip to the eye was 2.8 mm and from the eye to the nostril was 1.1 mm. The small, elliptical spiracle opening was dorsolateral, visible dorsally, and not immediately attached to the body. The distance from the snout tip to the spiracle was 5.3 mm. The vent tube was dextral and continuous with the ventral fin margin. The tail height was less than the head height, and the ratio of the height of the tail muscles at the base of the tail to the height of the trunk was 0.50. The tail tip was acute and the axis of the tail, extrapolated anteriorly, passes below the eye. The ratio of the tail length, which was 24 mm long, to the length of the head and the trunk was 2.7, although this ratio usually exceeded 3.2 in other specimens. The dorsal fin of the low tail starts at the intersection of the body and tail muscle, and was equal and nearly parallel to the ventral fin. The tail fins taper to the tip with nearly straight margins. The max fin height was 2.8 mm. The distance from the highest point on the fin to the tip of the tail was 7.0 mm (Channing and Willems 2018).
Unlike P. subpunctata, P. taenioscelis, P. keilingi, P. upembae, P. guibei, P. nilotica, and P. anchietae, P. mutinondoensis does not have contrasting longitudinal bands on the posterior thigh. The focal species’ feet are longer than half the snout-vent length, unlike P. schillukorum and P. mossambica. There is no triangle pattern on the snout like there is for P. subpunctata, P. obscura, P. anchietae and P. oxyrhynchus. Ptychadena mutinondoensis has three unwebbed phalanges on the fourth toe, so it does not have as much webbing as P. keilingi and P. bunoderma, but only less than one phalanx of the fifth toe is unwebbed, so it has more webbing than P. mossambica, P. keilingi, P. upembae, P. guibei, P. taenisocelis, P. bunoderma, P. ansorgii, P. porosissima, and P. obscura. Furthermore, while P. bunoderma has large median skin folds, the median skin folds of P. mutinondoensis are all similarly developed. Its snout lacks the skin folds found on P. uzungwensis and some P. broadleyi. The tibia in P. mutinondoensis lacks a light longitudinal line, which is found on P. subpunctata, P. keilingi, and P. porosissima and is sometimes present in P. mossambica, P. guibei, P. taenioscelis, and P. nilotica. While the fourth metatarsal in P. mutinondoensis has a row of flat white marks, it does not have a row of tubercles like P. mossambica, P. keilingi, P. upembae, P. guibei, P. bunoderma, P. ansorgii, P. uzungwensis, P. grandisonae, P. porosissima, and P. obscura. It has a snout-nostril distance greater than the internarial distance, and a longer snout than P. schillukorum, P. mossambica, P. keilingi, P. upembae, P. guibei, P. ansorgii, P. broadleyi, P. grandisonae, P. poroissima, P. nilotica, P. obscura, and P. anchietae. The foot is shorter than the tibia, unlike for P. subpinctata, P. keilingi, P. guibei, P. taenioscelis, P. ansorgii, and P. nilotica. For males, the gular pouch slits end below the arm insertion like P. grandisonae, but unlike P. subpunctata, P. schillukorum, P. mossambica, P. keilingi, P. upembae, P. guibei, P. taeniscelis, P. bunoderma, P. ansorgii, P. uzungwensis, P. broadleyi, P. porosissima, P. nilotica, P. obscura, P. anchietae, and P. oxyrhynchus. Finally, to distinguish the P. mutinondoensis further from its sister species, P. broadleyi, P. mutinondoensis has a longer and sharper snout in dorsal view. Additionally, the call of P. broadleyi has louder longer trills and longer call durations than P. mutinondoensis, with more pulses but a lower pulse rate. Both calls have a similar range in frequency, but P. broadleyi has a higher average frequency (Channing and Willems 2018).
In life, Ptychadena mutinondoensis has a beige dorsum with small dark brown patches along its skin folds. It has reddish-brown patches on its upper arm, as well as a thin white line highlighted by a broad beige band down its back. A dark brown line runs from the tip of the snout, over the nostril, down to the base of the eye, where the iris is also dark brown. By the eye, a beige triangle marks the transparent tympanum. The upper lip is lightly speckled brown. From the posterior region of the lower lip into the vocal pouch slit, P. mutinondoensis is black. The paired vocal sac is darkly pigmented through the vocal sac slit. The lateral skin fold and the gland from the upper lip to the arm insertion are white, and the flanks have an irregular brown and pale marbling. The arm is beige with dark brown bars traversing the lower arm. Females also had bright red infusions on the upper arm, above the eye, and on the flanks. The leg has eight transverse irregular darker bars over the upper leg, thigh and foot. The back of the thigh, however, has no markings other than some light speckling. The outer metatarsal tubercle looks like a small white bump. Finally, the venter is white, and the throat is yellow with some black speckles. When nuptial pads are present they are pale (Channing and Willems 2018).
In preservative, the pattern on the back of P. mutinondoensis is more clearly visible as a darker brown on a paler brown background (Channing and Willems 2018).
In life, the tadpoles are usually a mottled grey and yellow-brown, but can sometimes be a darker mottled black and grey. Along the tail muscle, there is a row of small white dots, which are also found on the base of the dorsal fin. The upper and lower jaw sheaths are finely serrated, and are deeply pigmented for more than half their height (Channing and Willems 2018).
In preservation, the pale spots disappear and the tadpole is pigmented dorsally. There are dark areas around the nostrils and between the eyes, and a dark line that runs from behind the eyes to the base of the tail. Along the caudal muscles and dorsal fin, there are fine black stipples in groups. There is no pigment under the trunk or on the anterior half of the base of the tail. The gut coils are also clearly visible through the ventral skin (Channing and Willems 2018).
The paratypes are proportionally similar to the holotype. However there is variation on the fourth toe; anywhere from 3 - 3.5 phalanges might not have webbing on either side (Channing and Willems 2018).
Distribution and Habitat
At the time of its description, the frogs were only found in the Mutinondo Wilderness Area in Northern Zambia, for which they are named. They have a distribution of 25 km2 at elevations between 1400 and 1460 m. They are associated with granite slopes thinly covered by 1 - 2 mm of slow-flowing water, usually seeping from higher vegetation (Channing and Willems 2018).
Life History, Abundance, Activity, and Special Behaviors
Adults are found in and under the grassy vegetation near films of water on granite slopes. The species is notable for being one of the few species with rupicolous eggs and tadpoles, meaning they are found on rocks, living in a film of water so thin that it does not fully cover them. All known oviposition sites are large single granite sheets that prevent water from seeping into the rock, sloped at angles between 10 - 40° (Channing and Willems 2018).
Calling was first observed in the 2016 - 2017 year on 22 December, the first warm evening following 48 hours of heavy rainfall and cold weather. The next morning, there were freshly deposited eggs at two of 10 known oviposition sites. No calling was observed during the next couple of dry days, but continued again after more heavy rainfalls on 2 January and continued throughout the wet season. Though calling activity usually peaks in the first hours after dusk, it was noticed at other times of both the day and the night during warm and wet conditions, but rarely noticed after multiple dry days (Channing and Willems 2018).
Based on 93 calls recorded on 14 January 2016 between 19h00 and 20h00, with a 24°C air temperature, the call consists of an irregular fast series of clicks followed by brief trills of five to seven pulses. The pulse rate is about 35 pulses per second, the fundamental frequency is 1.1 kHz with additional harmonics, and the duration of each trill varies from 0.1 - 0.3 seconds. The number of pulses varied from 1 to 8, with a mean of 3.7, and the duration of calls varied from 13 - 288 ms, with a mean of 100. The pulse rate varied from 10.5 - 58.1 pulses per second, with a mean of 29.2 per second, and the emphasized frequency varied from 1033 - 3531 Hz, with a mean of 1738 Hz (Channing and Willems 2018).
Eggs are deposited in films of water on top of rock faces with under overhanging vegetation, but the eggs are not completely covered by the water. After long periods without rain, the spongy soil and vegetation continued to provide a source of water. The eggs were found during or shortly after good rainfalls in December until March. Four clutches, found on 7 January 2016, contained 140, 180, 190, and 280 eggs. The average diameter of each egg was 9.3 mm (Channing and Willems 2018).
After hatching, metamorphosis of tadpoles takes about 1 - 2 months. Like other rupicolous tadpoles, P. mutinondoensis tadpoles develop hind legs early, allowing them to move more easily on the wet rock surfaces. In the 2015 - 2016 and 2016 - 2017 season, clear size cohorts between and within oviposition sites suggested multiple egg-laying periods during each season, likely linked to warm and rainy weather (Channing and Willems 2018).
At time of description, 1,050 tadpoles in total were counted on Lodge Rock and Campers Rock, spread over 20 seepage patches. There were an estimated 20 - 50 calling males just on the western flank of Klipspringer Rock, 50 tadpoles on Kankonde Rock, one juvenile on Charlie’s Rock and the Chipundu Stream, 50 juveniles in Mulino Falls, and 100 tadpoles and a single adult female in Mafonie. In general, the P. mutinondoensis seems restricted to a small area with a small population (Channing and Willems 2018).
One predator of P. mutinondoensis tadpole is a black 10 - 15 mm long species of terrestrial carabid beetle. Other possible predators include carnivorous water insects that share a habitat with the tadpoles, such as small dragonfly (Anisoptera) larvae (Channing and Willems 2018).
Trends and Threats
The main threat to P. mutinondoensis is habitat degradation. Because their habitat depends on a constant seepage of water from vegetation higher on the slope, loss of permanent vegetation and organic soil layers could lead to interrupted flows. While the Mutinondo Wilderness is a private reserve, a change in land-use could open the area to more fires, both man-made and natural, as well as cattle-grazing from an increasing number of private farms. These are the two main factors stimulating erosion and threatening vegetation in the Muchinga Highlands. Based on the small range and population size but the lack of evidence of population declines, the authors of the species' description recommended an IUCN Red List status of "Near Threatened" (Channing and Willems 2018).
Possible reasons for amphibian decline
General habitat alteration and loss
The species authority is: Channing, A. and Willems, F. (2018). “A new grass frog with rupicolous tadpoles from northern Zambia (Anura: Ptychadenidae).” Zootaxa 4462 (3): 349-366.
Based on Maximum Likelihood analysis of 16S rRNA sequences, the new species, P. mutinondoensis, differs by 5.9 - 13.1% from all other Ptychadena species. Its sister species is P. broadleyi (Channing and Willems 2018).
Ptychadena mutinondoensis is named after the Mutinondo Wilderness area where it was discovered (Channing and Willems 2018).
Although there are many available granite habitats in Africa, there are only two grass frog species that utilize these granite sheets for breeding (Channing and Willems 2018).
Channing, A., Willems, F. (2018). ''A new grass frog with rupicolous tadpoles from northern Zambia (Anura: Ptychadenidae).'' Zootaxa, 4462(3), 349-366.
Written by Rina Lu (rinaq AT berkeley.edu), UC Berkeley
First submitted 2018-10-16
Edited by Ann T. Chang (2018-12-19)
Species Account Citation: AmphibiaWeb 2018 Ptychadena mutinondoensis: Mutinondo Grass Frog <http://amphibiaweb.org/species/8904> University of California, Berkeley, CA, USA. Accessed Feb 18, 2019.
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Citation: AmphibiaWeb. 2019. <http://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 18 Feb 2019.
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