Arthroleptidae

Family Arthroleptidae

David C. Blackburn, University of Kansas
March 2011

General Introduction
Characteristics
Distribution
Ecology
Evolution
Classification
Similar Species
Conservation
Uses by Humans
References

Cardioglossa gracilis
Photo by David Blackburn
(Click for details)

1. General Introduction

The family Arthroleptidae is restricted to sub-Saharan Africa and associated near-shore islands (such as Bioko and Zanzibar). One species (Leptopelis palmatus) occurs on the distant oceanic island of Pr'ncipe; this family does not occur on Madagascar. Current taxonomy recognizes eight genera, two of which are monotypic, and some relationships between these genera remain unresolved. This family is sister to the Hyperoliidae, which is also restricted to sub-Saharan Africa and its islands (including Madagascar). Together, these two families (Arthroleptidae, Hyperoliidae) are part of a larger radiation of frogs endemic to sub-Saharan Africa that also includes the Brevicipitidae and Hemisotidae.

In general, arthroleptid frogs are terrestrial occurring in leaf litter or near streams. However, the most species-rich genus (Leptopelis) is scansorial and is often found in trees; even so, some species of Leptopelis are still frequently encountered on the ground and/or under leaf litter. Only two genera (Arthroleptis and Leptopelis) are found in eastern or southern Africa, whereas the others are restricted to western and central Africa. Several genera (Leptodactylodon, Nyctibates, Scotobleps, and Trichobatrachus) occur only in the Lower Guinean Forest Zone bordering the Gulf of Guinea. Diet specialization appears to be uncommon though some species may specialize on freshwater snails.

Several taxa within this family have morphological traits unique to vertebrates. In some species of Arthroleptis and Cardioglossa, males have hyperelongate third fingers (sometimes nearly 40% of body length), the function of which remains unknown. In Astylosternus, Scotobleps, and Trichobatrachus, both males and females have bony claws that pierce through the skin and are used in defense. Trichobatrachus is perhaps the most famous species of this family. Male "Hairy Frogs" exhibit thin extensions of skin that appear as hair; these have been proposed to increase the surface area available for cutaneous respiration, but also as structures that protect against wounds during male-male combat.

2. Characteristics

a. External Morphology and general body shape.
While the dorsal surfaces of many species have a range of drab coloration (e.g., brown, gray), some genera (Leptodactylodon, Cardioglossa, Leptopelis) can be very colorful. Some Leptopelis species have bright red irises. The coloration and patterns observed across the genus Cardioglossa makes this one of the most striking genera in continental sub-Saharan Africa. Skin texture varies from smooth to moderately tuberculate (most genera) to the presence of fine ridges (Astylosternus, Nyctibates). All genera have well-developed, single (i.e., not bifid) subarticular tubercles on the digits; the development of the palmar tubercles varies both within and between genera. No species have an outer metatarsal tubercle, though all genera have an inner metatarsal tubercle, which in some species (Arthroleptis stenodactylus) can be very large and developed as a digging spade. All species exhibit a tympanum, though it can be reduced in size and difficult to observe in some. Species in the genus Leptopelis generally have expended digit tips, as do some other species in other genera (i.e., Arthroleptis).

b. Ecomorphotype.
This family exhibits ecomorphological forms that are arboreal and scansorial (Leptopelis), terrestrial (Arthroleptis, Cardioglossa), "stream-dwelling" (Astylosternus, Nyctibates), and even perhaps semi-fossorial (Leptodactylodon).

c. Size.
The smallest taxa in the Arthroleptidae are species of Arthroleptis, in which some species can reach maturity as small as 15 mm snout-vent length. The largest species is Trichobatrachus robustus, in which males can attain lengths of up to 130 mm snout-vent length; T. robustus is the only arthroleptid species in which males regularly attain larger body sizes that females.

d. Growth and development.
While some is known of larval diversity in the Arthroleptidae, there are few studies of growth and development. A study of males of Arthroleptis stenodactylus revealed that growth of the third finger may occur rapidly coincident with the onset of vocal sac development (Schmidt & Inger, 1959); the same study found no indication that male secondary sexual traits regress seasonally. During growth, Arthroleptis xenodactyloides transitions from a diet dominated by springtails (Collembola) to one consisting largely of ants.

There are a variety of male secondary sexual traits in the Arthrolepidae: some species of Arthroleptis and Cardioglossa have hyperelongate third fingers; Astylosternus, Leptodactylodon, Nyctibates, and Trichobatrachus have nuptial excresences on the digits of the hand; species of Leptopelis may exhibit glandular pads on the forelimbs or ventral surface near the sternum.

e. Osteology and other anatomy.
Few specialized or unique bony features have been documented for the Arthroleptidae, but some examples do exist. In several genera (Astylosternus, Scotobleps, Trichobatrachus), species have bony (not keratinous) claws on the toes that when erected pierce through the skin to become functional; a recent study revealed a specialized soft tissue anatomy within the toe tip. Cardioglossa and some species of Arthroleptis lack teeth. Similar to species in the sister-family Hyperoliidae, species of Leptopelis have an intercalary element between the ultimate and penultimate phalanges; however, unlike all other neobatrachian frogs, this element is composed of dense connective tissue rather than bony tissue. Some species of Cardioglossa as well as some miniature Arthroleptis exhibit fusion of the first two vertebrae.

f. Behavior.
Little is known of the behavioral ecology for most species in this family. Some species of Leptopelis are known to have a defensive display in which the mouth is opened and the forelimbs rotated above the head. Arthroleptis are often referred to as "squeakers" because of their distinctive insect-like call. Based on scars on mature male specimens, Trichobatrachus robustus is suspected of engaging in male-male combate; male Arthroleptis may engage in some form of male-male aggressive interactions. At least some species are known to have axillary amplexus, though in many cases the form of amplexus is not documented.

g. Larvae
Tadpoles of Astylosternus, Cardioglossa, Leptodactylodon, Nyctibates, Scotobleps, and Trichobatrachus are typically found in streams or small rivers. Depending on the species, tadpoles of the genus Leptopelis can be found in a variety of habitats ranging from montane streams, small pools, or even marginally flooded leaf litter. Tadpoles of each genus often co-occur (for example, tadpoles of Astylosternus, Cardioglossa, Leptodactylodon, and Leptopelis can often be collected in the same dipnet; D.C. Blackburn, pers. obs.).

Tadpoles of each genus are morphologically distinctive. For example, Astylosternus tadpoles are large and carnivorous; Cardioglossa tadpoles are dorsoventrally compressed with small eyes and elongate spiracular tube, and are semi-fossorial, often partially submerged in sediment in streams; Leptodactylodon tadpoles are morphologically similar to the funnel-mouthed tadpoles of the megophyrid frog genus . Only one (Arthroleptis) of the eight genera lacks a free-living tadpole; all species of Arthroleptis are believed to have direct development in which a miniature froglet hatches from a terrestrial egg.

h. Genomics.
Arthroleptis exhibit the fewest number of chromosomes (n = 7) documented for a frog. Other arthroleptid genera exhibit a relatively wide range of karyotypes (Astylosternus, n = 27; Nyctibates, n = 14; Cardioglossa, n = 8 or 9; Leptopelis, n = 11, 12, or 15).

To explore genetic data for Arthroleptidae in GenBank, click here.

3. Distribution

a. Geographical distribution.
Found in forests, grasslands, and woodlands throughout much of sub-Saharan Africa, but noticeably absent from the Cape Floristic Region, a biodiversity hotspot with a number of endemic frog species.

b. Elevational range.
From near sea-level to above 9000 feet (~2700 m).

4. Ecology

a. Trophic biology
In general, these species consume a range of leaf litter arthropods. However, some species, such as Leptopelis brevirostris, are known to eat snails.

b. Type of microhabitat(s)
Species in all genera can be found within or on the leaf litter of the forest floor.

c. Life history

All species of Arthroleptis are believed to have direct development. Species of all other genera are believed to have tadpoles, though there is the possibility that some Leptopelis may also have direct development (L. brevirostris).

d. Diurnal and seasonal activity

Breeding seasons appear to be correlated with rainy seasons in most places, though this may not be the case for Trichobatrachus. Males of most species can be found calling at dusk or thereafter. Species of Arthroleptis, Cardioglossa, and sometimes Astylosternus can be found active in the leaf litter during the day. However, for the most part these species are most active at dusk and at night.

e. Reproduction
Females of at least some species of Arthroleptis are known to attend clutches of terrestrial eggs (all species of Arthroleptis are believed to have direct development). Males of Arthroleptis and Cardioglossa can be found calling from the leaf litter, on stones, or in some cases even perched on leaves. Cardioglossa are often found calling in close proximity to streams. Males of Leptopelis are typically found calling in bushes or trees a meter or more above the ground. Astylosternus, Leptodactylodon, Nyctibates, and Scotobleps can all be found calling on the leaf litter or under stones near streams. Little is known of the reproductive behaviors of Trichobatrachus.

f. Antipredator defenses
Astylosternus, Scotobleps, and Trichobatrachus have sharp bony claws used in defense; when the male or females frogs are grabbed, the frogs kick their legs and rake these claws against ones skin. These claws are the terminal pedal phalanges (i.e., the last bone in the toe) and become functional by piercing through the skin of the toe. It has recently been suggested that these bony claws may also be used in male-male combat in Trichobatrachus.

5. Evolution

a. Systematics.
There are 141 described species allocated to eight genera. Leptopelis and Arthroleptis are the most species-rich genera, followed by Astylosternus, Cardioglossa, and Leptodactylodon. Three genera are monotypic (i.e., containing only one described species): Nyctibates, Scotobleps, and Trichobatrachus.

b. Phylogeny.
The nearest relative of the Arthroleptidae is the Hyperoliidae. Based on molecular phylogenies in which the rate of evolution is calibrated by either paleobiogeographic scenarios or the placement of fossils of distantly related taxa, the Arthroleptidae and Hyperoliidae may have diverged from one another as early as the Late Cretaceous (i.e., > 65 million years ago).

c. Fossils.
There are no fossils assigned to the Arthroleptidae.

d. Character Evolution.
Several signficant morphological features of arthroleptid frogs may have been gained and lost multiple times. While Astylosternus and Trichobatrachus are sister taxa, and thus the distribution of bony claws in these genera can be related to their presence in their common ancestor, the genus Scotobleps, also with bony claws, may be more closely related to other arthroleptid genera. This would indicate either multiple gains or loss of this trait. The common ancestor of Arthroleptis and Cardioglossa likely had males with elongate third fingers, yet this trait has been lost multiple times (at least once in Cardioglossa and several times in Arthroleptis). Small body sizes (i.e., miniaturization) have evolved several times in this family, including species of Leptodactylodon and Arthroleptis. Analysis of body size evolution within Arthroleptis suggests that common ancestor of all Arthroleptis species was miniature and that this lineage subsequently attained larger body sizes.

e. Biogeography.
Central Africa, especially the Lower Guinean Forest Zone, appears to be a center of diversification for much of this family, though the historical biogeography of Leptopelis remains largely uninvestigated. The widespread genus Arthroleptis appears to have originated in Central Africa before dispersing elsewhere to eastern, southern, and western Africa.

6. Classification

a. Current placement.
The Arthroleptidae is a member of a larger radiation termed the Afrobatrachia (part of the Ranoidea), which also includes the Hemisotidae, Brevicipitidae, and Hyperoliidae. The Afrobatrachia is sister to the diverse and widely distributed family Microhylidae.

b. Alternative schemes.
At present, it seems that there is little disagreement regarding the higher-level classification of the Arthroleptidae. Previously, Leptopelis was often considered part of the Hyperoliidae, whereas Arthroleptis (and its now junior synonym Schoutedenella) and Cardioglossa were considered the sole members of the Arthroleptidae and Astylosternus, Leptodactylodon, Nyctibates, Scotobleps, and Trichobatrachus comprised the Astylosternidae. One current taxonomy places Leptopelis in its own subfamily Leptopelinae and the other seven genera in the Arthroleptinae, but it is not yet clear that these subfamilies form reciprocally monophyletic clades.

c. Brief overview of controversies.
During the mid-to-late 20th century, the genus Leptopelis was often considered a member of the Hyperoliidae, though as early as the 1940s it was considered, based on skeletal anatomy, to be more closely related to other genera now included in the Arthroleptidae. Though clear that they are part of the Arthroleptidae, the precise placement of Leptopelis and Scotobleps in the phylogeny of this family remains uncertain.

d. Inclusive clade.
Afrobatrachia

e. Sister clade.
Hyperoliidae

7. Similar Species

a. Potentially sympatric taxa.
In sub-Saharan Africa, the genus Arthroleptis is most commonly confused with the distantly related genus Phrynobatrachus, also a group of small-to-medium sized terrestrial frogs endemic to Africa. These genera can be most reliably differentiated by the presence of a tubercle in the middle of the tarsus in Phrynobatrachus. Arthroleptis can also be distinguished by the presence of a raised line of skin along the midline of the back. Other genera are not typically confused with other African frogs.

b. Convergent morphs worldwide.
Arthroleptis is morphologically similar to many other clades of small-to-medium sized terrestrial frogs, including species of the Strabomantidae and Eleutherodactylidae. Superficially, Cardioglossa resembles the dart-frogs historically placed in the Dendrobatidae; indeed, when first described, the genus Cardioglossa was considered part of this family. Nyctibates and some species of Astylosternus have a resemblance to the megophryid frog Leptobrachium.

8. Conservation

a. Diseases.
At present, there is only one unpublished report of infection by chytrid (in Astylosternus sp. from eastern Nigeria).

b. Habitat threats.
As many species, especially in Cameroon, are restricted to relatively small regions of montane forest, habitat loss and degradation is a pervasive threat for many arthroleptid genera.

9. Uses by Humans

a. Focus on particular species as exemplars.
While still considered Least Concern, Trichobatrachus robustus is perhaps the most heavily utilized species by people. It is hunted and eaten in a number of places in Cameroon, and also appears in the international pet trade.

b. Use by indigenous populations.
Adults of several genera (including Astylosternus and Trichobatrachus) are actively collected for food in Cameroon; tadpoles of some genera (Astylosternus, Leptodactylodon) are also eaten.

c. Contributions to technology advances.
None demonstrated. However, if wound-healing occurs subsequent to piercing of toe-tips by internal bony claws in Astylosternus and Trichobatrachus, this may be of biomedical interest.

d. International trade.
Trichobatrachus robustus and some species of Leptopelis are not uncommon in the international trade of frog species for hobbyists.

10. References

Note: Rather than focus on supplying references to all literature (or even all literature including species descriptions), the below list aims to capture diverse aspects of the biology of taxa in this family. In some cases, this includes significant taxonomic works, especially revisionary studies.

Amiet, J.-L.. 1970. Morphologie et developpement de la larve de Leptodactylodon ventrimarmoratus (Boulenger) (Amphibien Anoure). Annales de la Faculté des Sciences du Cameroun 1970: 53-71.

Amiet, J.-L. 1971. Le t'tard d'Astylosternus corrugatus Boulenger (Amphibien Anoure). Annales de la FacultÈ des Sciences du Cameroun 1971: 85-98.

Amiet, J.-L. 1972. Description de cinq nouvelles espèces camerounaises de Cardioglossa (Amphibiens Anoures). Biologia Gabonica 8: 201-231.

Amiet, J.-L. 1977. Les Astylosternus du Cameroun (Amphibia anura, Astylosterninae). Annales de la Faculté des Sciences de Yaoundé 23-24: 99-227.

Amiet, J.-L. 1980. Révision du genre Leptodactylodon Andersson (Amphibia, Anura, Astylosterninae). Annales de la Faculté des Sciences de Yaoundé 27: 69-224.

Amiet, J.-L. 1989. Quelque aspects de la biologie des amphibiens anoures du Cameroun. Annales de Biologie 28: 73-136.

Barbault, R., and M.T. Rodrigues. 1979. Observations sur la reproduction et la dynamique des populations de quelques anoures tropicaux. III. Arthroleptis poecilonotus. Tropical Ecology 20: 64-77.

Barbour, T., and A. Loveridge. 1928. A comparative study of the herpetological faunae of the Uluguru and Usambara Mountains, Tanganyika Territory with descriptions of new species. Memoirs of the Museum of Comparative Zoology 50: 87-265.

Barej, M.F., W. Bˆhme, S.F. Perry, P. Wagner, and A. Schmitz. 2010. The hairy frog, a curly fighter? - A novel hypothesis on the function of hairs and claw-like terminal phalanges, including their biological and systematic significance (Anura: Arthroleptidae: Trichobatrachus). Revue Suisse de Zoologie 117: 243-263.

Blackburn, D.C. 2008. Biogeography and evolution of body size and life history of African frogs: phylogeny of squeakers (Arthroleptis) and long-fingered frogs (Cardioglossa) estimated from mitochondrial data. Molecular Phylogenetics and Evolution 49: 806-826.

Blackburn, D.C. 2009. Diversity and evolution of male secondary sexual characters in African squeakers and long-fingered frogs. Biological Journal of the Linnean Society 96: 553-573.

Blackburn, D.C., J. Hanken, and F.A. Jenkins, Jr. 2008. Concealed weapons: erectile claws in African frogs. Biology Letters 4: 355-357.

Blackburn, D.C. and C.S. Moreau. 2006. Ontogenetic diet change in the arthroleptid frog Schoutedenella xenodactyloides. Journal of Herpetology 40: 388-394.

Bogart, J.P., and M. Tandy. 1981. Chromosome lineages in African ranoid frogs. Monitore Zoologico Italiano Supplement 15: 55-91.

Dubois, A. 1981. Liste des genres et sous-genres nominaux de Ranoidea (Amphibiens Anoures) du monde, avec identiﬁcation de leurs espèces-type: conséquences nomenclaturales. Monitore Zoologico Italiano Supplement 15: 225-284.

Dubois, A. 1992. Notes sur la classiﬁcation des Ranidae (Amphibiens Anoures). Bulletin Mensuel de la Société Linnéene de Lyon 61: 305-352.

Frost, D.R., T. Grant, J. Faivovich, R.H. Bain, A. Haas, C.F.B. Haddad, R.O. de S·, A. Channing, M. Wilkinson, S.C. Donnellan, C.J. Raxworthy, J.A. Campbell, B.L. Blotto, P. Moler, R.C. Drewes, R.A. Nussbaum, J.D. Lynch, D.M. Green, and W.C. Wheeler. 2006. The amphibian tree of life. Bulletin of the American Museum of Natural History 297: 1-291.

Gonwouo, L.N., and M.-O. Rˆdel. 2008. The importance of frogs to the livelihood of the Bakossi people around Mount Manengouba, Cameroon, with special consideration of the Hairy Frog, Trichobatrachus robustus. Salamandra 44: 23-34.

Idris, O. 2004. Taxonomy, phylogeny, and biogeography of the African treefrog species of the genus Leptopelis (Hyperoliidae). Unpublished Ph.D. Thesis, University of Texas, Arlington.

Lamotte, M., and M. Zuber-Vogeli. 1954. Contribution ‡ l'étude des batraciens de l'Ouest-africain. IIIóle developpement larvaire de deux espèces rheophiles, Astylosternus diadematus et Petropedetes natator. Bulletin de l'Institut fondamental d'Afrique noire, SÈr. A 16: 1222-1231.

Lamotte, M., and J.-L. Perret. 1963. Contribution ‡ l'Ètude des batraciens de l'Ouest Africain XIVóle développement direct de l'espèce Arthroleptis poecilonotus Peters. Bulletin de l'Institut fondamental d'Afrique noire, SÈr. A 25: 277-284.

Laurent, R.F. 1940. Contribution ‡ l'osteologie et ‡ la systematique des ranides africains, Première Note. Revue de Zoologie et de Botanique Africaines 34: 74-97.

Laurent, R.F. 1941. Contribution ‡ l'osteologie et ‡ la systematique des rhacophorides africains, Première Note. Revue de Zoologie et de Botanique Africaines 35: 85-111.

Laurent, R.F. 1942. Note sur l'ostÈologie de Trichobatrachus robustus. Revue de Zoologie et de Botanique Africaines 36: 56-60.

Laurent, R.F. 1954. Remarques sur le genre Schoutedenella Witte. Annales du MusÈe Royal du Congo Belge, 4, Sciences Zoologiques, Tervuren 1, 34-40.

Laurent, R.F., 1973. The natural classiﬁcation of the Arthroleptinae. Revue de Zoologie et de Botanique Africaines 87: 666-678.

Laurent, R.F. 1973. Le genre Leptopelis Gunther (Salienta) au Zaire. Annales du Musée Royal du Congo Belge, 8, Sciences Zoologiques, Tervuren 202: 1-62.

Laurent, R.F. 1978. L'appareil hyoÔdien des Astylosterninae et des Arthroleptinae (Amphibia). Revue de Zoologie Africaine 92: 233-240.

Laurent, R.F., and M. Fabrezi. 1985. Le carpe des Arthroleptinae. Alytes 4: 85-93.

Laurent, R.F., and M. Fabrezi. 1989-1990. Further data on carpal structure in ranoid frogs. Alytes 8: 41-50.

Lawson, D.P. 1993. The reptiles and amphibians of the Korup National Park Project, Cameroon. Herpetological Natural History 1: 27-90.

Maas, J.D. 1945. Contributions to the cranial morphology of the West African ranid Schoutedenella muta Witte. Annals of the University of Stellenbosch 23: 21-42.

Manzano, A.S., M. Fabrezi, and M. Vences. 2007. Intercalary elements, treefrogs, and the early differentiation of a complex system in the Neobatrachia. Anatomical Record 290: 1551-1567.

Mohneke, M., A.B. Onadeko, M. Hirschfeld, and M.-O. Roedel. 2010. Dried or fried: amphibians in local and regional food markets in West Africa. TRAFFIC Bulletin 22: 117-128.

Noble, G.K. 1925. The integumentary, pulmonary, and cardiac modifications correlated with increased cutaneous respiration in the Amphibia: a solution to the 'hairy frog' problem. Journal of Morphology and Physiology 40: 341-416.

Poynton, J.C. 1976. Classiﬁcation and the Arthroleptinae. Revue Zoologie Africaine 90: 215-220.

Poynton, J.C. 1985. Nomenclatural revision of southeast African treefrogs of the genus Leptopelis (Amphibia: Hyperoliidae). South African Journal of Science 81: 466-468.

Poynton, J.C. 2003. Arthroleptis troglodytes and the content of Schoutedenella (Amphibia: Anura: Arthroleptidae). African Journal of Herpetology 52: 49-51.

Richards, C.M., and W.S. Moore. 1996. A phylogeny for the African treefrog family Hyperoliidae based on mitochondrial rDNA. Molecular Phylogenetics and Evolution 5: 522-532.

Schmidt, K., and R.F. Inger. 1959. Amphibians, exclusive of the genera Afrixalus and Hyperolius. Exploration du Parc National de l'Upemba, Mission G.F. de Witte, Insitut des Parcs Nationaux du Congo Belge 56: 1-64.

Schi¯tz, A. 1975. The Treefrogs of Eastern Africa. Steenstrupia, Copenhagen.

Schi¯tz, A. 1999. Treefrogs of Africa. Edition Chimaira, Frankfurt am Main.

Toft, C.A. 1982. Community structure of litter anurans in a tropical forest, Makokou, Gabon: a preliminary analysis in the minor dry season. Revue d'Écologie (La Terre et la Vie) 36: 223-232.

Vences, M., J. Kosuch, F. Glaw, W. Bˆhme, and M. Veith. 2003. Molecular phylogeny of hyperoliid treefrogs: biogeographic origin of Malagasy and Seychellean taxa and re-analysis of familial paraphyly. Journal of Zoological Systematics and Evolutionary Research 41: 205-215.

Zimkus, B.M. and D.C. Blackburn. 2008. Distinguishing features of the sub-Saharan frog genera Arthroleptis and Phrynobatrachus: a short guide for field and museum-based researchers. Breviora (Museum of Comparative Zoology) 513: 1-12.