Family Hyperoliidae

Afrixalus spinifrons (Cope, 1862)

Natal Leaf-Folding Frog, Natal Spiny Reed Frog, Spinose Dwarf Leaf-folding Frog, Natalse Blaarvouendepadda (A)

By M. Pickersgill, M. Burger and P.J. Bishop

Currently accepted name: Afrixalus spinifrons (Cope, 1862)
Red listing status: Least Concern (2016)

Photo by Brouard Jean-Paul; Luke Durkan, 2011. URL: FrogMAP: 697

RED LIST SPECIES

Status: Vulnerable (VU) Criteria: B1+2ab(ii,iii,iv,v)

Identification

This species belongs to the dwarf Afrixalus complex, whose members are difficult to distinguish from one another morphologically, but whose advertisement calls are diagnostic (Backwell and Passmore 1991). A. spinifrons may be distiguished from sympatric Hyperolius species by the presence of a vertical pupil and tiny asperities on the skin.

The distribution and structure of the asperities is variable. In 60% of A. s. spinifrons males, there is a concentration of tubercular asperities on the snout, that is moderately to markedly swollen, almost bulbous. Along the Eastern Cape and KwaZulu-Natal coast, this character may be used to distinguish A. spinifrons from the other sympatric dwarf Afrixalus species, namely A. delicatus and A. aureus. However, at higher altitudes, populations assigned to A. s. intermedius lack a swollen snout and the asperities are distributed evenly over the dorsum (Pickersgill 1984, 1996, 2000).

In A. s. spinifrons, the dorsum is ivory to golden yellow with a broad, brown, median wedge that starts at the tip of the snout and broadens posteriorly, where it is split in two by a narrow light-coloured wedge over the urostyle. In A. s. intermedius, these dark markings are absent or confined to the posterior part of the dorsum. The light to dark, lateral band present in all dwarf Afrixalus species, is unmarked in A. spinifrons but speckled to a varying extent in A. aureus and A. delicatus. A dark, transverse band on the tibia distinguishes A. aureus from A. spinifrons. (Pickersgill 1984, 1996, 2000; Passmore and Carruthers 1995).

The advertisement calls of A. spinifrons and A. delicatus consist of two components, a short “zip” and a prolonged “trill”, while A. aureus produces only the trill component. It has been demonstrated that the trill component functions in female attraction in A. delicatus and A. aureus, and that females only respond to calls of their own species. A similar function is presumed for A. spinifrons (Backwell and Passmore 1991).

In A. s. spinifrons, the trill component has a lower emphasized frequency and a higher pulse rate than that of A. delicatus (Backwell and Passmore 1991; Passmore and Carruthers 1995; Pickersgill 1996). The published call data are based on small samples and exhibit a wide range of variation. Further investigation of the differences in advertisement call within the A. spinifrons complex is needed (see also comments under A. knysnae).

A. spinifrons bears a close resemblance to A. knysnae, but is slightly smaller (16.4–24.6 mm versus 22.0–25.2 mm) and has a larger gular disc (Pickersgill 1996, 2000). The ranges of the two species are separated by about 400 km.

Distribution

A. spinifrons is endemic to South Africa. The subspecies A. s. spinifrons occurs at low to intermediate altitudes from Cintsa Bay (3228CC) and Kei Road (3227DA) in Eastern Cape Province, to St. Lucia Village (2832AD) in KwaZulu-Natal. The subspecies A. s. intermedius occurs at altitudes above 1000 m in western KwaZulu-Natal, between Underberg (2929CD) and Hilton/Cedara (2930CB; Pickersgill 1996). In the course of the atlas survey, additional populations were discovered near Franklin (3029BC) and Umtata (3128DB). Populations in Eastern Cape and KwaZulu-Natal provinces, previously referred to A. knysnae by Poynton (1964), were assigned to A. s. spinifrons by Pickersgill (1996).

Unlike most frog species sharing its breeding habitat, A. spinifrons has a rather quiet call. This may be a factor contributing to the paucity of atlas records and the patchy distribution reflected on the map.

Habitat

A. spinifrons inhabits Coastal Bushveld-Grassland and Moist Upland Grassland. Armstrong (2001), using the Biosource Programme, modelled the potential distribution range of A. s. intermedius to include these vegetation types: Moist Coast Hinterland Ngongoni Veld, Moist Midlands Mistbelt, Moist Highland Sour Veld and Dry Highland Sour Veld (Harrison et al. 2001). In low-lying areas along the coast, A. s. spinifrons breeds in standing water, in dense sedge beds and inundated, grassy wetlands with abundant surface vegetation, while at higher altitudes, it inhabits marshes, dams, floodplains and riverbanks (Lambiris 1989a; Pickersgill 1996).

Life history

During the day, both subspecies retreat into the leaf axils of grasses, rushes and arum lilies Zantedeschia spp., particularly those standing in or immediately adjacent to water, and A. s. intermedius may also be found sunbathing in arum lily blooms. This habit is adopted almost exclusively by females and juveniles (Pickersgill 1996). In winter, A. s. spinifrons is often encountered in the leaf axils of Strelitzia and bananas.

Breeding activity has been reported August–February for A. s. spinifrons, and September–January for A. s. intermedius. Males call from emergent vegetation in ponds, dams and streams, in relatively high-density choruses of 4–20 individuals, between sunset and 03:00 (Backwell and Passmore 1991). The white eggs are deposited in longitudinally folded leaves of grasses, young Phragmites and herbaceous plants (particularly Commelina spp.), just below or above the water, and hatch 4–6 days later. Metamorphosis typically occurs within about six weeks (Backwell and Passmore 1991; Pickersgill 1996, 2000).

Predators include water snakes Lycodonomorphus spp., green snakes Philothamnus spp. and the Rhombic Night Adder Causus rhombeatus. Prey items include mosquitoes and other flies, spiders and moths (M.P. pers obs.).

Conservation

Status

During the atlas period, few records of this species were obtained (see table). Although not evaluated by Harrison et al. (2001), assessment for this publication placed it in the Vulnerable category on the basis of its extent of occurrence (<20 000 km²) and area of occupancy (<2000 km²), a severely fragmented range, a perceived continuing decline in area of occupancy, numbers of individuals and locations, and a decrease in the area and quality of its habitat.

The KwaZulu-Natal Nature Conservation Services has listed A. s. intermedius as a taxon of special significance because it is endemic to the province (Armstrong 2001).

Threats

The KwaZulu-Natal coast has several threatened frog species (Burger and Harrison 2002). This region has been extensively altered by urban and industrial developments, sugar cane plantations, timber plantations and infestations of alien vegetation, all of which have negatively affected biodiversity (Johnson 1987; Johnson and Raw 1987; Alexander 1990; Armstrong et al. 1998). These activities are the primary factors responsible for excessive loss and fragmentation of A. spinifrons habitat. Many of the surviving populations are small and localized and therefore more susceptible to catastrophic events that could result in local extinctions. Further threats to this species include pesticide pollution of breeding habitats, and trampling of habitat by livestock.

Recommended conservation actions

Historical localities should be revisited to establish the status of populations. In the course of such surveys, it is recommended that recordings of advertisement calls be collected along with specimens and tissue samples.

Armstrong (2001) noted that the species was inadequately conserved with only 2.8% of its predicted range falling within protected areas. A. spinifrons was predicted to occur in 12 KwaZulu-Natal protected areas, nine of which could potentially maintain viable populations, the largest being the uKhahlamba-Drakensberg Park (3864 ha). Although Armstrong’s (2001) assessment was based only on a subset of the species’ range, omitting the coastal populations, the species is clearly not adequately protected and it is therefore recommended that a series of known viable populations be evaluated with the aim of incorporating them into the network of protected areas. Protected areas in coastal KwaZulu-Natal, from which this species is known, include reserves at St. Lucia and Mtunzini. In Eastern Cape Province it is known from Hluleka and Silaka nature reserves.

Some general recommendations include habitat management, limiting factor management, monitoring, and public education. Future EIAs for urban, industrial, agricultural and forestry developments along the KwaZulu-Natal and Eastern Cape coast, should take cognisance of the conservation status of A. spinifrons and of potential impacts on its habitat, and should mitigate their plans accordingly.

Current distribution map

Undated records;  pre-1996;  1996 to 2002;  2003 to present