Family Hemisotidae

Hemisus Günther, 1859

shovel-nosed frogs, pig-nosed frogs, snout-burrowers, graafneuspaddas (A)

By G.J. Alexander

Currently accepted name: Hemisus sp.
Red listing status:

Description

The name derives from the Greek hemi = half, and ous = ear. It is unclear as to why Günther felt that “half-ear” was an appropriate name for this genus. There has been some controversy over the derivation of the name Hemisus (see Van Dijk 1978a; Poynton 1978; Poynton and Broadley 1985a; Frost and Savage 1987, 1988), which has led to confusion over the correct ending for specific names in this genus. Currently, the accepted ending for specific names is –us (rather than –um), although this is still not followed consistently by all authors (e.g. Ritter and Nishikawa 1995; Nishikawa et al. 1999).

These frogs are unmistakable. They have a squat and bloated appearance and the head is narrow and pointed with smallish but obvious, beady eyes. The pupil is vertical and there are no teeth on the upper jaw. The snout is spade-like with a sharp, hardened edge used for burrowing forwards. The toes are usually free of webbing or, at most, barely webbed.

Distribution

Hemisus is the sole genus of the African endemic family Hemisotidae. Hemisus occurs widely in sub-Saharan Africa, but is absent from all but the most northeastern parts of southern Africa. Of the eight species in this genus (Duellman 1993), three occur in the atlas region, including one endemic species, H. guttatus. The two historical records of H. guineensis broadleyi, in Limpopo Province, lie at the periphery of its range that extends to the north of the atlas region (Poynton and Broadley 1985a).

Habitat

Most species seem to prefer semi-arid to arid savanna habitats in tropical and subtropical climates, but some also occur in Grassland and Forest biomes. H. guttatus has a limited distribution in areas with a relatively high annual rainfall, occurring only in the vicinity of large pans and along rivers (Alexander 1990). Noble (1924) noted that H. marmoratus is associated with termitaria.

Life history

All Hemisus species are fossorial. Unlike most other burrowing frogs, shovel-nosed frogs burrow forwards, using their snouts to wedge open the soil with forceful up-and-down movements. The sturdy front legs are used to move soil aside during burrowing, while the hind legs push the frog forward along the burrow.

Their diet appears to consist primarily of ants and termites (Ritter and Nishikawa 1995) or earthworms (Wager 1986), and may be species specific. Ritter and Nishikawa (1995) and Nishikawa et al. (1999) described the unusual tongue of H. marmoratus, that is probably characteristic of all species in the genus. The tongue is elongated forwards during feeding by a muscular hydrostatic mechanism rather than the usual muscle-driven flipping of the tongue out of the mouth. This unique method of projection allows the tongue to be aimed with more accuracy than is attained in any other frog species (Ritter and Nishikawa 1995), and it may represent an adaptation to feeding underground, or for the extraction of termites and ants from their tunnels.

Breeding takes place in temporary bodies of water such as pans and vleis, as well as more permanent wetlands such as marshes. Males usually call from the surface but are also known to call from their burrows. The call is a long, insect-like trill similar in some cases to that of a mole-cricket (Gryllotalpa spp.).

Amplexus is inguinal, and the much-larger female burrows underground with the amplexing male in tow. She constructs a breeding chamber where the eggs are laid and fertilized. The female remains with the eggs and tadpoles until they leave the egg chamber to enter the water.

Owing to the temporary nature of their breeding sites, Hemisus species have evolved a reproductive strategy that gives them an advantage over many other species with which they co-exist: they lay their eggs in underground chambers, where the tadpoles are able to develop to quite an advanced stage before the main rains fall. Wager (1986) drew attention to the well-developed blood system on the surface of the body of tadpoles and suggested that this allows improved respiration if the tadpoles remain out of water for some time.

Unpredictable rainfall patterns have led to a number of behavioural adaptations that ensure that the tadpoles find their way to water. If the water level does not reach the nest, the female constructs a mudslide down which the tadpoles slither into the water or, if this is not possible, they cling to her body while she physically transports them to the water (Rödel et al. 1995; Kaminsky et al. 1999; Rödel 2000). These behaviours have been observed in H. marmoratus but not H. guttatus, for which little life history information exists.

Conservation

The endemic H. guttatus has been classified Vulnerable (this publication). This species is poorly known and infrequently observed, and should be targeted for research. H. marmoratus is not threatened and H. guineensis is peripheral to the atlas region.