Rare frog hasn't croaked yet

A new spiny-throated reed frog from Tanzania

Reed frog is the common name for any of about 150 species of the genus Hyperolius found in sub-Saharan Africa.  Within the genus is a small complex of species known as the spiny-throated reed frogs.  As the name suggests, they are distinguished, in part, by patches of tiny spines on the male gular flaps. The complex was unknown until the 1970s and what was initially thought to be one species have proven to be a complex of at least eight.

Male Hyperolius ukaguruensis in life, Ukaguru Mountains, Tanzania, by Lucinda P. Lawson.  Source:  L. P. Lawson et al. (2023) PLoS ONE 18: e0277535. https://doi.org/10.1371/journal.pone.0277535

The most recently discovered spiny-throated reed frog is Hyperolius ukaguruensis and, like the others, it lives in the Eastern Arc Mountains that stretch from Tanzania some 1450 kilometers to Kenya. It is small, golden-hued, with eyes smaller than those of related species. Spiny-throated reed frogs are documented from montane forests and grasslands in Tanzania, Malawi and Mozambique.  Their small size — the new one varies from a bit under to a bit over an inch in length — their morphological similarity, limited geographic distributions, and the fact that males do not vocalize with advertising calls makes locating and identifying them challenging.  Without mating calls, it is not clear how these frogs communicate, although pheromones are one possibility.

So far, the new species is known only from the Ukaguru Mountains of Tanzania.  It fits the pattern of other spiny-throated reed frogs being limited to a single mountain block or, at most, nearby blocks.  This geographic pattern differs from other Hyperolius.

Spiny-throated reed frogs are voiceless, or nearly so, with no documented calling behavior and only a single species, Hyperolius minutissimus, observed to emit a faint, raspy sound.  They prefer undisturbed, in-tact habitat, being far less common in marginal or seasonal ones. The species are significantly genetically diverged from one another, with the exception of one pair of sister-species separated by a small genetic distance in spite of being morphologically distinct.  The function of the gular spines has not been demonstrated, but spines on toes and legs of other male frogs are used to stimulate females during mating.  Other interesting behaviors of species in this complex include females guarding eggs and splashing them with water to keep them moist.

One species of the genus, Hyperolius viridiflavus, not part of the spiny-throated group, is noteworthy as the only documented example of protogyny in a frog, that is, a female producing eggs then becoming a functional male.  In amphibians, environmental conditions and hormones can alter sex determination.  But this kind of sequential hermaphroditism is a phenomenon previously limited to a few plants and certain invertebrates and fishes.  Seven of 24 females, under laboratory conditions, produced a clutch of eggs then became male without hormone exposure.  This included the appearance of male calling posture after about two months and the development of vocal sacs.  While this sequential sex transformation was unprecedented, frogs are noted for their genetic plasticity. For example, when the testes of the European toad, Bufo bufo, are surgically removed, the Bidder’s organ develops into functional ovaries.

Like so many species with limited habitats and narrow geographic distributions, the spiny-throated reed frogs are susceptible to environmental change and extinction.  It is concerning that some species have not been observed in the field recently.  The new voiceless species is evidence that, thankfully, these fascinating frogs haven’t croaked yet.

References

Grafe, T. U. and K. E. Linsenmair (1989) Protogynous sex change in the reed frog Hyperolius viridiflavus.  Copeia 1989 (4): 1024-1029.

Lawson, L. P., Loader, S. P., Lyakurwa, J. V., and H. C. Liedtke (2023) Diversification of spiny-throated reed frogs (Anura: Hyperoliidae) with the description of a new, range-restricted species from the Ukaguru Mountains, Tanzania.  PLoS ONE 18: e e0277535.