This is a large and complicated assemblage. The family Bufonidae is composed of 52 genera with a worldwide distribution, except for Australia. The only bufonid in Australia is the introduced Cane Toad (Rhinella marinus), which has become a pest and danger to local wildlife, due to its voracious appetite, large size, high fecundity, and high toxicity (it has pronounced negative effects on native Australian snakes). This family includes the stereotypical "toad" with shortened forelimbs, hindlimbs used for walking or hopping, dry warty skin, and parotoid glands behind large eyes. Development in this group is quite varied; there are species that lay eggs in water and produce aquatic larvae, terrestrial direct developers, and the only known viviparous species of frogs,Nectophrynoides andNimbaphrynoides occidentalis, which give birth to fully developed young toadlets. Bufonids have potent skin toxins, some concentrated in the prominent parotoid gland behind the eyes, and can be fatal to predators when ingested. However, there is evidence for powerful potential pharmaceutical uses of these bufonid compounds, from both folk medicinal practices and in scientific research in cancer and pain treatment. Some genera, like members ofAtelopus, lack a tympanum and the middle ear column, making them "earless" but nonetheless sensitive to high frequency sounds (Womack et al 2018), leaving many questions about their physiology and evolution. Some toad species are believed to be extinct, including the famous Golden Toad (Incilius periglenes) of Costa Rica, which was last documented in 1989. Other species listed as Extinct by IUCN includeAtelopus longirostris andAtelopus vogli. This specious group has been subject to many phylogenetic studies over the years, some taking a comprehensive anuran approach or others focusing on regional or specific clades in attempts to propose a supported monophyletic taxonomy for bufonids. However, the diversity of over 600 species of bufonids have been a challenge to find stable consensus. News Highlights:February 28, 2022: Harlequin toads (Atelopus) are renowned for their toxicity, but how much is really known about their chemical defense strategies and characteristics?Pearson and Tarvin (2022) reviewAtelopus chemical defense literature and find that only 16 of the 100 known species have been assessed for toxins. Furthermore, South American species – which make up the vast majority ofAtelopus diversity – have received disproportionately little investigation relative to Central American species. SurveyedAtelopus possess individual toxins (chiriquitoxin and zetekitoxin) and toxin combinations found nowhere else in nature, and the authors suggest that the genus contains further unidentified toxin diversity that – in the face of catastrophic population declines – may be lost to science. The authors also trace the development of toxin detection and quantification methods applied toAtelopus, noting impacts of historical methodological limitations on chemical defense data and identifying promising techniques for future studies. Lastly, given the limited information available regardingAtelopus predators, they speculate about possible immunological and ecological roles ofAtelopus toxins. Time to study many harlequin toad species may be limited, but Pearson and Tarvin make the case for continued chemical defense research on this genus. (Kannon Pearson) February 15, 2021: The mystery of anuran middle ear loss has new insights! Most frogs have a tympanic membrane and middle ear column (the columella) that help them hear airborne sound. Despite being well known for their calls, many frogs and toads are “earless”, lacking the tympanic membrane and columella. The factors contributing to more than 38 evolutionary losses of the anuran ear remain unknown. Recently,Stynoski et al. 2020 raised and examined two earless toad species and two closely-related eared toad species. To their surprise, they found the middle ear column started to develop right on-time in both eared and earless species! However, in earless species, the middle ear column abruptly arrests development and remains either stagnant and very small or is completely resorbed. This means that the genetic and developmental mechanisms that initiate middle ear differentiation are intact in earless species but perhaps changes in timing or expression of biochemical pathways that regulate the extension or differentiation of the columella after metamorphosis underlie convergent trait loss among toad lineages. Earless frogs are helping biologists understand the evolutionary and developmental mechanisms of trait loss; this discovery of developmental ear remnants further shows that lost traits are often not lost entirely. Their findings join many other studies that have found developmental remnants of lost traits, recently reviewed bySadier et al. 2021. Studying development reveals key insights into evolutionary processes and this is certainly the case with earless frogs. (DW) Written by AmphibiaWeb |
Relevant ReferencePough, F. H., R. M. Andrews, M. L. Crump, A. H. Savitzky, K. D. Wells, and M. C. Brandley. 2015. Herpetology. Fourth Edition. Massachusetts: Sinauer. Womack MC, Christensen-Dalsgaard J, Coloma LA, Hoke KL. 2018. Sensitive high-frequency hearing in earless and partially eared harlequin frogs (Atelopus). J Exp Biol. 221(Pt 10):jeb169664.doi: 10.1242/jeb.169664. |
