anurans

Tadpole diversity needs more attention if anurans are to be effectively conserved

October 1, 2025 by Admin

Roger Downie

Froglife and University of Glasgow

A quarter of a century ago, James Hanken (1999) drew attention to the ironical situation that at a time when the serious worldwide declines in amphibian populations were becoming recognised, the number of described amphibian species was increasing at a faster rate than for any other vertebrate class. In 1985, the number of species was 4003; given the current rate of new descriptions, Hanken expected the number to reach 5000 by the turn of the century (1.7% more per year). Now, half-way through 2025, Frost’s database lists 8883 species (actually 3% more per year since 1985), 7828 of them anurans (frogs and toads), with no sign that the rate of new species description is likely to decline soon. But sadly, as these numbers continue to rise, so too do the threats: the Global Amphibian Assessment (GAA) in 2004 concluded that 32.5% of species were threatened with extinction, the highest proportion for any vertebrate class; GAA2 in 2022 revised that figure upwards to 40.7% (although part of the rise results from a decrease in the proportion of species classed as Data Deficient, 22.5 to 11.3%). That figure can only be a tentative estimate, since the conservation status of the newly-described species is rarely known.

If we are to roll back the wave of amphibian extinctions, a key step is to determine how many species exist and where they live (to non-biologists, it is probably a surprise to realise that after centuries of species recording, the world species catalogue is nowhere near complete). However, if we are to protect species into the future, we need to know much more than is contained in a basic species description. Hortal et al. (2015) listed the ‘shortfalls’ (or deficiencies) in our current knowledge about biodiversity under a set of headings named after prominent scientists, for example:

Linnean: description and cataloguing of species
Wallacean: distribution of species
Prestonian: abundance and population dynamics
Darwinian: relationships and variability

Faria et al.(2020) identified another class of shortfall: Haeckelian, named after the prominent 19^th century German embryologist Ernst Haeckel. This class encompasses knowledge of a species life-history stages. Now, a group of mainly Argentinian biologists has applied this idea to frogs and toads (Vera Candioti et al., 2023; Nori et al., 2025): they note that for more than 50% of species, we have only adult descriptions, with no information on embryonic, larval and juvenile stages, including on their habitats and habits. This clearly matters for species with biphasic life histories where the larval/tadpole stage is fundamentally different from the adult in form, habits and habitat, since measures aimed at conserving adults may be ineffective in assisting earlier stages. The authors note that the IUCN species assessment process does not yet take this information shortfall fully into account, finding that many species where knowledge of larval stages is entirely lacking are currently listed as of Least Concern.

Nori et al.’s (2025) paper describes a strategy for reducing the Haeckelian shortfall for anurans, using geographical and other data to identify the areas where focussed fieldwork should be capable of finding and documenting the missing larval stages, habits and habitats: these are parts of the tropical Andes, eastern Brazil, tropical Africa, India, southeast Asia and New Guinea.

In the UK, with our very limited anuran fauna, including limited larval diversity (all species having pond-dwelling algal browsers), it is hard for us to appreciate the vast diversity of larval forms found elsewhere, and to understand that much diversity remains to be discovered. A few recent examples illustrate this point:

Dias et al. (2024a) analysed the food processing mouth-parts of several so-called ‘sand-eating’ tadpoles from Madagascar: the ruffled ridges used for separating organic material from sand-grains are described as ‘astonishingly novel’ and not seen previously in any anuran larvae.
Dias et al. (2024b) described the adhesive oral discs of two Andean toad tadpoles as so different from those of relatives that they justified assignment to a new genus Adhaerobufo despite the adults being quite similar to relatives.
Romero-Carvajal et al.(2023) describe early development of a ‘plump toad’, one of eleven species found in the Andes, where early stages are previously unknown. Small numbers of large terrestrial eggs are produced, with no free-living tadpole stage; however, the pre-hatching stages include an elongated tail, unlike the findings for other direct-developing species.

Dias et al. (2024a) analysed the food processing mouth-parts of several so-called ‘sand-eating’ tadpoles from Madagascar

When I started research on the amphibians of Trinidad and Tobago in the 1980s, I was lucky that the existing account (Kenny, 1969) included illustrations and natural history notes on most of the tadpoles, and we were able to extend these to cover some new species, including the glassfrog and stream-frog tadpoles of Tobago. But much remains to be done. We made observations on some key variables: for example, the time taken from hatching to metamorphosis, as little as 12 days in some cases; the time taken for the period of metamorphosis itself, and the behaviour of newly metamorphosed froglets. All these are crucial to the success of the anuran life-cycle, but remain hugely under-researched for most species. Let’s hope that the focus provided by Vera Candioti and colleagues provides an impetus for the needed research.

Acknowledgement

Thanks to Robin Bruce for drawing my attention to Nori et al’s paper.

References

Dias, P.H.S. et al. (2024a). Stranger things: on the novel buccopharyngeal anatomy and functional morphology of ‘sand-eating’ Malagasy tadpoles. Zoological Journal of the Linnean Society 202, zlae127.

Dias, P.H.S. et al. (2024b). The remarkable larval morphology of Rhaebo nasicus with the erection of a new bufonid genus and insights into the evolution of suctorial tadpoles. Zoological Letters10, 17.

Faria, L. et al. (2021). The Haeckelian shortfall, or the tale of the missing semaphoronts. Journal of Zoological Systematics and Evolution Research 59, 359-369.

Hanken, J. (1999). Why are there so many new amphibian species when amphibians are declining? Trends in Ecology and Evolution 14, 7-8.

Hortal, J. et al. (2015). Seven shortfalls that beset large-scale knowledge of biodiversity. Annual Review of Ecology, Evolution and Systematics 46, 527-549.

Kenny, J. S. (1969). The amphibia of Trinidad. Studies on the Fauna of Curacao and other Caribbean Islands 29, 1-78.

Nori, J. et al. (2025). Global key areas for anuran tadpole discovery. Biological Journal of the Linnean Society 145, blaf017.

Romero-Carvajal, A. et al. (2023). Direct development or endotrophic tadpole? Morphological aspects of the early ontogeny of the plump toad Osornophryne occidentalis. Journal of Morphology 284, e21582.

Vera Candioti, F. et al. (2023). Global shortfalls of knowledge on anuran tadpoles. npj Biodiversity 2, 22.

Toe tapping frogs: is it a lure?

October 1, 2024 by Admin

Written by Dr. Laurence Jarvis (external contributor)

Anurans (frogs and toads) are well known for their vocal abilities, particularly during the breeding season. However, visual cues are less studied and understood. Toe tapping is an unusual and interesting behaviour exhibited by a range of amphibian species. Also known as toe twitching, wiggling or trembling amphibians will typically raise or wave one or more legs or feet in the air. This may be slow or very fast. The behaviour appears to be performed by a range of amphibian species as a recent study found reports of toe tapping from 42 amphibian species from 12 families¹. There is currently no consensus on the exact reason for toe tapping that can be applied to all amphibian species. The behaviour often appears to occur in response to the presence of prey and may be linked to prey capture. It has also been noted to occur in relation to mate attraction and courtship, such as in the Southern Hispaniola Crested Toad (Peltophryne guentheri), as well as aggression. However, toe tapping may vary widely within a species with individuals sometimes performing the behaviour and not at other times². When the movements are slow with a raised limb, this is referred to as pedal luring and this is thought to attract prey through mimicking vibrations produced by the prey, as observed in the South American horned frogs (Ceratophrys spp.). However, Sloggett & Zeilstra (2008) proposed that instead, toe-tapping may cause additional vibrations that keeps the prey moving in order to allow detection by the frog³. In support of this hypothesis, a recent study of poison dart frogs (Dendrobatidae) found that individual frogs increased their toe tapping behaviour when prey were inactive which resulted in increased prey movements and more effective detection².

Figure 1. Dyeing poison frogs (Dendrobates tinctorius) may tap their toes to enable prey capture.

Dyeing poison frogs (Dendrobates tinctorius) of South America have often been observed to tap their posterior toes⁴. This tapping can be incredibly rapid with reports of up to 500 taps per minute, which is near the limit of vertebrate muscular vibration⁴. Adults of this species feed on a range of small and fast-moving prey such as flies and other arthropods so effective prey capture is essential. Research carried out by Parrish and Fischer (2024), found that frogs performed toe tapping more whilst feeding and when on a surface that would carry the vibrations, such as smooth leaves⁴. The frogs performed toe tapping much less when sitting on soil which transmit vibrations more slowly. The researchers also found that individuals increased their toe-tapping whilst in the presence of a breeding partner. The reasons for this are unclear but shows that this species may adapt its tapping behaviour depending on the presence of other individuals, particularly breeding partners. Overall, this study supports the work of Sloggett and Zeilstra (2008) and suggests that toe tapping may agitate small arthropods by substrate vibrations and thereby help the individuals to detect their prey⁴.

Figure 2. The Yellow-striped Poison Frog (D. truncates) exhibits accelerated toe tapping.

The Yellow-striped Poison Frog (D. truncatus) is a poison dart frog native to Colombia. In an attempt to further understand the function of toe tapping in this species Vergara-Herrera et al. (2023) designed an experiment to examine the vibrations produced during toe tapping⁵. The results confirmed that toe tapping was performed in relation to prey capture. In addition, the researchers found that tapping rate accelerated during their sequence of attacks on prey during foraging⁵. This is analogous to the foraging behaviour of species including bats, dolphins and sperm whales, all of which increase call rate as they approach their prey. In these species it is thought that the increase in call rate immediately prior to capturing their prey helps to confirm its location, speed and movement and thus aid in capture. It is not known whether this is the reason for increasing tapping rate in the Yellow-striped Poison Frog but the acceleration is likely to aid in final prey capture. If the toe tapping serves to further agitate the prey, as proposed by Sloggett & Zeilstra (2008), then more rapid toe tapping would make the prey more detectable, and thus easier to capture by the frogs.

Figure 3. The Southern Corroboree Frog (Pseudophryne corroboree) is a critically endangered species native to Australia.

Toe tapping is an under recorded behaviour but is being noted in an increasing number of species worldwide. The genus Pseudophryne consists of 14 endemic Australian species, all of which are small, cryptic ground-dwelling species. Toe tapping has only previously been recorded in one other species within the genus⁶. The Southern Corroboree Frog (Pseudophryne corroboree) is a critically endangered species, restricted to areas above 1300 m elevation in New South Wales, Australia. Recently it was not thought to exhibit toe tapping behaviour. However, during captive breeding, McFadden et al. (2008) observed rapid toe tapping prior to being fed crickets⁶. In this species, for the vast majority of individuals, the tapping foot was raised off the substrate and waved as in the South American horned frogs. This is in contrast to poison frogs, which usually tap their feet whilst in contact with the substrate. The behaviour of the Southern Corroboree Frog would suggest that the moving foot is used to attract prey, rather than trigger movement. Alternatively, as stated by the researchers of the study, it is possible that the tapping could be simply an excited response to the presence of prey.

Overall, toe tapping is an interesting behaviour which has been observed in a wide range of amphibian species. Increased observations and studies on a wider range of species are demonstrating that the behaviour is more widespread than previously thought. Although the exact functional significance is not always fully understood, and may vary between species, it demonstrates the capacity for varied visual cues to be utilised by frog and toad species.

References

¹Erdmann J.A. (2017). The function of toe movement in feeding by the gulf coast toad (Incilius nebulifer). Masters thesis, Southeastern Louisiana University.

²Claessens L.S.A., Ganchev N.O., Kukk M.M., Schutte C.J., and Sloggett J.J. (2020). An investigation of toe-tapping behaviour in anurans by analysis of online video resources. Journal of Zoology, 312 (3): 158–162. https://doi.org/10.1111/jzo.12815.

³Sloggett J.J. and Zeilstra I. (2008). Waving or tapping? Vibrational stimuli and the general function of toe twitching in frogs and toads (Amphibia: Anura). Animal Behaviour, 5: e1–e4.

⁴Parrish T.Q. and Fischer E.K. (2024). Tap dancing frogs: Posterior toe tapping and feeding in Dendrobates tinctorius. Ethology, p.e13465.

⁵Vergara-Herrera N., Cocroft R. and Rueda-Solano L.A. (2023). Eating to the beat of the drum: vibrational parameters of toe tapping behavior in Dendrobates truncatus (Anura: Dendrobatidae). Evolutionary Ecology, 1-17. https://doi.org/10.1007/s10682-023-10277-x

⁶McFadden M., Harlow P.S, Kozlowski S. and Purcell D. (2010). Toe-twitching during feeding in the Australian myobatrachid frog, Pseudophryne corroboree. Herpetological Review, 41(2): 153-154.

Croaking Science: Vocal communication in anurans- call matching and masking

February 27, 2020 by Admin

Males of most frogs and toads typically produce a small number of stereotyped, repetitive calls that have a restricted number of functions. Identified calls include the advertisement, aggressive, distress, warning and release calls (in Narins et al., 2000). The advertisement call often consists of a single note or a series of notes that is repeated frequently for many hours and can be highly stereotyped in its structure. These characteristics are highly advantageous when frogs call in high assemblages and with a high level of background noise. However, an increasing number of research studies have demonstrated that the calls of anurans are not necessarily as simple as once thought and in some species can be highly complex.

***Figure 1****. Several frog species have been observed to emit complex advertisement calls such as the Panama cross-banded tree frog, Simlisca sila.* [Photo credit: Brian Gratwicke, https://commons.wikimedia.org/wiki/File:Simlisca_sila.jpg]

The male Emei music frog (Babina daunchina) from southwest China calls from an underground burrow that serves as a nest for egg laying and tadpole development. Its call typically consists of 4-6 repeated notes with a 150 millisecond gap in between. Research by Yue et al. (2017) showed that the first note in a series of calls conveys more information and is most important than subsequent notes. The researchers examined the brain wave response of females and found that the first call note of the male elicited a greater electromagnetic response compared to the following call notes. The importance of the first call note has been noted in other species from different taxa including zebra finches and some gecko species, where the initial call contains subtle information about the caller which is used in mate selection. The first call note in Emei music frogs may therefore contain information which is crucial for species recognition, individual discrimination and call type identification (Yue et al., 2017). Producing calls is potentially dangerous as it increases predation risk. Therefore encoding greater information in the initial call is biologically advantageous as it conveys most information without increasing predation risk. However, this situation may become more complex as a male may perform call masking which aims to interfere with the call of a neighbouring male whilst enhancing their own call. For example in Kassina fusca, a frog species from the African savannah, an individual male may try to produce its second call very close to the first call of another rival male. Known as backward masking, this results in the second call of the initial male sounding more like a first call and masks that of its neighbour. Call masking is little known in frogs and further research is required to understand its importance in mate selection.

Call matching is another form of competitive vocal interaction between males, which has been described in a few species of frogs. In these species, the number and complexity of calls that are produced by a male are approximately matched by its neighbour (Gerhardt et al., 2000). Since calling is energetically costly and increases risks of predation, males may match the number and type of call of a neighbouring male. The Australian quaking frog, Crinia georgiana is a species with a complex mating system. Males produce calls that typically consist of between one and 11 pulsed notes. Research by Garhardt et al. (2000) found when a male emitted 2-4 calls a neighbour would match the number of notes, but at a higher number of calls e.g. 8, call matching was less often employed. This suggests that the risk of predation at a higher number of calls is too risky and it is not worth males matching the number of calls of a rival.

***Figure 2.*** *The Australian quaking frog, Crinia georgiana exhibits call matching to optimise finding a mate.* [Photo credit: Jean-Marc Hero, https://commons.wikimedia.org/wiki/File:Crinia_georgiana03.jpg]

The Madagascar bright-eyed tree frog Boophis madagascariensis is endemic to Madagascar. It is a large brown tree frog with conspicuous ‘spines’ on its elbows and heels. During the day these frogs hide in the leaf-axils of large plants but at night males emerge to call from shallow water. Initial studies on this species have shown that it has a surprising range of calls and the sounds emitted by one male tended to trigger the same call notes in neighbouring males (call matching). In addition, small groups of male frogs form choruses of similar calls which are initiated by one chorus leader. Research by Narins et al. (2000) recorded 28 different call types by males which is the largest known call repertoire of any amphibian (Narins et al., 2000). Morphological analysis of the frogs did not reveal any unusual structures which could produce such varied sounds. Instead, the researchers suggest that different neural wiring in this species may allow greater complexity of calls. For example, removing the stereotyped order of calls which are produced by some tropical species, increases call complexity. The function of so many calls remains untested but Narins et al. (2000) suggest that it may be linked to call matching as males often call in choruses consisting of similar calls. Counter call matching is when, for example, a male emits a longer, presumably more attractive call, and his neighbour is forced to increase the duration of his call to match or exceed that of the competitor. This may result in evolutionary pressure for males to develop more varied and complex calls.

***Figure 3.*** *The Madagascar bright-eyed tree frog Boophis madagascariensis has an extensive call repertoire, consisting of 28 different calls.* [ Photo credit: Charles J. Sharp, https://commons.wikimedia.org/wiki/File:Madagascar_bright-eyed_frog_(Boophis_madagascariensis)_Ranomafana.jpg]

References

Gerhardt, H.C., Roberts, J.D., Bee, M.A. & Schwatz, J.J. (2000) Call matching in the quacking frog (Crinia georgiana). Behavioral Ecology & Sociobiology, 48:243-251.

Narins, P.M., Lewis, E.R., McClelland, B.E. (2000) Hyperextended call note repertoire of the endemic Madagascar treefrog Boophis madagascariensis (Rhacophoridae). Journal of Zoology, London., 250: 283-298.

Yue, X., Fan, Y., Xue, F., Brauth, S.E., Tang, Y. & Fang, G. (2017) The first call note plays a crucial role in frog vocal communication. Scientific Reports, 7: 10128, Doi: 10.1038/s41598-017-09870-2.

anurans

References

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