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Croaking Science: Venomous Amphibians

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Venomous animals are able to inject their toxins into another organism while poisons are ingested, inhaled and absorbed. The ability to deliver venom into another animal has distinct evolutionary advantages such as in defence, prey capture and even sexual selection.

Amphibians secrete a wide variety of compounds from their skin glands. Generally, mucous glands help provide a moist coating on their skin to facilitate cutaneous respiration while granular glands secrete substances that amphibians use as a chemical defence against predators (eg. toxic and noxious compounds) and microorganisms (eg. antimicrobial peptides). This is sometimes displayed with bright, aposematic colouration (Duellman and Trueb 1996). 

Many different toxic secretions have been found in amphibian skin which act in numerous ways to disrupt the physiology of potential enemies (Daly et al. 2005, Savitzky et al. 2012).  For example, newts in the genera Taricha and Notophthalmus synthesise tetrodotoxins in high concentrations and are co-evolving in an evolutionary arms race with Thamnophis garter snakes where toxicity is a selective pressure (Brodie et al. 2005, Mailho-Fontana et al. 2019, Hague et al. 2020).  Fire salamanders secrete samandarin alkaloids through their paratoid glands which can cause convulsions, hypertension and respiratory paralysis in potential predators.  Frogs can sequester an array of toxins such as the potent neurotoxin Zetekitoxin in the Panamanian golden frog and the batrachotoxins in highly toxic Phyllobates poison frogs (Duellman and Trueb 1996).  The bright yellow Australian corroboree frogs of the genus Pseudophryne synthesise their own pseudophrynamine toxins as well as sequestering pumiliotoxins from their environment to deter predators (Smith et al. 2002). Fossorial caecilians are also known to produce defensive toxins with poison glands being discovered on the tails of Siphonops annulatus ringed caecilians as a possible defence against predators as they burrow into the soil (Jared et al. 2019). 

Venom, however, is rare in amphibians with only a few species possessing a system to deliver their toxins into another organism. 

The Iberian ribbed newt (Pleurodeles waltii) is a fascinating salamandrid from Spain and Portugal with an incredible defence behaviour. They have the ability to use their ribs to protrude through the skin to envenomate and ‘sting’ a predator. When distressed these newts can flatten themselves or arch their backs in an antipredator posture. They will then rotate their ribs 65° forwards which increases the angle of the spine to allow its ribs to penetrate through the skin wall and project as ‘spines’. This allows them to coat their ribs with a viscous, milky substance from their skin tubercles and inject it into the mouth of the predator making them unpalatable and allowing them to escape (Heiss et al. 2010). This defence mechanism doesn’t cause any permanent damage as antimicrobial peptides are able to prevent infection in the lacerated skin and their tissue is able to regenerate remarkably quickly. The tip of the ribs are also covered with a periosteum layer which is also thought to prevent microbial infection when the ribs puncture the skin.

The Echinotriton genus of crocodile newts are a sister group to the sharp ribbed newt and are also able to use their ribs to pierce their body wall when attacked by predators (Brodie et al. 1984).  

Sharp ribbed newt (Pleurodeles walti)

Brazil is home to two tree frogs which have incredible cranial morphologies and venomous defensive mechanisms. The Greening’s frog Corythomantis greeningi live in the semi-arid caatinga ecosystem of Eastern Brazil and conceal themselves in tree hollows and rock crevices to stay moist and evade predators. Bruno’s casque headed frog Aparasphenodon brunoi is another endemic Brazilian hylid with a fascinating skull morphology. They inhabit lowland tropical forests and shrubland and like C. greeningi, will hide in water-filled tree or bamboo hollows and bromeliad phytotelmata. 

Both of these peculiar frogs have flattened, casqued heads with their skin co-ossified with underlying bones. They use their heads to aid in a behaviour known as phragmosis (Jared et al. 2006, Blotto et al. 2020).  Phragmosis occurs when an animal enters a hole and blocks the entrance with their head to defend themselves from predators. In the lab, these frogs will exhibit this behaviour by entering test tubes backwards and blocking themselves off with their casqued heads. This phragmotic behaviour along with their venomous spines means these frogs have never been observed being predated in the wild. It is also thought that cranial ossification and phragmosis also indirectly reduces water loss and prevents desiccation by creating a humid microclimate within their tree holes (Jared et al. 2006). 

These frogs have bony spines, ridges and protrusions on their skulls in areas with high concentrations of granular and mucous glands which secrete a potent venom. Their mobile heads allow the frogs to deliver the venom into animals via head-butting and jabbing with their spines which are coated with the toxic secretions as the spines pierce their venom glands. This provides a highly effective chemical defensive mechanism as the toxin coupled with the wound caused by the head spines ensures would-be predators have a bad time when they attempt to ingest these frogs (Jared et al. 2015). Their cutaneous secretions include alkaloids and steroids which can induce oedema and intense pain in predators (Mendes et al. 2016). The venom contains both proteolytic and fibrinolytic agents as well as hyaluronidase which aids the toxins in diffusing around their enemies’ bodies (Jared et al. 2015). 

The venom of Greening’s frog is thought to be twice as lethal as fer-de-lance snakes of the genus Bothrops while Bruno’s casque headed frog secretes a venom 25x as toxic as these notorious neotropical vipers with an LD50 of 94.8µg in mice. A single gram of A. brunoi venom could kill 300,000 mice or 80 humans (Jared et al. 2015)!  However, A. brunoi has smaller spines and granular glands than C. greening and so may not be able to inject as much venom when defending against a predator. 
Top: Bruno’s casque headed frog (Aparasphenodon brunoi) L: Renato Augusto Martins R: Carlos Jared 
Bottom: Greening’s frogs (Corythomantis greeningi) Carlos Jared 

 

There are many other frogs with complex cranial morphology including immense variation in skull shape and hyperossification which often relate to their interesting and diverse ecologies (Paluh et al. 2020). With an array of other anurans having mineralised and spiny skulls, it is possible that there are a few more venomous frogs which are waiting to be studied. Contenders include the fascinating shovelhead tree frog Triprion, the crowned tree frog Anotheca spinosa and Polypedates ranwellai (Jared et al. 2015). 

In a recent paper by Mailho-Fontana et al. 2020, a new set of specialised dental glands were discovered in Brazilian ringed caecilians (Siphonops annulatus) that may produce venomous enzymes – but further research is needed to confirm this. These enzymes were demonstrated to have gelatinolytic, caseinolytic and fibrinogenolytic properties. This incredible discovery may allow researchers to rethink the evolution of venom in vertebrates (since it could have evolved independently in both amphibians and reptiles) and inspire new studies about caecilian toxinology.

Through histological and biochemical analysis of saliva samples, researchers found A2 phospholipase enzymes which could mean that some fossorial caecilians inject venomous saliva via these dental glands into their earthworm prey in order to incapacitate and digest them. These enzymes are found in many other venomous creatures such as scorpions, snakes and insects. It is also worth noting that many venoms have originated as saliva such as in komodo dragons, shrews, bats and slow lorises making the prospect of venomous gymnophiones very exciting!

Other caecilians including the basal genus Rhinatrema showed similar dental glands to the ringed caecilians which could suggest that caecilians evolved to inject oral venom early on in their evolution (Jared et al. 2020). 

Written by Xavier Mahele

References

Blotto, B.L., Lyra, M.L., Cardoso, M.C., Trefaut Rodrigues, M., R. Dias, I., Marciano‐Jr, E., Dal Vechio, F., Orrico, V.G., Brandão, R.A., Lopes de Assis, C., Lantyer‐Silva, A.S., Rutherford, M.G., Gagliardi‐Urrutia, G., Solé, M., Baldo, D., Nunes, I., Cajade, R., Torres, A., Grant, T., Jungfer, K.‐H., da Silva, H.R., Haddad, C.F. and Faivovich, J. (2020) The phylogeny of the Casque‐headed Treefrogs (Hylidae: Hylinae: Lophyohylini). Cladistics

Brodie, E.D., Feldman, C.R., Hanifin, C.T. et al. (2005) Parallel Arms Races between Garter Snakes and Newts Involving Tetrodotoxin as the Phenotypic Interface of Coevolution. Journal of Chemical Ecology 31, 343–356.

Brodie, E., Nussbaum, R., & Marianne DiGiovanni. (1984) Antipredator Adaptations of Asian Salamanders (Salamandridae). Herpetologica, 40(1), 56-68.

Daly, J. W., Spande, T. F. & Garraffo, H. M. (2005) Alkaloids from Amphibian Skin:  A Tabulation of Over Eight-Hundred Compounds. Journal of Natural Products 68, 1556–1575 

Duellman, W. E. & Trueb, L. (1996) Biology of Amphibians. McGraw-Hill.

Hague, M.T.J., Stokes, A.N., Feldman, C.R., Brodie, E.D., Jr. and Brodie, E.D., III. (2020) The geographic mosaic of arms race coevolution is closely matched to prey population structure. Evolution Letters 4: 317-332.

Heiss, E., Natchev, N., Salaberger, D., Gumpenberger, M., Rabanser, A. and Weisgram, J. (2010), Hurt yourself to hurt your enemy: new insights on the function of the bizarre antipredator mechanism in the salamandrid Pleurodeles waltl. Journal of Zoology 280: 156-162.

Jared, C., Antoniazzi, M.M., Navas, C.A., Katchburian, E., Freymüller, E., Tambourgi, D.V. and Rodrigues, M.T. (2005) Head co‐ossification, phragmosis and defence in the casque‐headed tree frog Corythomantis greeningi. Journal of Zoology 265: 1-8.

Jared C, Mailho-Fontana PL, Antoniazzi MM, Mendes VA, Barbaro KC, Rodrigues MT, Brodie ED (2015) Venomous Frogs Use Heads as Weapons. Current Biology Volume 25, Issue 16,

Jared, C., Mailho-Fontana, P.L., Marques-Porto, R. et al. (2018) Skin gland concentrations adapted to different evolutionary pressures in the head and posterior regions of the caecilian Siphonops annulatus. Scientific Reports 8, 3576.

Mailho-Fontana, P.L., Jared, C., Antoniazzi, M.M. et al. (2019) Variations in tetrodotoxin levels in populations of Taricha granulosa are expressed in the morphology of their cutaneous glands. Scientific Reports 9, 18490.

Mailho-Fontana, P. L. et al. (2020) Morphological Evidence for an Oral Venom System in Caecilian Amphibians. iScience 23, 101234.

Mendes VA, Barbaro KC, Sciani JM, Vassão RC, Pimenta DC, Jared C, Antoniazzi MM. (2016) The cutaneous secretion of the casque-headed tree frog Corythomantis greeningi: Biochemical characterization and some biological effects. Toxicon Volume 122.

Paluh D, Stanley EL, Blackburn DC. (2020) Evolution of hyperossification expands skull diversity in frogs. Proceedings of the National Academy of Sciences 117 (15) 8554-8562.

Savitzky, A.H., Mori, A., Hutchinson, D.A. et al. (2012) Sequestered defensive toxins in tetrapod vertebrates: principles, patterns, and prospects for future studies. Chemoecology 22, 141–158.

Smith, B. P. et al. (2002) Evidence for Biosynthesis of Pseudophrynamine Alkaloids by an Australian Myobatrachid Frog (Pseudophryne) and for Sequestration of Dietary Pumiliotoxins. Journal of Natural Products 65, 439–447.l

 

Croaking Science: How many amphibian species are there, how do we know, and how many are threatened with extinction?

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I’m sure most Froglife supporters and Croaking Science readers will be aware that the world’s amphibian species are in crisis. However, you may not know just how bad the crisis is, and how we know about it. This Croaking Science article aims to show how strong the evidence is, how it is gathered, and how difficult it is to keep up to date.

Herpetologists began to become aware in the 1990s that, although all kinds of wildlife were in trouble all over the world, amphibians were a special case. Simon Stuart and colleagues put numbers to this feeling in 2004. Their Global Amphibian Assessment estimated that 32.5% of amphibian species were globally threatened (i.e. they fitted into the IUCN Red List categories Critically Endangered, Endangered or Vulnerable: see later for definitions) compared to 12% of birds and 23% of mammals. Further, 22.5% of amphibians fell into the Data Deficient category i.e. too little was known about their status to make an assessment, a substantially higher proportion than for birds and mammals whose status tends to be better known. This meant that only about 44% of amphibian species could be classed as of Least Concern.  Stuart and colleagues discussed possible reasons for amphibians being in worse shape than the other mainly terrestrial vertebrate groups (at that time, too little was known about the status of reptiles to make a similar estimate for them). Although anthropogenic habitat loss and change were the underlying main causes for all wildlife declines, amphibian populations seemed to be declining even in good quality habitat, for what Stuart et al. called ‘enigmatic’ reasons. Within a few years, it was clear that the main cause of the ‘enigma’ was the spread of the fungal disease chytridiomycosis against which many amphibian species had no or only very limited resistance. It is interesting in a time of a global pandemic affecting the human population to reflect on the causes and effects of a novel disease affecting wildlife.

Stuart et al.’s assessment was based on the then total number of described amphibian species, 5743. However, James Hanken (1999) had earlier noted the irony that, at a time when amphibian species were in severe decline, the number of known species was rapidly increasing. This trend has continued. The Amphibian Species of the World website (Frost, 2020) currently (October, 2020) lists 8226 species, a 43% increase on the number assessed by Stuart et al. in 2004. Over the last 10 years, the number of described species of amphibians (mostly anurans: frogs and toads) has increased on average by 150 per year.

How has this happened? The biggest discoveries of new species are in the tropics where amphibian diversity is highest, and where, until recently, resources and expertise for amphibian research were very limited. Amphibians are mostly small, mainly active at night, and often quite localised, all of which create difficulties in cataloguing biodiversity.  It also turns out that external appearances can conceal underlying differences, so that it is only recently, with the availability of DNA sequencing, that herpetologists can work out that some species, previously thought to have extensive ranges, should really be split into several distinct species. Over my time studying the frogs of Trinidad and Tobago, new species descriptions of this kind have occurred in several cases. For example, the stream frogs of northern Venezuela, Trinidad and Tobago (see Croaking Science September 2020 for an account of colour changes in these frogs) were earlier thought to belong to one species: now they are three, Mannophryne trinitatis, M. olmonae and M. venezuelensis. Will this process of finding new amphibian species come to an end? Presumably it will, but currently there is no sign of the new discovery trend slowing down.

How do we establish the conservation status of a species? The task of keeping tabs on amphibians falls to the IUCN Species Survival Commission’s Amphibian Specialist Group (ASG). Their aim is to ‘provide the scientific foundation to inform effective amphibian conservation around the world’. A key part of their work is the compilation and revision of the Amphibian Red List i.e. an assessment of all species relative to their status in the wild. The main Red List categories (IUCN, 2001), with their definitions, and the current proportions of amphibian species in each category (from a total of 6932 species) are shown below:

Category, definition, percentage of amphibian species  

Critically endangered (CR): extremely high risk of extinction in the wild, 8.9%

Endangered (EN): very high risk…, 14.4%

Vulnerable (VU): high risk…., 9.8%

Near threatened (NT): close to qualifying as threatened, 5.6%

Least concern (LC): widespread and abundant, 41.5%

Data deficient (DD): inadequate information available to make an assessment, 19.3%

There is a final category of ‘not evaluated’: these are mainly newly identified species for which information on their population status is often very limited. Where new species have been named from the splitting of a widespread species, this causes a particular problem for the Red Listing process. For example, the widespread small tree frog Dendropsophus minutus is an abundant neotropical LC species; the population in Trinidad is now classed as a Trinidad endemic, D. goughi, so its status now needs assessed separately.

Clearly, a species’ status can change with time, so the Red List needs regular updating. In addition, as noted above, the identification of new species means there is a constant need for new assessments. At present, the ASG is nearing completion of a major re-assessment, due for release in December 2020: it will be fascinating to find how amphibians as a whole have fared since the last major revision in 2010.

What sort of evidence goes into determining a species conservation status? The Red Listing process is as objective as possible and relies on input from experts around the world. Key questions are: have populations changed, and if so, by how much? What is the geographic range of the species, and how much of this range does it occupy? How large or small is the population?  These can be quite difficult questions to answer with any degree of certainty. Consider the UK, with its considerable resources and abundant wildlife experts. We only have a small number of amphibian species. How good is our knowledge of their population sizes and trends? Even our presence/absence distribution maps are very variable in quality, and knowledge of populations is very patchy, even for the species that are of conservation concern, like natterjack toads and great crested newts.

Now consider Venezuela: a large country in political and economic turmoil with only a small number of dedicated herpetologists, but over 300 species of amphibians. The task of Red Listing here is immense and likely to be much based on educated guesswork. I have had some involvement over the last year in re-assessing the conservation status of Trinidad and Tobago’s much smaller number (35) of species, including three previously classed as threatened, and which share their ranges with Venezuela. The process has involved several herpetologists knowledgeable about the amphibians of the three territories pooling their information to complete the IUCN’s detailed evaluation forms, and coming to a judgement. The three species are:

  • Phytotriades auratus, the golden tree frog (see Figure). Formerly CR on the basis of occurrence only on two separate Trinidad mountain peaks, where it lives among the leaves and in the water tanks of huge arboreal bromeliads. Recent discoveries of populations in northern Venezuela and on another Trinidad mountain have led us to revise the assessment to EN.
  • Hyalinobatrachium orientale, the eastern glass frog (see Croaking Science, July 2020). Found along forest streams in northern Venezuela and in north east Tobago, but not Trinidad. Previously assessed as VU. Because of its limited range in Tobago and threats from deforestation in Venezuela, we retain VU as its Red List status.
  • Flectonotus fitzgeraldi, the dwarf marsupial tree frog (see Croaking Science October 2020). Found on forest and forest edge vegetation that holds pools of water, such as bromeliads and Heliconia, in northern Venezuela, Trinidad and Tobago. The previous assessment was EN, but when we examined the evidence, it became clear that this was a case where the previous assessors had made a judgment based on very limited data. Fortunately, we had been involved in extensive surveys of the presence/absence of this species in Trinidad and Tobago, as well as population estimates at a few locations. Added to data on the species’ range in Venezuela, we have been able to publish a paper (Smith et al. in press) and to assess this species as LC.

In these three cases, we have two reductions in the estimated threats. In both cases, these are not linked to active conservation effort, but rather to better data on the numbers and distribution of species. Much of the hard work has been done, not by experts, but by students and amateur naturalists taking part in actions like Bioblitzes. Mobilising citizens in this way has been important in the UK for taxa likes birds and butterflies. It needs to be done all over the world for more taxa.

References

Frost, D.R. (2020). Amphibian Species of the World, version 6.0. Available online.

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

IUCN (2001). IUCN Red List Categories and Criteria, version 3.1. Available online.

Smith, J. et al. (in press). The distribution and conservation status of the dwarf marsupial frog (Flectonotus fitzgeraldi; Anura, Hemiphractidae) in Trinidad, Tobago and Venezuela. Amphibian and Reptile Conservation.

Stuart, S. et al. (2004). Status and trends of amphibian declines and extinctions worldwide. Science 306, 1783-1786.

Croaking Science: More than meets the eye- cryptic frogs are more colourful than you think

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Mannophryne is a genus of 20 frog species found in the forests of Venezuela and Trinidad and Tobago. They belong to the family Aromobatidae, the sister family to the well-known Dendrobatidae, or poison dart frogs. Unlike poison dart frogs, that are brightly coloured to advertise their toxicity, aromobatids use cryptic colouration to blend into their surroundings as they do not have toxic compounds in their skins. In addition, they have a very effective escape response and can identify and reach cover quickly. This has led to one of their common names of rocket frogs, as well as being known as stream frogs, because of their normal habitat of stream margins.

In Trinidad and Tobago, we studied the endemic Mannophryne trinitatis (Trinidad) and M. olmonae (Tobago). Previous assessments of both species for IUCN indicated that they are threatened with extinction on account of habitat loss or degradation. An additional worry was the finding that both species were infected with chytrid fungus, the cause of numerous amphibian population declines across the world, although Mannophryne seemed not to suffer from the infection. We conducted population surveys of M. trinitatis in the Northern Range of Trinidad and found them to be numerous, no longer infected with chytrid, and with no sign of significant habitat loss. In Tobago, M. olmonae are very restricted in their distribution and are found at much lower numbers along streams in the northeast of the island. However, they also showed a lack of chytrid infection and no serious loss of habitat. We contributed to a review of IUCN’s frog assessments and recommend that M. trinitatis now be classed as Least Concern, and M. olmonae as Vulnerable.

Mannophryne are small mottled brown frogs, only a few centimetres in length, which are active during the day close to small mountain streams in primary and secondary forests. They feed on small  arthropods such as beetles, flies and spiders present on the abundant rotting fruit. During the rainy season (June to December), calling males can be heard morning and afternoon trying to attract a mate with their quick “weep, weep, weep” whistling calls. If you follow the calls and look carefully, they can be seen perched on rocks, roots, plants or at the mouths of small caves but will jump quickly away to safety if approached. Meanwhile, females hold a territory, which they defend from other females. When a female has chosen a mate, she follows the male into his cave and lays up to 14 eggs on the damp leaf litter. The males provide parental care, defending and cleaning the eggs until they hatch. This takes around 21 days and then each tadpole flips onto its father’s back and he begins the journey to look for a safe pool of water for his tadpoles to develop in. When a pool free from predatory fish or crustaceans and with a good supply of food is located, the father deposits some of the tadpoles and then searches for another. Tadpoles from a single clutch may be deposited in several bodies of water and will take around 2 months to develop to the stage of metamorphosis.

Figure 1: Calling male (left) and non-calling male (right) M. trinitatis.

One of the most interesting and recently studied aspects of this genus is their colouration. Sexual dichromatism is the difference in colouration between the males and females of a species. This is often used in communication as a visual signal to warn off competitors or highlight quality to potential mates. Sexual dichromatism can be split into two categories. Dynamic dichromatism only occurs in males and involves the development of temporary colouration, often as part of a courtship display, whilst ontogenetic dichromatism can occur in males or females and involves the development of a permanent colour difference as they mature.  Mannophryne are a very rare example among frogs in showing both, where males turn a jet-black colour when they call, and females develop a yellow patch on their throats as they mature.

This is noteworthy, because both forms of dichromatism are energetically expensive, stressing their importance. As male Mannophryne are active and calling during the day, their colour change seems counter- productive as it makes them more conspicuous. Additionally, it appears that open areas are a preferred calling site, and in turn attract a mate, further increasing their predation risk. Therefore, both signals (visual and audio) must be essential in attracting a mate. The role of the yellow throat in females is likely a signal of quality. We found that throat colour is individually variable in the extent of the colour patch and in its hue, ranging from pale yellow to bright orange. Larger and heavier females have more orange throats. Animals often use bright or bold colours to signal their quality to others and to avoid physical confrontation. In M. trinitatis, females can be see raising their heads to show their pulsing throat patch when confronted by another female. This signal shows that the defending female has the additional energy to dedicate to the “expensive” orange colouration and often provides enough incentive to dissuade the challenger from further conflict. Yellow and orange colours are based on carotenoid pigments, and there is evidence from many kinds of animals that carotenoids signal health and quality, as they are related to immune system functions. As a test of this idea that colour differences in the females are a sign of quality, we tested their escape responses in a semi-natural experiment: frogs with the brightest yellow throats showed more effective escape responses than the others.

Figure 2: Female (yellow throat) and male (grey throat) M. trinitatis

An extra facet to throat colour use by Mannophryne females  may be signalling their quality to males. As males take such exhaustive care of their clutch through to deposition of tadpoles, it is unlikely that they could manage more than one at a time. Therefore, it is important that they choose a high-quality female. Mate selection on both sides may be a more significant and understudied aspect to sexual selection in animals where males take all or the majority of parental care responsibilities.

Another fascinating piece of the puzzle is the timing of these colour changes. We observed the colour change on a male M. trinitatis, with the whole process taking around 20 minutes. The male was located by erratic calling and found to still be in its mottled brown colouration. As the calling became more consistent and frequent, the colouration changed to become darker until its change was complete. This is interesting because colour change in frogs is generally quite slow, mediated by hormones, and it will be interesting to discover if there is a special mechanism in Mannophryne. We found through rearing juveniles for several months after metamorphosis, that the yellow colouration develops very gradually in females, well before they reach sexual maturity.

Even though they do not have the bright colours or toxicity of their dart frog cousins, these little frogs are a lot more interesting and special than you might think at first glance.

Further Reading

Downie, J. R., Livingstone, S. R., & Cormack, J. R. (2001). Selection of tadpole deposition sites by male Trinidadian stream frogs, Mannophryne trinitatis (Dendrobatidae): an example of anti-predator behaviour. Herpetological Journal 11, 91-100.

Downie, J. R., Robinson, E., Linklater‐McLennan, R. J., Somerville, E., & Kamenos, N. (2005). Are there costs to extended larval transport in the Trinidadian stream frog, Mannophryne trinitatis (Dendrobatidae)?. Journal of Natural History 39, 2023-2034.

Greener, M. S., Hutton, E., Pollock, C. J., Wilson, A., Lam, C. Y., Nokhbatolfoghahai, M., … & Downie, J. R. (2020). Sexual dichromatism in the neotropical genus Mannophryne (Anura: Aromobatidae). PloS ONE 15(7), e0223080.

Greener, M. S., Shepherd, R., Hoskisson, P. A., Asmath, H., & Downie, J. R. (2017). How many Trinidad stream frogs (Mannophryne trinitatis) are there, and should they be regarded as vulnerable to extinction?. Herpetological Journal 27, 5-11.

Jowers, M. J., & Downie, J. R. (2005). Tadpole deposition behaviour in male stream frogs Mannophryne trinitatis (Anura: Dendrobatidae). Journal of Natural History 39, 3013-3027.

Lehtinen, R. M., Mannette, R. P., Naranjit, K. T., & Roach, A. C. (2007). Ecological observations on the critically endangered Tobago endemic frog Mannophryne olmonae. Applied Herpetology 4, 377-386.

Manzanilla, J., Jowers, M. J., La Marca, E., & García-París, M. (2007). Taxonomic reassessment of Mannophryne trinitatis (Anura: Dendrobatidae) with a description of a new species from Venezuela. Herpetological Journal 17, 31-42.

Contributing authors : Mark S. Greener and Roger Downie

Croaking Science: Mud-packing frogs: new approaches to protecting eggs

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The amphibian family Nyctibatrachidae forms one of the three oldest frog families and these species are found only in India and Sri-Lanka. Within the genus Nyctibatrachus there are currently 36 species, many of which have unique reproductive behaviours (see Croaking Science May 2019: https://www.froglife.org/2019/04/). Three closely related species within the genus occupy similar habitats on the forest floor, close to streams. Two of the species, Jog’s Night Frog (Nyctibatrachus jog) and the Kempholey Night Frog (N. kempholeyensis) both lay small clutches of eggs on leaves or branches overhanging slow-moving or still water bodies. The male then guards the eggs and provides water to prevent them drying out (AmphibiaWeb, 2011). However, the recently discovered Kumbara night frog (Nyctibatrachus kumbara) has a unique strategy for protecting its eggs. After laying a small clutch of between 4 and 6 eggs on a branch over-hanging water, the male collects mud and covers the eggs (Figure 1). This is thought to help protect the eggs from predators and prevent them from drying out (Gururaja et al., 2014). Covering eggs with mud in this way has not been recorded in any other species of frog and represents a unique method of protection (Gururaja et al., 2014). After covering the eggs with mud, the males will call to attract females which lay further clutches nearby. The male remains close to the egg clutches for several days until the eggs hatch. By exhibiting an alternative reproductive strategy, this species reduces competition between closely related species which occupy similar ecological niches.

Figure 1. The male Kumbara night frog (Nyctibatrachus kumbara) covers its eggs with mud. Left: a male starting to cover eggs with mud. Right: the male leaving the eggs once they have been covered with mud.  [Photo credit: Gururaja et al., 2014.]

References

AmphibiaWeb (2011) Nyctibatrachus jog: Jog’s Night Frog <http://amphibiaweb.org/species/7715> University of California, Berkeley, CA, USA. Accessed Jan 3, 2020.

Gururaja, K.V., Dinesh, K.P., Priti, H. & Ravikanth, G. (2014) Mud- packing frog: a novel breeding behaviour and parental care in a stream dwelling new species of Nyctibatrachus (Amphibia, Anura, Nyctibatrachus). Zootaxa, 3796: 33-61.

Case Study: North of England Wildlife Tunnels

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Background

Between 2014 and 2018, Froglife carried out camera monitoring in two tunnels at a new development in the north of England. Prior to development, the site contained several great crested breeding ponds. As part of mitigation during development of the site into a shopping complex, multiple new receptor ponds were created along with additional water management ponds. To aid in dispersal of great crested newts and other amphibian species between the ponds, four tunnels were installed on either side of a main road (Figure 1).

Figure 1. Location of the tunnels and newly developed ponds. Great crested newts were translocated to the amphibian mitigation zone.

Image adapted from: Jarvis, L.E., Hartup, M. & Petrovan, S.O. (2019) Road mitigation using tunnels and fences promotes site connectivity and population expansion for a protected amphibian. European Journal of Wildlife Research, 65:27-38.

Froglife developed two unique time lapse cameras (Figure 2), with LEDs to allow continuous 24-hour recording of activity in the tunnels. The cameras were set to record during two monitoring periods, spring and autumn, each year over the five year period. The aim was to determine the success of the tunnels for great crested newts as well as common toads, frogs and smooth newts which also occurred on the site.

Figure 2. Time lapse camera installed into one of the tunnels.

Results

Over the five years we recorded five species of amphibian moving through tunnels (Figure 3):

  • 243 adult great crested newts
  • 322 juvenile great crested newts
  • 67 adult smooth newts, 161 juvenile smooth newts
  • 69 adult common frogs
  • 189 common toads

Figure 3. Amphibian species observed moving through tunnels: a) great crested newt; b) common toad and juvenile great crested newt; c) juvenile smooth newt; and d) adult and juvenile great crested newt.

Image adapted from: Jarvis, L.E., Hartup, M. & Petrovan, S.O. (2019) Road mitigation using tunnels and fences promotes site connectivity and population expansion for a protected amphibian. European Journal of Wildlife Research, 65:27-38.

On average, 74% of adult great crested newts made a complete journey through the tunnels, with the remainder turning around and exiting through the same entrance. Results were similar for other amphibian species (Figure 4). In addition, population modelling of newts in the surrounding ponds indicated a significant increase in population size over the monitoring period. This is encouraging and demonstrates that at this site the tunnels were successful in promoting population expansion and movement of great crested newts and other species.

Figure 4. Percentage success rate of amphibian species making complete journeys through tunnels.

For full details and results please see our research paper at: https://link.springer.com/article/10.1007/s10344-019-1263-9

Sign our petition: Give Wildlife the Green Light – Build Wildlife Tunnels to save the Common Toad

Croaking Science: Artificial light at night- a problem for amphibians?

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Light pollution from industrialization, urban and suburban development is spreading rapidly across the world. It is estimated that 20% of land on earth is polluted by artificial light (Cinzano et al., 2001). An increasing range of wild animal species are being exposed to levels of night-time light higher than ever before. It is estimated that the average amount of light reaching the ground from one street lamp is 50 lux, compared to 0.1 lux of bright moonlight (Bennie et al., 2016) (Figure 1). Car headlights may reach over 1,000 lux, some 10,000 higher than natural night time light exposure. These levels of artificial light have been shown to affect a range of animal taxa from mammals to birds, reptiles and insects. The impacts of artificial light on amphibians appear to be varied, depending on the species and their ecology (reviewed in Dutta, 2018). For example, the calling behaviour of many frog species appears to be affected, with individuals calling less frequently and moving more often. This has the potential for decreasing mating opportunities and negatively impacting on subsequent spawning success. However, certain species, such as the cane toad (Bufo marinus) appear to benefit from street lights, foraging more often on the insects which congregate beneath them at night. On the contrary, red-backed salamanders (Plethodon cinereus) from North America forage less under artificial light, hiding in the leaf litter. This may have consequences on an individual’s ability to effectively forage and feed at night. Road mortality may be increased in areas of artificial light as has been shown in the American toad (Bufo americanus), which is attracted to street lighting and is more likely to cross roads (Mazerolle, 2004). Indirect effects of artificial light may include increased detection by predators and subsequent mortality of amphibians.

Figure 1. The amount of light generated by one street light can be several hundred or even several thousand times brighter than moonlight.
[Photo credit: Hackspett1265, https://commons.wikimedia.org/wiki/File:Night_light_behind_tree.jpg]

Artificial light may impact a range of amphibian life stages including the growth, development and activity of larvae, juveniles and adults. Our understanding of how artificial light may impact each life-stage is not fully understood. Dananay & Benard (2018) carried out experiments to determine the impacts of artificial light on larval and juvenile American toads. The researchers did not find any significant impact of artificial light on larval growth or behaviour, but juvenile American toads were affected. Juvenile toads under artificial light treatment were more active than those under dark treatments and had growth rates 15% lower than those in dark treatments. This increased nocturnal activity by juveniles under artificial light conditions appears to have resulted in increased energy expenditure and thus reduced growth rates (Dananay & Benard, 2018). This reduced growth may result in delayed reproductive maturity, lower fertility and reduced survival. Combined with other stressors, such as climate change, this could lead to population declines in many of our common amphibian species.

Figure 2. Juvenile toads under artificial light at night experienced lower growth rates which may result in delayed reproductive maturity, lower fertility and reduced survival (Dananay & Benard, 2018).
[Photo credit: Fungus Guy, https://commons.wikimedia.org/wiki/File:Eastern_American_toad_(Sudden_Tract).jpg]

Habitats restored for recreational purposes, as well as for wildlife, including amphibians, are often situated close to towns and cities. The light intensity reaching wetland areas close to cities may be greater than the brightest full moon (Secondi et al., 2017). Common toads (Bufo bufo), may be particularly affected by increased levels of artificial light as they have a very short breeding season and may use light to orient towards ponds and aid in synchronicity in breeding. Touzet et al. (2019) carried out research on the common toad in France to examine toad behaviour under artificial light generated by street and outdoor lighting in semi-urban areas. After 20 days of nocturnal exposure during the breeding period at 5 lux the total time spent active by male common toads decreased by more than half; at 20 lux activity levels dropped by 73%. This was due to male toads being less active during nocturnal periods (Touzet et al., 2019). In addition, common toads decreased their active energy expenditure by 18% at 5 lux and 38% at 20 lux, probably due to increased stress (Touzet et al., 2019). The authors conclude that the alteration of both activity and energy metabolism could have negative impacts on common toad reproduction and ultimately lead to a reduction in survival.

The impacts of artificial light on amphibians may not always be negative and some species seem to be resistant to anthropogenic light sources at night. Underhill & Höbel (2018) tested the effects of artificial light on the breeding behaviour of female eastern gray treefrogs (Hyla versicolor). Contrary to expectation, the researchers found no effects of artificial light on mating preferences and breeding behaviour. In this species, increased levels of artificial light should not affect population persistence nor affect mate choice. This is in contrast to túngara frogs (Engystomops pustulosus) which changed their behaviour under different light conditions in a way that suggested that they felt safer under darker conditions (Rand et al., 1997). Frog species vary in their sensitivity to light and the degree that they use visual cues for orientation and reproduction. The eastern gray treefrog does not rely heavily on visual cues for mate selection which may explain the lack of significant impacts of artificial light on their breeding behaviour (Underhill & Höbel, 2018).

Figure 3. The eastern gray treefrog (Hyla versicolor) from North America appears resistant to the effects of artificial light.
[Photo credit: Cliff, https://commons.wikimedia.org/wiki/File:Grey_Tree_Frog_(Hyla_versicolor)_(3151990943).jpg]

It appears that there is no consistent and universal impact of artificial light on amphibians. The response seems to vary by species depending on their ecology and breeding biology and their reliance on visual cues. In addition, responses by individual populations are likely to vary depending on location and the amount of artificial light. However, in many cases it appears that artificial light may have negative impacts on amphibian populations. Further research is required at a population level to determine the long-term impacts of artificial light and possible synergistic interactions with other environmental stresses such as habitat loss, fragmentation, pollutants and climate change.

References

Bennie, J., Davies, T.W., Cruse, D. & Gaston, K.J. (2016) Ecological effects of artificial light at night on wild plants. Journal of Ecology, 104: 611–620.

Cinzano, P., Falchi, F., & Elvidge, C.D. (2001) The first world atlas of the artificial night sky brightness. Monthly Notices of the Royal Astronomical Society, 328: 689–707. https://doi.org/10.1046/j.1365-8711.2001.04882.x

Dananay, K.L. & Benard, M.F. (2018) Artificial light at night decreases metamorphic duration and juvenile growth in a widespread amphibian. Proceedings of the Royal Society, London B, 285:

20180367. http://dx.doi.org/10.1098/rspb.2018.0367.

Dutta, H. (2018) Insights into the impacts of three current environmental problems on Amphibians. European Journal of Ecology, 4 (2): 15-27, doi:10.2478/eje-2018-0009

Feuka, A.B., Hoffmann, K.E., Hunter Jr, M.L. & Calhoun, A.J.K. (2017) Effects of light pollution on habitat selection in post-metamorphic wood frogs (Rana sylvaticus) and unisexual blue-spotted salamanders (Ambystoma laterale × jeffersonianum). Herpetological Conservation and Biology, 12 (2):470–476

Mazerolle, M.J. (2004) Amphibian road mortality in response to nightly variations in traffic intensity. Herpetologica, 60 (1): 45-53.

Rand, A. S., Bridarolli, M. E., Dries, L., & Ryan, M. J. (1997). Light levels influence female choice in túngara frogs: Predation risk assessment? Copeia, 1997, 447–450. https://doi.org/10.2307/1447770.

Secondi, J., Dupont, V., Davranche, A., Mondy, N., Lengagne, T. & Théry, M. (2017) Variability of surface and underwater nocturnal spectral irradiance with the presence of clouds in urban and peri-urban wetlands. PLoS One, 12: e0186808. doi:10.1371/journal.pone.0186808.

Touzot, M., Teulier, L., Langagne, T., Secondi, J., Théry, M., Libourel, P., Guillard, L. & Mondy, N. (2019) Artificial light at night disturbs the activity and energy allocation of the common toad during the breeding period. Conservation Physiology, 7 (1): coz002; doi:10.1093/conphys/coz002

Underhill, V.A. & Höbel, G. (2018) Mate choice behavior of female Eastern Gray Treefrogs (Hyla versicolor) is robust to anthropogenic light pollution. Ethology, 124: 537–548. doi:10.1111/eth.12759