Croaking Science

Croaking Science: Frogs & toads at the coast

September 28, 2018 by editor

Frogs and toads at the coast: salt tolerance in amphibians

It is generally understood that amphibians breed and associate with freshwater habitats such as ponds, lakes and other small waterbodies. Many scientific studies have demonstrated that amphibians are particularly vulnerable to saline conditions at embryo, larval and adult stages. The most vulnerable life stage is the embryo which often experience high mortality when in salt environments (e.g. Beebee, 1985; Hua & Pierce, 2013). This is because salt water tends to disrupt the ionic and water exchange across permeable membranes. However, an increasing number of studies have found amphibian species that are able to adapt to and tolerate salt water habitats, especially those in coastal marshes. In his observations of the lack of amphibians on islands and in saline habitats, Charles Darwin in 1872 noted that only “one Indian species” was able to tolerate salt water (Darwin, 1872 in Hopkins & Brodie, 2015). Since then a range of scientific observations and studies have vastly increased our knowledge of the number of amphibians that are able to tolerate salt water in the natural environment. In a recent review of all scientific studies relating to amphibians and their ability to resist salt water Hopkins and Brodie (2015) identified 144 amphibian species from 28 amphibian families across the world from anurans (frogs and toads), salamanders and caecilians that are able to inhabit salt habitats. Salt tolerance in amphibians has been recorded from all continents except Antarctica.

The majority of studies which have found amphibians in salt water habitats have recorded them from coastal areas, which get flooded predictably by sea water. For example, mangrove swamps are flooded daily by tidal fluctuations which results in predictable levels of salt water in surrounding standing water. The most well-known frog to inhabit such environments is the crab-eating frog (Fejervarya cancrivora) of southeast Asia (Figure 1). This unique species can live in freshwater as well as 75% seawater and can adapt from one to the other in a matter of hours (Ren et al., 2010). It is able to achieve this by rapidly changing levels of urea in its body tissues which avoids excessive water loss through the skin in salty conditions (Ren et al., 2010). In addition, it has three glands within its skin: mucous glands, mixed glands, and vacuolated gland which help buffer the additional salt which would otherwise enter its body (Amphibiaweb, 2018). Other studies on amphibians which are able to tolerate salt water environments have also noted similar changes in physiology.

Figure 1. Crab-eating frog (Fejervarya cancrivora) from southeast Asia which can tolerate daily influxes of seawater. By Bernard DUPONT from FRANCE – Crab-eating Frog (Fejervarya cancrivora), CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=40168133

The exact mechanisms behind the evolution of salt tolerance in amphibians is still under debate but may be due to a number of genetic variables. All amphibians have highly permeable skin, which is prone to desiccation. Most amphibians are efficient at water regulation through their skin and retaining salts through effective transport of sodium and chlorine ions. Therefore, there may be a genetic predisposition to adapting to salt water environments through altering the physiology of ion transfer across the skin. Several amphibian species which are tolerant of highly arid conditions are also able to resist salt environments. The green toad (Bufotes viridis) of Central Europe is able to tolerate extremely high temperatures of 40°C and withstand up to 50% body water loss (Amphibiaweb, 2018) (Figure 2). It aestivates (hibernates) during the hottest summer months and has the same adaptations to avoiding water loss as species which inhabit saline environments, i.e. accumulation of urea in body tissues. The same occurs in the tiger salamander (Ambystoma tigrinum) of North America which uses a similar physiological mechanism to inhabit saline habitats. Some biologists have therefore proposed that the ability to tolerate saline environments has evolved through a pre-adaptation to living in arid environments. However, this does not appear to be the case for all amphibians inhabiting salt water. The natterjack toad (Epidalea calamita) is known to inhabit both freshwater and brackish habitats both in the UK and across northern Europe (Beebee, 1985; Gomez-Mestre & Tejedo, 2005) (Figure 3). Through genetic studies Gomez-Mestre & Tejedo (2005) found no support for this hypothesis and suggested that salt and aridity tolerance has evolved independently. They conclude that the mechanisms for resisting salt are fundamentally different in embryos and larvae compared to juveniles and adults. Therefore, amphibians which are able to resist salt whilst in the embryonic or larval stages require unique physiology which has not evolved in arid tolerant species (Gomez-Mestre & Tejedo, 2005). Other theories for the evolution of salt tolerance in amphibians are selection pressures from freshwater predators, forcing amphibians into new habitats or diversification of amphibians into new habitats to allow them to utilise unique prey (Hopkins & Brodie, 2015).

Figure 2. The green toad (Bufotes viridis) of Central Europe is able to tolerate extremely high temperatures using a similar physiological mechanism as species inhabiting salt water. By Umberto Salvagnin – originally posted to Flickr as European Green Toad, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=11010130

Figure 3. Typical coastal habitat of the natterjack toad. Breeding ponds in certain areas can easily become saline. As a result, populations can adapt to living in salt water at low concentrations. Inset: male natterjack toad (Epidalea calamita). Natterjack toad habitat: By Gary Rogers, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=13571929 Inset image: By Thomas Brown – Natterjack Toad (Epidalea calamita). Uploaded by mgiganteus, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=27451130.

In Australia, many amphibian species are threatened with a range of anthropogenic factors including habitat loss, fragmentation and pollution. Salinization of freshwater habitats has been cited as being one of the factors in causing declines of Australian amphibian species but evidence is lacking (Kearney et al., 2012). In experimental studies, Kearney et al. (2012) examined the effects of elevated salinity on growth, metamorphosis and survival of the tadpoles of three species of native Australian frog. Kearney et al. (2012) found that tadpoles of the green and golden bell frog (Litoria aurea) raised in salt conditions had higher survival than those in freshwater. In contrast, a number of studies on other anuran larvae have found that larvae suffered abnormalities when raised in saline conditions. The authors propose that L. aurea has adapted to salt water by rapidly increasing development when in these conditions to avoid the potentially harmful effects of salt. However, other amphibians native to Australia such as the spotted grass frog (Limnodynastes tasmaniensis) and the painted burrowing frog (Neobatrachus sudelli) are unable to exhibit such plasticity in development and consequently are negatively affected by increases in salt water in the environment (Kearney et al., 2012).

Overall, it appears that an increasing number of amphibian species are being recognised as being able to tolerate salt water in the natural environment. However, species vary hugely in their tolerance of salt water habitats and our knowledge of the degree to which amphibians have adapted to salt water is poorly understood, especially those in human dominated habitats. Further in-depth investigations are required to increase our understanding of the mechanisms behind the evolution of salt tolerance in amphibians and their response in a potentially changing environment.

References

AmphibiaWeb (2018) http://amphibiaweb.org. University of California, Berkeley, CA, USA. Accessed 11 Sep 2018.

Beebee, T.J.C. (1985) Salt tolerances of natterjack toad (Bufo calamita) eggs and larvae from coastal and inland populations in Britain. Herpetological Journal, 1: 14-16.

Gomez-Mestre, I. & Tejedo, M. (2005) Adaptation or exaptation? An experimental test of hypotheses on the origin of salinity tolerance in Bufo calamita. Journal of Evolutionary Biology, 18: 847–855.

Hopkins, G.R. & Brodie Jr, E.D. (2015) Occurrence of amphibians in saline habitats: a review and evolutionary perspective. Herpetological Monographs, 29: 1-27.

Hua, J. & Pierce, B.A. (2013) Lethal and sublethal effects of salinity on three common Texas amphibians. Copeia, 2013 (3): 562-566.

Kearney, B.D., Bryrne, P.G. & Reina, R.D. (2012) Larval tolerance to salinity in three species of Australian anuran: an indication of saline specialisation in Litoria aurea. PLOS ONE, 7 (8): e43427.

Ren, Z., Zhu, B., Ma, E., Wen, J., Tu, T., Cao, Y., Hasegawa, M. & Zhong, W. (2015). Complete nucleotide sequence and gene arrangement of the mitochondrial genome of the crab-eating frog Fejervarya cancrivora and evolutionary implications. Gene, 441: 148-155.

Croaking Science: The toad fly Lucilia bufonivora in common toads

August 29, 2018 by editor

Croaking Science: The toad fly Lucilia bufonivora in common toads

Blow-flies are dipteran flies that evolved 105 million years ago and there are now over 1,000 species occurring in 150 genera in a range of countries worldwide (Figure 1) (McDonagh, 2009). Within the family Calliphoridae there are 80 species which are known to cause myiasis, which is the infestation of live human and vertebrate animals, where the fly larvae feed on the host’s dead or living tissue, liquid body substances, or ingested food (Zumpt, 1967). There are three main types of blow-fly that cause myiasis. The first are known as saprophages and simply infect animal carcasses that are already dead and decaying. These species do not initiate myiasis but rather take advantage of animals that are already decaying. The second group of blow-flies are generally ectoparasites (i.e. parasites that attach to the skin or fur of an animal) that occasionally initiate myiasis by feeding on damaged tissue of their host. These flies sometimes feed on dead decaying matter, like the saprophages. The last group of blow-flies are known as obligate parasites, only feeding on the living tissue of their host and initiating myiasis as a result (Stevens & Wall, 1997). It has been proposed that the obligate myiasis-causing blow-fly parasites have evolved from an ancestral saprophageous stage, with flies occasionally being attracted to dead or decaying tissue or wounded animals. This later involved into flies which relied more on living tissue until obligate parasites evolved (Zumpt, 1967; McDonagh, 2009).

*Figure 1. Blow-flies belonging to the genus Lucilia range from feeding on flower nectar to liquefied dead vertebrate tissues.*

The blow-fly genus Lucilia comprises approximately 27 species, all of which are similar in appearance. The larvae of the majority of these species are saprophageous, feeding on dead matter. However, there are several species which are occasionally ectoparasites on large mammal species and which may cause myiasis, particularly in sheep (Stevens & Wall, 1997). One species, Lucilia silvarum, sometimes feeds on dead amphibian species, but the toad fly, Lucilia bufonivora, is a highly specialised blow-fly and is an obligate parasite of live prey. Adult L. bufonivora make their first emergence in May to July and seek out living common toads (Bufo bufo) and their lays eggs on the back and flanks of an amphibian host. The larvae then hatch and migrate to the head and nasal cavities of the toad where they continue to feed on the live host (Figure 2). Infected toads tend to change their behaviour and instead of hiding away in sheltered areas, move out into exposed locations where they are susceptible to further infestation by blow-flies. The nasal cavities and head are gradually consumed until eventually the infected toad dies. Once the toad has died, the larvae continue to feed on the flesh of the toad before dropping off to pupate in the soil. Here they undergo metamorphosis and hatch into new adult flies a few weeks later. Toad flies are capable of having three generations each year, depending on the weather and availability of toads.

*Figure 2. A common toad (Bufo bufo) infected with blow-fly larvae from Lucilia bufonivora. The larvae have migrated to the nostrils where they will continue to consume the living flesh.*

Toad-flies have a wide distribution across Europe, North Africa and Asia and are particularly common in the Netherlands where between 15 and 70% of common toads may be affected each year, with adults being most commonly affected (Weddeling & Kordges, 2008). Despite this high level of infection, there is no evidence of toad flies causing decline in the common toad. In the UK, toad flies are relatively uncommon and the number of reports each year is low.

L. bufonivora and another blow-fly species L. silvarum are highly similar in appearance. In Europe, L. silvarum tends to only feed on carrion with a preference for dead toads. However, in North America, the species has been recorded as infecting living toads with larvae found in the neck, legs and parotid glands (Eaton et al., 2008). Research by the University of Bristol and Exeter, in collaboration with RAVON, has looked at how closely related L. bufonivora and L. silvarum are to each other. The two species are so similar in appearance it is possible that the eggs found on toads in Europe are from one or both species. Using genetic analysis, the researchers found that the two are sister species, being genetically distinct, but are very closely related and have only recently diverged as separate species. In addition, the researchers found that L. silvarum blow-flies in North America are more closely related to toad flies L. bufonivora, than they are to their own L. silvarum species in Europe. This suggests that obligate parasitism in Lucilia blow-flies may have evolved independently several times and originally diverged from L. silvarum (Arias-Robledo et al., 2008). The obligate parasite traits of L. bufonivora may have evolved as the two species diverged. The findings from this research also show that in Europe and the UK common toads are only infected by L. bufonivora and L. silvarum has yet to become an obligate parasite in these countries (Arias-Robledo et al., 2008). Further research is required to determine the evolutionary status of other closely related blow-fly species such as L. elongata, which is relatively poorly understood.

References

Arias-Robledo, G., Stark, T., Wall, R.L. & Steven, J. R. (2018) The toad fly Lucilia bufonivora: its evolutionary status and molecular identification. Medical and Veterinary Entomology, doi: 10.1111/mve.12328.

Eaton, B. R., Moenting, A. R., Paszkowski, C. A. & Shpeley, D. (2008) Myiasis by Lucilia silvarum (Calliphoridae) in Amphibian Species in Boreal Alberta, Canada. Journal of Parasitology, 94 (4): 949 – 952.

McDonagh, L. M. (2009) Assessing patterns of genetic and antigenic diversity in Calliphoridae (blowflies). PhD thesis, University of Exeter.

Stevens, J. & Wall, R. (1997) The evolution of ectoparasitism in the genus Lucilia (Diptera: Calliphoridae). International Journal of Parasitology, 27 (1): 51-59.

Wellling, K. & Kordges, T. (2008) Lucilia bufonivora-Befall (Myiasis) bei Amphibien in

Nordrhein-Westfalen – Verbreitung, Wirtsarten, Ökologie und Phänologie. Zeitschrift für Feldherpetologie, 15: 183–202.

Zumpt F. & Ledger J. (1967) A malign case of mylasts caused by Hemipyrellia fernandica (Macquart) (Diptera Calliphoridae) in a cape hedgehog (Erinaceus frontalis A. Smith). Acta Zoologica et Pathologica Antverpiensia, 43: 85-91.

Croaking Science: Overwintering in Frog Tadpoles

July 27, 2018 by editor

Overwintering in frog tadpoles

During June and July in the UK the majority of Common Frog (Rana temporaria) tadpoles metamorphose into juveniles which leave ponds for terrestrial habitats. They will spend the rest of the summer and autumn foraging and feeding on small invertebrates in preparation for the winter. However, a number of common frog tadpoles each year will remain in ponds and spend the winter in the water, metamorphosing into juveniles the following spring (Figure 1). What causes some tadpoles to metamorphose and others to remain in the water over the winter? Often, ponds drying out or freezing over can limit the capacity for frog tadpoles to overwinter. However, there are many ponds in the UK where this does not occur, which allows for overwintering of tadpoles. Our understanding of the exact factors which trigger overwintering in tadpoles are not fully understood, but research by Walsh et al. (2016) found that temperature and food availability were key factors. In addition, the decision on whether to over‐winter as a tadpole appears to be made relatively early in the season (Walsh et al., 2008). Under laboratory conditions, lower temperatures and reduced food availability during the summer resulted in a higher proportion of individuals remaining as tadpoles during the winter (Walsh et al., 2016). These findings suggest that cold weather conditions in the early autumn may affect tadpole development, perhaps by affecting endocrine function. However, temperature alone does not appear to be enough to trigger overwintering in tadpoles; food availability also appears to be important. Although these two factors play an important role in triggering overwintering in common frog tadpoles, there are other factors, not fully understood, that appear to be involved.

***Figure 1.*** *Common Frog Rana temporaria tadpoles may spend the winter in their natal ponds under certain environmental conditions. (Photo credit: John Roston)*

Living in areas with variations in altitude provides challenges for the development of common frog tadpoles. Higher altitude ponds experience lower temperatures and food availability and these may promote a higher incidence of overwintering in tadpoles. However, Muir at al. (2014) found that in tadpoles living at high altitude in Scotland, individuals had a lower resting metabolic rate which allowed for more energy to be allocated to growth. This is likely to allow individuals to grow faster under cooler environmental conditions and allow tadpoles to metamorphose earlier than expected and prevent the necessity to overwinter. However, if tadpoles developing at high altitudes do need to overwinter, they are able to better tolerate the colder winter temperatures. Muir et al. (2014) found that the tadpoles of Common Frogs at high altitude were able to tolerate freezing for short periods of time. This adaptation provides common frog tadpoles with the ability to withstand the cooler pond conditions at high altitude over the winter and therefore lead to greater survival.

Across Central Europe the incidence of overwintering tadpoles varies by species and is more typical in later breeding species with large tadpoles such as the Midwife Toad Alytes obstetricans, Spadefoot Toad Pelobates fuscus, and water frogs (Pelophylax species) (Gilbert & Harmsel, 2016). The incidence of overwintering in Common Frogs Rana temporaria is rare and was reported for the first time in the Netherlands in 2013 (Gilbert, 2016). This was likely to be due to the unusual weather conditions of 2013/14 along with the sheltered site where this was observed. Overwintering in the Edible Frog (Pelophylax esculentus complex) is a relatively rare phenomenon across Central Europe (Figure 2). Research by Péntek et al. (2018) found that overwintering by tadpoles only seems to occur in unusual weather conditions. In one particular incidence cold temperatures over the winter resulted in late breeding by adults which led to tadpoles getting a late start to their growth and development. Cool conditions early in the autumn (October) appeared to trigger halting of development and the subsequent mild winter promoted successful overwintering by tadpoles of this species (Péntek et al., 2018). Although still relatively rare, overwintering may become more common across Central Europe with changing environmental conditions causing increased variability in seasonal temperatures.

**Figure 2.** Overwintering by the tadpoles of the Edible Frog is relatively rare across Central Europe.

In the United States, 15 species of Ranid frogs occur but only five of these species are known to overwinter as larvae (North American Bullfrog Rana catesbeiana, Pig Frog R. grylio, River Frog R. heckscheri, Red-legged Frog R. aurora, and Southern Mountain Yellow-legged Frog R. muscosa). It appears that pond desiccation is one of the main factors limiting Ranid frog species in North America from overwintering as tadpoles. The Southern Leopard Frog Rana sphenocephala is a common frog inhabiting freshwater ponds across the Northern United States (Figure 3). Tadpoles of this species will remain in the larval stage until reaching a critical metamorphic size and if this is not reached by the end of the autumn, then the tadpoles will overwinter (Pintar & Resetarits, 2018). If winter conditions are suitable, tadpoles will remain in the pond for several years until the critical size for metamorphosis has been reached. This is different to Common Frogs in the UK which do not need a critical size to metamorphose but do require specific environmental conditions. These results highlight the variations that occur across Ranid species in their requirements for metamorphosis.

**Figure 3.** Tadpoles of the Southern Leopard Frog from the United States have to attain a critical body size before they can metamorphose. This can result in the tadpoles spending several years in one pond.

References

Gilbert, M.J. & Harmsel, R. (2016) Hibernating larvae of the common frog (Rana temporaria) in the Netherlands. Herpetology Notes, 9: 27-30.

Muir, A.P., Biek, R., & Mabel, B.K. (2014) Behavioural and physiological adaptations to low-temperature environments in the common frog, Rana temporaria. BMC Evolutionary Biology, 14: 110.

Péntek, A.L., Sárospataki, M., & Zsuga, K. (2018) Larval overwintering of the Pelophylax esculentus complex in a sodic bomb crater pond near Apaj, Hungary. North-western Journal of Zoology, 2018: e1775023/9.

Pintar, M.R. & Resetarits Jr., W.J. (2018) Variation in pond hydroperiod affects larval growth in Southern Leopard Frogs, Lithobates sphenocephalus. Copeia, 106 (1): 70–76.

Walsh, P.T., Downie, J.R. & Monaghan, P. (2016) Factors affecting the overwintering of tadpoles in a temperate amphibian. Journal of Zoology, 298 (3): 183-190.

Walsh, P.T., Downie, J.R. & Monaghan, P. (2008) Larval over‐wintering: plasticity in the timing of life‐history events in the common frog. Journal of Zoology, 276 (4): 394-401.

Croaking Science: Kin Recognition

May 31, 2018 by editor

Kin recognition – when recognising relatives is important

During the spring in temperate countries tadpoles of frogs and toads often develop in a range of water bodies from small ponds to lakes. Swimming around in the open water, tadpoles are highly vulnerable to predation so in many species such as the common toad (Bufo bufo), tadpoles swim in large groups or shoals (Figure 1). By living in groups the tadpoles gain advantages such as decreased risks of predation and increased access to food. However, there are costs to group living such as an increase in the risk of transmitting infectious disease and intraspecific competition. Research has shown that tadpoles further increase the benefits of group living by associating with relatives, or close kin. If forming shoals reduces the risk of predation, then swimming with relatives who possess similar genes will increase the chances that these genes will survive to the next generation. The exact mechanism of kin recognition in anuran tadpoles is not clear but studies suggest that frog and toad tadpoles recognise each other through chemical cues (Eluvathingal et al., 2009). The persistent dense swimming shoals of tadpoles of many amphibians from the genera Rana and Bufo (e.g. wood frog (Rana sylvatica); common toad (Bufo bufo); and boreal toad (Bufo boreas)) have been shown to consist primarily of associations of closely related kin (Blaustein & Waldman, 1982). However, in the red-legged frog (Rana aurora), Schneider’s toad (Duttaphrynus scaber) and Günther’s golden-backed frog (Indosylvirana temporalis), tadpoles only exhibit kin recognition early in their development when they form dense shoals. As they mature, tadpoles disperse and kin recognition drops (Rajput et al., 2014). During late development the risks of pond desiccation are high and individual tadpoles seek isolated patches of water. Under these conditions the risks of suffocation through desiccating water is higher than the benefits of associating with relatives. Further to this, recent research has shown that the persistence of associating with kin varies considerably even within a single species. For example, larvae of the wood frog (Rana sylvatica) from North America exhibits kin recognition and individuals often choose to swim with relatives. However, Halverston et al. (2006) have shown that the level and occurrence of swimming with relatives is not consistent but depends on a range of factors including levels of competition, predation and parasitism. Tadpoles from two adjacent ponds exhibited different levels of association with kin due to localised differences in these factors (Halverston et al., 2006). This demonstrates that associating and swimming with relatives only occurs under certain environmental conditions and groups of tadpoles may show variations in their degree of kin association.

*Figure 1. Several species of Bufo tadpole form shoals which may consist of close relatives.*

Many amphibians within temperate zones are herbivorous, however there are a number of tropical species of tadpole which are carnivorous. This provides the opportunity for cannibalism to evolve where larger tadpoles may predate smaller ones within the same species. In these species it may be advantageous for tadpoles to recognise kin to avoid potential predation on relatives who carry the same genes. Research has shown that cannibalism generally occurs when the nutritional benefits gained from eating relatives outweigh the disadvantages such as risks of injury, transmitting infectious diseases and losing potential members of the same species who carry similar genes. As a result, cannibalism has evolved in some carnivorous amphibian larval species, but not others, depending on the environmental conditions and the relative advantages and disadvantages of consuming close kin.

Many tropical frogs in the genus Dendrobates lay their eggs in terrestrial habitats that are then transported by males to small water-filled water bodies formed in the axils of tree leaves (phytotelmata). Females of the poison dart frog Dendrobates auratus only lays a few eggs per year, so the number of tadpoles developing within any given water body is low. In some species of Dendrobates frogs, the female lays non-fertile eggs into the water to feed the tadpoles. However, female D. auratus does not perform this behaviour so the tadpoles rely completely on external food sources e.g. fallen insects, for survival. Starvation within such small water bodies is very high and therefore cannibalism has evolved to allow some of the tadpoles to survive. In this species the larger tadpoles predate the smaller ones which provides additional nutrition which is crucial for survival. However, male D. auratus may bring tadpoles from a number of clutches so there is also the potential for kin recognition i.e. larger tadpoles may choose to consume non-relatives over relatives since the latter may share the same genes. However, experiments carried out by Gray et al. (2009) suggest this is not the case and that larger tadpoles indiscriminately predate kin from non-kin. The authors hypothesise that since food sources are so scarce for developing tadpoles then the benefits of individual survival outweigh the costs of losing genetic relatives.

Additional research by Poelman & Dicke (2007) has shown that in the poison dart frog Ranitomeya ventrimaculata, females exhibit a plasticity in where she lays her eggs, which depends on environmental conditions (Figure 2). In this species, like D. auratus, tadpoles are highly cannibalistic, consuming both kin and non-kin. At the beginning of the breeding season, when water-filled phytotelmata do not contain many tadpoles, the female will spread her eggs widely amongst different water bodies, avoiding those which already contain a tadpole. This reduces the chances that her eggs will get consumed by another tadpole (which may or may not be her own). However, nearer to the end of the breeding season, the female changes behaviour and actively starts laying more eggs in phytotelmata which already contain large tadpoles. Poelman & Dicke (2009) hypothesise that this is because at the end of the breeding season there are few free phytotelmata left and by provisioning those that already contain tadpoles with fertilised eggs, she is providing food to the tadpoles which may be her own. This sacrifice of eggs will indirectly improve the survival of her existing tadpoles, the eggs of which she laid early in the breeding season. This plasticity in behavioural response allows a greater number of tadpoles to survive when the risk of desiccation of water bodies is high.

Figure 2. The poison dart frog Ranitomeya ventrimaculata exhibits different egg-laying strategies depending on the environmental conditions.

The green and golden bell frog (Litoria aurea) was once a common species widely distributed in south eastern Australia (Figure 3). It has also been introduced into New Zealand and surrounding islands. However, the extreme sensitivity of these species to the infectious chytrid fungus has been the major cause of the species’ decline in its native range and the species is currently restricted to small isolated populations. The tadpoles form groups where individuals gain protection from predators, increased foraging efficiency and better growth. Associating with relatives is advantageous since individuals would be indirectly protecting those carrying similar genes. Pizzatto et al. (2016) examined the behaviour of tadpoles of the green and golden bell frog in relation to kin recognition. The authors found that in the bell frog L. aurea tadpoles did indeed distinguish kin from non-kin and thus formed shoals containing mainly relatives. The authors suggest the advantages to this species include increased growth and increased resistance to disease since related members are likely to carry the same disease resistance genes.

Figure 3. Tadpoles of the green and golden bell frog (Litoria aurea) from Australia congregate in groups containing related individuals.

References

Blaustein, A.R. & Waldman, B. (1982) Kin recognition in anuran amphibians. Animal Behaviour, 44: 207 -221.

Eluvathingal, L.M., Shanbhag, B.A. & Saidapur, S.K. (2009) Association preference and mechanism of kin recognition in tadpoles of the toad Bufo melanostictus. Journal of Bioscience, 34 (3): 435–444.

Gray, H.M., Summers, K. & Ibáñez, R.D. (2009) Kin discrimination in cannibalistic tadpoles of the Green Poison Frog, Dendrobates auratus (Anura: Dendrobatidae). Phyllomedusa, 81 (1): 41-50.

Halverson, M.A., Skelly, D.K. & Caccone, A. (2006) Kin distribution of amphibian larvae in the wild. Molecular Ecology, 15: 1139–1145.

Poelman, E.H. & Dicke, M. (2007) Offering offspring as food to cannibals: oviposition strategies of Amazonian poison frogs (Dendrobates ventrimaculatus). Evolution and Ecology, 21: 215–227.

Pizzatto, L., Stockwell, M., Clulow, S., Clulow, J. & Mahony, M. (2016) How to form a group: effects of heterospecifics, kinship and familiarity in the grouping preference of green and golden bell frog tadpoles. Journal of Herpetology, 26: 157–164.

Rajput, A.P., Saidapur, S.K. & Shanbhag, B.A. (2014) Kin discrimination in tadpoles of Hylarana

temporalis (Anura: Ranidae) and Sphaerotheca breviceps (Anura: Dicroglossidae): influence of hydroperiod and social habits. Phyllomedusa, 13 (2): 119–131.

Croaking Science: Miniaturisation in amphibians

March 26, 2018 by editor

Miniaturisation in amphibians – evolutionary specialisation

Amphibians exhibit vast ranges in body size ranging from just 7 mm long in the smallest known species (Paedophryne amanuensis) to 33 cm in the largest frog (Conraua goliath), which represents a 250 fold increase in size. Different body sizes come with distinct advantages and disadvantages. Large amphibians have fewer predators, a lower metabolic rate, and they can more easily maintain their temperature and hydration than small amphibians. Smaller amphibians, although being more prone to predation, are better equipped to hide more easily, exploit alternative food sources, use physically smaller niches, and attain reproductive ability at an earlier age (Leavy & Heald, 2015). By being very small, individuals are able to fully utilise habitat patches and consume the smallest prey, which are also the most numerous. However, living at such small sizes puts constraints on biological processes and as a result, miniature amphibians have evolved unique features to enable them to function effectively. Many miniaturised frogs have a reduced number of skull elements, fewer digits on their limbs, fewer vertebrae and a reduction in webbing on the feet. Many miniature frogs resemble juveniles and retain of their features. Indeed, the frog genus Paedophryne, discovered in 2010 from Papua New Guinea, means ‘juvenile form’ and contains some of the world’s smallest frogs measuring just 10.1–11.3 mm (Kraus, 2010). Most miniature frogs from a range of families are usually found in wet tropical forests and live on the forest floor under leaf litter feeding on minute termites, small ants and similar species (Almeida-Santos et al., 2011).

Miniaturisation within the amphibians has evolved independently several times and in many species adults only grow to 20 mm in length. For example, miniaturisation has evolved 8 times in the puddle frogs (genus Phrynobatrachus) of Sub-Saharan Africa (Zimkus et al., 2012). The unusual reproductive mode of many tropical amphibian species has promoted the evolution of small body size. Miniature amphibian species are usually fully terrestrial with individuals laying a small number of relatively large eggs within the leaf litter which hatch into fully metamorphosed juveniles. For example, the Strabomantid frogs of the Peruvian Andes are fully terrestrial and do not require water to breed. This means that individuals do not need to undergo breeding migrations to water bodies. This promotes the evolution of small body size since individuals are able to occupy very small home ranges (Lehr & Catenazzi, 2009). In addition, the unique physiology of amphibians has allowed the evolution of small body sizes even at high altitudes, especially in the tropics. In many groups of vertebrates there is a relationship between body size and temperature (Bergmann’s Rule) where individuals at higher altitudes or latitudes attain larger body sizes, often as a protection from colder temperatures. However, due to their physiology which allows them to alter their metabolism for activity at lower temperatures, small amphibian species are able to occupy high altitudes. This removes the constraint of temperature on the evolution of body size and allows species to occupy a greater range of ecological niches even at high altitudes.

*Figure 1. The pumpkin toadlet (Brachycephalus ephippium) from the Atlantic Forest of southern Brazil is one of the smallest known frog species.*

In recent years our knowledge and understanding of miniature amphibians has increased due to discoveries of new species and research into their unique ecology. Frogs within the genus Brachycephalus are all endemic to the Atlantic Forest in Brazil (Figure 1). There are currently 35 known species (Frost, 2018) and 19 of these have been discovered in the past six years. Also known as flea-frogs, these poorly understood species inhabit the moist tropical forest floor within this biodiverse hotspot. Growing to less than 20 mm in length, Brachycephalus didactylus, occurs only in Rio de Janeiro state where it is threatened by encroaching human developments and habitat fragmentation. During the breeding season, females lay just two eggs, probably laying each on a different day (Almeida-Santos et al., 2011). Each egg is laid amongst the leaf litter where it undergoes complete development into a fully metamorphosed juvenile frog with no free-swimming larval stage. The eggs of this species are also relatively large, allowing newly metamorphosed juveniles to emerge at a larger size which offers greater protection from predators.

*Figure 2. The miniature Malabar night frog, Nyctibatrachus major, from the Western Ghats, India.*

Night frogs, genus Nyctibatrachus, live in close association with mountain streams or marshes in forests of the Western Ghats, India. Species within this genus are extremely small ranging 10 – 77 mm. In 2017 Garg et al. (2017) discovered seven new miniaturised night frog species, four of which are among the smallest frogs in India measuring just 12.2 to 15.4 mm in length. All members of the new species were found under leaf litter or in wet grass near to streams or waterfalls. One male was found clasping a clutch of 10 eggs (Garg et al., 2017). The authors believe that more surveys will discover further new species which will also aid our understanding about the evolutionary advantages of miniaturisation and adaptation to terrestrial life within these and other miniature frogs

References

Almeida-Santos, M., Siqueira, C.C., Van Sluys, M. & Rocha, C.F.D. (2011) Ecology of the Brazilian flea frog Brachycephalus didactylus (Terrarana: Brachycephalidae). Journal of Herpetology, 45 (2): 251-255.

Frost, D.R. (2018) Amphibian Species of the World: an Online Reference. Version 6.0 (09 March 2018). Electronic Database accessible at http://research.amnh.org/herpetology/amphibia/index.html. American Museum of Natural History, New York, USA.

Garg, S., Suyesh, R., Sukesan, S. & Biju, S.D. (2017) Seven new species of night frogs (Anura,

Nyctibatrachidae) from the Western Ghats biodiversity hotspot of India, with remarkably high diversity of diminutive forms. PeerJ, 5:e3007; DOI 10.7717/peerj.3007.

Kraus, F. (2010) New genus of diminutive microhylid frogs from Papua New Guinea. ZooKeys 48: 39-59.

Leavy, D.L. & Heald, R. (2015) Biological Scaling Problems and Solutions in Amphibians. In: Additional Perspectives on Size Control in Biology: From Organelles to Organisms. Eds R. Heald, I.K. Hariharan and D.B. Wake. Cold Spring Harbor Laboratory Press, doi: 10.1101/cshperspect.a019166.

Lehr, E. & Catenazzi, A. (2009) A new species of minute Noblella (Anura: Strabomantidae) from Southern Peru: the smallest frog of the Andes. Copeia, 2009 (1): 148–156.

Zimkus, B.M., Lawson, L., Loader, S.P. & Hanken, J. (2012) Terrestrialization, miniaturization and rates of diversification in African puddle frogs (Anura: Phrynobatrachidae). PLoS ONE, 7 (4): e35118. doi:10.1371/journal.pone.0035118.

Croaking Science: Amphibians and Climate Change

February 22, 2018 by editor

Amphibians and climate change

Over the past few decades rising global temperatures and associated climate change have been of increasing concern due to rises in atmospheric carbon dioxide and other greenhouse-gas emissions. Over the past century global temperatures have risen on average by 0.7°C and these have been greatest in the last few decades (IPCC, 2007). This has resulted in a range of global climatic changes which have had varied impacts on amphibian species. For example, several species of amphibian have become extinct in southeast Brazil due to the occurrence of frost, and the decline in several tropical amphibian species has been attributed to unusually dry conditions (Blaustein et al. 2010). In the UK an increase in the prevalence of mild and wet winters has negatively affected the common toad Bufo bufo (Reading, 2007) (Figure 1). Range shifts, where species are either forced to higher latitudes or altitudes are a particular problem for many species within Central and South America where there are a high number of range-restricted species. Here, temperatures are predicted to become hotter and drier and may result in species extinctions if there is no other suitable habitat available to expand into. Other potential direct impacts of climate change on amphibians are decreased survival such as that reported in the common toad (Reading, 2007), changes in developmental rates of eggs and larvae, and changes in behaviour of adults and larvae (Blaustein et al. 2010).

**Figure 1** The common toad Bufo bufo is susceptible to changing winter temperatures in the UK. (Photo ©Froglife)

Changes in the timing, or phenology, of amphibian breeding have been intensively studied in a range of species across the globe. Within the UK, many studies have shown a trend for earlier breeding in the common frog Rana temporaria, common toad Bufo bufo, natterjack toad Epidalea calamita and two species of newts (smooth newt Lissotriton vulgaris and great crested newt Triturus cristatus). However, findings from several independent studies reveal that the impacts of a changing climate on breeding times may be more complex. For example, Beebee (2002) reported results of thirty years of studying populations of the common frog and found no trend for earlier breeding. Beebee (2002) reported that the two explosive breeding amphibians in the UK, common frog and common toad, showed relatively minimal changes in breeding date. However, the protracted breeders, natterjack toad and newt species showed significantly earlier breeding times (Figure 2). Beebee (2002) predicted that this could have important community effects. Earlier arrival of newts at ponds may increase predation on common frog spawn and this in turn may result in local population declines of the common frog. Similar findings have been found by Reading (1998) who found no evidence of earlier breeding in a population of the common toad. More recent studies have concluded contrasting results. For example, Scott et al. (2008) studied the phenology of the common frog and did find a trend for earlier breeding. However, the authors noted no trend for earlier hatching dates. In addition, Carroll et al. (2009) reported findings from a study from the UK Phenology Network which collected nearly 70,000 records of the first spawn dates of the common frog across the UK between 1998 and 2007. On average, the pattern of spawning followed a consistent pattern across the UK with first spawning dates earliest in the south west and progressively later in the north and east. Although there was variation in spawning dates between years, on average there was a trend for the common frog to breed 10 days earlier at the end of the period. In a more recent study carried out by Loman (2016) in Sweden, local variation in pond conditions was of key importance in determining spawning date of the common frog and in particular pond water temperature, as oppose to air temperature. In addition, Loman (2016) found that the number of frogs breeding at a pond had a significant impact on breeding date. Common frogs spawned earlier in ponds with a higher, rather than lower, number of breeding pairs. For example, a pond with only five breeding individuals spawned 7.2 days later than ponds with 500 individuals (Loman, 2016). Therefore population sizes, as well as climatic changes, may need to be considered when investigating the potential impacts on amphibian breeding phenology.

**Figure 2** Great crested newts Triturus cristatus are responding to milder winters by arriving at ponds earlier in the year. (Photo © Froglife)

In addition to the above direct effects of climate change, there are a number of potential indirect effects, such as variations in larval food availability. The larvae of many species of anurans depend on plentiful supplies of algae but rising temperatures in water bodies may trigger early blooms of detrimental filamentous cyanobacteria which may prove detrimental to feeding and growth. In addition, many salamanders and newts rely on small aquatic invertebrates and research by Durance & Ormerod (2007) has found that in the UK, spring macroinvertebrate abundance in headwater streams might decline by 21% for every 1 °C rise in water temperature, which highlights the sensitivity of headwater stream ecosystems to climate change. Finally, emerging infectious diseases have had wide-ranging and negative impacts on amphibian populations across the globe. In particular, chytrid fungus caused by Batrachochytrium dendrobatidis (BD) has been implicated as the proximate cause for crashes in many members of the tropical genus of Atelopus frogs. Pounds et al. (2006) have suggested that changing climatic conditions in mountainous areas of Central and South America have resulted in night time temperatures that have shifted closer to the optimal temperature for BD, while increased daytime cloudiness prevents frogs from finding thermal refuges from the pathogen. This has increased the prevalence of the disease and precipitated declines in many Atelopus species.

The impacts of climate change on amphibian populations are complex with many potential direct and indirect impacts on amphibians at individual and population scales. Global temperatures are predicted to increase by between 1.1 and 6.4 °C by 2100 (IPCC, 2007), so coherent research and conservation measures are required to preserve populations of amphibians across the globe.

References

Beebee, T.J.C. (2002) Amphibian phenology and climate change. Conservation Biology, 16 (6): 1454-1455.

Blaustein, A.R., Walls, S.C., Bancroft, B.A., Lawler, J.J., Searle, C.L. & Gervasi, S.S. (2010) Direct and indirect effects of climate change on amphibian populations. Diversity, 2: 281-313.

Carroll, E.A., Sparks, T.H., Collinson, N. & Beebee, T.J.C. (2009) Influence of temperature on the spatial distribution of first spawning dates of the common frog (Rana temporaria) in the UK. Global Change Biology, 15 (2): 467-473.

Durance, I. & Ormerod, S.J. (2007) Climate change effects on upland stream macroinvertebrates over a 25-year period. Global Change Biology, 13: 942-957.

IPCC. (2007) Climate change 2007: the physical science basis. In Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Eds: S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Avery, M. Tignor & H.L Miller. Cambridge University Press, Cambridge, UK.

Loman, J. (2016) Breeding phenology in Rana temporaria. Local variation is due to pond temperature and population size. Ecology and Evolution, 6 (17): 6202-6209.

Pounds, A., Bustamante, M.R., Coloma, L.A., Consuegra, J.A., Fogden, M.P.L., Foster, P.M., La Marca, E., Masters, K.L., Merino-Viteri, A., Puschendorf, R., Ron, S.R., Sánchez-Azofeifa, G.A., Still, C.J., Young, B.E. (2006) Widespread amphibian extinctions from epidemic disease driven by global warming. Nature, 439: 161-167.

Reading, C.J. (1998) The effect of winter temperatures on the timing of breeding activity in the common toad Bufo bufo. Oecologia, 117 (4): 469-475.

Reading, C.J. (2007) Linking global warming to amphibian declines through its effects on female body condition and survivorship. Oecologia, 151: 125-131.

Scott, A.W., Pithart, D. & Adamson, J.K. (2008) Long-term United Kingdom trends in the breeding phenology of the common frog, Rana temporaria. Journal of Herpetology, 42 (1): 89-96.

Croaking Science

Footer

Contact us