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Roger Downie

Croaking Science: See-through frogs are worth a look

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Glass frogs comprise a family (Centrolenidae) of 158 species found in the forests of the neotropics: Central America and northern South America. They get their name from the transparency of their belly skin, which allows the internal organs to be easily seen. They are adapted to life in the trees, possessing pads on the tips of their digits that allow them to adhere to leaves. These structures appear to have evolved independently in several lineages of frogs, since molecular phylogenetic results show that the main families where these pads occur (Hylidae, Rhacophoridae, Centrolenidae) are not closely related.

One of the special features of glass frogs is their mode of reproduction. Clutches of eggs, as flat round sheets, each egg encased in jelly, are laid on the surfaces of leaves overhanging streams. Once the eggs have developed into larvae, they hatch and fall into the stream below, often from a considerable height. Glass frogs are divided into two main sub-families: the Centroleninae (121 species) which lay on the upper sides of leaves and then leave the eggs to develop on their own; and the Hyalinobatrachinae (35 species) which lay their eggs on the lower sides of leaves; in these species, the father cares for the eggs, sometimes up till the point of hatching.

Figure 1: Several egg clutches on a single leaf

We studied the glass frog Hyalinobatrachium orientale which has distinct populations in northern Venezuela and northeast Tobago: this distribution is a bit of a puzzle. Northeast Tobago is quite distant from Venezuela and between these two locations is the large island of Trinidad, which has abundant forests, but no glass frogs. Walking along the forest streams of Tobago at night, once the rainy season (June to December) has started, you soon hear the high-pitched peeping of male glass frogs, located on the huge leaves of Heliconia bihai that overhang the water. With the aid of a good torch, you can locate the calling frogs; often, near them, you can spot the little patches of eggs. If you are lucky, you may locate a mating pair. We observed the behaviour of a mating pair. It took them about four hours to complete their clutch of around 30 eggs, laid as a spiral pattern, starting at the centre, with the pair turning as they proceeded. Once egg-laying was complete, the female departed, but the male stayed close to the clutch. Often, when searching for egg clutches, we found the father sitting on top of his eggs. We also found that some fathers, presumably good quality males, were looking after more than one egg clutch at a time. These are at different stages of development, so clearly produced on different nights. It is not entirely clear what functions male egg attendance performs, particularly given that it is not constant. However, observers have seen males driving away egg predators such as wasps and ants; it is also likely that males keep the eggs hydrated by reducing evaporation, simply by sitting on them, or by emptying their bladders over the eggs (this is established in some cases of frog parental care). But this raises another mystery. If paternal care is helpful to incubation success in the Hyalinobatrachinae, why does it not occur in other glass frogs, especially when they lay their eggs on leaf upper surfaces, where you would guess that desiccation and predation would be higher risks.

In the Tobago glass frog, hatching occurs around nine days after laying, although the actual time is variable, allowing tadpoles to be earlier or later stages of development when entering the water. Such variability may be quite common in frog development and represents a trade-off. Early hatchers are less well developed and more vulnerable when they enter the water; but it may be better to risk this than to be predated while still in the nest. It therefore pays the developing larvae to monitor conditions: if the father has deserted his clutch, or hot sun and no rain are risking desiccation, better to hatch early and hope for a stream with few predators.

Figure 2: Metamorphosing glass frog tadpole on a leaf; with pen for scale

The streams where glass frog tadpoles are found are fast-flowing when it rains, and are heavily populated with predatory fish and crustaceans. They are also shaded, making plant productivity low. As a consequence, glass frog tadpoles spend much of their time hidden under rocks, reducing the risks of predation and of being swept away by currents (unlike the tadpoles of some species that inhabit fast streams, glass frog tadpoles lack the kind of suctorial mouthparts that can help cling on to rocks). The tadpoles have long muscular tails, indicating an ability to move rapidly, and are relatively unpigmented, associated with a concealed lifestyle. Their behaviour limits foraging opportunities, so growth is slow. Glass frog tadpoles can take a year to reach metamorphosis, very slow by tropical frog standards, where many species reach that stage in a few weeks. We have been able to locate metamorphosing glass frogs near the edges of streams. They climb up on to the upper surfaces of leaves close to the ground, and take about four days to complete the process, reducing their tail to a stump. To our surprise, we found that they do not remain in one place through this process, but occasionally move around, possibly to confuse potential predators.

The transparency of glass frogs has long puzzled biologists. Recently, a research team from Bristol, Canada and Ecuador has tested an explanation. Their evidence suggests that it is not so much transparency that matters, but translucence. When a glass frog is at rest with its limbs tucked in, the translucence of the limbs blurs the edges of the body, and makes detection by predators more difficult.

Further reading

Barnett et al. Imperfect transparency and camouflage in glass frogs. PNAS (2020).

Byrne et al. The behaviour of recently hatched Tobago glass frog tadpoles. Herpetological Bulletin 144, 1-4 (2018).

Byrne et al. Observations of metamorphosing tadpoles of the Tobago glass frog. Phyllomedusa (in press) (2020).

Delia et al. Glass frog embryos hatch early after parental desertion. Proceedings of the Royal Society B 281, 2013-2037 (2014).

Downie et al. The tadpole of the glass frog Hyalinobatrachium orientale from Tobago. Herpetological Bulletin 131, 19-21 (2015).

Nokhbatolfoghahai et al. Oviposition and development in the glass frog Hyalinobatrachium orientale. Phyllomedusa 14, 3-17 (2015).

Contributing authors : Roger Downie, Isabel Byrne, Chris Pollock.

Book Review: Wild Child

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Wild Child: coming home to nature

Patrick Barkham, Granta (2020). Hardback, £16.99. 342 pp.

This is a lovely book, and could be read for enjoyment and instruction by anyone who works with younger children, at home or elsewhere. Patrick Barkham is a nature writer, author of four previous books (on butterflies, badgers, coasts and islands) and a regular contributor to The Guardian. This is not a ‘how to’ book, although it includes many experiences of exploring nature with children. It is more a reflection on how children learn about everything, and nature in particular. The focus is Barkham’s own family (the children are non-identical twin girls and a younger boy), but he also interacts with many other children while volunteering at his local outdoor nursery, and visiting forest schools in other parts of England.

His children first discover nature in the overgrown cemetery beyond the garden of their first house in Norwich. As the children grow, the family moves to more space in a village outside the city, and the children attend a nearby, highly innovative outdoor nursery. They are fortunate with the headteacher of the local primary school who allows the children one day at the outdoor nursery even after they are school age.

Some of the book’s chapters describe the changing seasons at the nursery: spring, summer, autumn and a muddy, snowy winter (it’s the year of the ‘beast from the east’). Barkham volunteers there throughout the year, but not on the days his own children attend. Other chapters cover different aspects of nature: birds’ nests and chicks; ponds and dipping for tadpoles, beetles and newts; caterpillars and butterflies. The children learn in a matter-of-fact way about death, and bury dead birds, or take good specimens to the local taxidermist. There’s a running debate about collecting and keeping specimens, and whether wildlife can be pets.

Amongst lively and colourfully worded descriptions of what children do and say (with many verbatim quotations), Barkham discusses in a very straightforward way the many studies which demonstrate the positive impacts on child development of interacting with nature. He also discusses the negative effects of too much time indoors, interacting with screens and anxious about the outside world. For those readers who wish to look further into such studies, there is a full set of references, arranged by book chapter at the end.

Some readers may be thinking: rural Norfolk, Guardian– writer, typical middle-class elitist with no idea of the problems ‘real people’ face. But two of the forest schools Barkham visits belie that stereotype. ‘Wild things in Nottingham works with ‘English as a second language’ children, mostly refugees, many of them coping with horrific past experiences, and few of them speaking much English as yet. Being in nature provides them with experiences for which spoken communication is non-essential. ‘Wilderness schooling’ in Northumberland helps under-achieving children to regain motivation for learning, while experiencing the outdoors and wildlife, and the organiser is accumulating data to prove it works.

Barkham does not hide the fact that children are individuals and not always 100% motivated to learn from wild nature all the time. However, he does insist that ‘however imperfect our lives and our homes, we can add doses of daily nature in a way that enriches us all’. To make this happen for everyone, we need to re-think how our towns, especially our roads are organised. The book was written before the covid19 crisis, but the message fits well with much discussion of how reduced traffic is improving air quality in cities and how provision needs to be made for the resurgence in cycling. He also urges reform of the national curriculum and the emphasis on testing children: modern schools too often stifle the natural creativity and self-motivation that children show. If I have a criticism of the book, it’s the relative lack of engagement with the extensive literature on alternative ways to organise educational systems. This seems a pity given that the original ‘free -school’, A.S. Neill’s Summerhill, is not far from Norwich (in east Suffolk) and is about to celebrate its centenary.

Although I noted that this is not a ‘how to’ book, it ends with a helpful appendix: ‘sixty-one things to do and ways of being with children outdoors’ and a bibliography of helpful and relevant books. The book has no photographs, but includes some maps and delightful chapter-title line drawings. Highly recommended.

Roger Downie

Froglife Trustee; honorary lecturer, University of Glasgow.

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.

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

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

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

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

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

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

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

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

References

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

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

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

Croaking Science- Foot flagging: an alternative method of communication in frogs

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Traditionally, frogs and toads are considered to communicate primarily by using acoustic cues, with males typically calling to attract females. However, in noisy tropical rainforests by fast flowing streams, acoustic communication becomes more problematic. In these environments male frogs have evolved an array of visual cues which complement acoustic cues to communicate between each other and to also attract females. Visual communication has evolved independently in several lineages of frog species, mainly in diurnal species inhabiting noisy, stream-side environments.

Figure 1. Male frogs using foot-flagging in male-male communication. [Photo credit: Amézquita & Hödl, 2004.]

Foot-flagging has been observed in 16 frog species (Preininger et al., 2013). The male will typically arch and rotate its back foot in the air, giving a conspicuous visual signal (Figure 1). Males of the Bornean frog genus Staurois have evolved a unique foot-flagging behaviour which males use in a variety of contexts. The Sabah splash frog (Staurois latopalmatus) is found exclusively close to waterfalls where it can be observed regularly on exposed perches. Within this habitat, males signal in close vicinity to each other near to the water (Preininger et al., 2009). Preininger et al. (2009) have identified three types of visual displays: foot flagging, arm waving and vocal sac displays. Foot-flagging displays are mainly performed in the direction of a male opponent and appear to be used in territorial defence. Preininger et al. (2009) hypothesise that foot-flagging behaviour in this species has evolved from physical male-male combat where each male tries to push the other off its perch. The foot-flagging behaviour may be a ritualization of this aggressive combat, avoiding the need for physical contact. The other visual displays, arm waving and throat display appear to be used less frequently than foot-flagging during male-male aggressive encounters.

Figure 2. The Bornean rock frog (Staurois parvus) (left) from Borneo and the small torrent frog (Micrixalus saxicola) (right) from the Western Ghats of India both exhibit foot-flagging behaviour. [Photo credits: (left) Mohamad Jakaria, https://commons.wikimedia.org/wiki/File:Staurois_parvus.jpg; (right) L. Shyamal, https://commons.wikimedia.org/wiki/File:MicrixalusSaxicola1.jpg]

The Bornean rock frog (Staurois parvus) from Borneo and the small torrent frog (Micrixalus saxicola) from the Western Ghats of India belong to different frog families (Figure 2). Males of both species use complex signalling involving high pitched calls, foot flagging, and tapping (foot lifting) to defend perching sites against other males (Preininger et al. 2013). The Bornean rock frog has conspicuous white feet, whereas the small torrent frog has feet which are the same colour as its body. In a study to examine the differences in the behaviour of the two species, Preininger et al. (2013) found that in the Bornean rock frog, foot-flagging achieved a 13 times higher contrast against their visual background than the feet of the small torrent frog. In addition, the Bornean rock frog primarily responded to stimuli with foot flagging, whereas the small torrent frog responded mainly with calls but never foot-flagging on its own (Preininger et al. 2013). The authors propose that in the small torrent frog foot-flagging is in a transient state, evolving from its current use in physical fighting behaviour.

The colouration on the feet of foot-flagging species may signal more than male presence. Research by Stangel et al. (2015) has found that in two species (Staurois parvus and S. guttatus) the brightness of the feet increases with age. The peak brightness seems to coincide with sexual maturity and may be linked to androgen hormone levels in the males. The foot-flagging in these species may therefore convey information on the status of the male and his receptiveness to mate. This may be useful both to intruding males and also females which may be in the area.

African puddle frogs of the genus Phrynobatrachus are unique to Africa and are named after their breeding strategy of often laying large numbers of eggs in slow-moving or stagnant water bodies. In East African mountain streams lives a day time active frog, krefft’s river frog (Phrynobatrachus krefftii). Like other members of its genus males are dull brown in colour but also possess a striking bright yellow vocal sac (Figure 3). Instead of using foot-flagging for visual communication, males of this species communicate using a combination of acoustic cues and exhibiting their bright yellow vocal sac. Sometimes males will utter a call, accompanied by exhibiting their yellow vocal sac, whereas at other times no call will be emitted. Hirschmann & Hödl (2006) propose that the use of their brightly coloured vocal sac in this way has evolved to indicate the aggressive motivational state in the male.

Figure 3. Male krefft’s river frog (Phrynobatrachus krefftii) (right) possess a bright yellow vocal which they use in visual communication. Females (left) lack this colouration. [Photo credit: Hirschmann & Hödl, 2006.]

Further research into foot-flagging and other visual displays at different life stages will help increase our understanding of the function, development and evolution of visual signals in amphibians. Foot-flagging is only confined to a relatively few number of frog species and these are unable to survive in habitats modified for human use. Habitats where many of these unique species occur are increasingly threatened with habitat loss and fragmentation. Understanding the ecology of these species is therefore crucial in identifying and protecting key habitats in the wild.

References

Amézquita, A. & Hödl, W. (2004) How, when and where to perform visual displays: the case of the Amazonian frog Hyla parviceps. Herpetologica, 60 (4): 420–429.

Hirschmann, W. & Hödl, W. (2006) Visual signaling in Phrynobatrachus krefftii Boulenger, 1909 (Anura: Ranidae). Herpetologica, 62 (1): 18–27.

Preininger, D., Boechle, M. & Hödl, W. (2009) Communication in noisy environments II: visual signaling behavior of male foot-flagging frogs Staurois latopalmatus. Herpetologica, 65 (2): 166–173.

Preininger, D., Boechle, M., Sztatecsny, M. & Hödl, W. (2013) Divergent receiver responses to components of multimodal signals in two foot-flagging frog species. PLoS ONE, 8 (1): e55367. doi:10.1371/journal.pone.0055367.

Stangel, J., Preininger, D., Sztatecsny, M. & Hödl, W. (2015) Ontogenetic change of signal brightness in the foot-flagging frog species Staurois parvus and Staurois guttatus. Herpetologica, 71 (1): 1–7.

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.

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