Croaking Science

Endotrophic tadpoles: a widespread occurrence?

April 1, 2026 by Admin

Written by Dr. Laurence Jarvis (external contributor)

The larval form of frogs and toads, the tadpole, is a well-recognised developmental stage. Globally, tadpoles appear in a range of forms, or ecomorphotypes. According to McDiarmid and Altig (1999), there are 21 tadpole ecomorphotypes. However, there are still many anuran species where the ecomorphotype is still unknown. In a recent review of the then 7,665 known anuran species, Nori et al. (2025) found that an astonishing 4,502 anuran species (approx. 59%) still have their larval form yet to be described. This highlights the shortfall in our basic understanding of the larval ecology of many amphibian species worldwide.

Perhaps the most familiar ecomorphotype is the exotrophic tadpole which are generally free-swimming and feed on a range of prey from macroinvertebrates to microscopic algae. These may occur in a vast range of water bodies including freshwater ponds, rivers, fast-flowing streams and phytotelm-dwelling (living in small pockets of water formed within the cupped leaves of terrestrial plants). They may also occur in terrestrial habitats such as damp moss, soil and even sand. Not surprisingly, free-living, aquatic tadpoles are the most common, occurring in approximately 6,550 of anuran species (Nori et al., 2025). However, this is not the only type of anuran tadpole ecomorphotype. Less common are the endotrophic tadpoles. These are essentially non-feeding, instead relying on the yolk derived from the egg or their parents. Free-living forms typically lack the keratinised mouth parts of their feeding counterparts and possess a relatively large volume of yolk in their intestines. They use this to fuel their growth and their metamorphosis into the adult form.

Figure 1. Endotrophic tadpoles (Blythophryne beryet) showing yolk in their intestines.

Figure 2. Mouthparts of an endotrophic tadpole (Blythophryne beryet) showing lack of teeth and keratinised mouthparts.

Photo credit for Figures 1 and 2: Chandramouli SR, Vasudevan K, Harikrishnan S, Dutta SK, Janani SJ, Sharma R, Das I, Aggarwal RK, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

There are five main groups, or guilds, of endotrophic tadpole (McDiarmid and Altig, 1999). Nidicolous endotrophic tadpoles are generally free-living but often remain in a nest created by the parent. Three guilds involve the froglet either birthing from the mother’s oviducts or from other parts of the body (e.g. stomach or skin surface). Lastly, there are the direct developers, which develop within the egg and hatch as tiny juveniles (Nori et al., 2025). The larvae belonging to the latter four groups are often referred to as embryos, rather than tadpoles, as they lack a free-living form. Free-living endotrophic tadpoles are rare, with just 226 species of anuran known to have this ecomorphotype (Miñarro et al., 2024). However, scientists suspect that in reality up to 30% of anuran tadpoles may live in this form, but have yet to be described (Vera Candioti et al., 2023). In addition, among the 68 genera with unknown tadpoles, around 60% are likely to be endotrophic (Vera Candioti et al., 2023). Currently, most anurans with free-living, endotrophic larvae are concentrated in South and Central America, Madagascar, Southwest India, Borneo and New Guinea (Vera Candioti et al., 2023). It is likely that there are a greater number of anurans with this tadpole ecomorphotype within these, and surrounding, regions. These distributional patterns of endotrophic tadpoles are perhaps not unsurprising but highlight where future research needs to be focussed for greater understanding of tadpole ecomorphology across a range of anuran taxa.

The Philippine flat-headed frog (Barbourula busuangensis) is an enigmatic species belonging to an ancient lineage of amphibian. Until recently virtually nothing was known of its tadpoles, behaviour or ecology. However, thanks to tireless work by field researcher Marta Miñarro and colleagues, we now understand more of this species’ fascinating ecology. The Philippine flat-headed frog is nocturnal, living within rocky, fast-flowing streams within rainforests and is consequently very difficult to observe (Miñarro et al., 2024). Since 1954 this frog was suspected to have direct development since it lays a small number of large eggs under stones, and no tadpoles had ever been observed. However, in 2023, tadpoles were discovered for the first time, clinging to rocks close to the eggs (Miñarro et al., 2024). These tadpoles had a typical form for an endotrophic tadpole, lacking specialised mouthparts and having an abundance of yolk in their intestines (Miñarro et al., 2024). It appears, contrary to expectation, that the female lays her small clutch of around 100 eggs in a nest attached to an underwater boulder. These eggs are guarded either by the male or female until they hatch into specialised non-feeding endotrophic tadpoles. These tadpoles possess an effective oral sucker enabling them to cling to the rocks in the fast-flowing waters. Of particular interest, and perhaps unique to this species, is that these tadpoles lack a row of keratinised teeth around its oral sucker (Miñarro et al., 2024). Most other tadpoles living in fast-flowing waters have these teeth for feeding and to help secure them to the rocks. Since the Philippine flat-headed frog does not feed, it lacks these teeth and the oral sucker has a unique and modified design. Miñarro et al. (2024) suspect that suction alone, without the aid of gripping teeth, enables these tadpoles to cling to the rock surface.

Photo credit: Pierre Fidenci, CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons

The South American genus of frog Adenomera, often referred to as thin-toed frogs, currently comprises 32 species, but this number changes due to new discoveries and taxonomic revisions (AmphibiaWeb, 2026). Frogs within this genus have an interesting mode of reproduction, laying their eggs in a terrestrial underground chamber. Males create a small depression in the ground using their snouts, in which the females lay their eggs. Kokubum and Giaretta (2005) studied an unknown species of Adenomera, describing its tadpole form and function. The number of eggs laid was around 20, which is similar to other species within the genus. Once hatched, the relatively large endotrophic tadpoles created a foam nest by blowing bubbles through their mouths and writhing around. These tadpoles lived within the foam, not feeding, until metamorphosis. There has been much discussion as to the exact function of the foam nest but it may serve multiple discussions. Hypotheses include: protection from predators (Menin and Giaretta, 2003); absorbing and maintaining moisture; aiding in respiration and gas exchange (Downie and Smith, 2003); and avoiding compaction of the tadpoles at the bottom of the basin (Kokubum and Giaretta, 2005 quoting Silva et al., in prep). Whichever of these is the most critical for this species of Adenomera is not known but a combination is likely to ensure effective survival to metamorphosis.

Figure 4. Adult Adenomera saci, a newly described species in 2013, endemic to northwest Brazil.

Photo credit: Reuber Brandão, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

These case examples demonstrate the fascinating ecology of species with endotrophic tadpoles. Unfortunately, many of the species lacking formal tadpole description live in remote and difficult to study locations and often face multiple threats such as habitat loss, fragmentation and disease. Given the suspected number of anuran species with this ecomorphotype, further research is vital to address this lack of knowledge and to enable effective conservation strategies for these species.

Click here for references

AmphibiaWeb (2026) <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 17 Mar 2026.

Downie J.R. and Smith J. (2003) Survival of larval Leptodactylus fuscus (Anura: Leptodactylidae) out of water: developmental differences and interspecific comparisons. Journal of Herpetology, 17: 107-115.

Kokubum M.N.D.C. and Giaretta A.A. (2005). Reproductive ecology and behaviour of a species of Adenomera (Anura, Leptodactylinae) with endotrophic tadpoles: systematic implications. Journal of Natural History, 39 (20): 1745-1758.

McDiarmid R.W. and Altig R. (1999) Research: materials and techniques. In: McDiarmid R.W., Altig R., editors. Tadpoles: the biology of anuran larvae. Chicago: The University of Chicago Press. p 7-23.

Menin M. and Giaretta A.A. (2003) Predation on foam nests of Leptodactyline frogs (Anura, Leptodactylidae) by larvae of Beckeriella niger (Diptera, Ephydridae). Journal of Zoology (London) 26: 1-5.

Miñarro M., Burrowes P.A., Lansac C., Afuang L. and De La Riva I. (2024). Mystery solved: unravelling the life history of the enigmatic and ancient Philippine frog Barbourula busuangensis (Anura: Bombinatoridae). Salamandra, 60 (4).

Nori J., Baldo D., Pereyra M., Grosjean S., dos Santos Dias P.H., Müller H., Cordier M., Huais P.Y., Tomba A. and Vera Candioti F. (2025) Global key areas for anuran tadpole discovery. Biological Journal of the Linnean Society, 145 (1), p.blaf017.

Vera Candioti F., Baldo D., Grosjean S., Pereyra M.O. and Nori J. (2023). Global shortfalls of knowledge on anuran tadpoles. Biodiversity, 2 (1): 22.

Croaking Science: Basking frogs: a puzzle

March 1, 2026 by Admin

Roger Downie, Froglife and University of Glasgow

It is well known that many species of reptiles bask i.e. sit in the sun with their dorsal surfaces exposed, absorbing solar radiation in order to raise body temperature. This behaviour is common in lizards and snakes, especially those inhabiting temperate regions where night-time temperature can be low even in spring and summer. The heat generated allows the active temperature of these animals to be significantly greater than ambient, and it can also promote food digestion and absorption. The potential disadvantage is that lying still in the open increases the risk of predation: this risk is often mitigated by colour change that helps conceal the animal. Basking at night can help tropical region reptiles to cool off (Kidman et al, 2024).

However, basking in frogs seems an unlikely behaviour. Their highly permeable skins allow a high rate of evaporative water loss in dry air, especially at higher temperatures in open sunshine. We therefore think of frogs as mainly active at night and in humid conditions where water loss is low. However, some frog species are mainly diurnal, and some do bask: why, and how do they survive this seemingly lethal behaviour?

The best-known basking frogs are in the genera Chiromantis ( African foam-nesting treefrogs: 4 species); Phyllomedusa (neotropical leaf frogs; 67 species); and Hyperolius (African reed frogs: 228 species). Where they have been assessed, it turns out that these frogs have evaporative water loss rates 10-100 lower than in typical frogs. This is achieved in several ways: the skin may contain reflective crystals that reduce light absorption; in others, the skin is covered with water resistant waxes and lipids. In Phyllomedusa, the lipids are secreted by localised glands and spread over the skin surface by wiping movements of the long hind limbs.

Bokermannohyla alvarengai spends hours during the day sitting on lichen-covered rocks exposed to the sun.

Not all basking frogs fit this picture. Tattersall et al (2006) described a South American hylid Bokermannohyla alvarengai (30 species described in this poorly known genus) which inhabits a montane meadow environment in southeastern Brazil. This frog spends hours during the day sitting, in water-conserving posture, on lichen-covered rocks, rocks exposed to the sun. When the frogs warm up, they turn pale (from their overnight grey-brown mottled colour) and can be easily seen. Tattersall’s experiments showed that the colour change is linked to both increasing light and temperature, with the pale colour developing within minutes of light exposure. The natural history of this species is poorly known, but Tattersall et al suggest that basking may increase food digestion rate, gonadal development and responses to infection. The pale skin colour should reduce excessive radiation absorption. Observations showed that the undersides of frogs on rocks were damp, suggesting that in the early morning, humid air was trapped below the frog, providing some protection against desiccation. However, these frogs did not have particularly low levels of evaporative water loss, and could not be counted amongst the ‘waterproof’ frogs.

Another case is the Trinidad and Tobago frog Kenny (1969) named Hyla crepitans: he noted that during the day, this medium-sized treefrog can often be observed lying exposed to the sun on garden walls or leaves or tree trunks. At night, the frog’s upper surface colour is mottled golden brown with a distinct dark cross, but during the day, it is pale grey to white. Murphy (1997) agreed with these colour change and behaviour observations, but added that, at night in the wet season, the male frogs, spaced out 1.2-1.8 metres apart, call from floating vegetation in man-made ponds and ditches.

The Trinidad and Tobago population was re-assigned to Boana platanera.

Like many other neotropical frogs, Hyla crepitans has recently undergone significant taxonomic revision. As Hyla crepitans, this species was thought to have a huge range, from Brazil north to Venezuela and Colombia. First, the generic name was revised to Hypsiboas, then to Boana (Dubois, 2017). Then Orrico et al (2017) found that the southerly Brazilian population was distinct from that in the north, and renamed the northern species Boana xerophylla. Most recently, the northern population has been found to be composed of two separate species, with the Trinidad and Tobago population being re-assigned to Boana platanera ( Escalona Sulbaran et al, 2021).

During expeditions to Trinidad (2013-15) University of Glasgow staff and students found several sites along the road to the Caura valley in the Northern Range mountains where Boana platanera adults were easily observed. Each site had a nearly vertical rock face pointing essentially eastwards and close to the road, with a concrete drainage ditch between road and rock. The rocky faces were about 5 metres high, with scrubby vegetation on top, backed by secondary forest. At several points, the drainage ditches were almost horizontal and often contained mud and rocky debris and vegetation in heaps which blocked the passage of water. The result was that these ditches often retained water several centimetres deep for considerable periods after rain fell, so they acted as elongated temporary pools. These pools supported the breeding of several frog species, including B. platanera.

The dorsal markings of the frogs were distinct enough that by taking digital photographs, we were able to recognise each individual. In our most detailed study in 2014, we were able to recognise 60 frogs over the four separate sites studied, with each frog being faithful in its attendance at a particular site. All the frogs seen basking on the rock surfaces were males. Out of 20 surveys made over a span of 30 days, the presence of 10 male frogs on the walls averaged 7.5 times out of the 20, with a range of 2-14: i.e. basking was a highly variable activity. When a basking frog was seen in the morning, it was generally in the same position in the afternoon.

At dusk (darkness falls very quickly in Trinidad), one of the males on the wall would start to call, soon followed by the others, and they would leave the wall and jump into the water. Their colour would rapidly change from near white to golden brown and they would begin to jostle for position in the ditch. Usually if one frog aggressively approached another, one would quickly retreat, but we did also see active grappling occur. The genus Boana is commonly known as ‘gladiator frogs’ because of the male-male combat behaviour seen prior to mating. By 2026, 101 species of these frogs had been described, and combat behaviour has certainly not been observed in them all, but the behaviour we have seen in B. platanera fits the description. After a while, some of the male frogs were calling regularly, more or less evenly distributed 50-60 cm apart along the pool of water in the ditch, while a few others were displaced, also calling, on the concrete of the ditch or on the muddy blockage at the end. At some point, females arrived and we did observe some pairs in amplexus. Next morning, the number of spawn clumps seen at any one site was never more than three, irrespective of the number of males that had been seen basking and calling.

In the tropics, where long wet seasons can occur, and high water temperatures can allow rapid development of tadpoles, the explosive spring seasonal breeding of amphibians seen in temperate regions is not the norm. Breeding occurs when individual females are ready and appear at a breeding site. In the leaf-frog Phyllomedusa we found that this creates a problem for the males (Boyle et al, 2021) : attending the breeding site very regularly increases the opportunities to find a mate, but has the potential risks of high predation (frog calls allow predators to locate their frog prey) and lost opportunities for feeding; whereas attending the site sporadically has the opposite set of costs. In practice, site attendance by males was very variable, some attending most nights, others only occasionally. Our observations on the Caura site suggest a similar situation for B. platanera: variable male attendance and low female availability. The extra feature here is the basking behaviour, apparently restricted to males. At our site, the basking wall was very close to the aquatic breeding site, so rapid access to the site could be an advantage. However, this is unlikely to offset the potential hazards of dehydration (there is no evidence that these frogs are water-proofed), and visibility to predators. Perhaps the higher body temperatures achieved through basking would help digest food rapidly, providing the energy needed for combat. Or there may be other advantages we have not yet considered.

It is clear that these preliminary observations, not yet formally published, need further work in order to determine what advantage these frogs get from their intriguing basking behaviour.

I acknowledge several members of the University of Glasgow expeditions in 2013-15 for contributing to the observations outlined here, especially Tom Burns.

Click here for references

Boyle et al (2021). Breeding site attendance in Phyllomedusa trinitatis. Phyllomedusa 20, 53-66.

Dubois (2017). The nomenclatural status of Hysaplesia, Hylaplesia, Dendrobates and related nomina. Bionomina 11, 1-48.

Escalona Sulbaran et al (2021). Integrative taxonomy reveals a new treefrog hidden under the name Boana xerophylla. Zootaxa 4981, 401-448.

Kenny (1969). The amphibia of Trinidad. Studies on the fauna of Curacao and other Caribbean islands 29, 1-78.

Kidman et al (2024). How turtles keep cool: basking in tropical turtles. Journal of Thermal Biology 121, 103834.

Murphy (1997). Amphibians and Reptiles of Trinidad and Tobago. Kreiger Publishing.

Orrico et al (2017). Integrative taxonomy supports two species within Hypsiboas crepitans. Salamandra 53, 99-113.

Tattersall et al (2006). Skin colour and body temperature changes in basking Bokermannohyla. Journal of experimental biology 209, 1185-1196.

Croaking Science: Giant Tortoise Conservation: some success stories

February 1, 2026 by Admin

Roger Downie, Froglife and University of Glasgow

The Reptile Database (September, 2025) lists 366 species of turtles and tortoises (Order Testudines). According to the most recent assessment (Rhodin et al., 2018), an alarming 51.9% of these species are ‘threatened’ with extinction (i.e. are in IUCN categories Critically Endangered, Endangered or Vulnerable).

In this group, the highly terrestrial, herbivorous tortoises have evolved on isolated islands multiple times into giant long-lived species (up to 1.3 m long; 400 kg in mass; some individuals are known to have lived over 200 years). Unfortunately, several characteristics of these animals have made them highly vulnerable to human exploitation: they are slow-moving and easy to catch; often unafraid of humans because of the isolation of their habitats; they can live for months without feeding, making them very convenient as living fresh meat stores on long sea voyages in pre-refrigeration times; and their meat is highly palatable. It is estimated that the Galapagos Islands held about 250,000 giant tortoises in the 16^th century, but only 15,000 by the 1970s. This massive decline is not simply the result of over-exploitation. The Galapagos Islands lacked permanent human inhabitants until the 1830s, but once humans settled, they brought other species that competed with the tortoises; goats consumed the vegetation; black rats, which arrived earlier from visiting ships, ate the eggs and hatchlings. Even scientists contributed to the declines: the California Academy of Sciences captured 385 mainly adult specimens for museum collections in 1906.

Currently, two distinct populations of giant tortoises exist: in the Galapagos Islands and in the Seychelles, including Aldabra Atoll. Considerable efforts are now being made to conserve and enhance the extant populations of giant tortoises. There are two main motivations for this (in addition to the ethical impulse to avoid extinctions caused by people):1) giant tortoises are charismatic species which attract tourists, and therefore generate income for their island homes; 2) giant tortoises provide ecological services such as seed dispersal and vegetation control. In this article, I report on some recent conservation-related studies on both populations.

Chelonoidis, the Galapagos Giant Tortoise. The Reptile Database and Wikipedia consider this a single species with historically 15 subspecies, three of them now extinct. But some researchers (see Gray et al., 2022) describe 14 separate species, all threatened with extinction. The saddest case was ‘Lonesome George’, the last remaining individual of the Pinta Island population, who died in captivity in 2012, after a long life and many unsuccessful attempts to provide him with a suitable mate.

Evelyn Jensen (Newcastle University) has led a programme of research into the genetic make-up of the Galapagos species, using new (and rapidly developing) DNA technology to compare the genomes of extant populations with those of museum specimens collected 100 years ago or more.

The uninhabited island of Fernandino held the only population of C. phantasticus. The only sighting had been in 1906, until, surprisingly, a live ‘young’ female, named Fernanda (about 50 years old) was discovered in 2019. DNA analysis has shown that its genome is essentially the same as that of the specimen collected in 1906 (Jensen et al., 2022) and quite distinct from the other Galapagos species. The existence of this individual suggests there must have been parents fairly recently, but surveys in the challenging terrain of the island have found no evidence (it is volcanically the most active of the archipelago). The fate of Fernanda will the same as that of Lonesome George, unless other specimens are discovered.

The species on Floreana, C. niger, was reported extinct by 1840. However, it was recently found that on the largest island, Isabela, which has five distinct species of giant tortoise, each inhabiting the slopes of a different volcano, that some individuals on Wolf mountain had unusual characteristics. Again, a comparison of these individuals’ genomes with those of museum specimens showed that 89 of the Wolf population are Floreana hybrids (ships’ records show that tortoises collected on one island were sometimes released or escaped on to other islands, so a known dispersal route exists). Gray et al.(2022) have tried to sort out which of these hybrids could provide the best breeding stock for an attempted recovery of C. niger. In 2023, the NGO Island Conservation accomplished a successful eradication of the rats on Floreana, and the plan is to release on the island 300 captive-bred tortoises, genetically as close as possible to the original C.niger, in 2026.

The tiny island of Pinzon holds C. duncanensis. A small adult population exists, but they have had no breeding success for about 50 years because of rat predation on eggs and hatchlings. Since the adults are long-lived and can remain reproductively active, it has been possible to collect eggs, incubate them artificially and rear the hatchlings in captivity to a size where rats cease to be a threat (a procedure known as ‘head-starting’). This had been done successfully for decades, but, without removing the rats, it was an unsustainable strategy. Rats were eliminated in 2012 and by 2014, wild-born young were being found. Jensen and colleagues were concerned that genetic variability might have been reduced during the head-starting programme, but comparison with DNA from specimens collected in 1906 showed that no decline invariability.

Aldabrachelys gigantea, the Aldabra (and Seychelles) Giant Tortoise. Another perspective on giant tortoise conservation comes from Sheffield University where Seychellois Alessia Lavigne is studying reproductive failure in the Aldabra Giant Tortoise. This species is native to Aldabra Atoll and a number of the Seychelles islands in the Indian Ocean, with three subspecies currently extant. Evidence suggests that this population diverged from the (now extinct) Madagascar Giant Tortoise 4.5 million years ago. Aldabra Atoll lies 400 km north of Madagascar, with the Seychelles a further1000 km northeast (Aldabra is now part of the Republic of the Seychelles). Conservation has a long history on Aldabra, with Gunther at the Natural History Museum successfully campaigning for it to be declared a nature preserve. However, in the 1960s, the UK government was eyeing up Aldabra as a site for a RAF base. Julian Huxley led the opposition to this idea, with David Stoddart leading a Royal Society study of the atoll’s fauna and flora in the 1970s. The Royal Society gave way to the Seychelles Islands Foundation and, in addition to its nationally protected status, the atoll was declared a UNESCO world heritage site in 1982. The British government transferred its plans for a military base to the Chagos islands, with devasting consequences for the islanders (Sands, 2022), only recently resolved.

Anecdotal evidence suggests that there is currently very little recruitment of new individuals to the Aldabra Giant Tortoise populations, suggesting that, while the populations remain stable, this is the result of the longevity of individuals, not the health of the population. Lavigne suggests that these populations suffer from ‘extinction debt’ where the limited ability of long-lived individuals to adapt to changing environments and produce viable offspring can result in a sudden population collapse. Lavigne et al. (2025) developed a method for assessing whether unhatched turtle eggs are the result of fertilisation failure, or embryonic death at some stage or other. In a comparison of unhatched marine turtle and giant tortoise eggs, they found fertilisation failure was low in the marine turtles but much higher in their sample of Aldabran Giant Tortoise eggs. A follow-up study (Lavigne et al., 2025?) collected data on unhatched eggs from 24 clutches from six separate populations of Aldabra Giant Tortoise. Sixteen clutches showed 100% failure to hatch; the hatching success per clutch ranged from 0-91%. The Aldabra Atoll population showed 46% success, but there was 0% success in four populations. Fertilisation failure was an issue in some populations, but in most, the failure was in embryo development after fertilisation. Sample sizes were small (3-7 clutches from the different populations), but the results are nevertheless alarming. Lavigne and colleagues suggest that the underlying problem is climate change. A recent news item (Watts, 2025) reports that Lavigne has resorted to artificial incubation of a clutch of eggs (from Cousin Island, zero hatching success in the earlier study) with 13 hatchlings resulting from 18 eggs, although no details are yet available on the incubation conditions provided. Although artificial incubation is clearly not a long-term solution to the problem of reproductive failure in giant tortoises, it may provide some much-needed time for good sustainable solutions to be found.

Click to read references

Gray et al.(2022). Temporal monitoring of the Floreana Island Galapagos Giant Tortoise captive breeding programme. Integrative and Comparative Biology 62, 1864-1871.

Jensen et al. (2022). The Galapagos Giant Tortoise C. phantasticus is not extinct. Communications Biology 5, 546.

Lavigne et al. (2025). Understanding early reproductive failure in turtles and tortoises. Animal Conservation 28, 353-364.

Lavigne et al.(2025?). The illusion of stability in long-lived species: poor early-life survival threatens the persistence of Giant Aldabra tortoise populations. (published on-line)

Rhodin et al. (2018). Global conservation status of turtles and tortoises (Order Testudines). Chelonian Conservation and Biology 17, 135-161.

Sands, P. (2022). The last colony: a tale of exile, justice and Britain’s colonial legacy. Weidenfeld and Nicolson, London.

Watts, J. (2025). Thirteen rare giant tortoises hatch thanks to artificial incubation trial. The Guardian 11^th October, 2025.

Dinosaurs, and a new fossil lizard on the Isle of Skye, Scotland

January 1, 2026 by Admin

Roger Downie, Froglife and University of Glasgow

The Jurassic Coast of Dorset and east Devon is internationally known: a UNESCO World Heritage Site, designated for its wealth of fossil-rich rocks. Less well known is that the Isle of Skye is also a key site for fossils.

Although local people had long known about the occasional occurrence of what looked like giant footprints on the shores of the Isle of Skye, scientists had not seen good evidence of dinosaur remains on the island, despite the sedimentary rocks being of the right sort of age, until the 1970s.

Michael Waldman, who died this year, but then a geology teacher, was leading a school field trip and discovered what turned out to be a rich fossil site at the village of Elgol. Over the years, Jurassic mammals, fish, amphibians and reptiles have been found at the site including one named after Waldman as Eileanchelys waldmani (Waldman’s island turtle). In the Jurassic period, 199-145 million years ago, the land that became Skye was part of the vast continent of Pangaea and located south of the Equator with a climate similar to modern Florida. The Jurassic was the time when dinosaurs were the dominant land animals.

On Skye, intense investigation followed the discovery of a massive sauropod dinosaur femur in 1994, and new discoveries are now made most years. Dinosaur footprints have been discovered at several beaches: An Corran, Score Bay, Brothers’ Point and Duntulm. These are so accessible that commercial guided tours are advertised for visitors to see them. White & Ross (2020) have reviewed the history of these discoveries.

The Jurassic fossils found on Skye are not only of dinosaurs. They include small mammals, turtles, marine invertebrates, lizards and amphibians. The early scientific work was led by Dr Neil Clark of Glasgow University’s Hunterian Museum, latterly joined by Professor Steve Brusatte and colleagues, University of Edinburgh. Examples of recent discoveries include:

2016: A tiny lizard, 6 cm long, named Bellairsia gracilis after the eminent British herpetologist Angus Bellairs.

2017: a remarkably well-preserved large pterosaur

2018: a mouse-like rodent

2020: new deltapodus at Brothers’ Point (a deltapodus is the fossilised footprint of a stegosaur).

Bellairsia gracilis. The 6cm long lizard.

This year, two major scientific papers have been published based on fossil reptile discoveries on Skye. First, Blakesley et al.(2025) have reported on an extensive set of dinosaur trackways from Prince Charles’s Point (named after the location where Bonnie Prince Charlie is thought to have escaped capture following his defeat at Culloden in 1746) on the northwest coast of the Trotternish peninsula.

Fieldwork at the site, only possible at low tide, extended over several seasons, 2019-24. The team recorded 131 dinosaur trackways (each having at least three tracks), 65 of them being made by bipedal three-toed carnivorous theropods, most likely Megalosauripus, and 58 by quadrupedal herbivorous sauropods, likely Breviparopus (with a few of the trackways unidentifiable). The site had been studied in the 1980s, but the researcher involved had concluded that the marks he observed in the rocks were from ancient fish resting burrows.

The present authors have undertaken extremely detailed measurements and photographic recording of the depressions to arrive at a very different conclusion. The evidence of different directions of movement indicates a population milling around at the shallowly submerged margin of a lagoon. The high proportion of carnivores is a surprising result from the study. Trackways like these can rarely be preserved, as they will eventually be eroded away, so it is important that scientists record them thoroughly while it remains possible.

Second, Benson et al.(2025), a multinational team from the USA, UK and France, have described Breugnathair elgolensis (Breugnathair is derived from Gaelic, meaning ‘false snake’; and elgolensis is from Elgol, the location on Skye first worked on by Waldman). The specimen (NMS G. 2023.7.1) was discovered in March 2015 at the Elgol Coast Site of Special Scientific Interest, and extracted as a block of limestone 22x18x15 cm. The block was submitted to a wide range of modern imaging techniques (CT scanning; X-ray tomography), whole and in parts, and some of the fossil bones were sectioned. Their highly technical results have been put into context along with another paper by Marke et al. (2025) by Zaher (2025).

Zaher frames his commentary using Antoine de Saint Exupery’s story from The Little Prince: his childish drawing was interpreted by adults as a hat, while he himself claimed that it showed a boa constrictor that had swallowed an elephant! Message: things are not always what they seem, and snakes have remarkable ability to consume whole prey much larger than their heads. This ability derives from several evolutionary innovations, mainly the mobility of the head skeleton, and of sharp-pointed curved teeth. Marke et al.’s results push the divergence of squamates (snakes and lizards, now more than 12,000 species) from Rhynchocephalians (now only one species, New Zealand’s tuatara) further back than previously thought, to the early middle Triassic, 247-241 million years ago.

Benson et al. provide the most complete description so far of a parviraptorid, from the mid Jurassic, 167 million years ago: previous descriptions of incomplete specimens of this enigmatic group, which existed for 20 million years from the mid Jurassic to early Cretaceous, have revealed such a mix of characters that investigators concluded that the fossils contained more than one species. However, Benson’s results show these characters belong to a single species. Their analysis does not resolve the issue of whether parviraptorids are early snakes, early lizards or a side-line with no long-term outcome, hence their generic name meaning ‘false snake’. Zaher concludes: ‘the data reframe the debate and highlight the limitations of drawing far-reaching conclusions from fragmentary material…evidence suggests that the key snake characters, such as hallmark dental features, might have evolved multiple times, or might have been more widespread among early squamates than was previously thought’.

One thing is clear from all this: the Isle of Skye is likely to be a fruitful site for exploring reptile evolution for some time to come. However, if you are tempted to rush off to Skye, please read the JNCC’s (1997) Fossil Collecting Policy Statement first, detailing permissions needed and best practice guidelines.

Click for references

Benson et al. (2025). Mosaic anatomy in an early fossil squamate. Nature 1/10/25

Blakesley et al. (2025). A new middle Jurassic lagoon margin assemblage of theropod and sauropod dinosaur trackways from the Isle of Skye, Scotland. PLOS ONE 20(4), e0319862.

Marke et al. (2025). The oldest known lepidosaur and origins of the lepidosaur feeding adaptations. Nature 10/9/25

White & Ross (2020). Jurassic Skye: dinosaurs and other fossils of the Isle of Skye. Pisces Publications.

Zaheer (2025). Mix-and-match fossils tell the tale of snake and lizard evolution. Nature News and Views 1/10/25

Seismic waves: an untouched method of communication?

December 1, 2025 by Admin

Written by Laurence Jarvis (external contributor)

Communication in frogs and toads (anurans) has been studied by numerous researchers for decades. Despite this, new methods of communication in anurans, especially the use of multiple cues, have recently been discovered. For example, recent research has demonstrated unique and novel methods of communication among anurans including the use of ultrasonic calls (Feng et al., 2006), multiple modes of communication such as foot waving and twitching (Zina and Haddad, 2007), as well as cryptic mating systems involving complex female mate choice (Bruning et al., 2010).

One method of communication that has received relatively little attention is the use of seismic waves. Seismic communication may be defined as the “exchange of information using self-generated vibrational signals transmitted via a substrate such as the soil, a plant stem, or even a blade of grass” (Narins, 1990). Studies have shown that an increasing number of anurans may use seismic communication to supplement vocal cues, particularly in mate choice and acquisition.

Lewis and Narins (1985) were two of the first researchers to note the potential use of seismic communication in their investigations of the white-lipped frog (Leptodactylus albilabris) which inhabits the Luquillo Mountains and surrounding lowlands of Puerto Rico. They initially observed that isolated calling males alter their calling patterns in response to distant, very light footfalls which suggested that these frogs have a heightened sensitivity to vibrations transmitted through the soil (Lewis and Narins, 1985).

They subsequently found that whilst males performed mating calls using their vocal sacs, these would contact with the soil or grass creating small, but detectable, seismic waves. The receiving males would respond to these seismic waves by shifting and changing their call structure. Subsequent studies have demonstrated further use of seismic waves by anurans which may also influence mate choice in females.

Figure 1. White-lipped frog (Leptodactylus albilabris).
Photo credit: USGS – [http://cars.er.usgs.gov/pics/frogsandtoads2/frogsandtoads2/frogsandtoads2_22.html] {{PD-USGov-USGS}}

The Menwig Frog (Physalaemus albonotatus) and Cope’s Swamp Frog (Pseudopaludicola mystacalis) are two poorly known frogs of Brazil which breed in small puddles and small water bodies. During the breeding season males use their vocal sacs to help project their calls to alert other males, as well as attract females. Males establish small territories, which they defend against intruding males.

Forti & Encarnação (2012) have observed both species using their vocal sacs to send water-waves across the water surface. In P. albonotatus, a resident male was observed to send water-waves when an intruding male approached. The intruding male was not deterred and a physical encounter resulted. The authors suggested that the production of water-waves by the resident male served to alert the intruding male of his presence and to augment his vocal cues. In addition, Forti & Encarnação (2012) observed similar behaviour in P. mystacalis. On this occasion, the resultant water-waves towards an intruder led this male to turn away and avoid the resident male. Forti & Encarnação (2012) proposed that the use of seismic communication may supplement the calls of males and aid in spacing of males across their small breeding ponds.

Golden rocket frogs (Anomaloglossus beebei) are native to the Kaieteur National Park in Guyana. Like many dendrobatid frogs, they breed in small bodies of water which have accumulated in the leaves of bromeliads. During the breeding season males defend suitable water bodies from intruding males by sitting and calling from the bromeliad leaves.

Narins et al. (2018) tested the hypothesis that males not only use vocal cues but also rely on vibrations sent through the leaves on which they perched. To test this the researchers sent small vibrations through leaves towards those of a resident calling male. These vibrations mimicked the calls of an approaching intruding male. Narins et al. (2018) found clear responses in the resident male who turned towards the vibrations as well as altering the timing and duration of their own call. These experiments demonstrate that these frogs may well use multiple cues to deter intruders using both sound and vibrations to communicate their presence.

Figure 2. The golden rocket frog Anomaloglossus meansi, a close relative of A. beebei.
Photo credit: Kok et al (2018). ZooKeys 759: 99-116. https://doi.org/10.3897/zookeys.759.24742

Similar experimental findings have been found in the red-eyed treefrog Agalychnis callidryas from Central America. This popular and much studied tropical species forms large mating aggregations during the breeding season. Males defend small patches, occupying leaves over-hanging water bodies. Males will aggressively chase off intruders and make loud mating calls. In addition, males perform ‘tremulations’, which involves raising their bodies off the leaf and rapidly contracting and extending their hind limbs (Caldwell et al., 2010).

These create unique vibrations which are transmitted through the leaves on which they perch. Caldwell et al. (2010) performed mimicking experiments and found that, like golden rocket frogs, the males of the red-eyed treefrog responded to the vibrations of calling males. In addition, the vibrations conveyed information relating to the status of the male. More victorious and successful males produced more vigorous tremulation vibrations. Male size did not itself alter the vibrations produced (Caldwell et al., 2010).

It appears that intruding males do not just respond to any vibrations as these occur frequently due to the mass movement of frogs during breeding. Instead, it appears that males respond specifically to the vibrational cues conveyed in tremulation displays. This study adds to the current research that frogs may use multiple modes of communication during the breeding season to deter intruders and potentially to attract females.

Figure 3. A red-eyed treefrog Agalychnis callidryas from Central America.
Photo credit: Bernard DUPONT from FRANCE, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons

As well as producing seismic cues during communication whilst breeding, research has shown that some amphibians may detect forthcoming seismic events. Research by Grant & Halliday (2010) suggests that the common toad (Bufo bufo) may detect tiny cues produced prior to an earthquake.

During fieldwork at a common toad breeding pond in Italy in April 2009, the researchers observed over 95% of the population to abandon breeding, five days before a large earthquake hit the region. This behaviour is highly unusual for common toads, which normally remain at breeding sites for several days until spawning has been completed.

At this site, the population of common toads had only just started. The sudden abandonment of breeding by the common toads was therefore very unusual. The researchers suggest that the common toads were possibly responding to small perturbations to the gravitational field in the ionosphere, which alter preceding an earthquake (Grant & Halliday, 2010).

Why the toads should abandon breeding is not clear, but could potentially act as an ’early warning system’ for an approaching seismic event. Although this behaviour has not previously been documented in amphibians, unusual behaviour prior to seismic activity has been observed in several species of bird and mammal (see Grant & Halliday, 2010). Further research will show if this was an isolated phenomenon of whether other amphibian species may exhibit such interesting behaviour.

Click here for references

Bruning B., Phillips B.L. and Shine R. (2010). Turgid female toads give males the slip: a new mechanism of female mate choice in the Anura. Biol Lett., 6: 322–324.

Caldwell M.S., Johnston G.R., McDaniel J.G. and Warkentin K.M. (2010). Vibrational signaling in the agonistic interactions of red-eyed treefrogs. Current Biology, 20 (11): 1012-1017.

Feng A.S., Narins P.M., Xu C.H., Lin W.Y., Yu Z.L., Qiu Q., Xu Z.M. and Shen J.X. (2006). Ultrasonic communication in frogs. Nature, 440: 333–336.

Forti L. and Encarnação L. (2012). Water-wave production in the Neotropical frogs Physalaemus albonotatus and Pseudopaludicola mystacalis: a seismic signal. Salamandra, 48 (3): 181-184.

Grant R.A. and Halliday T. (2010). Predicting the unpredictable; evidence of pre‐seismic anticipatory behaviour in the common toad. Journal of Zoology, 281 (4): 263-271.

Lewis E.R. and Narins P.M. (1985). Do frogs communicate with seismic signals? Science, 227 (4683): 187-189.

Narins P.M. (1990). Seismic communication in anuran amphibians. Bioscience, 40(4): 268-274.

Narins P.M., Meenderink S.W., Tumulty J.P., Cobo-Cuan A. and Márquez, R. (2018). Plant-borne vibrations modulate calling behaviour in a tropical amphibian. Current Biology, 28 (23): R1333-R1334.

Zina J. and Haddad C.F.B. (2007). Courtship behavior of two treefrog species, Aplastodiscus arildae and A. leucopygius (Anura: Hylidae), from the Atlantic rainforest, Southeastern Brazil. Herpetol Rev., 38: 282–285.

Review: The lives of frogs, a new introduction to and celebration of their diversity

November 1, 2025 by Admin

Authors: Jim Labisco and Richard Griffiths. Publishers: Princeton University Press 2025; 288 pp, hardback, ISBN 978-0-691-25501-9, £30. 

Roger Downie, Froglife and University of Glasgow

Princeton University Press is well-known for its series of lavishly-illustrated WildGuides to the biodiversity of Britain and Europe. This book is not in that series, but is a similarly beautiful ‘introduction’ to and ‘celebration’ of the diversity and lives of frogs. The authors are Jim Labisco, a young lecturer at University College London and not-so-young Richard Griffiths, emeritus professor at the University of Kent and one of the UK’s most eminent herpetologists. In addition, the production team at Princeton University Press, acknowledged at the end, are responsible for much of the arrangement of the book, and such decisions as the use of American rather than British English, and the inclusion of imperial as well as metric measures.

The book is in hardback, 288 pages long and listed at £30: what do you get for your money? First, a lot of beautiful colour photographs of frogs. As an example, the 12-page introductory chapter is more than 50% illustrations, including three full page photographs. Following the introduction, which summarises frog diversity and tackles the age-old question (what’s the difference between frogs and toads?), nine chapters cover different aspects of frog biology: evolutionary origins; life on land and in freshwater; communication and reproduction; development and parental care; activity; nutrition; defences against predators and disease; population fluctuations; and, finally, the frog population decline crisis and what is being done about it. At the end of each chapter, there is a set of five species accounts, with a full-page photograph and a page of text describing the species, its distribution and key features. The species are chosen to illustrate some aspect of the preceding chapter. For example, the Lesser Antillean Frog Eleutherodactylus johnstonei which likely originated on the Caribbean island of Montserrat and has now spread throughout the region (largely by anthropogenic means), is used to illustrate the chapter on frog movements. I have had some involvement in documenting its spread in Trinidad and Tobago. Each account includes a world map in grey, showing the frog’s distribution in green (oddly missing for E. johnstonei). Following the topic chapters, there’s a glossary defining some technical terms, a list of resources and an index.

The book has high production standards. Each photograph has a clear legend including an arrow pointing to the illustration (rather than a figure number). In addition to photographs, there are many other graphics: I particularly liked the two-page illustration of the frogs inhabiting an Amazonian floating meadow (pp. 56-7). Sources of original illustrations are given in the end acknowledgements section, but in a very concentrated format: I would personally have preferred them in the figure legends. Frog names are mostly given as capitalized common names and italicised scientific names, although scientific names are omitted from some of the graphics. I found very few typographical errors. One minor problem: the text includes frequent cross-references to pages where more information on a topic can be found; some of these are to chapter numbers, but the contents page does not number the chapters, and you need to work out for yourself that the introductory chapter is not counted as chapter one.

Who is the book aimed at? An ‘introduction’ might seem to be for people who don’t already know much about frogs, but who would like to find out, or for people who know about their local frogs, but not about any others. The book’s coverage is very wide-ranging, quite detailed, pretty up to date and mostly uses non-technical language, so there is something here even for people who know quite a lot already. For example, the truly viviparous African frog Nimbaphrynoides occidentalis was new to me, as were the Brazilian frogs whose bony spines on the skull allow them to inject toxins into attackers, making them the only truly venomous frogs. I do however have some grumbles and reservations:

The description of North American frogs that survive extremely cold winters claims that their blood contains anti-freeze proteins. The research I’ve read denies this: rather, these frogs tolerate freezing by greatly increasing circulating levels of glucose which acts as a cryoprotectant, preventing damage to frozen cells.

About frogs that develop directly to froglets, without a tadpole stage in water, we read that ‘tadpoles complete their development within the egg capsule and hatch as fully metamorphosed froglets’. There is a wide range of direct developing frogs, but at least in those I have looked at, it is misleading to think of the early stages as tadpoles, because they are so highly modified, with the tail bud developing into a hugely expanded respiratory organ.

There are some examples of what I call inappropriate ‘speciesist’ writing, where a trait is said to have evolved in a particular species, but where it is actually characteristic of a whole group, genus or family. Examples are the occurrence of disc-shaped suckers in torrent-dwelling tadpoles (this is a feature of the multi-species genus Amolops, not of a single species); and the lack of a tongue in the Surinam Toad is, again, a feature of pipids in general.

It is claimed that Cane Toads locate Tungara Frogs (as prey) by detecting their calls. I’d like to see the evidence for this. Michael Ryan’s work showed that predatory bats locate Tungara Frogs by their calls, and that the frogs simplify their calls when bats are present, increasing their survival chances, but at the cost of reducing mating opportunities, because females prefer the complex calls.

I also felt that there are some odd omissions or example choices. Variable tadpole morphology in response to environmental factors (polyphenism) is exemplified by Mallorcan Midwife Toads, but not by David Pfennig’s stunning description of the carnivore/omnivore morph system in spadefoot toads. Salt tolerance is mentioned in the American Green Treefrog, but the best known and most complete example of this, the Crab-eating Frog Fejervarya cancrivora is surprisingly omitted. Similarly, there is a brief mention of one treefrog species reducing water loss by means of mucus skin secretions, but the fascinating skin-wiping behaviour, spreading a waxy secretion over the skin in phyllomedusine frogs is absent. The farming of frogs as a source of human food is briefly mentioned, but the conditions under which these frogs live and are slaughtered is not discussed: the welfare of frogs remains a little considered subject.

Next, a frustration. The text omits citations to sources, so how can you follow up on points of interest? The book list is a good resource (although, outrageously, Altig and McDiarmid’s seminal Tadpoles is omitted), but only two of the books were published after 2015. You can find some of the examples in the ‘selected’ list of research papers, but without clear citations, it is a labour, and for the general reader the book is aimed at, not used to working through research papers, quite intimidating.

The final chapter appropriately covers the worldwide decline in amphibian populations, reviewing causes (I feel that herpetologists have yet to factor in the likely links with the parallel declines in insect populations) and progress towards solutions, both small-scale community-based work such as Toads on Roads campaigns, and large-scale efforts such as the Amphibian Ark, which focuses on the captive breeding of threatened species. I was surprised not to see any comment on such efforts to save Darwin’s Frog and the Mountain Chicken (both are covered under the parental care section, but with no comment on their dire state in the wild). The list of organisations at the end includes many of the organisations involved in frog conservation, but oddly omits Froglife (responsible for Toads on Roads in the UK) and Save the Frogs. In conclusion, despite the reservations listed above, I thoroughly recommend this book. It truly is a celebration of frog diversity, and I endorse the message at the start that the main motivation behind conservation efforts should not be based on notions of services frogs may provide to us, but simply because they are wonderful.

Croaking Science

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