Archives for March 2018
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.
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.
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
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.
Amphibian and reptile declines – UK perspective
The UK supports a range of iconic mammal species including hedgehogs, water voles, badgers and several bat species. However, in recent years, research by various conservation bodies has found startling declines in many of these species. Research led by the Wildlife Trusts indicates there has been a 30% decline in water vole populations since 2006, which represents an approximately 3% loss in populations per year1. Of more concern, is the dramatic decline in the hedgehog which is estimated to have declined by 66% over the past 13 years (5% decline per year)2. Increased agricultural intensification, use of pesticides, habitat loss and fragmentation have all attributed to the decline which has been reported by the British Trust for Ornithology2. In addition, several of the UK’s iconic bat species are in decline, as reported by Bat Conservation, including the brown long-eared bat which has declined by 31.3% since 1999 (2.2% decline per year)3.
Amphibians and reptiles are generally less understood by the public who often perceive these species differently to iconic mammals. However, many populations of our once common amphibian species are in decline. Common frog, common toad and natterjack toad populations have been reported as being in decline since the 1970s4,5. Recent research in 2016 by Froglife and the University of Zurich has shown that common toad populations have declined across the UK by 68% over the past 30 years, which approximates to a 2.26 % decline per year. This value is comparable to the declines in many of our iconic mammal species and highlights that significant declines may be widespread across our native fauna. The reasons for the decline in the common toad are similar to those affecting hedgehogs including habitat loss and fragmentation, pollution and climate change. The adder, being the only native venomous snake in the UK, often has a poor perception from the public. However research by Natural England and Froglife in 2002 has indicated significant population declines in the adder, especially from the Midlands6. In this study one third of adder populations were estimated to consist of less than 10 individuals which puts them at high risk of extinction. These declines are likely to be attributable to poor habitat management including agricultural intensification as well as public pressure (recreation) and persecution6. At Froglife we aim to raise awareness of the declines in our native amphibian and reptile species, especially those which were once common. We wish to highlight that widespread amphibian and reptile species, such as the common toad, are suffering declines equivalent to iconic mammals such as the water vole and hedgehog. At Froglife we carry out extensive practical habitat creation and restoration each year, both locally and nationally, with the aim of improving environmental conditions for our native amphibians and reptiles and aiding to conserve populations of our valuable herpetofauna.
1Wildlife Trusts (2018). http://www.wildlifetrusts.org/news/2018/02/26/new-report-points-30-decline-water-vole-distribution. Accessed on 13th March 2018.
2BTO (2018) https://www.bto.org/science/monitoring/hedgehogs. Accessed on 13th March 2018.
3Bat Conservation (2018) http://www.bats.org.uk/pages/species_population_trends.html. Accessed on 13th March 2018.
4Beebee, T.J.C. (1973) Observations concerning the decline of the British Amphibia. Biological Conservation, 5 (1): 20-24.
5Beebee, T.J.C. (1977) Environmental change as a cause of natterjack toad (Bufo calamita) declines in Britain. Biological Conservation, 11: 87-102.
6Baker, J.R., Suckling, J. & Carey, R. (2004) Status of the adder Vipera berus and slow-worm Anguis fragilis in England. English Nature Research Report 546, English Nature, Peterborough, UK.
This weekend (17-18th March) another cold spell of weather has been forecast by the Met Office with further outbreaks of snow and widespread freezing conditions. By now, many common frogs, common toads as well as newts will have made their way to breeding ponds and are at risk from sudden periods of cold weather. Adults in ponds, as well as those migrating towards breeding areas, are prone to winterkill which has already caused a high incidence of mortality in common frogs this year. In addition, spawn laid by common frogs and toads is prone to freezing in hard frosts. Reptiles are at less of a risk since the majority have not yet emerged from hibernation and breeding does not commence until later in the year.
To help common frogs and toads during periods of cold weather there are a few actions that you can take. First, if you have a garden pond, periodically break any ice on the surface to promote oxygen exchange. This will allow amphibians to survive in the water beneath the ice. Second, you can try floating a small object e.g. tennis ball, in the water which prevents ice formation. However, this only works in moderate frosts and in severely cold weather, breaking the ice is the only option. Third, provide piles of leaves or areas of dense vegetation and scrub close to your garden pond as this will provide areas for amphibians to take refuge during periods of cold weather. If you have frog spawn, the upper portions may freeze, but the spawn which is underwater should survive. However, if you have a very small pond and/or it is shallow and prone to freezing throughout, you can temporarily place your frog spawn into a bucket of water and place in a garage, or similar place, out of the freezing conditions. Once the cold weather has passed, ensure that you return the spawn to the original place within the same pond to allow it to continue to develop.
NB: The majority of reptiles have not yet emerged from hibernation