The Fascinating Frogs of Tobago's Main Ridge Reserve

Are you following this series on Tobago's special and unique creatures? This is the second in the series by Amy Deacon, Lecturer in the Department of Life Sciences at The University of the West Indies, St Augustine and Secretary of the Trinidad and Tobago Field Naturalists’ Club. This week Amy takes a closer look at some of the more hidden – but no less valuable – biodiversity of Tobago’s Main Ridge: the frogs. 
 
Frogs, toads, newts and salamanders are all amphibians, as they live underwater for at least some of their lives, and on land the rest of the time. Tobago is home to 15 different frog species, many of which are found inside the Main Ridge Reserve. Four of these are not found on Trinidad, and three are found nowhere else in the world.

The cartoon-like Tobago Glass Frog. Photo courtesy Renoir Auguste

The Tobago Glass Frog is one of the most unusual. This tiny (2cm) frog has a Kermit-like, cartoon appearance as it is bright green with big eyes. However, if you were to turn it over, you would see how it got its name: its belly is completely transparent, meaning that its heart, liver and all other internal organs are plain to see. Remarkable or repulsive, depending on your perspective!
The other astonishing thing about this species is how it reproduces. First, a male calls to attract multiple females to the underside of a carefully chosen Heliconia leaf by the side of a stream. A single male may mate with many females, each of which lays their eggs on the leaf before disappearing. The male then remains for several days to loyally guard them from predators – such as insects, spiders and crabs. Eventually the bright green tadpoles hatch and simply plop into the stream below. Amazingly, the presence of a predator can actually trigger the early hatching of the tadpoles, as a clever escape mechanism.

The next species we will look at may be less pretty, but is no less interesting. The Tobago Stream Frog is endemic to Tobago – in other words, it is found nowhere else in the world. They are widespread within the Main Ridge forest but you are unlikely to spot them as they are about the same size as the glass frog and tend to be dull brown in colour from above. However, on closer inspection, females have a bright yellow throat and – strangely – males turn black when they are calling to females. They are extremely good at jumping, and almost impossible to catch, even if you are keen-sighted enough to spot one! Once again, it is the fathers that are responsible for looking after the babies. After spawn is laid and hatches in forest leaf litter, the male frog actually carries the young tadpoles on his back to a suitable pool of water - a very strange sight, and exemplary parenting behaviour.

The Tobago Stream Frog isn’t the only frog endemic to the island; two other Main Ridge frogs that are found nowhere else in the world are the Charlotteville Litter Frog and Turpin’s Litter Frog. ‘Litter’ frogs are so-called for their preference for leaf-litter along the sides of streams, not because of a preference for garbage, although in some places they are forced to put up with plastic trash left by those who pass to collect water from the reserve’s clean supply.

These frogs eluded scientists for centuries, perhaps due to their incredible camouflage abilities, perfectly blending in among dead leaves. The Charlotteville Litter Frog was first scientifically documented in 1995 and Turpin’s Litter Frog in 2001. The former was originally discovered in the north-east of the island (hence the name), but has now been sighted almost everywhere aside from the Scarborough and Crown Point areas. The latter appears to be restricted to the North Coast road and parts of the Main Ridge. However, very little is known about either species in terms of how they breed, what they eat, and their exact distribution on Tobago. Such knowledge will be essential if we are to effectively conserve them.
The cryptically-coloured Charlotteville Litter Frog. Photo courtesy Renoir Auguste

The Turpin's Litter Frog is known only from the Northern parts of the Main Ridge Photo courtesy Renoir Auguste

Frog conservation becomes even more important when we consider that the health of the Main Ridge frog populations can tell us a lot about the health of the whole ecosystem. Frogs are especially sensitive to environmental change; they need both healthy forests and clean water to complete their life cycle, so if either is compromised this will be reflected in their numbers and distribution. Also, their breathable damp skin quickly absorbs toxins from the environment, so any pollution will affect frogs first, giving us a warning sign. These reasons also mean that frogs, and amphibians in general, are in trouble globally, as they are especially vulnerable to the effects of climate change, disease and human development.

The Tobago Glass Frog in particular has extremely specific habitat needs and tends to be found in small, isolated populations – which means that the removal of streamside vegetation for agriculture or development could wipe out a population overnight. For this reason it is classified as a ‘vulnerable’ species by the International Union for Conservation of Nature.

We must learn more about our native frogs and ensure their survival – especially those that we have sole responsibility for as they are found nowhere else on the planet. If we don’t – who will?

The male Tobago Glass Frog guarding his precious eggs Photo courtesy Renoir Auguste

(For more information on T&T’s amphibians and reptiles, see: www.trinidad-tobagoherps.org and keep an eye out later in the year for a new Field Guide published by the Trinidad and Tobago Field Naturalists’ Club.)



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