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Poison Frogs and Convergent Evolution

Mantella baroni, common names include: Baron's Mantella, Painted Mantella, Harlequin Mantella, Variegated Mantella

Mantella baroni, common names include: Baron’s Mantella, Painted Mantella, Harlequin Mantella, Variegated Mantella

Although it is late in the season for finding Mantella frogs in Ranomafana, after a couple of hours of searching we found one. I say we but the hard work was all down to my guide Lova who knew exactly where to look. Mantella, even though brightly coloured, are hard to find as they tend to sit in refuges by streams and under leaf litter. The most reliable way to find them is by their call, a  short click noise, however they only tend to do this in the wet season when temperatures are higher so we just had to rely on Lova’s knowledge, a bit of luck and a couple of hours spent poking around in holes with a stick.

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At only 2.5cm these are small frogs!

The most interesting thing about these small brightly coloured frogs is that they share such striking similarities with neotropical Dendrobatids or Poison Dart Frogs.  Although genetically unrelated, Mantella and some Dendrobatid frogs share many of the same features: toxic skin chemicals, terrestrial eggs, small body size, toothless jaws, a specialist diet composed largely of ants, active diurnal foraging, and aposematic (warning) coloration. All of which are considered to have evolved completely separately, a process called convergent evolution.

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They also do not sit still making them very difficult to photograph!

Convergent evolution is defined as the process by which similar features evolve separately in species with separate lineages. In other words evolution creates animals with attributes that have a similar form or function that were not present in the last common ancestor of each species. These traits usually enable animals to fill very similar ecological niches.

In both Mantella and Dendrobatids, defensive chemicals appear to be closely associated with the evolution of aposematism (warning colouration) as a visual warning of their toxicity to potential predators, and active diurnal foraging, a behaviour that is generally rare in frogs.

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M. baroni is very similar to M. madagascarensis however it lacks the red flash marks on the legs and horseshoe shape under the chin.

Although wild poison frogs retain toxic skin alkaloids for several years in captivity, they do not seem to be able to produce their own toxic alkaloids in fact they accumulate these toxins from their diet. Toxic alkaloids have been shown to be absent in Dendrobatid and Mantella frogs when raised in captivity on a diet of fruit flies however these individuals will readily accumulate alkaloids when added to their diet.

The foraging behaviour and diet of Mantella is not yet well documented relative to Dendrobatids however ants are known to dominate the diet of Mantella. A study conducted in Ranomafana National Park collected Mantellid frogs and arthropods with the potential to be sources of alkaloid chemicals. The study found that 13 of the 16 alkaloid compounds found in the arthropods of Ranomafana are also known in other ants, beetles, and frogs endemic to the neotropics and that 9 of the 12 alkaloids found in Mantella are also found in Dendrobatids.

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Because ants are a common or even dominant part of poison frog diets the presence of suitable toxic alkaloids in ants may have been the essential requirement for the evolution of chemical defence in both Malagasy and Neotropical poison frogs. The finding that endemic Malagasy ants contain these toxic alkaloids is evidence to support convergent evolution. Furthermore the convergence seen between the two groups of frogs might itself have been driven by convergent evolution in the unrelated ant groups found in Madagascar and the Neotropics.

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Reference:

Clark, V. C., Raxworthy, C. J., Rakotomalala, V., Sierwald, P., & Fisher, B. L. (2005). Convergent evolution of chemical defense in poison frogs and arthropod prey between Madagascar and the Neotropics. Proceedings of the National Academy of Sciences of the United States of America, 102(33), 11617-11622.

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Frog Diversity in Ranomafana

Boophis madagascariensis

Boophis madagascariensis

In the past couple of weeks I have been going out photographing one of my favorite subjects, frogs! Ranomafana has one of the highest amphibian diversities of any national park in the country with only Andasibe offering any competition. Of the 300+ described species of frog in Madagascar all except two are endemic and at least 120 of those have been found in Ranomafana including 8 potential species endemic to the park.

Spinomantis aglavei

Spinomantis aglavei

Spinomantis aglavei

Spinomantis aglavei

According to the Global Amphibian Assessment (GAA) in 2004, Madagascar ranks as the country with the 12th highest amphibian species richness however it probably should be higher than this as over 200 candidate species have been identified since. This would make the estimated total number of frog species over 500 and would mean Madagascar is home to roughly 4.3% of the world’s amphibians whilst occupying less than 0.5% of the worlds land area.

Gephyromantis tschenki

Gephyromantis tschenki

Boophis reticulatus

Boophis reticulatus

The largest family of frogs in Madagascar is the Mantellidae which is very diverse and is estimated to have colonised Madagascar around 58 million years ago. Some species of the Mantella genus share convergent features with neotropical dendrobatids (poison dart frogs) with their sharply contrasting aposematic (warning) colouration being associated with toxic skin.

Mantella baroni, common names include: Baron's Mantella, Painted Mantella, Harlequin Mantella, Variegated Mantella

Mantella baroni, often called Painted Mantella or Harlequin Mantella

Boophis is another endemic genus of frogs belonging to the family Mantellidae. This genus is known for its brightly coloured eyes which it has been hypothesised act as an anti-predator startle response. The brightly coloured sections usually remain hidden during the day when the frog is resting but suddenly become visible when the frog opens its eyes.

Boophis tasymena

Boophis tasymena – Polka-Dot Bright-Eyed Frog

Boophis tasymena

Boophis tasymena – Polka-Dot Bright-Eyed Frog

Almost all Malagasy rainforest frogs are either terrestrial and diurnal or arboreal and nocturnal. This is probably because of alternating predation pressure from birds in trees during the day and snakes, tenrecs and other predators on the ground at night.

Mantidactylus femoralis

Mantidactylus femoralis?

Plethodontohyla notosticta

Plethodontohyla notosticta

As with amphibians elsewhere in the world Malagasy frogs face numerous threats. Ongoing habitat destruction has already led to destruction of 90% of the original vegetation and threatens most species to some extent.

There are recent reports of the arrival of Asian common toads (Duttaphrynus melanostictus) in Madagascar, most likely arriving inside shipping containers from Asia. This species’ relative, the cane toad (Rhinella marina), has caused widespread ecological destruction in Australia, and there is now concern that an invasion in Madagascar will have disastrous impacts. One positive aspect is that so far this species has never been recorded as a carrier of the Chytrid fungus, Batrachochytrium dendrobatidis (Bd).

Unlike the majority of the world’s amphibian populations Madagascar has so far escaped the catastrophic declines associated with Bd. However Between 2010 and 2014, Bd was recorded in five different areas of the country, including Ranomafana, although the virulence of the strain is as yet unknown and no die-offs have been reported.

Boophis elenae

Boophis elenae

Boophis elenae

Boophis elenae

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