Eyelash Viper and Tropical Frogs

Local weather conditions may differ drastically from year to year and during 2009 the Osa Peninsula experienced a particularly rainless dry season.  Normally, from December through to April, there is at least some rain, not much; it is after all the dry season, but last year, barely a drop.  This was then followed by a very dry wet season.  When the rains should have arrived in May, they did not, and this condition carried on unabated throughout June and July.  For visitors to the Osa Peninsula, the conditions could not have been more perfect, lots of hot, sunny days, perfect vacation weather.  But that interruption in the natural rain cycle was having a profound effect on the fauna and flora of the area.

Rain forest trees are very dependent on their water source and if this becomes limited in supply, they begin to suffer environmental stress.  The first obvious sign is that they will shed leaves.  Should the stress continue, they begin to shed branches.  Finally, if we end up, as we did in 2009, with a drought in a rain forest, the trees divert a lot of energy into reproduction to ensure the passing on of their genetic material lest they themselves die.

That is exactly what we saw in 2009.  Visitors were treated to a canopy high display of flowering, rarely equaled, with a veritable kaleidoscope of color; yellows, reds, purples and pinks painted like a pastel infusion among the normal mix of greens at the treetops.  Some of the trees did not just flower once; they produced blooms two or three times throughout last season.  The exuberant flowering display led in turn to a profusion of fruiting, the trees were quite literally hanging with fruit.

This series of events led in turn to a very good reproductive year for those animals whose diet depends to a greater or lesser extent on nectar or fruit, such as Spider and Howler Monkeys, Toucans, Hummingbirds and Manakins.  This year visitors have been treated to the sight of huge flocks of toucans and a great many of the female Spider and Howler Monkeys carrying young ones.

But what a difference a year makes.  In a complete reversal of last year, 2010, so far, has been a wetter than normal year.  Rather than the tired looking yellows and browns evident in last year’s vegetation, this year has remained luxuriously verdant and that bodes well for another group of animals; the amphibians.  Despite a myriad of problems causing global declines in amphibian populations, here on the Osa Peninsula, the frogs are still holding their own.

Mating pair of Red-eyed Green Tree Frogs, (Agalychnis callidryas)

There are certain species that one can almost rely upon to be present right throughout the year; Marine Toads and the picture postcard child of Costa Rica, the Red-eyed Green Treefrog.  With the onset of the rains though, all those species that have been noticeable by their absence for the previous five months emerge once again, sometimes in numbers so large that it begs the question, “Where has such a biomass of amphibian life been hiding for almost half the year?”

When they do emerge, it is with only one thing in mind; reproduction.  Around the breeding pools, the cacophony of the calling males renders it all but impossible to make yourself audible.  The initial heavy rains of the season immediately encourage a response from Costa Rica’s largest treefrog, the Milky Frog.  Within hours on the evening of the first downpour a full breeding choral congregation of males, in numbers upwards of fifty will have accumulated in the pond.  The deafening din of their ardor can be heard from great distances.  After several nights of this repeat performance, they once again decline in numbers until it is only a very occasional chance sighting that you will experience during the remaining rainy months.

Now is the time of the frog though and during the next seven months we are treated to a nightly chorus of serenading males of different species, some of them more melodious than others; the jackhammer mating call of the Marine Toad, the chuck of the Red-eyed Green Treefrogs, the sound of little metal bells tinkling from the Tink Frogs, clipped chirps of Banana Frogs and what you would swear sounded like two blocks of wood being knocked together that announce a Gladiator Frog male is looking for a mate.

Banana Frog, (Dendropsophus ebreccatus)

Yet there is one more amphibian phenomenon that only a lucky few experience; the explosive breeding display of Parachuting Red-eyed Green Tree frogs.  This is the second species of Red-eyed Green Tree frog to be found on the Osa Peninsula.  They are rarely, if ever seen, living as they do at the tops of the trees, but several times during the rainy season following several days of concerted torrential rain out they come, and they come in their hundreds.  They receive the parachuting part of their name from having large hands and feet that are extensively webbed.

Parachuting Red-eyed Green Tree Frogs, (Agalychnis spurelli). Mating pair

Taking the monsoon-like conditions as their cue, they launch themselves from above, opening the hands and feet, spreading the webbed fingers and toes, allowing them to effectively ‘glide’ to the lower levels.  The males and females pair up and then not long before dawn they start to lay eggs, everywhere, covering the surfaces of the leaves overhanging the ponds.  The eggs develop for seven to ten days, liquidize, slip off the leaves and plop into the water which is where the normal tadpole stage will then take place.  The explosive breeding frenzy has to end before sunrise; the frogs need to take shelter from the drying effect of the sun’s harmful rays on their delicate skin.

If you are out early enough you will be awestruck by the sight of hundreds of pale green frogs trying to climb up into shadowy parts on the underside of leaves.  Sometimes the weight of such huge numbers will snap the vegetation in which they are attempting to seek refuge.  It really does have to be seen to be believed.

Eyelash Viper Feature

Eyelash Viper, (Bothriechis schlegelii). Gold phase – Oropel

In all my years living on the Osa Peninsula I have seen the elusive Eyelash Viper, (Bothriechis schlegelii), on only four occasions.  It is not necessarily because they are rare but more to do with the fact they are very cryptically colored, the point of which being, to avoid being seen by both predator and prey.  Although they come in several color forms, the ones I have seen at Cabo Matapalo have all been the green mottled morph. The camouflage coloration blends the contours of the body perfectly against the moss covered branches of the plants where they sit as ambush predators awaiting the approach of a small lizard, frog, bird or rodent on which they feed.

The most distinguishing feature of this small pit viper are the two spine-like scales protruding over the eye giving it the name Eyelash Viper.  There is no definitive answer as to why it has these scales, possibly to push vegetation out of the way which may scratch the snake’s unlidded eyes.  One other eponymous feature  to the pit vipers is the small hole on the front of the face between the nostril and the eye.  These house acute sensory apparatus that detect very small changes in background infrared radiation allowing the snake to detect the presence and location of it prey.

Eyelash Viper, (Bothriechis schlegelii). Motteld phase- Bocaraca

The Eyelash Viper is a small snake, especially given the size some other vipers can grow to.  It is a nocturnal predator and will take a variety of prey ranging from frogs, lizards, birds and mammals. Unlike terrestrial vipers which envenomate their prey, release it, then seek it out after it has died, the arboreal vipers have a strongly prehensile tail and tend to hold on to their prey after biting.

Eyelash Vipers are distributed from Mexico down through Central America into Venezuela.  They prefer shrubby vegetation found in secondary growth.  In Costa Rica the yellow morph is know as the oropel, (golden skin), while the mottled morph is known as the bocaraca.

Due to its size, cryptic coloration and preference for resting during the day in the lower areas of vegetation it is responsible for biting those people who work in those areas.  Although deaths from the bite of an Eyelash Viper are rarely fatal, there can be some long term issues with impairment of movement particularly if bitten on the hands.  Viper venom varies between species and can be quite complex in chemical make-up.  Generally they contain a cocktail of fast acting proteolytic enzymes that cause a multitude of effects including tissue necrosis and death.

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