Frog defense: hiding, fighting, or both

June 4, 2016 • 7:30 am

JAC: Instead of “Readers’ Wildlife” today, we’ll have a report on frogs by Greg Mayer, who’s just visited Costa Rica.


by Greg Mayer

Although quite cryptic on the forest floors it calls home, the smoky jungle frog (Leptodactylus pentadactylus) in the photo below is too obvious to be a candidate for “spot the frog”. This large species of frog (reaching over a 150 mm in snout-vent length) sports what in the military would be called “defense in depth”– a series of defensive behaviors and adaptations that help the frog avoid becoming someone else’s meal. The one in the photo below I encountered at the Lapa Rios Ecolodge, near the tip of the Osa peninsula, on my recent trip to Costa Rica.

Smoky Jungle Frog (Leptodactlyus pentadactylus), Lapa Rios Ecolodge, Osa, Costa Rica
Smoky Jungle Frog (Leptodactlyus pentadactylus), Lapa Rios Ecolodge, Osa, Costa Rica

It was a large individual (well over 100 mm), and we found it at night in the rainforest. Its first line of defense is that it’s quite hard to see against the variegated mixture of brownish leaves, twigs, and mud of the forest floor. (The red shine of the eye is more noticeable, but fortunately for the frog, natural predators don’t carry flashlights!) When first seen, the frog was sitting up at attention, but when we approached, it pushed itself down flat against the substrate, and as I moved around in front of it for a picture, it really pushed its face into the ground, making itself less noticeable.

Smoky Jungle Frog (Leptodactlyus pentadactylus), Lapa Rios Ecolodge, Osa, Costa Rica
Smoky Jungle Frog (Leptodactlyus pentadactylus), Lapa Rios Ecolodge, Osa, Costa Rica

Since all we wanted was pictures, the frog did not move to its further lines of defense. Had we provoked it, it would have assumed an elevated defense posture, with the back raised, also inflating its body and expelling air to make a hissing sound, similar to what is seen in some toads (Leptodactylus is not a true toad).  I haven’t seen (or at least can’t recall seeing) this in Leptodactylus— the behavior was described in this species by Jaime Villa (1969)– but I have seen it in giant toads.

Elevated defensive posture of Leptodactylus pentadactylus (Villa, 1969: Fig. 6).
Elevated defensive posture of Leptodactylus pentadactylus (Villa, 1969: Fig. 6).

This of course draws a potential predator’s attention to the frog– having hidden, why would it now face up to its foe? This is where the next lines of defense come in. First, the frog is big, and this behavior makes it look even bigger. For some predators, the frog is a mouthful too far. Next, if the frog is touched, it exudes a copious and toxic mucus. This mucus induces a strong allergic response in humans, and presumably others, at least mammals if not all other vertebrates- intense sneezing, watery and itching eyes– the unpleasantness of which I can attest to from personal experience. It is said that people merely in the vicinity, who have not touched the frog, can, through aerial transmission of toxin droplets, get the same symptoms. The mucus can irritate the skin, and cause pain to any scratches or open wounds (which I fortunately did not have when catching the frogs). And the frog will also emit a loud, piercing shriek, which might well startle a predator into releasing its grip. Norm Scott reported that caimans were attracted to this cry, and even speculated that that was its function– to attract caimans to dispatch the frog’s predator– sort of like a bugle call to the cavalry!

More straight forward than the multi-layered defenses of the smoky jungle frog is the defense of poison dart frogs– aposematic, or bright, warning coloration, accompanied by very toxic skin secretions. We encountered two species at Lapa Rios. Phyllobates vittatus, with bright orange stripes, is a member of the genus which contains the three species of the poison dart frog family, Dendrobatidae, that are actually used by Indians to make poison darts.

Poison Dart Frog (Phyllobates vittatus), Lapa Rios Ecolodge, Osa, Costa Rica
Poison Dart Frog (Phyllobates vittatus), Lapa Rios Ecolodge, Osa, Costa Rica

We found three of them, during the day, along the Rio Carbonero. We also found three Dendrobates auratus along the paths at the Lodge itself, wandering about during the day, bold as brass, as is their wont. I’ve seen them quite abundant in other parts of Costa Rica, but we saw only three during 4.5 days at Lapa Rios. For neither species of dart frog was I able to get a very good picture; there’s a better picture of auratus in an earlier post, and, in another earlier post, more details and references on poison dart frogs. BBC Earth has a nice explainer on poison dart frogs, with links to interesting papers

Poison Dart Frog (Dendrobates auratus), Lapa Rios Ecolodge, Osa, Costa Rica
Poison Dart Frog (Dendrobates auratus), Lapa Rios Ecolodge, Osa, Costa Rica

Savage, J.M. 2002. The Amphibians and Reptiles of Costa Rica: A Herpetofauna between Two Continents, between Two Seas. University of Chicago Press, Chicago

Scott, N.J. 1983. in D.H. Janzen, ed. Costa Rican Natural History. University of Chicago Press, Chicago

Villa, J. 1969. Comportamiento defensivo de la “Rana Ternero”, Leptodactylus pentadactylus. Revista de Biología Tropical 15:323-329. pdf

The crab is spotted

June 2, 2016 • 10:30 am

by Greg Mayer

I think most readers probably spotted the crab (which happens to be spotted) rather quickly, in part because it was in the dead center of the photo, which is how I deliberately composed the shot. More than a challenge to readers’ spotting ability, I wanted to illustrate the delightfully exact camouflage that the crab’s spotted pattern achieved on the sand background. If you look at the original post, you’ll note that the crab has stopped near the edge of the crest of the little dry sand hillock it has ascended. Because the crab’s front side is raised higher than its posterior, the light, coming from behind the crab, casts a slight shadow to the front of the crab. This shadow would ordinarily make the crab stand out a bit as a dark spot. But because it is near the crest, its shadow connects with the shadow that the crest casts in the lee of the light, and the crab is less visible. I wondered whether it chose that position deliberately to aid its camouflage, but my observations were insufficiently prolonged for me to determine whether its position was intentional or accidental.

After allowing me to take a number of pictures of it atop the dry sand hillock, the crab moved off to wet sand, and its camouflage is nearly as good.

Crab on wet sand, Playa Pan Dulce, Osa Peninsula, Costa Rica
Crab on wet sand, Playa Pan Dulce, Osa Peninsula, Costa Rica

The lightness of the crab’s underparts makes it a bit more visible on the darker wet sand, but it seems to my eye that the dark spots of the crab have darkened somewhat, enhancing its camouflage. Again, my observations were not extensive enough to resolve the question of color change, but blowing both pictures up, and comparing them side by side, the spots do indeed look darker on wet sand.

More common than the well-camouflaged crabs were hermit crabs (Coenobita sp.), most of which were in the shells of periwinkles, live specimens of which were common lower in the intertidal zone.

Hermit crabs (Coenobita) on Playa Pan Dulce, Osa Peninsula, Costa Rica
Hermit crabs (Coenobita) in periwinkle shells on Playa Pan Dulce, Osa Peninsula, Costa Rica

Finally, in this last picture, I’m not sure if the camouflaged crab, or one of its companions, is present– it’s a real spot the crab problem, since I don’t know if there is one. (The hole, and accompanying excavated hillock of dry sand, I suspect is the work of a larger, unseen crab.)

Playa Pan Dulce, Osa Peninsula, Costa Rica
Playa Pan Dulce, Osa Peninsula, Costa Rica

If anyone can help identify the camouflaged crab, the periwinkle (?Littorina sp.), or the species of the hermit crab, please weigh in.

Spot the crab!

June 1, 2016 • 10:00 am

by Greg Mayer

I’ve just returned from Costa Rica, and hope to regale WEIT readers with further reports, but we’ll begin with a “spot the ____”.  This is from Playa Pan Dulce, near the tip of the Osa Peninusla in southwestern Costa Rica.

Playa Pan Dulce, Osa, Costa Rica, 24 May 2016
Playa Pan Dulce, Osa, Costa Rica, 24 May 2016

Summer reading and summer vacation: anoles

August 23, 2009 • 11:19 am

by Greg Mayer

Anoles are the neatest of all animals, and if you don’t believe me, take it up with my friend here– she’ll set you right!

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An agitated anole

But rather than tangle with her, you can convince yourself by reading my friend and colleague Jonathan Losos’s new book, Lizards in an Evolutionary Tree: Ecology and Adaptive Radiation of Anoles, which was published at the end of July. Anoles are a group of 300 or so species found in the southeastern US, Central and South America, and throughout the West Indies.  Although they may be fairly described as, on average, diurnal, arboreal insectivores, they exhibit a great range in behavior, structure, and ecology: some are aquatic, some terrestrial, Lizards in an Evolutionary Tree, by Jon Losossome engage in carnivory and frugivory, and some live in deserts, and others in rainforests. They are perhaps most remarkable for the evolution of convergent multi-species communities on the islands of the Greater Antilles (Cuba, Hispaniola, Jamaica, Puerto Rico).  This is not just the usual (but still remarkable) convergence in features between, say sabertoothed tigers and sabertoothed marsupial tigers, or dolphins and icthyosaurs. It is convergence in the whole set of species living together in a community.  Thus each of the four Greater Antilles has a large, green anole that lives in the canopy of trees, a medium green anole with short legs that lives in the tree crown and on the trunk, a whitish, very short-legged anole which lives on twigs in the crown, and a medium brown anole with long legs that lives on the trunks and bases of trees; and there are several other inter-island correspondences among species. The corresponding species, however, are not, in general, related to one another; rather, on each island a more or less independent adaptive radiation has produced similar ecological sets of species. There are lots of other neat things about anoles, but I’ll leave you to read about them in Jon’s book, which you need to add to your summer reading lists.

Many anoles are marvelously colored, and the book is beautifully illustrated and well-produced. My pictures here are of anoles from my trip earlier this summer to Estacion Biologica La Suerte, Costa Rica, where I taught a field course in tropical herpetology.

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Anolis capito, Estacion Biologica La Suerte, Costa Rica
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Anolis oxylophus, Estacion Biologica La Suerte, Costa Rica