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Meet the Frogs
 The
Dendrobatidae family includes 170 species spread across 7 genera. There
are many color variations and patterns both within and across species.
Most of them have aposematic coloring to warn potential predators and
all of them are vibrant and eye-catching. Sizes range from 12mm (Dendrobates minutus) to 60mm (Dendrobates auratus). They are endemic to Central and South America, however there is a population of Dendrobates auratus in Hawaii. The Hawaiian D. auratus were introduced over sixty years ago from Isla Tabago, an island off the coast of Panama.
Toxic skin secretions are not unique to
Dendrobatidae and can be found in Bufonidae, Myobatrachidae, and
Mantellidae. These frogs are very different in their ecology yet they
all obtain their toxins through their diet. Between the three groups,
there are over 500 lipophilic alkaloids that span across 20 structural
classes.
Among the mucous glands dispersed throughout the
skin are granular glands that secrete toxins. Toxins serve two
purposes: 1. to avoid being eaten by producing a burning sensation,
numb feeling, or horrible taste in the would-be predator's mouth and 2.
to prevent bacteria and fungi from colonizing the frog's permanently
moist skin. (Heselhaus, 1992). The degree of toxicity varies across
species, some causing mild discomfort and some being deadly. For
example, hens can safely eat Dendrobates silverstonei and large spiders are known to attack some frogs.
The most dangerous frog of all is not completely safe from predation; Liophis epinephelus can eat Phyllobates terribilis
without devastating side effects. These snakes appear to have a very
high metabolic rate. I presume that this enables them to eat P. terribilis; rapid degneration of the primary toxin protects the snake and the toxin metabolities are not harmful. | 
Liophis epinephelus
© 2003 Twan Leenders |
Not all frogs in the Dendrobatidae family are toxic. Genera Colostethus and Aromobates are commonly called rocket frogs and skunk frogs, respectively, and do not accumulate toxins. Aromobates is probably the most primitive dendrobatid and produce a volatile, foul-smelling, mercaptan-like compound.
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What's in a name?
The Dendrobatids are commonly called
poison dart frogs, dart poison frogs, or arrow poison frogs. This is
because some species were and are still used to poison the tips of
blow-pipe arrows/darts. Columbian Indians favored the use of Phyllobates aurotaenia, P. bicolor, and P. terribilis.
P. terribilis produces the most deadly toxin of the three and
was used by the southern Emberį Chocó Indians. The darts are wiped
across the backs of live specimens.
P. bicolor and P. aurotaenia are not as potent and were
used by the northern Noanama Chocó Indians. The method of poison
extraction was much more brutal as described by Captain Charles Stuart
Cochrane in 1823-1824:
"Those who use this
poison catch the frogs in the woods, and confine them in a hollow cane,
where they regularly feed them until they want the poison, when they
take one of the unfortunate reptiles, and pass a pointed piece of wood
down his throat, and out one of his legs. This torture makes the poor
frog perspire very much, especially on the back, which becomes covered
with a white froth: this is the most powerful poison that he yields,
and in this they dip or roll the points of their arrows, which will
preserve their destructive power for a year. Afterwards, below this
white substance, appears a yellow oil, which is carefully scraped off,
and retains its deadly influence for four to six months, according to
the goodness (as they say) of the frog. By this means, from one frog
sufficient poison is obtained for about fifty arrows.... A tiger when
hit, runs ten or a dozen yards, staggers, becomes sick, and dies in
four or five minutes. A bird is killed as with a bullet; and the arrow
and wounded part of the flesh being cut out, the reminder is eaten
without danger."
P. aurotaenia
Kokoe Poison Frog | 
P. terribilis
Golden Posion Frog | 
P. bicolor
Black-Legged Poison Frog |
Today Dendrobates are a popular pet among hobbyists. On a varied diet of wingless fruit flies (Drosophila),
ants, moth larvae, and pinhead crickets, captive-raised specimens are
free of the famed toxins but the name "poison dart frogs" is still
widely used.
^^
You are what you eat!
Dendrobatids obtain their toxins from
myrmicine ants, coccinellid beetles, millipedes, termites, free-living
mites, mosquitoes, and probably from currently unknown prey items. (The
termites under suspicion are of the genus Kalotermes and Neotermes connexis.) These anurans are called "ant-specialists" and use ants as the base of their wild diet.
Alkaloid Classes of Prey Item Origin
Myrmicine Ants
- pumiliotoxins
- 2,5-disubstituted pyrrolidines
- 2,6-disubstituted piperidines
- 3,5-disubstituted pyrrolizidines
- 3,5-disubstituted indolizidines
- 4,6-disubstituted quinolizidines
- 2,5-disubstituted decahydroquinolines
- 3,5-disubstituted lehmizidines*
- histrionicotoxins*
- gephyrotoxins*
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Beetles
- tricyclic coccinelline
- batrachotoxin
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| Siphonotid Millipedes
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* alkaloids are strongly suspected to be of ant
origin. Their structures are analogous to other ant alkaloids
(unbranched carbon skeleton and terminal acetylene)
There are other alkaloids that the frogs
possess but are not found in prey items. It is possible that the prey
providing those toxins have not yet been discovered yet or that the
frogs are able to obtain toxin precursors from the prey and convert the
compounds by themselves. Another paradox is the discovery of certain
arthropod alkaloids in leaf litter which did not contain the suspected
arthropod. Daly et. al. (2002) speculated that the alkaloids are formed
by a symbiotic organism or possibly a microscopic mite that produces
the alkaloids in question which can use various arthropod hosts rather
than different groups of arthropods developing different biosynthetic
alkaloid-yielding enzymes.
Toxin production depends heavily on prey items,
thus the quantity and variety of dendrobatid toxins depends on a few
factors (Daly, 2000). The first factor is availability and
sustainability of alkaloid-containing prey items. This depends on the
season and local vegetation. The second factor is prey selection by
frog species, which relies on their habitat location. Many species are
isolated in small centralized areas. Each area may have different
populations of prey species available. The last factor is selectivity
and degree of expression of alkaloid sequestering system by each frog
species. Dendrobatid frogs have the ability to selectively accumulate
dietary alkaloids.
The last point is exemplified by the Monomorium pharaonis,
also called the Pharaoh ant or sugar ant. These ants offer momomorine-I
(a 3,5-dialkylindolizidine) and
trans-2-heptyl-5-(5-hexenyl)pyrrolidine. However, when D. aurus
are fed a diet of these ants, only monomorine-I was accumulated and
there was no trace of pyrrolidine. The monomorine-I in frog secretions
differed in sterochemistry from its isomer found in the ants.
Daly et. al. (2002) stated that, " The failure
to detect some of the major/minor frog-skin alkaloids in potential
leaf-litter prey items cold result from many factors including seasonal
variations in the availability of such arthropods and the occurrence of
random hatches or migrations (i.e., some of the frog-skin alkaloids may
have been sequestered and retained from arthropods eaten months or even
years before [studies])."
The
alkaloids don't seem to be stored anywhere in the body as the muscles
and other internal organs are free of any toxin. Surprisingly, frogs
maintain high levels of alkaloids after one to six years when their
natural diet was replaced by a diet of fruit flies. Retention of
alkaloids is enabled by eating shed skin, so basically the frogs are
'recycling' the toxins. The shed skin and epidermal mucous is also a
good source of protein. | 
Dendrobates histrionicus (Harlequin Poison Frog) eating its shed skin. |
It recently been discovered that the origin of
pumiliotoxins are formicine ants (Saporito et. al. 2004). Pumiliotoxins
are found in all species of frogs that are protected by lipophilic
alkaloids and are obtained by consumption of Brachymyrmex and Paratrechina ants. The toxin-bearing ants are often associated with Heliconia plants. Coccinellid beetles provide coccinellines and Choresine beetles (Melyridae) are strongly suspected to be the source of batrachotxin (Dumbacher et.al 2004). A millipede called Rhinotus purpureus (Siphonotidae) provide spiropyrrolizidine oxime 236.
Brachymyrmex longicornis | 
Paratrechina steinheili |
^^
About the Toxins
The toxins are lipophilic alkaloids. The
structures are based upon a piperidine ring, a six membered ring
composed of five carbons and one nitrogen.
 | Pumiliotoxins are found in Dendrobates and Phyllobates.
They are an alkaloid family that contain over 80 members and can be
subdivided into pumiliotoxin A, B, and C. Pumiliotoxin A and B are more
potent than C. Small doses cause hyperactivity and difficulties in
locomotion and hypersensitivity to stimuli. Injections of 100mg of the
former two into a mouse causes locomotion difficulties, partial
paralysis of the hind limbs, salivation, extensor movements, clonic
convulsions, and death in fewer than 10mins. Injections of 20mg of
pumiliotoxin B caused death in less than 20 mins (Patocka et. al.
1999). Pumiliotoxin is more toxic than decahydroquinolines,
pyrrolizidines, indolizidines, quinolizidines, lehmizidines, and
histrionicotoxins. It works on voltage-dependent sodium channels in the
heart or other muscle and forces the release of intracellular calcium
ion stores. The muscle contracts for a prolonged time because
pumiliotoxin also hinders re-accumulation of calcium ions.
Pumiliotoxins are 100 to 1000 times less potent than batrachotoxins. |
Batrachotoxins are found only in Phyllobates. A wild-caught, adult specimen of P. terribilis may contain 700 to 1900µg (usually 1100µg) of batrachotoxin and homobatrachotoxin. LD50
of batrachotoxin in mice (subcutaneous injection) is 0.2µg/kg and
minimal lethal doses range from 0.01 to 0.02 µg/kg. Homobatrachotoxin
is lightly less toxic (0.04 to 0.06 µg/kg). They work on
voltage-dependent sodium channels and forces the channels to remain
open. The cell membrane becomes depolarized after the high influx of
sodium ions and render the heart/muscle tissue useless. The victim may
experience strong muscle contractions, violent convulsions, salivation,
and labored breathing. Heart arrhythmia, fibrillation, and ultimately,
failure may result at high doses. One gram of batrachotoxin can kill
1000 full grown humans. Because it is so potent, one wonders how the
South American natives can use the skin secretions of the most toxic
frogs to kill their own food and still be alive and faring well after
dinner. The answer is that the oral potency of batrachotoxin is much,
much lower. The tiny, ingested amount of toxin may be denatured during
cooking and the metabolites are not poisonous. |  |
 | Histrionicotoxins
prevents action potentials jumping from nerves to muscle cells by
blocking acetylcholine and end-plate receptors. They also work on
potassium channels and force them to remain in an open or closed state.
As a result, action potentials are prolonged and lengthen the time of
muscle contractions. |  |
Epibatidine has a similar structure to nicotine.
At high doses, it depolarizes ganglionic nicotinic receptors. It is
naturally found with histrionicotoxins promoting their activity.
Spiropyrrolizidine oxime works by strongly blocking nicotinic receptors, especially at ganglionic subtypes.
^^
Toxins are Good for Humans, Too!
Toxins, though often associated with
illness, deformities, and death, are not always harmful. Like any other
chemical, its good/bad value comes from what we know about it and how
we can use it. This is true for dendrobatid toxins as well.
Epidobates tricolor
Phantasmal Poison Frog |
In 1993, epibatidine was isolated in the Phantasmal poison dart frog (Epidobates tricolor).
Epibatidine is only found in Epipedobates and is a relatively minor
toxin. It may be derived from anabasine, a Solanaceae ant alkaloid. It
is a very potent painkiller; on weight-for-weight basis it is 200x
stronger than morphine and is nonsedating and non addictive. Since it
affects nicotine receptors instead of opiate receptors, it is also not
addicting. It is also nonsedating which makes a very valuable candidate
for synthetic reproduction in the pharmaceutical market. The FDA
approved testing of synthetic versions in 1999.
Batrachotoxin is employed in neurology research, aiding in the study of local anesthetic and anticonvulsant interactions.
Pumiliotoxins are cardiotonics; they strengthen
cardiac muscle contractions. This results in more forceful and
effective heartbeats. In other words, the heart can pump more blood
with fewer beats. This can become a potent medicine for patients with
heart problems such as congestive heart failure and arrhythmia. They
can also serve as pharmacological probes in stress tests. |
The minor alkaloids (i.e. decahydroquinolines,
gephyrotxins, izidines, and pyrrolidines) have potential to be
effective local anesthetics because they are non-competitive blockers
of nicotinic channels.
^^
Bibliography
- Daly JW, Garraffo M, Jain P, Spande TF, Snelling RR, Jaramillo C,
Rand S. "Arthropod-frog connection: decahydroquinone and pyrrolizidine
alkaloids common to microsympatric myrmicine ants and dendrobatid
frogs." Journal of Chemical Ecology 26:1 (2000) pg. 73-85
- Daly JW, Secunda SI, Garraffo HM, Spande TF, Wisnieski A, Cover Jr.
JF. "An uptake system for dietary alkaloids in poison frogs
(Dendrobatidae)." Toxicon 32:6 (1994) pg.657-663
- Daly JW, Secunda SI, Garraffo HM, Spande TF, Wisnieski A, Nishihira
C, Cover Jr. JF. "Variability in alkaloid profiles in neotropical
poison frogs (Drendrobatidae): genetic versus environmental
determinants." Toxicon 30:8 (1992) pg.887-897
- Dumbacher JP, Wako A, Derrickson SR, Samuelson A, Spande TF, Daly
JW. "Melyrid beetles (Choresine): a putative source for the
batrachotoxin alkaloids found in poison-dart frogs and toxic passerine
birds." Proc Natl Acad Sci U S A. 101:45 (2004) pg. 15857-60
- Grenard, Steve. Amphibians: Their Care and Keeping. New York: Howell Book House, 1999.
- Heselhaus, Ralf. Poison-Arrow Frogs: Their Natural History and Care in Captivity. London: Blandford, 1992.
- Myers CW, Daly JW. "Dart-Poison Frogs." Scientific American 248:2 (1983) pg.120-133.
- Patocka J, Wulff KS, Palomeque MVM. "Dart Poison Frogs and Their Toxins." The ASA Newsletter 5:74 (1999) Applied Science and Analysis, Inc. <http://www.asanltr.com/ASANews-99/995frogs.htm>
- Pough FH, Andrews RM, Cadle JE, Crump ML, Savitzky AH, Wells KD. Herpetology Third Ed. New Jersey: Pearson Prentice Hall, 2004.
- Saporito RA, Garraffo HM, Donnelly MA, Edwards AL, Longino JT.
"Formicine ants: an arthropod source for the pumiliotoxin alkaloids of
dedrobatid poison frogs." Proc Natl Acad Sci U S A. 101:21 (2004) pg. 8045-50
- Stewart Sean. The True Poison-Dart Frog: The Golden Poison Frog
Phyllobates terribilis. Dart Den - Poison Dart Frog Resource and Forums. Retrieved 2005 March 12, from http://www.dartden.com/cs_terribilis.php
- Walls Jerry G. Jewels of the Rainforset - Poison Frogs of the Family Dendrobatidae. New Jersey: T.F.H. Publications, 1994.
Photo Credits
- Map: Pough et. al. Herpetology, 2004
- Frogs: Walls JG. Jewels of the Rainforset - Poison Frogs of the Family Dendrobatidae, 1994
- Ants: Saporito et. al. "Formicine ants: an arthropod source for the pumiliotoxin alkaloids of dendrobatid poison frogs." Proc Natl Acad Sci U S A, 2004
^^
Elaine Kung
Animal Science 625: Nutritional Toxicology, Spring 2005
Cornell University, Department of Animal Science
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