The chemistry of poison in amphibian skin
Amphibian’s integumentary system is a system providing protection
from injury and abrasion as well as the deterring of the predator. Amphibians
do have 2 types of glands in their skin: the mucus gland and the granular
gland. Mucus gland is needed to secrete mucus in order to keep the amphibian’s
body moist and this helps in the respiration, in the slipping from predators,
and as a technique to prevent the dehydration of the skin. However, granular
glands produce noxious or even toxic secretions. Such secretions benefit their
possessors by making them unpalatable to some predators. These glands often
occur in masses and give a roughened texture to the skin. The warts and the
parotoid glands of toads (Figure 1) and the dorsolateral ridges of ranid frogs
are examples. Secretions of these integumentary glands consist of amines such
as histamine and nor-epinephrine, peptides, and steroidal alkaloids.
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| Figure 1: showing the integuimentary system of amphibian. (a) A skin section of amphibian. (b)Shows the mucus and the poison gland. (c) & (d) Warts and parotid gland |
The Alkaloids
Amphibians are mostly noted to have poison in their skin glands. Poisons are common in nature serving in chemical defense for most of the organisms. Amphibians mostly have in their skin amines, proteins, peptides, steroids, water-soluble and lipid soluble alkaloids. Alkaloids are stored in skin glands and produced during predation. These alkaloids are known to be the vital elements for protection in amphibians by which they play an important role in the chemical defense against predatory. Till now a wide variety of alkaloids have been discovered and classified into different structural classes according to their chemical structure.
Chemical defense mainly results from a series of adaptations to
avoid predation. Animals can synthesize their chemical defense or acquire it
from external sources which can include symbiotic relationships with other
organisms or sequestration from dietary sources. Dietary specialization is very
important in organisms possessing chemical defense from dietary sources.
A great variety of alkaloids have been detected over many years of
research, this variation was linked to the difference in habitats, in climate
and in the lifestyle of every amphibian. Later in this chapter we will discuss
the factors affecting alkaloid diversity.
Source of Alkaloids
Poisons are common in nature they are either
produced de novo or raised from dietary sources. Amphibian skin provided a wide
range of biologically active alkaloids. More than 400 alkaloids of 20 different
structural classes have been detected over 30 years ago. These alkaloids
include the batrachotoxins, histrionictoxin, pumiliotoxin, epibatidine and many
other important alkaloids. Researchers have found that these alkaloids arise
from the dietary sources, mainly the arthropod containing alkaloids.
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| Figure 2: Pumiliotoxin-containing ants |
Pyrrolidines, piperidines, decahydroquinolines,
pyrrolizidines, indolizidines, and quinolizidines appear likely to be derived
from ants example the Formicine ant (Figure 2).
The idea beyond which alkaloids arise from dietary sources came from many experiments revealing that the arthropods in the stomach of some frogs contain same kind of alkaloid as its predator.
Variation of alkaloids:
1- Prey abundance and foraging techniques:
Variation of alkaloids has been detected while
studying the alkaloid extracts of amphibians living in different geographic and
temporal areas. This is mainly due to the difference in the availability of
arthropods. So, alkaloid variation is dependent on prey consumption which will
be affected by foraging strategies as well as spatial and temporal variations.
Foraging allow frog to spend more time outside their shelter, so
they are more susceptible for predation. Thus species following the active
foraging are more colorful, poisoness, and more cryptic that ambushing species.
2- Sex difference:
Alkaloid variation was detected between male and
female (Figure 3). female tend to have larger quantities of
alkaloids than the male. However, this male and female were of the same
body size. Researchers related this difference to the different dietary source
among male and female. Studies show that female consumes more arthropods than
the male. In 1991 Donnelly stated that the difference in behavior leads to the
difference in alkaloid composition between sexes. Also researchers proposed
that the causes of such difference are that the male has a smaller home range
than the female which allow it to have limited amount of arthropods.
3- Body size:
Variation in alkaloid composition is related to body size since
Juvenile frog has lower quantities of alkaloids than the adult frogs. This was
proved by a study done on Juvenile A. Bracata and adult A. Bracata. This study
reveals that Juvenile eat more Acani (very small in size) than the adult frog.
Since adult frog requires larger prey this what causes the diversity in
alkaloids based on different prey sizes. Researchers reveal that ontogenetic
changes in the diet of anurans involve changes in the spectrum of prey sizes
available to predators of different body sizes. Dietary ontogenetic changes
might prevent intra-specific competition between small and adult individuals
(Forty, 2011).
Toxicity of alkaloids
Alkaloids are the major chemical produced by the
amphibian as a way to defend them and to be able to increase their survival
rate. However, not all alkaloids are toxic. The toxicity of alkaloids varies depending on the type of the alkaloid so that the alkaloids in pumiliotoxin family are much toxic than izidine, decahydroquinolines, and histrionicotoxins.
Some alkaloids act as a warning signal so that
they give a bitter taste in order for the predator not to complete eating its
meal. Many alkaloids produced by the amphibian skin are noxious rather than poisoness, but at high concentrations all alkaloids are poisoness. Alkaloids are very specific in inducing their actions. Every type of alkaloid is seen to be active on a certain tissue, molecule, or receptor.
Correlation between Skin Coloration and Toxicity
Aposematism is
one of the great mysteries of evolutionary biology. The evolution of aposematic
coloration is poorly understood, but even less understood is the evolution of
polymorphism in aposematic
signals. Aposematism is a widespread trait in invertebrate taxa, but, in
vertebrates, it is mostly evident in amphibians, reptiles, and fishes. Poison
frogs (Dendrobatidae) are one of the most well known examples of the
co-occurrence of warning coloration and toxicity. Warning signals may inform a predator that the
intended prey is toxic, unpalatable, or generally not worth the predator's
effort. Aposematism is the association of unprofitability with a warning
signal, such as bright or conspicuous coloration. Although aposematism evolves
as a predator deterrent, its chance of establishment in a population is
predicted to be low, because it would lead to an increased probability of
predation. There are many models proposed to explain the origin of aposematism.
Aposematism is correlated with the toxicity and this is mainly seen in the
poison dart frog (Figure 4).
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| Figure 4 showing a poison dart frog |
Conclusion
Life style of the animals forces them to acquire some changes in
their body physiology in order to be able to increase their survival rate.
Amphibian’s poison secretion is as a result of arm race coevolution. The
production of poison by amphibians has a major important role in deterring the
predator, so that the amphibians are capable of protecting themselves against
predator. Toxicity and types of alkaloids does change from place to place
indicating that according to the area inhabited by a certain different types of
predator amphibians could as a result of evolution, evolve their alkaloids.
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