By
the A.M. Costa Rica wire services
Brazilian scientists
have discovered that the strong odor released by some
amphibian species is produced by bacteria and that
attracting a mate is one of its purposes. The bacteria
in question are a noteworthy example of symbiosis as
they assist in the animal's mating process. A paper
recounting the discovery of this role of microorganisms
isolated from the skin of frogs has been published in
the journal Proceedings of the National Academy of
Sciences (PNAS).
"Frogs emit a pungent odor. Sometimes a particular
species can be recognized by its scent, but until now,
the function of this odor was unknown. It was typically
assumed to be an aposematic smell, meaning a chemical
warning sign that served to repel predators, as in the
case of skunks [Mephitis mephitis] among mammals, for
example," said Célio Haddad, a professor at São Paulo
State University's Rio Claro Bioscience Institute
(IBRC-UNESP) in Brazil and a coauthor of the article.
According to Haddad, who is also affiliated with the
university's Aquaculture Center (CAUNESP) in
Jaboticabal, this hypothesis was considered plausible
because many amphibian species, especially when
poisonous, are brightly colored, and this serves as a
visual alert to frighten predators. "We thought odor
might play a similar role among anurans [frogs and
toads]," he said.
The new study resulted from the postdoctoral research of
Argentinean biologist Andrés Eduardo Brunetti,
supervised by Professor Norberto Peporine Lopes.
Conducted at the University of São Paulo's Ribeirão
Preto School of Pharmaceutical Sciences (FCFRP-USP), the
research was supported by FAPESP.
"The importance and originality of Brunetti's research
is that for the first time it shows a pronounced
difference in the odors emitted by frogs of opposite
sexes," Haddad said. "No other studies of anurans have
ever described this type of behavior. The results
suggest that the odor serves to permit mutual
recognition between males and females of the same
species for mating purposes."
The research was also supported by the FAPESP Research
Program on Biodiversity Characterization, Conservation,
Restoration and Sustainable Use (BIOTA-FAPESP) and by
the University of São Paulo (USP), the National Council
for Scientific and Technological Development (CNPq) and
Brazil's Coordination for the Improvement of Higher
Education Personnel (CAPES).
"In anurans, you often see different species sharing a
lake or marsh. In such places, there are 30 male frogs
for every female of the same species on average. The
question is how the females recognize males of their own
species among a multitude of males belonging to several
species while they're all vocalizing at the same time,"
Brunetti said.
"It's well-known that the function of the call of anuran
males is to attract females and that every species has a
characteristic song. Our findings suggest that odor
appears to play a similar role, serving as an olfactory
signal that enables females to recognize males of their
own species."
Biologists were also unaware of a difference in the
scents of male and female frogs. Brunetti discovered
this difference during his research, whose primary goal
was to understand the chemical composition of the
volatile components emitted by the skin of various frog
species.
His working hypothesis suggested that smell was a
chemical warning sign that served to repel predators. To
verify the hypothesis, Brunetti conducted field surveys
at several sites in São Paulo state and Rio de Janeiro
state, collecting specimens of the tree frog Boana
prasina.
"It's very hard to collect females in the wild.
Initially, we managed to collect only males. When we
noticed what appeared to be a sexual difference in their
odors, I went into the field again with the specific aim
of capturing females for comparison," he said.
"During my doctoral research at the Argentinian Natural
Science Museum in Buenos Aires, while investigating the
volatile compounds in two other frog species, I
discovered that the secretions were made up of a blend
of 35 to 42 compounds in nine different chemical
classes. We then realized that some of the compounds had
the specific signature of compounds produced by
bacteria."
Brunetti came to Brazil to investigate whether the
selected tree frogs had skin bacteria that produced the
characteristic odor of each species, and if so, which
compounds they produced. His laboratory research
proceeded on two fronts: analysis of the volatile
compounds released by the skin of these frogs and
identification of the bacteria on their skin.
Brunetti and colleagues used gas chromatography and mass
spectrometry to analyze the diversity of the volatile
components secreted by the skin of B. prasina. They
found that adult males and females secrete a blend of
60-80 compounds, including alcohols, aldehydes, alkenes,
ethers, ketones, methoxypyrazines, terpenes and
thioethers.
|
A.M.
Costa Rica wire services photo
This
hypothesis was considered plausible because
many amphibian species, especially when
poisonous, are brightly colored, and this
serves as a visual alert to frighten
predators
The compounds were exactly the same in
both males and females, but the
researchers were surprised to find a
pronounced sexual difference in the levels
of methoxypyrazines, terpenes, and
thioethers.
"These
three components were responsible for the
difference between males and females. Thioethers
and methoxypyrazines are typically produced by
microorganisms," Brunetti said.
They decided to determine whether microorganisms
were the source of these compounds in B.
prasina. To do so, they isolated, cultivated and
identified bacteria associated with the skin of
these frogs and analyzed their volatile
components.
No fewer than 128 different components were
detected. Analysis of each component revealed
that four methoxypyrazines present in males and
females were produced by a single bacterium of
the genus Pseudomonas.
In B. prasina, Brunetti discovered,
methoxypyrazines were much more abundant in
females than in males. Two of the four types of
methoxypyrazines were measured at higher levels
in females, while two were found at higher
levels in males.
"The interesting thing about Pseudomonas sp. is
that these bacteria live on the skin of males
and females, where they metabolize the same
volatile compounds but at different levels of
concentration according to the sex of the host,"
Brunetti said.
The levels of methoxypyrazine measured in these
frogs, he added, suggest the existence of a
complex mechanism of metabolic interactions that
creates a different environment on the skin of
males and females, favoring the synthesis of
characteristic methoxypyrazines in each sex.
"These frogs and bacteria have a symbiotic
relationship. In exchange for the service
provided by the bacteria, entailing sexual
differentiation by scent, the frogs provide an
environment -- their own skin -- on which the
bacteria can proliferate," he explained.
The function of this sexual difference in
methoxypyrazine levels is unknown. "However, we
assume that the difference in scent helps male
frogs of this species recognize females of the
same species in places inhabited by other frog
species," Brunetti said.
"We know that many anurans use visual
communication [bright skin colors] to repel
predators as well as acoustic communication
[vocalization] to attract female mates. Perhaps
B. prasina uses a form of olfactory
communication for the same purpose."
Brunetti will attempt to confirm this hypothesis
in future research. If correct, it will have
major repercussions. "Only one anuran, in
Madagascar, is currently known to communicate by
odor. Among amphibians, salamanders, which are
distant relatives of anurans, are known to use
this form of communication," Haddad said.
"If B. prasina uses scent as a form of
communication, it may well be the case that
other species also use olfactory communication,
given that each species has a characteristic
odor. Brunetti's discovery, if confirmed, opens
up a new field of investigation in herpetology,
which will now focus on olfactory communication
among anurans, rather than just visual and
acoustic communication."
---------
Editor's note: Original
study can be reached at Sao Pablo Research
Fundation at http://www.fapesp.br/en/
|
