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Deciphering The Communicative Rosetta Stone
By
MURRAY
S. BLUM
system, especially since it emphasizes our ignorance of
the roles of natural products in plant and animal biology.
From an evolutionary standpoint the consistent production of specific compounds in a plant indicates that they
must be of selective advantage to those species which produce them. Schmalhasen (1949) emphasized this point
when he discussed the retention of characters by animals.
Furthermore, since there are compelling reasons for regarding the presence of natural products in plants as of
great adaptive significance (Fraenkel 1959), no cogent
grounds remain for designating these compounds with a
secondary rank. To continue to do so is as biologically
unsound as it is illogical.
Although it has been known for years that plants and
animals are remarkable natural product chemists, it has
only been in the last few decades that biologists have developed some awareness of the significance of this biosynthetic versatility. Among animals, a dazzling array of
organic compounds function as external stimuli which
regulate a multitude of intraspecific behavioral responses.
These have been heretofore described mostly in anecdotal
terms. Among the invertebrates, these chemical agents,
the pheromones, have been documented for so many
groups that it may not be premature to speculate that the
presence of these compounds constitutes an ubiquitous
characteristic of these animals. Furthermore, even at this
early stage in our comprehension of the modus operandi
of these chemical stimuli, it is obvious that we now have
a particularly powerful additional tool with which to
study the evolution of animal behavior.
Recently, plant and animal natural products have been
divided into three categories-pheromones,
allomones, and
questionably, kairomones-based
on the adaptive value
that
these
compounds
may
possess
for
individuals
in-
volved in either intra- or interspecific interactions (Brown
et a!. 1970). Pheromones-intraspecific
chemical stimuli
-are considered adaptive for an emitter individual as a
result of favorable behavioral or developmental responses
produced in a receiver individual of the same species.
Allomones also produce reactions in the perceiver individuals that are adaptively favorable to the emitters, but
these compounds, in contrast to pheromones, function in
interspecific contexts. Allomones, often involving both
plants and animals, play roles in both mutualistic and
agonistic relationships. The compounds that attract pollinators and secd disseminators to flowers and fruits are
allomones, as arc the gustatory stimulants that may induce these same individuals to feed on floral nectar or the
fruity pulp. On the other hand, antagonistic allomones
are identified either with the defensive secretions of animals or the natural products of plants which render the
tissues unpalatable to a potentially predatory species.
Characteristically, the allomonal functions of a wide range
of compounds exploit the environment in terms eminently
favorable to the producer organism.
MURRAY
CHEMICAL
SIGNALS
S.
Brown et a1. (1970) have labeled a third class of compounds as kairomones or transpecific chemical messengers,
the adaptive value of which falls on the perceiver rather
than the emitter. Kairomones are considered to be nonadaptive or maladaptive to the producing individuals.
These compounds include the phagostimulants that act as
feeding triggers for specific herbivores and attractants
which are responsible for attracting predators and parasites. However, it does not follow that such natural products represent a class of chemical signals distinct from
allomones and pheromones. Rather, the "so called" kairomones appear to be pheromones and ailomones which have
evolutionarily backfired.
BLUM
AND TERMINOLOGY
The inexactitude of the terminology applied to natural
products primarily reflects our lack of knowledge of the
functions of these compounds. For example, many compounds produced by plants are sometimes referred to as
secondary plant substances or metabolites, in order to
contrast them with the omnipresent lipids, carbohydrates,
and proteins. However, this description assigns an unnatural priority to these compounds, and as a consequence,
there is no justification for utilizing such a classificatory
The phagostimulants which induc£' a limited number of
herbivores to feed on a plant appear to have been originally evolved to deter the vast multitude of herbivorous
species in an environment from feeding on the same plant
(Fraenkel 1959). These natural products certainly arc
allomones which are highly favorable to the species. To
consider them nonadaptive or maladaptive because a few
species feed in their presence ignores their critical role in
insulting the plant from indiscriminate predation by herbivores. Similarly, the same volatile natural products of
plants or animals which may betray them to predators or
1 The
1973 Founders'
Memorial Award Lecture.
Presented
at the meeting of the Entomological
Society at Dallas,
TX,
Nov. 26, 1973.
30
Table I.-The
insect species.
utilizatiun
of arthropod
defensive compounds as pheromones by non-social, presocial, and eusocial
Source Taxa
Compound
Family
Order
..
-
.-
Genus
Formic acid
Lepidoptera
Coleoptera
Hymenuptera
!\'otodontidae
Carabidae
Furmicidae
Dicrallura
Dicaclus
Formica
II-Pentanoic
acid
Hemiptera
Coleoptera
Coleoptera
Coreidae
Carabidae
Elateridae
2-IIeptanone
Blattaria
IIymenoptera
Hymenoptera
Blattidae
Apidae
F ormicidae
ll-Dodeceny I
aet'tate
Lepidoptera
Lepidoptera
II-L:ndecane
Coleoptera
Hymenoptera
Cossidae
Noctuidae
Staphylinidae
Formicidae
PtcYllistria
PromccodcYlls
LimOllius
Platy::ostcria
Ap';s
Iridomyrmcx
Zcu::cra
Diparopsis
Drusilla
Campollolus
------------
Authority
Function
-- ---.------
._---
parasites are of great selecti"e value to their producers as
phl'ronHlIles, playing key roles as species recognition
agl'nts. sex attractants. aggregative stimuli, etc. Therefon" unless it can be demonstrated that compounds which
attract pn'dators or parasites to plants and animals are
not pheromonl's or allomones, it seems wise to simply
n'gard thcm as cvolutionary boomerangs.
Defensive
Defensive
Defensive &
alarm pheromone
Defensive
Defensive
Sex attractant
Defensive
Alarm pheromone
Alarm pheromone
Defensive
Sex attractant
Defensive
Orientationaggregation
pheromone
Poulton (1888)
McCullough (1967)
Maschwitz (1964)
Baker & Kemba\1 (1967)
Moore and Wallbank (1968)
Jacobson et al. (1968)
Wallbank & Waterhouse (1970)
Shearer & Boch (1965)
Blum et al. (1963)
Marchesini et al. (1969)
Nesbitt et at. (1973)
Brand et at. (1973)
Ayre and Blum (1971)
PHEROMONAL
ORIGINS
While the presence of pheromones in plants and animals has been well established, the origins of these communicative stimuli have remained relatively obscure.
Pheromones generally are low molecular-weight
compounds possessing a fairly high vapor pressure and highly
stimulatory olfactants. Significantly, defensive compounds
have the same properties, and a plethora of evidence suggests that these antagonistic allomones may have often
been direct pheromonal antecedents.
\Vhik the utilization of a pheromone by members of
thc same species clearly constitutes communication, some
caution should be exercised in extending this definition
to intl'rspl'cific. interactinns based on allomonal stimuli.
To equatc communicatively the defensive discharge that
a pcntatomid directs at a predatory bird with the sex attractant which brings a male moth to its ca\1ing female
"C'n<krs the \\'hole concept of communication so diffuse as
to be biologically meaningless.
As Burghardt
(1970)
!ll'rsuasi\'c1y emphasizes, to regard the defensive spray of
a skunk as chemical communication would then extend
the concept of chemical communication to the use of
l'ither kar gas or 11ace. Burghardt concludes that communication must possess the element of intent. Intent
implies that it is to the real or perceived ad\'antage of the
signaler or the signaler's group to transmit its message
to anothcr individuaL Thus, information transfer would
ha\'c becomc adaptively favorable to the transmitter or
his group through natural selection.
The widespread distribution of the same defensive compound in the Arthropoda demonstrates that the capacity
to biosynthesize it has arisen de novo frequently in Unrelated groups of invertebrates.
Thus, many of the enzymes required for the biogenesis of specific defensive
compounds must be present in a variety of arthropod
lines, and the biosynthesis of these a\1omones may have
required relatively few mutations in order to complete
the required enzymatic pathway. The subsequent use of
defensive compounds as pheromones by nonsocial, presocial or eusocial species would become possible as a result of selection for emitting them, and being preprogrammed to respond to them adaptively, in specific behavioral contexts. Furthermore,
since these compounds
now possess both a pheromonal and defensive function,
their production by a species is exceptionally adaptive.
Consequently, the interception of a signal by an individual of another species would not constitute true communication if the subsequent behavior of the receivers did
not prove favorable to individuals of the emitter species.
Thus, since the attraction of predatory clerid beetles to
thc aggregation pheromones produced by male Ips COllil/sl/s
(Wood et al. 1968) is poorly adaptive for the bark
bectles, it cannot be classified as communication. On the
other hand, interspecific communication can occur in certain contexts, as for example when a threatened tenebrionid beetle Jlostures on its head prior to discharging the
quinonoidal mixture in its pygidial glands (Eisner 1970).
This stereotyped posturing behavior constitutes a highly
distinctive signal for a predator, especially since it invariably precedes the accurate discharge of a highly irritating secretion.
Table 1 lists some of the defensive compounds of some
nonsocial species which are also utilized as pheromones
by pre-social or eusocial species. These are representati ve
of a multitude of defensive compounds identical to the
pheromones produced by a wide variety of arthropod species. It also illustrates the diversity in function that these
compounds possess when they are utilized as communicative stimuli. Formic acid, which is produced by lepidopterous, coleopterous, and hymenopterous species, is of
special interest because it accumulates in very high concentrations in the glandular reservoirs of these species.
However, the enzymes required to synthesize lO-formyltetrahydrofolate, the immediate precursor of formic acid
(Blakley 1969), are present in a wide variety of insects
(Grzelakowska-Sztabert
and Zielinska 19(7) which do
not store this compound. It seems appropriate to ask why
31
Initially, plant natural products may have constituted
nothing more than general membrane irritants for animal
chemoreceptors.
This seminal condition was probahly
necessary for the ultimate evolution of highly specific
receptors. Subsequent selection for chemoreceptors capable of being triggered by a narrower spectrum of compounds would result in olfactory or gustatory generalists
which could perceive chemical cues emanating from food
sources. Further selection for highly specific receptors
which were triggered only by specific compounds would
result in the evolution of olfactory or gustatory specialists
-the chemoreceptors that are only fired by the attractants or phagostimulants utilized by many insects to identify their specific food plants. Obviously the evolution of
a hierarchy of behavioral responses, mediated through the
central nervous system as a result of perceiving these
plant-derived stimuli, would occur simultaneously with
selection for increasingly spccialized chemoreceptors.
these species do not accumulate large amounts of formic
acid as do notodontid larvae (Table 1). The highly selective distribution of formic acid-rich defensive glands in
insects must reflect evolutionary adaptations that have
made both the large-scale synthesis and accumulation of
this compound possible. Significantly, this synthetic emphasis may have only required a single mutation in order
to produce an enzyme which forms formic acid from
10-formyltetrahydrofolate,
an end result which might be
achieved through 10-formyl THF deacylase (Osborn et al.
1957). Whatever the ultimate morphological and enzymatic raison d'etre of formic acid in the defensive exudates of insects, a wide variety of species that do llOt
utilize this compound as a defensive product already
possess most of the biosynthetic machinery to do so.
It has also been suggested that some pheromones may
have been derived from steroidal hormones.
Haldane
( 1954) has adumbrated the 'possible evolution of vertebrate pheromones from sex hormones and more recently,
Kittredge and Takahashi (1972) have discussed the significance of a crustacean molting hormone, crustecdysone,
functioning as a sex attractant.
Kittredge et al. (1971)
have demonstrated that traces of this steroid, which release typical pre-copulatory behavior in males, leak from
the female crab only during the ultimate molt. Since
males already possess chemoreceptors for the hormone,
selection for both externalization of the receptors and a
preprogrammed responsiveness (attraction) to crustecdysone would guarantee the functionality of this pheromonal
system (Kittredge and Takahashi 1972).
CO-E\'OLUTION
Chemical communication systems predicatcd on using
defensive compounds which functioncd as pheromones may
have evoh"ed in a manner essentially identical to that suggested for the evolution of spccializcd chemoreceptors for
plant natural products. After the ability to biosynthesize
a defensive compound has evolved by mutation, selection
for more specific. but still broadly tuncd chemoreccptors
(generalists),
would occur. At this juncture, relatively
small numbers of these receptors would be present, resulting in a rather limited signal and consequently, a lo\\'
sensitivity to the stimulus. The concurrent evolution in
the species of a responsivencss to this chemical stimulus
would result in selection for new behavioral traits adaptively favorable for the species. Subsequent selection for
very specific chemoreceptors (specialists) in large numbers would culminate in a maximally sensitive receiver
which is able to discriminate between the pheromone and
even closely related compounds.
OF THE SIGNAL AND THE RECEPTOR SYSTEM
Kittredge et al. (1974) have emphasized the improbability of an animal co-evolving the ability to both synthesize a compound and exhibit a stereotyped reaction
after perceiving it with great olfactory specificity. Based
on the utilization of crustecdysone as a sex pheromone,
they persuasively suggest that chemical communication
evolved in a stepwise manner. This conclusion is evolutionarily sound, but the specific example of crustecdysone
cannot be compared with the cases of most pheromones
whose geneses and chemoreceptor sensitivities do not appear to have been derived from any known endocrine
products. Perhaps the main driving force for the evolution of a variety of pheromonal chemoreceptors was supplied by the natural products of plants.
ADAPTIVE CHARACTERISTICS
OF THE RECEPTOR SYSTE~l
The functionality of the pheromonal L'mwelt of arthropods reflects their ability to process rapidly the information encoded in highly psecific molecules which impinge
on their batteries of specializcd chcmoreceptors
. The
remarkable adaptiveness of such a communicative system
is especially evident when the characteristics of sex pheromone perception by male silkworms are analyzed in
some detail. A recent discussion (Schneider 1970) of the
biophysical events that occur when a threshold concentration of the sex pheromone, bombykol, is detected by
the olfactory receptors on the male antennae illuminates
some of the adaptive qualities of this chemosensory system. Each antenna contains 25,000 specialist receptors
and these cells provide an intrinsic input (background)
of 1,600 impulses/second to the hrain. At a behavioral
threshold level of 3 X 10-6 p.g of bombykol, 120 additional
impulses/second/
antenna are theoretically required to
overcome this background signal. However, it has been
determined that 300 cells hit by bombykol molecules actually generate 200 impulses at a behavioral concentration
(Schneider 1970). Extraordinarily,
it can be calculated
that at a signal to noise ratio of 0.075 (120 bombykol
impulses/1,600 background impulses/antenna),
a behavioral threshold is reached, whereas present day electronic
detectors used in analytical instrumentation usually optimize on a signal to noise ratio of 2: 1 to minimize detection time while obtaining statistically valid data.
The frequent congruity of plant and animal natural
products is striking, as is the fact that the same classes
of low molecular weight compounds are often emphasized
by both groups of organisms. Thus, mono terpene aldehydes and hydrocarbons, quinones, Ct, ,B-unsaturated aldehydes, and aliphatic acids have a widespread distribution
among both plants and animals. In plants, these compounds generally function as feeding deterrents, while in
specialized instances, they act as gustatory or olfactory
stimulants. In animals these same compounds are generally utilized as weapons of defense, but some species
employ them as key elements in their communicative systems. If the chemoreceptors of animals have been constantly challenged by the natural products of plants, then
animals have had an extended period of time in which to
have evolved specialized chemoreceptors capable of reacting specifically to these compounds while simultaneously
develOping a preprogrammed
responsiveness.
Significantly, if animals subsequently produce the same or similar compounds, the archetypal chemosensory elements for
their discrimination and transduction may already be
present.
Thus, at a behavioral threshold, the bombykol receptor
system of the male silkworm possesses adaptive characteristics which do not appear to have a parallel in any
32
pods, notwithstanding their relatively weak organoleptic
properties. Indeed, the independent evolution of n-undecane as a chemical deterrent by species of phalangids and
insects in several orders, provides strong evidence for
regarding hydrocarbons as exocrine products of high
adaptiveness. It seems evident that the members of this
class of compounds possess some physiological properties
that justify their widespread occurrence in defensive
exudates.
detector system which has been similarly analyzed. The
specialist cells which detect the sex pheromone molecules
exhibit greater elllciency at very low concentrations of
homhykol; the male antennae must constitute an extraordinarily selective filter. At the very low signal to noise
ratio of 0,075. the male moth is nevertheless able to behaviorally respond, notwithstanding
the fact that few
bombykol impulses are present at a negligible confidence
len'!. However, the remarkable adaptive favorabi1ity of
this detector system becomes particularly evident when
viell'ed as a function of the vi rtually instantaneous reaction time of the male in the presence of so few integratahle impulses. Indeed, a male moth will exhibit a sexual
response 100 milliseconds after detecting as little as 20
bomhykol hits. The exhibition of such a rapid behavioral
response in the presence of such a negligible sensory
input must be regarded as a major evolutionary developnwnt. Consequently, if the pheromonal receptor system
of the male silkll'orm is typical of that of other arthro]lOd spl'cies, it indicates that the relatively high resting
signal frequencies (background)
associated with the inordinately large number of evolved specialist cells do not
l'lll1stitute a nl'urophysiological liability.
Blum and Brand (1972) have suggested that hydrocarbons may function as antagonistic allomones by temporarily altering the generator characteristics of the olfactory chemoreceptors present on a predator's antennae,
In essence, this defensive modus operandi of hydrocarbons
could permit the molested arthropod to "hide" from its
aggressor, since the disruption of the normal olfactory
processes of the latter could effectively neutralize its
predatory behavior. Hydrocarbons may thus constitute
some of the cryptic odors of Haldane (1955), and m••y
represent a large number of defensive compounds that
"jam" the chemosensory systems of predators. Kittred!;'e
et a!. (1974) regard these cryptic odors as either "negative odors", which alter the dendritic membrane potenti ••1
and block its generator potential, or as the chemical
equivalent of a "white noise" capable of generating an
"uncoded" array of spikes in the chemosensory neuroml.
Significantly, the complex defensive secretions of many
arthropods may constitute ideal agents with which to
overstimulate the chemoreceptors and thus trigger bursts
of uncoded information into the central nervous system.
Certainly, the Dufour's gland secretion of the formicine
Camponotus ligHiperda, enriched with at least 41 compounds including alkanes, alkenes, esters, alcohols, and
ketones (Bergstrom and Lofqvist 1971), should be eminently capable of stimulating
simultaneously
several
classes of olfactory generalists, thereby generating a sensory input which is very maladaptive for the predator.
Consequently, the chemical diversity identified with so
many defensive secretions may actually represent a cryptic
odor unleashed at the chemosensory neurons of an antagonist in the form of a highly disruptive chemical
overload.
It may II'dl be that the sophistication of arthropod olfaction not only reflects the existence of olfactory specialists, but also the presence of these cells on two antennae
instead of one. Hangartner (1967) has demonstrated that
whl'n a worker of the formicine ant Lasius !uligiHosus
folloll's a chemical trail, each antenna is alternately moved
in and out of the plane of the odor field. Significantly,
the antl'nnae may be utilized as a nulling device, so that
wl1l'n one antenna moves out of the odor field, it nulls
the signal to the other antenna, At the same time, the
sensitivity of the "out" antenna may be increased as it
falls to bl'lolI' threshold in the absence of a strong stimulus. The alternate insertion of the antennae in and out of
the odor fidd would also diminish the possibility of residual olfactory effects and thus guarantee the maintenance
of maximal sensitivity of the target cells to the pheromone
molecuks, Such a nulling system would also be particularly adaptive since it would provide an orientation axis
to the signal path which might not be achieved if both
antennae lI'ere continuously maintained in the odor field
and habituation of the specialist receptors occurred. Since
Farkas and Shorey (1973) have presented evidence that
moths, orienting to a sex pheromone, similarly alternately insert an antenna in and out of the odor plume
generated by the chemical stimulus, it is possible that
antennal nulling may represent a common device for maximizing the efficiency of the olfactory chemoreceptor
system.
CRYPTIC DEFENSIVE
THE SPECIFICITY IS THE BLEND
While the chemical variegation of defensive secretions
may effectively "conceal" arthropods, diversity in pheromonal secretions may constitute the primary means by
which individuals of the same species facilely locate each
other. Although many species of arthropods synthesize
common pheromones, it appears that their chemical signals are generally insulated from deciphering by foreign
species. This is because these pheromones are secreted
in admixture with other compounds which confer an element of great specificity on the chemical signal. It thus
appears that the specificity is the blmd. Precise intraspecific communication appears to reflect the evolution of
a complex of olfactory specialists capable of discriminating multicomponent pheromones, the function of which is
to modulate the informational content encoded in a chemical message.
CO~IPOt.:NDS
Although the specialist receptors can usually be fired
only by the highly specific molecules to which they react,
the same cannot be said for the cells which constitute the
so-called olfactory generalists.
The latter sensory eletlll'l1ts exhibit varying degrees of specificity to a spectrum
of compounds and it is probable that the deterrent effectiveness of many antagonistic allomones is a consequence
of their ability to stimulate these generalist receptors.
Thus, whereas the highly stimulatory lJuinones and shortchain carboxylic acids characteristic of many defensive
sl'cretions would seem to constitute ideal repellents, a
priori, many of the other major antagonistic allomones
could hardly be considered as potent olfactants. Hydrocarbons, for example, have been emphasized as major
constituents in the chemical defense arsenals of arthro-
Many sympatric lepidopterous species produce the same
primary sex pheromones (Roelofs and Comeau 1971), but
the presence of additional secondary components is often
required for atlractancy to be exhibited (Hummel et al.
1973) and in some cases, these minor concomitants may
actually inhibit the response of related species (Ganyard
and Brady 1971). Kullenberg et al. (1970) have demonstrated that the territorial pheromones utilized by 19 spe-
33
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B., and Z. M. Zielinska.
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Kittredge, J. S., M. Terry, and F. T. Takahashi.
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Sex pheromone activity of the molting hormone,
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Kullenberg,
B., G. Bergstrom, and S. Stallberg-Stenhagen. 1970. Volatile components of the cephalic
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T. 1967. Compounds in the defensive
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cies of male bumblebees, while they contain many common
components, constitute qualitatively distinct blends that
can serve as highly specific chemical messengers. Similarly, while closely related species of Trigona produce
trail pheromones enriched with complex mixtures containing many identical compounds, the specific integrity
of these blends appears to be guaranteed by their qualitative distinctiveness (Blum 1970). Vertebrate pheromones
are also characterized by the presence of blends (Michael
et a!. 1971) as further testimony to the widespread utility
of multicomponent chemical signals in the animal kingdom.
Thus, if we are to comprehend both the evolution and
elegance of chemical communication, it will be necessary
to analyze these topics in considerably more depth than
heretofore. Although a compound isolated from a female
moth may readily attract males of the same species, this
phenomenon hardly illuminates the communicative mechanisms employed by this species to maintain its reproductive integrity in the presence of sympatric forms that may
produce the same compound. On the other hand, a recognition of the potential richness of the vocabulary of pheromonal signals may eventually enable us to decipher the
communicati ve Rosetta Stone whose message contains
both the behavioral and neurophysiological legacies of a
species.
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CITED
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