Allomone

Allomone (coined in 1968 by William L. Brown Jr. and Thomas Eisner, and formalized in 1970 by Robert H. Whittaker and Paul P. Feeny from the Greek allos, "other," and hormon, "to excite") is a class of interspecific chemical signaling compound (allelochemical) released by an individual of one species that induces a behavioral, physiological, or developmental response in a receiving individual of a different species that is adaptively favorable to the emitting organism (+/- or +/0 interaction). In the overarching classification of semiochemicals (information-bearing chemical compounds), allomones are contrasted with pheromones (which act intraspecifically between members of the same species) and with two complementary classes of interspecific allelochemicals: kairomones (which benefit the receiver at the expense of the emitter, such as host body odors used by Aedes aegypti mosquitoes or parasitoid wasps to locate prey) and synomones (which benefit both emitter and receiver, such as herbivore-induced plant volatiles that recruit predatory mites to consume leaf-eating caterpillars). Across terrestrial and marine ecosystems, allomones mediate chemical warfare, plant–plant competition (allelopathy), aposematism, and deceptive pollination or predatory lures.

Chemical ecologists categorize volatile and non-volatile semiochemicals according to whether the interaction is intraspecific or interspecific and which participant gains a Darwinian fitness advantage:

Importantly, a single chemical molecule can function simultaneously as a pheromone, an allomone, and a kairomone depending on the ecological context (semiochemical parsimony). For example, the pine bark beetle (Ips pini) emits ipsdienol as an intraspecific aggregation pheromone, which simultaneously acts as an interspecific allomone inhibiting competing bark beetle species (Ips paraconfusus) while serving as a kairomone that guides predatory clerid beetles (Thanasimus dubius) directly to the bark beetle gallery.

The most widespread functional class of animal allomone consists of repugnatorial secretions synthesized in specialized exocrine glands to deter predators, competitors, and microbial pathogens:

• Bombardier Beetle Exothermic Quinone Reactor (Brachinus and Stenaptinus): As elucidated by Thomas Eisner and Daniel J. Aneshansley (1969, 1999), carabid bombardier beetles store an aqueous solution of 25% hydrogen peroxide (H₂O₂) and 10% hydroquinones (1,4-dihydroxybenzene and 2-methylhydroquinone) inside a compressible reservoir chamber separated by a muscular sphincter valve from a thick-walled reaction chamber lined with catalase and peroxidase enzymes. When pinched by an ant, frog, or spider, the beetle squeezes reservoir fluid into the vestibule: catalase decomposes H₂O₂ into O₂ gas and water while peroxidases oxidize the hydroquinones into irritant p-benzoquinones, releasing ΔH ≈ −202.8 kJ mol⁻¹ of heat. Within milliseconds, the mixture boils and is ejected through a rotatable abdominal turret as a 100 °C pulsating spray (500 pulses per second) that scalds and chemically blinds the attacker.
• Aldehydes, Cyanogenesis, and Formic Acid: Pentatomid stink bugs eject trans-2-decenal and trans-2-octenal from metathoracic scent glands; polydesmid millipedes (Apheloria) hydrolyze mandelonitrile in two-chambered ozadene glands to release hydrogen cyanide (HCN) gas and benzaldehyde; and formicine ants spray 60% formic acid (HCOOH) from their poison gland reservoir while mandibular glands secrete citral, 2-heptanone, and undecane alarm allomones.
• Marine Nudibranch and Sponge Terpenoids: Shell-less dorid nudibranch mollusks sequester or synthesize sesquiterpene and diterpene isonitriles, furanoterpenes, and macrolides (latrunculin A, which depolymerizes actin microfilaments) in mantle dermal formations, deterring reef fish feeding at micromolar concentrations.

In botany and plant community ecology, allomones released by plants through foliar volatilization, rain leachates, root exudation, or litter decomposition to suppress the germination and root elongation of neighboring plant competitors are termed allelochemicals (mediating allelopathy, a term coined by Hans Molisch in 1937):

1. Juglone in Black Walnut (Juglans nigra): Leaves, hulls, and roots of the eastern black walnut synthesize the colorless, non-toxic glycoside 1,4,5-trihydroxynaphthalene-4-β-D-glucoside (hydrojuglone glucoside). Upon leaching into the soil rhizosphere, microbial hydrolysis and oxidation convert hydrojuglone into the potent naphthoquinone allomone juglone (5-hydroxy-1,4-naphthoquinone). Juglone inhibits plasma-membrane H⁺-ATPase activity, disrupts mitochondrial electron transport, and induces reactive oxygen species in susceptible understory species (such as Solanaceae, pines, and birch) at concentrations as low as 10⁻⁵ M.
2. Sorgoleone in Sorghum bicolor: Specialized root hairs of sorghum exude hydrophobic droplets composed of 85–90% sorgoleone (a lipid benzoquinone), which blocks Photosystem II electron transfer between Q_A and Q_B in competing weed seedlings by binding to the D1 protein pocket alongside inhibiting hydroxyphenylpyruvate dioxygenase (HPPD).
3. Volatile Monoterpenes in Coastal Sage Scrub and Chaparral: Aromatic shrubs such as Salvia leucophylla (purple sage) and Artemisia californica volatilize 1,8-cineole (eucalyptol) and camphor, which adsorb onto dry summer clay soils around the shrub canopy and inhibit mitotic cell division in the radicles of annual grass seedlings during autumn rains, contributing to bare vegetation-free halos.

Rather than repelling enemies, a specialized class of allomones—termed propaganda allomones or aggressive chemical mimics—exploits the innate sensory receptors of a target species to lure prey, usurp social insect colonies, or manipulate pollinators without offering any reward:

• Bolas Spiders (Mastophora spp.): Adult female bolas spiders abandon orb-web construction and hunt exclusively at night by swinging a single silk thread tipped with a viscid adhesive droplet (the bolas). Rather than waiting passively for random insects, the spider synthesizes and releases volatile acetate and aldehyde blends—such as (Z)-9-tetradecenyl acetate and (Z)-11-hexadecenal—that exactly match the female sex pheromone ratios of specific noctuid moth species (Spodoptera frugiperda and Tetanolita mynesalis). As a result, 100% of the spider's captured prey consists of sexually aroused male moths flying upwind into the sticky bolas.
• Slave-Making Ants (Polyergus and Formica sanguinea) and Myrmecophiles: During colony raids on Formica host nests, Formica subintegra and Polyergus slave-making workers discharge vast quantities of decyl, dodecyl, and tetradecyl acetates from hypertrophied Dufour's glands. These esters act as a propaganda allomone that mimics and overwhelms the alarm pheromone channels of the defending Formica workers, throwing the defenders into panicked dispersal and mutual fratricide while the raiders carry off the host pupae. Similarly, parasitic Maculinea (Phengaris) rebeli butterfly caterpillars synthesize cuticular hydrocarbons that match Myrmica schencki ant larvae and emit acoustic stridulations mimicking the queen ant, tricking nurse workers into carrying the caterpillar into the brood chamber and feeding it via trophallaxis ahead of their own brood.
• Sexual Deception in Ophrys Bee Orchids: Mediterranean Ophrys sphegodes orchids produce zero nectar; instead, their velvety labellum petals visually resemble a female solitary bee (Andrena nigroaenea) and emit an exact stoichiometric blend of C₂₁–C₂₉ straight-chain alkanes and (Z)-7-, (Z)-9-, and (Z)-11-alkenes identical to the virgin female bee's sex pheromone—inducing male bees to attempt copulation (pseudocopulation) with the flower and transfer pollinia before learning to avoid that floral bouquet.

1. Brown, W. L., Eisner, T., & Whittaker, R. H. (1970). Allomones and kairomones: transspecific chemical messengers. BioScience, 20(1), 21–22. https://doi.org/10.2307/1294753
2. Whittaker, R. H., & Feeny, P. P. (1971). Allelochemics: chemical interactions between species. Science, 171(3973), 757–770. https://doi.org/10.1126/science.171.3973.757
3. Eisner, T., & Aneshansley, D. J. (1999). Spray aiming in the bombardier beetle: photographic evidence. Proceedings of the National Academy of Sciences, 96(17), 9705–9709. https://doi.org/10.1073/pnas.96.17.9705
4. Schiestl, F. P., Ayasse, M., Paulus, H. F., Löfstedt, C., Hansson, B. S., Ibarra, F., & Francke, W. (1999). Orchid pollination by sexual swindle. Nature, 399(6735), 421–422. https://doi.org/10.1038/20829