Table of Contents
Pheromones are chemical signals that are released by one individual and received by another individual of the same species, where they trigger a specific, relatively stereotypical physiological or behavioral response. Unlike hormones (which act within an organism via the bloodstream or other internal pathways), pheromones act between organisms via the environment, usually air or water.
In this chapter, the focus is on what distinguishes pheromones, the major types that have been identified, and how the pheromonal communication system is built and used in different groups of organisms.
Basic Features of Pheromones
External Chemical Messengers
Pheromones share some key features:
- They are produced by specialized cells or glands and released to the outside (skin, cuticle, cloaca, urogenital tract, exocrine glands, etc.).
- They travel through the environment (as gases, aerosols, or dissolved in water; sometimes deposited on surfaces).
- They are detected by chemoreceptors (primarily olfactory and gustatory organs; in some groups via specialized systems like the vomeronasal organ).
- They trigger species-specific responses with relatively little conscious processing in animals that possess such mental states.
The amounts involved are often extremely small; many pheromones are active at concentrations comparable to a few molecules per liter of air or water.
Chemical Nature
Pheromones belong to various chemical classes:
- Small volatile molecules (e.g., short-chain acids, alcohols, aldehydes, esters, terpenoids) – usually airborne, typical in insects and mammals.
- Peptides and proteins – often used in vertebrates and some invertebrates, usually less volatile, detected at short range or by direct contact.
- Steroid-derived compounds – important in vertebrate reproductive pheromones.
- Long-chain hydrocarbons – common in insects as cuticular hydrocarbons serving in recognition and trail marking.
The chemical structure is crucial for specificity: small changes (e.g., position of a double bond, stereochemistry) can turn an effective pheromone into an ineffective one—or even one that repels instead of attracts.
Types of Pheromones and Their Functions
Pheromones are often grouped by the kind of effect they produce. A common distinction is between releaser pheromones and primer pheromones, with additional functional subclasses.
Releaser Pheromones
Releaser pheromones cause rapid, immediate behavioral responses.
Sex and Mating Pheromones
These are among the best-studied pheromones:
- Attractants: guide potential mates toward one another.
- Example: Many female moths emit highly specific blends of unsaturated alcohols or acetates that males detect over hundreds of meters.
- Courtship modulators: adjust courtship intensity, posture, or song.
- Copulation signals: may indicate receptivity or trigger copulatory behavior.
Key points:
- Often sex-specific in production and detection.
- Allow mate finding even when individuals are spatially separated or mostly stationary.
- High specificity reduces costly interspecies mating attempts.
Alarm Pheromones
Alarm pheromones signal danger and trigger escape or defensive behaviors.
- Typical in social insects such as ants, bees, and aphids.
- Released when an individual is attacked or disturbed.
- Nearby conspecifics may:
- Flee or drop from the plant (aphids).
- Rush to defend the nest (ants, bees).
Alarm pheromones usually:
- Are highly volatile (rapid spread, but short-lived).
- Act at short to moderate range.
- Help coordinate group defense or evasion without visual contact.
Trail and Orientation Pheromones
Many social insects use pheromones to mark paths:
- Trail pheromones allow ants or termites to mark routes to food sources or new nest sites.
- Workers deposit small amounts from special glands onto the substrate.
- Following individuals reinforce good trails, creating positive feedback.
- Orientation marks: bees, wasps, and others may mark entrances or important landmarks.
Trail pheromones need to:
- Persist long enough for others to follow.
- Fade slowly to prevent outdated routes from dominating.
- Often be less volatile than alarm pheromones but still detectable at close range.
Aggregation Pheromones
These cause conspecifics to gather at a particular location:
- Seen in beetles (e.g., bark beetles) to mass-attack trees.
- Used by some bugs and larvae to form protective groups.
- May be emitted by both sexes or one sex only.
Benefits include:
- Improved defense against predators.
- More efficient exploitation of patchy resources.
- Enhanced opportunities for mating.
Territorial and Spacing Pheromones
Pheromones can signal occupied territory or personal space:
- Many mammals mark territory with urine, feces, or gland secretions.
- The chemical marks advertise:
- Owner identity.
- Sex.
- Reproductive status.
- Dominance or health.
In some species, pheromones deposited at boundaries help reduce direct conflicts by allowing individuals to avoid or time encounters.
Primer Pheromones
Primer pheromones change the physiology of receivers, often via the endocrine system, and act over a longer timeframe than releasers.
Reproductive Physiology Modulators
In many mammals and social insects, pheromones:
- Alter puberty onset, estrous cycles, or fertility.
- Influence spermatogenesis or hormone levels.
- Synchronize or suppress reproduction within groups.
Examples (at the conceptual level):
- In rodent groups, pheromonal signals can synchronize estrus cycles of females or accelerate puberty in young animals.
- In some species, pheromones from dominant individuals can suppress reproduction in subordinates, stabilizing the social structure.
Caste Determination in Social Insects
In eusocial insects (ants, bees, some wasps, termites), primer pheromones contribute to determining who becomes a reproductive vs. worker:
- Queen pheromones are secretions from the queen that:
- Inhibit ovary development in workers.
- Suppress the rearing of new queens when a fertile queen is present.
- This chemical control helps maintain a stable division of labor and colony organization.
Primer pheromones often act by:
- Being distributed through grooming, trophallaxis (mouth-to-mouth food exchange), or contact.
- Affecting hormone production and gene expression in receivers.
Production and Detection of Pheromones
Glands and Release Mechanisms
Pheromones are synthesized in specialized exocrine glands and released via ducts to the body surface or into secretions.
Examples of secretion sites and modes:
- Insects:
- Pheromone glands in the abdomen, thorax, or legs.
- Trail pheromones from glands at the tip of the abdomen or on the feet.
- Cuticular hydrocarbons covering the exoskeleton for contact signals.
- Mammals:
- Scent glands near the anus, face, feet, or genitals.
- Urine and feces as carriers.
- Saliva or skin gland secretions.
- Aquatic animals:
- Pheromones dissolved directly into the water from skin, gills, gonads, or urine.
Release patterns can be:
- Continuous, low-level (e.g., colony identity odors).
- Triggered by specific events (e.g., injury, mating readiness).
- Under hormonal control, linking internal endocrine state to external chemical signals.
Sensory Systems for Pheromones
Pheromone detection largely uses chemosensory systems.
Olfactory and Gustatory Receptors
- Insects possess antennae and mouthparts specialized with numerous chemosensilla.
- Individual neurons can be finely tuned to specific pheromone components.
- Vertebrates use:
- Main olfactory epithelium for volatile pheromones.
- Taste buds and other oral receptors for nonvolatile compounds.
Signal transduction generally involves:
- Binding of pheromone molecules to receptor proteins.
- Activation of G-protein-coupled receptor pathways.
- Generation of electrical signals that the nervous system interprets.
Vomeronasal Organ and Accessory Systems (Vertebrates)
Many terrestrial vertebrates have a vomeronasal organ (VNO):
- Located in the nasal cavity or roof of the mouth.
- Specialized in detecting nonvolatile or low-volatility pheromones.
- Often engaged via specific behaviors (tongue flicking, flehmen response).
Input from the VNO typically projects to brain regions that control innate social and reproductive behaviors, rather than conscious odor perception.
Pheromones in Different Groups of Organisms
Insects
Insects have some of the most elaborate pheromonal communication systems:
- Sex pheromones: long-range female signals in moths; contact pheromones in flies and beetles.
- Trail pheromones: central to foraging in ants and termites.
- Alarm pheromones: mobilize defense in bees and ants.
- Colony recognition pheromones: cuticular hydrocarbon blends that identify nestmates.
Important characteristics:
- Often multicomponent blends; the ratio of components encodes species and sometimes colony identity.
- High specificity limits interference between different species, even in dense communities.
Vertebrates (Especially Mammals)
In mammals, pheromones play roles in:
- Mate choice: signals of genetic compatibility, health, and reproductive condition.
- Parent-offspring bonding: recognition between mothers and young via scent.
- Territorial marking and social status: scent marks encode identity and dominance.
- Reproductive synchronization and suppression in group-living species.
The relative importance of pheromones varies:
- Very high in rodents and many carnivores.
- Less obvious but still present in many ungulates and primates.
- Human involvement is debated; some chemical cues may influence mood or social perception, but clear, stereotypical pheromonal effects are difficult to demonstrate.
Aquatic Organisms
In water, chemical communication is particularly effective:
- Fish, amphibians, and many invertebrates release pheromones that:
- Coordinate mass spawning events.
- Signal sexual maturity and readiness.
- Mark territories or home ranges.
- Because water dissolves and carries chemicals well, pheromones can act over considerable distances, but currents and mixing strongly influence signal distribution.
Plants and Microorganisms: Borderline Cases
Although “pheromone” is often used for animals, similar principles apply in other groups:
- Some fungi release chemical signals to coordinate mating types.
- Some unicellular eukaryotes and bacteria use quorum-sensing signals to regulate group behaviors (e.g., biofilm formation, virulence).
In these non-animal systems, the term “pheromone” is sometimes used, sometimes reserved for multicellular animals, and sometimes replaced by other terms such as “mating factors” or “autoinducers.” The underlying idea—chemical communication between individuals of the same species—is shared.
Pheromones, Endocrine System, and Behavior
Although pheromones act outside the body, they are tightly linked to the endocrine system:
- Production of many pheromones depends on internal hormonal status:
- Sex steroids regulate sex pheromone production.
- Social status hormones (e.g., androgens, stress hormones) influence scent marks.
- Reception of pheromones often leads to hormonal changes in receivers:
- Altered gonadotropin secretion.
- Adjusted stress responses.
- Shifts in maternal behavior hormones.
Thus pheromones function as an interface between internal endocrine states and external social signals, enabling individuals to coordinate reproduction, social hierarchy, and defense in a way that links physiology and behavior across members of a population.
Applications and Implications
Pheromones in Agriculture and Pest Control
Knowledge of insect pheromones has been used to develop environmentally friendly pest control methods:
- Monitoring traps: synthetic sex pheromones lure males into traps, allowing early detection of pest presence and abundance.
- Mating disruption: saturating a crop with synthetic pheromone makes it difficult for males to locate females, reducing successful matings.
- Attract-and-kill systems: combining pheromones with insecticides can target specific pests with minimal impact on non-target species.
Advantages:
- High species specificity.
- Low toxicity for humans and beneficial organisms.
- Can reduce the need for broad-spectrum insecticides.
Pheromones in Animal Management and Conservation
Pheromones or pheromone-like cues are explored for:
- Managing reproduction in captive breeding programs (e.g., encouraging mating, synchronizing estrus).
- Reducing human–wildlife conflict (e.g., using chemical deterrents or territory marks).
- Guiding migratory fish during restocking or conservation measures, using natural or synthetic cues.
Ethical and Practical Considerations in Humans
Claims about human pheromones are common in cosmetics and marketing. Scientifically:
- Humans do emit complex body odors influenced by genetics, diet, health, and microbiota.
- Some odor components can affect perception, mood, and social judgments in subtle ways.
- However, a small number of clearly identified human chemicals that function as classic pheromones (with stereotypical, specific behavioral or hormonal effects) remains controversial.
This uncertainty has implications for:
- How strongly we should interpret commercial claims.
- Ethical questions about attempting to manipulate social or sexual behavior via chemistry.
Summary
Pheromones are specialized chemical signals that operate between individuals of the same species, shaping behavior and physiology across social and ecological contexts. They:
- Function as external complements to internal hormones, linking individual endocrine states to group-level coordination.
- Take many forms, from rapid releaser pheromones (e.g., sex, alarm, trail) to slow-acting primer pheromones (e.g., those influencing reproduction and caste).
- Are central to social organization and communication in many animals, particularly insects and mammals.
- Provide practical tools in areas such as pest management and wildlife conservation, while raising intriguing questions about chemical communication in humans.