Chapter 6 Summary

Olfaction is the oldest method of communication, having evolved from chemical mechanisms that primitive organisms used to identify food and locate mates. The sender’s ability to control the transmission of chemical signals is limited, which explains the evolution of more sophisticated signaling modalities in more advanced animals. Individual signal molecules must move the entire distance from the sender to the receiver via current flow or diffusion, or the receiver must approach and make direct contact. In comparison to light and sound transmission, olfaction is considerably slower, its stimulus field contains no directional information, temporal patterning is not possible, and it possesses no linear array of variants equivalent to the spectra of sound and light frequency. Chemicals that facilitate communication between species are called allelochemicals. Chemicals used for communication between conspecifics are called pheromones. Combinations of several chemicals in a specific ratio are called pheromone blends, and variable mixes of many chemicals are called pheromone mosaics or odor signatures.
Pheromones are organic compounds differing in size, shape, composition, and polarity. Airborne odorants must be sufficiently volatile to vaporize, and waterborne pheromones must be water soluble. Pheromones are produced in four ways. Secretory glands are well-defined structures near the body’s surface that manufacture, store, and release highly specific chemical products. Glands are classified by secretion mechanism: merocrine, holocrine, or apocrine. The deposition of these gland products is usually associated with specific behaviors and social circumstances, leaving little doubt as to the general function of the signal. Waste metabolites in urine, feces, saliva and sweat released from body orifices and organs associated with digestion and reproduction often provide useful information about gender, condition, reproductive stage, and dominance status. Internal gland products may be transported to these externally released fluids with special binding proteins. A few species derive pheromones from plant secondary compounds and from bacterial breakdown products.
Odors can be released into the medium in three basic ways: by passive exposure of glandular tissue, by piggy-backing on another activity, and by specialized release behaviors. Any of these mechanisms may be employed to deliver odors to three types of locations: direct release into a fluid medium, deposition onto the sender’s own body, and deposition onto other solid surfaces. Some common dissemination methods include spraying, hair dispersal, self anointing, self-generated current, footprints, and deposited marks.
In the absence of any current flow, chemical odorants can be transmitted from sender to receiver via diffusion. If the volatility of the chemical is known, the spread of the odorant with time can be precisely modeled for a single instantaneous puff, continuous emission, and moving source emission. If the threshold concentration of detection by receivers is also known, the active space can be described. Diffusion is likely to operate only over short distances. It is probably most useful for small organisms, such as ants and termites, that live within the viscous boundary layer of a substrate and for some small aquatic organisms that live in relatively still water.
Long-distance transmission of chemical odorants must be coupled with environmental or sender-generated current flow. If the flow is slow and laminar, an elongated active space with a smooth concentration gradient is formed. However, most flows are turbulent, characterized by a complex, unpredictable, and variable pattern of eddies, whirls, and vortices. The stimulus field of an odorant in a turbulent flow is a patchwork of filaments and plumes that spreads in a wedge from the source. Odorant dispersal is also affected by vegetation, weather conditions, humidity, and topographical features of the landscape.
Chemoreception can be subdivided into two general categories: olfactory reception of airborne or waterborne chemicals from a distance source and contact reception. Olfactory reception begins by drawing odorant-laden medium into the chemoreception organ with mechanisms such as sniffing air into the nasal cavity, pumping water into nares, or flicking antennae. For contact reception, the receiver approaches the source and makes direct contact with the chemical.
Chemoreceptor sensory cells form a sheet of epithelium within chemosensory organs. External chemicals must pass through a mucus layer surrounding the sensory cells. The mucus layer contains binding proteins for transporting important odorants as well as compounds that defend the sensitive tissue and neurons against toxins and pathogens. The sensory cells are bipolar neurons derived from ciliary or microvillous cells that form projections into the mucus. Anchored on the membranes of these projections are G protein–coupled receptors that temporarily bind with certain odorant ligands. Binding activates a chemical cascade and nerve impulse. A limited range of odorant molecules will stimulate a given sensory cell. There are a large number of sensory receptor cell types, each sensitive to different chemical features and encoded by a different gene from multigene families.
At the first olfactory processing region in the brain, all of the axons from sensory cells of the same type converge on a few spherical glomeruli. Second-order neurons pass the information from each receptor cell type on to higher processing centers. A given chemical is recognized by a combinatorial code of stimulated glomeruli. Some animals have dual pathways to different parts of the brain, one primarily for analysis of the huge array of general environmental chemicals and one primarily for analysis of specific odorants, usually pheromones. Many terrestrial vertebrates possess two separate chemosensory organs, the olfactory organ for detection of airborne odorants and the vomeronasal organ for detection of contact and waterborne odorants.
The purpose of long-distance olfactory signals is to attract or repel receivers. If the signal is transmitted by diffusion or laminar flow, receivers can employ either klinotaxis (sequential sampling from two locations) or tropotaxis (simultaneous comparison from separated receptors) to determine the direction of the concentration gradient, and orient their movement accordingly. If the signal is transmitted in a turbulent flow field, receivers will couple detection of the odorant with assessment of the current flow direction to move up-current toward the source. Substrate-walking animals can use mechanosensors to detect current flow direction. Flying or swimming animals must assess their movement relative to visual landmarks and will initially make wide zigzagging exploratory movements before approaching the source.