lec24 reading - Chapter Chemical Signals CHEMICAL SIGNALING...

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Unformatted text preview: Chapter Chemical Signals CHEMICAL SIGNALING Is THE OLDEST METHOD OF COMMUNICATION. From the earliest days of life in Earth’s oceans, single-celled organ- isms possessed the ability to detect and selectively take in different classes of chemicals needed for cellular metabolism. The detection of food was and still is the primary function of most chemical re- ception organs. At the same time that organisms are taking in food, they are eliminating metabolic waste products. Once such organ— isms evolved the ability to distinguish between the chemical com— pounds emanating from conspecifics and the chemical components of food, a primitive social organization and communication system existed. Early metazoans relied exclusively on chemical communi— cation to synchronize gamete release and mediate fertilization be~ tween conspecific sperm and eggs, and some colonial species even signaled alarm in the presence of a predator. More advanced organ— isms quickly evolved two types of chemical detection systems: smell and taste (in human terms). In this chapter we shall examine the types of chemicals and production methods used for chemical communication, the transmission properties of chemical odorants 279 280 Chapter 10 in different environments, and the evolution of chemoreception organs for the 1 detection of chemical signals. GENERAL FEATURES OF CHEMICAL COMMUNICATION To transmit a chemical signal, individual molecules have to move the entire distance between sender and receiver. How can such movement be achieved? There are three mechanisms. (1) Senders can use the current flow in air or water to carry the molecule to the receiver. (2) In the absence of a current, the molecule can only move by diffusion. Molecules naturally move along a con— centration gradient, from a point of high concentration to a point of low con- l centration, but this requires a certain amount of time. (3) The receiver can i move toward the signal and pick up the molecule directly by contact, so that l the molecule doesn’t have to move at all. Contrasts between the Propagation of Olfactory, Auditory, and Visual Signals The propagation of olfactory signals differs in major ways from the propaga— tion of auditory and visual signals. DIRECTIONALITY. Both sound and light travel as orderly waves in a relatively straight direction away from the source of emission. Chemical signals also spread out from their source, but the movement of odorant molecules from a point of high to low concentration follows an irregular path. At any given mo— ment, at diffusing molecule may move either away from or towards the source. SPEED. Although both sound and diffusion require molecular motion, sound is propagated at a much higher speed than odors. In a sound far field, only the disturbance is propagated, not individual molecules; in diffusion, the in- dividual molecules must be propagated. Typical delay times between sender and receiver are milliseconds for sound, but seconds, minutes, or even days for odors. TEMPORAL PATTERN. Sound and light retain their temporal patterns as they propagate (although there may be some distortion over long distances). Nei— ther diffusion nor current flow can sustain an initial pattern of modulation be- cause molecules do not move in synchrony. Any temporal pattern imposed on an olfactory signal during emission is lost within a short distance from the source (Bossert 1968). SPECTRUM. The spectrum of olfactory signals (i.e., the different chemical com— pounds) cannot be arrayed in one linear dimension as can the frequency spec- tra of sound and light. This means we cannot use Fourier analysis to character— ize olfactory signals or to determine how they are generated, propagated, and received. We thus need a different method of analyzing olfactory signals. Forms of Chemical Communication Because the selective detection and uptake of chemicals is a fundamental process of all living cells, chemical communication in a broad sense occurs at Chemical Signals many biological levels. Chemicals that operate internally and facilitate com- munication between the brain and organs involved in growth, digestion, and reproduction are called hormones. Chemicals that facilitate communication between conspecifics are called pheromones and are, of course, the main focus of this chapter. Chemicals that are transmitted and detected between species, such as predators and prey or sympatric competitors, are called al- lomones. For the two latter types of external communication, two different modes of detection may be employed: olfactory reception, which involves the detection of airborne or waterborne chemicals from a distant source (e. g., by smell), and contact reception, which requires direct contact of the receptors with the chemical source (e.g., by taste). Both olfactory and contact receptors may be used for the detection of food and conspecifics. As we shall see in the section on reception, many animals possess three separate chemical sensory systems, one for detection of diffusing or current-borne chemicals, another for identification of contacted food, and a third for contact reception of social signals. PRODUCTION OF OLFACTORY SIGNALS In this section we shall examine the range of chemicals used by animals to communicate with conspecifics, the sources of these chemical odorants, and the ways in which the chemicals are released by senders. Types of Chemicals Used for Intraspecific Communication The array of chemicals identified as pheromones is vast. All are, of course, organic compounds with a basic carbon skeleton. The major constraints on the chemical composition of pheromones are determined by the type of transmission, i.e., diffusion, current, or contact, and by the medium, i.e., air or water. Airborne odorants must be volatile in air; that is, they must evaporate easily (Wilson and Bossert 1963, Wilson 1970, Wheeler 1977). Volatility is pri— marily a function of molecular size and weight—larger, heavier molecules have lower volatility. The upper size limit for airborne pheromones is a mol- ecular weight (MW) of about 300. Most airborne odorants contain between 5 and 20 carbon atoms. Molecules larger than this size are both expensive to produce and too large to diffuse effectively. Pheromones with less than 5 car— bons may be rare because they are too volatile and possess too few options for species-specific structural variants. Within this size range, chemical odor— ants show a great deal of variation in shape and type of functional group. A few examples are shown in Figure 10.1. The majority contain a single func- tional group with one oxygen atom, such as an alcohol, aldehyde, or ketone. Some pheromones are acids or esters with two oxygen atoms, and others are alkanes and alkenes with no oxygen. Pheromones vary greatly in the posi- tion of the functional group, the position of double and single carbon-carbon bonds, the occurrence of branches and rings in the carbon chain, and some contain other atoms such as nitrogen or sulfur. These variants have only minor effects on volatility, but they greatly affect the shape of the molecule 281 ~ 7 swam. a? a“ .... \ (x; ‘ r :e. Jé‘fir % 282 Chapter 10 W HHHHHrrrrrtrrrrr l l | I l H-C~C*C=C-C=C-$~$~$-$-$“$~t-$-$*$—OH l I | H H H H H H H H H H H H H H H H ' H H (B) H \C_C, I-I\C/ (C) H\ \ / /H *C l /\ l\ / \H C/C \ __ _ H = H \ / C H H $ C\ H‘/C C\ /H \C<\ HH />C/ H C'C C\ H / H H \ /H / \ \C ,C H H H y H \ H-‘(i IC=O /C/H H\C\ H \/H H\ / H C HHHI—I c H/ \c/ I \ \c/ \H /\C—C/\ H l l H H H e rrrtrrrrt H—“C—C—C*Ci3*‘CII*$~C=C—C~OH | | H H H H H (E) H H H H (F) H H \ / \ / | l /C C\ H—c-—s—s—~C—H H \c=c—c H l ' H / \ /\ /H H H \ c C C / \ / /\ H H H H H ttt onnne H H H Figure 10.1 Some examples of airborne Chemical odorants. (A) Silkworm moth (Bombyx) sex attractant, (B) a common termite alarm substance, (C) civet (Civettictis civetm) sex attractant, (D) honeybee (Apis mellifera) queen substance, (E) cockroach (Periplaneta amert'cana) sex attractant, and (F) hamster (Mesocricetus aumtus) mounting pheromone. In many cases, the pheromone is a mixture of very similar chemicals. (After Wilson 1963; Moore 1968; Johnston 1977.) and thus the molecule’s detection by receptor cells (Morse and Meighen 1986). - The size restriction does not apply for waterborne and contact phero- mones. Organic compounds composed of primarily carbon (MW = 12) and hydrogen (MW = 1) can be less dense than water composed of oxygen (MW = Chemical Signals 16) and hydrogen, and hence float regardless of their size. Large organic com— pounds such as lipids and proteins can therefore be used as pheromones in these circumstances. Waterborne odorants must be water soluble to disperse effectively and be detected by olfactory receptor organs. Contact pheromones in terrestrial environments are even less restricted by size constraints, and a larger Variety of chemical compounds is therefore available to senders (Carr 1988). Production Sites Pheromones can be expelled from two fundamentally different sources: (1) well-defined secretory glands that empty their products onto the outside of an animal’s body, and (2) body orifices and organs involved in digestion and reproduction such as the mouth, anus, cloaca, penis and vulva. The phero— monally active chemical components are considerably easier to identify in the case of gland secretions compared to excretory products. The bodies of both vertebrates and invertebrates contain numerous glands composed of secretory cells that produce specific chemicals. Endocrine glands empty their secretory products into the blood stream; these chemicals are the hormones that regulate internal body metabolism. Exocrine glands, on the other hand, are those glands located either externally on the skin / integu- ment or internally with ducts leading to the exterior of the body. Their func— tion is to maintain the condition of the body covering and/ or to produce chemical communicatory signals—pheromones and allomones. Exocrine glands can be divided into two types based on their appearance and manner of secretion (Figure 10.2). Sebaceous glands are flask-shaped or lobed. The basal layer of the gland’s epithelium continually produces new cells, forcing the old cells into the center of the gland. As the old cells approach the lumen they become rich in lipids and then disintegrate completely to become a thick, oily product called sebum (Albone 1984). Volatile and nonvolatile pheromone chemicals are embedded in this sebum matrix, which can greatly affect the transmission properties of the pheromone. Secretion rates of sebaceous glands are always slow. Sudoriferous glands look like coiled tubules. The secretory cells in such glands are not layered and continuously renewed as in sebaceous glands, but form an orderly lining around the inside of the tubules. The secre- tory products are collected in droplets or vacuoles Within the cells and then emptied into the tubular lumen, Where the chemicals are stored. The secretory product is always liquid and can consist of relatively pure pheromone. Secre- tion rates are much faster than for sebaceous glands. The development and secretory activity of both types of glands are often controlled by endogenous hormones (Ebling 1977). Release of the secretion from the gland is much more likely to be under nervous control in sudoriferous glands, however. Some pheromones appear to be produced over the entire external surface of some animals by many small sebaceous and / or sudoriferous glands located throughout the skin or integument. Vertebrate cutaneous (skin) glands are an excellent example (Quay 1977; Flood 1985). In mammals, each hair follicle has associated with it a pair of sebaceous glands and a single sudoriferous gland. 283 284 Chapter 10 (A) (B) Hair follicle Epidermis Sebaceous gland Dermis Hypodermis Sudoriferous gland Figure 10.2 Cutaneous glands in mammals. (A) Mongolian gerbil (Mariones unguiculatus) cheek gland. (B) Human skin. (After Kivett 1978; Flood 1985.) The sebaceous glands secrete sebum that maintains the condition of the skin and hair, while the sudoriferous gland produces sweat for thermoregulatory purposes. Pheromones can be added to either secretion. In the garter snake, for example, gravid females secrete a series of large, nonvolatile methyl ketones from their cutaneous glands that are deposited on the ground whenever they travel (Mason et a1. 1989). Males can follow the female trail with the use of their contact chemoreceptors. Parental cichlid fish secrete a protinaceous mucus from their bodies that not only maintains contact with the brood of free-swimming fry but also provides food for the young (Barlow 1974). Many vertebrate and invertebrate animals possess one or more major ex~ ocrine glands that produce species—specific pheromones. The chemical secre- tions from different glands send specific types of communication messages. The glands may be located in a variety of positions on the body where the se- cretion is most usefully emitted. Figure 10.3 shows some vertebrate examples, and Figure 10.4 shows the typical battery of glands present in hymenopteran insects. Body orifices associated with digestion and reproduction are obvious lo- cations for the leakage of chemicals out of an animal’s body. Waste products such as amines and the byproducts of steroid hormones that are eliminated in urine and feces can provide important information to other individuals that can detect them. Potentially important volatile compounds are delivered to the digestive tract by the liver via the bile. Exocrine glands associated with the digestive tract whose primary function is to secrete digestive Chemicals can also secondarily produce important pheromones. For example, the saliva Chemical Signals (A) (B) Caudal gland Cheek Tarsal gland , . gland : / lnterdigital " ' ' ?/ Plreogbital gland\ g an Urine (C) (D) Mental Femoral gland Underside of head Figure 10.3 Location of glands in vertebrates. (A) Reindeer (Rangifer tarandus) produce airborne scents from their caudal gland, tarsal gland, and urine. The tarsal gland also marks the deer’s resting sites, the interdigital gland leaves marks on the ground along the animal’s path, and the preorbital gland is rubbed on upright twigs for territorial marking. (B) Ground squirrels (Spermophilus) mark their burrows with the dorsal gland and objects in their territories with the cheek gland. (C) Salamander males (Aneicles) mark females during courtship with mental gland secretions. (D) Iguanid lizards mark rocks on their territories with femoral gland secretions. (After Stebbins 1966; Brown and Macdonald 1985; Gosling 1985.) of male boars and hedgehogs is the source of pheromones used during courtship (Signoret 1970; Perry et a1. 1980). The urogenital system can also provide external chemical information about internal metabolic events. The kidneys, like the liver, process cellular waste products and transfer volatile and non-volatile compounds from the blood to the urine. Steroid hormones in particular are concentrated by the urine. Chemical products from the genitals themselves, and in particular the vagina and vulva of female vertebrates, may also be expelled into the exterior environment (Michael et al. 1971; Michael and Bonsall 1977). Microorganisms provide another source of volatile chemical odorants in some animals. Small pockets or cavities of skin that retain moisture or urine 285 286 Chapter 10 Thorax labial gland Metapleural Hindgut Postpharyngeal Anal Jr» _ gland gland -. ~ 3 t ‘ -- 5,! . , — Maxillary gland 5mg “Mandible ‘3 Dufour’s Poison gland gland Pavan’s gland Figure 10.4 Glands of a worker ant (Iridomyrmex humilis). (After Wilson 1971.) provide an ideal growth chamber for bacteria, which in turn produce a vari— ety of small volatile metabolic products (Albone et al. 1977). Methods of Dissemination Animals use a variety of methods to release odorous chemicals. Some of these are illustrated in Figure 10.5. The method obviously depends on the viscosity of the secretory substance, the type and location of the gland or other source, and the target or recipient of the chemical signal. It is often difficult for human ob— servers to know when an olfactory signal has been produced, since we may not be able to smell it ourselves. Many olfactory signals, however, are accompanied by specific behaviors, visual signals, postures, structures and even auditory sig- nals, and in these cases we can be more certain about signal production. Liquid. secretions can be released in a brief forcible stream and therefore directed at a specific target individual or location (Figure 10.5A,B). This re— lease requires neuromuscular control over the gland. Alarm, threat, and de— fensive olfactory signals are typically disseminated in this way in ants, skunks and bombadier beetles. Urination can be similarly controlled and used for marking mates, as in the South American mara, or used for marking specific locations on a territory, as in dogs and many rodents. Some arboreal mam- mals urinate on their feet and mark the branches in their territory as they move around (Charles-Dominique 1977 ). Liquid secretions can also be re- leased slowly into a current, as in the mate—attraction pheromones of moths and other insects (Elkington and Carde 1984). Gooey sebaceous gland secre— tions, on the other hand, must be rubbed onto a surface, object, or target indi— vidual. Examples include marking stems or posts with anal or facial glands (Figure 1050, marking females with chin or facial gland secretions by many male mammals, and marking rocks with femoral gland secretions by territor— ial lizards (Figure 105]). Some animals carefully spread sebaceous gland se— cretions over their own bodies during grooming (Figure 10.5D). Anal, cloacal, and preputial glands add their secretions to feces and urine and are therefore spread along with the excreta. Chemical Signals Certain structures and behaviors are used to help transmit chemical odor— ants. Hairs are frequently specialized to disseminate scents (Figure 10.5G,I-D. They may be used like a brush to deposit secretions on objects or they may be modified with rough surfaces to form a substrate for the chemical. Hairs in- crease the surface area for vaporization of both liquid and oily chemical secre- tions. A variety of behaviors and movements enhances the transmission of chemicals by spreading the odorants or creating current flows. For example, odorant may be rubbed onto the tail and the tail waived toward the target, and secretions may be released into air or water currents generated by the sender (Figure 10.5E,F). Hairs, structures and movements also add a Visual component to an olfactory signal, as in the maned rat, lizard, and butterfly ex- amples (Figure 10.5H—D. In most of the examples described above, there is a close association in time between the release of the odorant by the sender and its reception by the target. The behavioral responses of receivers may also be sufficiently rapid and obvious that we can ascertain the function of the signal (Figure 10.6). However, for many chemical signals such as territorial scent marks there is a significant delay between deposition by the sender and reception by the re— ceiver. The marks are intended to last a long time and the sender may be quite far away when they are detected. This means that the signal and the sender may be dissociated in both space and time. TRANSMISSION OF CHEMICAL SIGNALS In this section we shall quantitatively examine the transmission properties of olfactory signals in several different environmental circumstances. The first subsections will describe simple diffusion processes in still air and water. We will then analyze how current flow affects the transmission of chemicals. General Rules for Diffusion Molecules move down their concentration gradients. This movement occurs not because of some form of repulsion among similar molecules, but because of ...
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