Chemical senses are probably among the first senses to have evolved and all cellular life forms from bacteria to animals are sensitive to chemical information, whether it comes from potential food, predators, the environment, or other members of the same species.
Chemicals from outside the organism that provide information are termed semiochemicals (Figure 1.1). With the chemical senses in place it was perhaps inevitable that chemical communication would evolve. Pheromones are evolved chemical signals between members of the same species. The molecules are emitted by an individual and received by a second individual of the same species, in which they cause a specific reaction, for example, a stereotyped behavior or a developmental process (Wyatt 2010, after Karlson & Lüscher 1959). The word pheromone was coined from the Greek pherein, to carry or transfer, and hormōn, to excite or stimulate. While Karlson and Lüscher (1959) proposed the word after the first pheromone had been identified in an insect, the silk moth Bombyx mori, they suggested the term would apply to chemical signals in all types of animal from crustaceans to fish to terrestrial mammals. Equally, while the silk moth pheromone was a single molecule, pheromones consisting of many molecules together were not excluded. Karlson and Lüscher anticipated that different species might share some of the same molecules. No requirement was made for pheromone responses to be innate. I emphasize these points only because some authors writing about mammals raise these as objections to applying the term to mammal pheromones (e.g., Doty 2010; Petrulis 2013). As I argue below, pheromones are found across the animal kingdom and while mammal pheromones are hard to study, we do have many examples that seem robust (see for example Schaal et al. 2003; Schaal,Chapter 17). Even so, criticisms of some claims in the literature are justified. We should also be careful to distinguish phenomena mediated by pheromones from olfactory phenomena that have more to do with differences between individuals’ chemical profiles, allowing individuals to be distinguished (see below).
As well as pheromones, animals also receive chemical information about the identity of another individual. The receiving animal may learn a signature mixture: a variable chemical mixture (a subset of the molecules given off in an animal’s chemical profile) learned as a template by other conspecifics and used to recognize an animal as an individual (e.g., lobsters, mice) or as a member of a particular social group such as a family, clan, or colony (e.g., ants, bees, badgers) (Figures 1.1 and 1.2) (Wyatt 2010, 2014).
The signature mixture is the mix of molecules (and likely, their relative ratios) that are learned. The template is the neural representation of the signature mixture stored in the memory of the learner (after van Zweden & d’Ettorre 2010). There are two distinguishing characteristics of signature mixtures: first, a requirement for learning, and second, the variability of the cues learned, allowing other individuals to be distinguished by their different chemical profiles (Wyatt 2014).
There are many biological systems where distinguishing between pheromones and learned highly variable signature mixtures can help our understanding. For example, each ant colony has different combinations of molecules, largely cuticular hydrocarbons, which can be learned as signature mixtures allowing ants to discriminate between colony members and noncolony members (Bos & d’Ettorre d’Ettorre 2012). Every queen of the species, however, has the same queen pheromone (Liebig 2010). In the ant Lasius niger, the queen pheromone is the cuticular hydrocarbon 3-methylhentriacontane, missing from worker profiles (Holman et al. 2010). Similarly, explaining a phenomenon such as the Bruce effect in mice (Brennan 2009; Mucignat-Caretta,Chapter 11) is easier if we distinguish between the individually distinct odors of different males and the pheromone(s) that all male mice produce. The female’s memory of the signature mixture of the particular male she mated with has the effect of blocking any stimulus from later contact with his male pheromones whereas the stimulus from the same male pheromones of other, unfamiliar males passes through to the hypothalamus, triggering the Bruce effect.
Pheromones and the molecules learned as signature mixtures appear in the cloud of molecules that make up the chemical profile of an organism (Figure 1.2). Much of the chemical profile is highly variable from individual to individual. The sources of the molecules in the chemical profile include the animal’s own secretions as well as its environment, food, bacteria, and other individuals. It is this complex background that makes identifying pheromones so challenging in many organisms.
Semiochemicals can also be significant to individuals from other species and these are termed allelochemicals (Figure 1.1) (Nordlund & Lewis 1976). Broadcast signals can be eavesdropped, as kairomones. Some of the most spectacular examples of using such kairomones come from predatory beetles homing in on the aggregation pheromones released by their prey, bark beetles (Raffa 2001). Eavesdropping can occur across taxa. Nematode-trapping fungi detect the pheromones of their prey nematodes and produce more traps in response (Hsueh et al. 2013). Aggressive chemical mimicry (allomones) can exploit the responses of organisms to their own pheromones (Vereecken & McNeil 2010). For example, most orchids offer no nectar reward but instead, by mimicking the female pheromones of the insect, they attract bee and wasp males to pollinate them. Bolas spiders lure male moths by producing the moths’ female sex pheromone.
In this introductory chapter, I will touch on many classic systems such as moths, Drosophila, and mice, which are explored in more detail in the other chapters of this book, but I will also take the opportunity to illustrate points with the pheromones of other animal taxa such as nematodes, mollusks, and fish. For this introductory chapter, I have usually chosen recent references that will lead you to the relevant literature. More detail on the topics covered in this chapter can be found in Wyatt (2014).
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