Alarm pheromones

Alarm pheromones are either produced upon detection of a predator or passively released after wounding following a predator attack. In both cases, the cue warns receiving conspecifics, thereby evoking either defensive or escape behavior. Due to the obvious advantages of being able to interpret alarm signals also of other species, alarm pher-omones are normally not very species specific: the same pheromone may evoke alarm responses in two phylogen-etically distant species. From an evolutionary point of view, alarm signals are perplexing, because the signaler becomes even more conspicuous and thus increasingly threatened with predation than a silent neighbor.

Benefits to close kin may account for the evolution of these apparently altruistic signals.

Indeed, actively produced alarm pheromones are exceedingly common in many clonal (e.g., aphids, sea anemones), presocial (e.g., tadpoles, deer, rodents), and eusocial organisms (e.g., ants, bees, termites). For example, when parthenogenetically reproducing aphids are attacked by a predator or parasitoid, they release droplets that contain an alarm pheromone whose main and sometimes only component is the sesquiterpene (£)-/3-farnesene. The droplet induces escape behavior in the receiving clonemates such as running away or dropping off the plant. In eusocial insects, alarm pheromones are primarily involved in the disturbance of nests. Alarm behavior in ants, bees, wasps, and termites is often accompanied by the liberation of pheromones that attract nestmates, which aggressively attack foreign assailants. The intruder is warded off by biting and stinging and frequently marked with alarm pheromones to become a chemical beacon for the frenetic workers to attack.

A general characteristic of actively produced alarm signals is their short fade-out time. This is reflected by their relatively low molecular weight that usually ranges between 100 and 200 u.

Chemical releasers of escape behavior, however, can also be found among unrelated individuals such as fish, amphibians, or marine invertebrates. In these cases, injuries caused by predatory attack often entail the passive release of chemical compounds from internal body tissues of the prey. These chemicals serve as alarm cues and induce in nearby conspecifics antipredator behavior such as flight, increased shelter use, shoaling, or freezing. Hermit crabs (Clibanarius vittatus), for example, detect stimuli from injured conspecifics and respond by either fleeing or investigating the source of the chemicals. Fleeing removes the predation risk and inspecting the source of the chemicals opens the opportunity to acquire the shell of the victim, which may be larger than their own. Damage-released signals, however, benefit the receiver rather than the sender (i.e., the victim). A situation in which the sender could benefit from the release of alarm pheromone is if the pheromone attracted secondary predators that disrupt a predation event and allow the prey to escape. Such a case has been reported for fathead minnows (Pimephales promelas). After being captured by a northern pike (Esox lucius), fathead minnows release alarm pheromones from broken club cells in their skin. The released pheromone involved hypoxanthine-3-^-oxide and other nitrogen-oxide-containing compounds, attracted a second pike that interfered with the first one, and increased the chances of the bitten minnow to escape.

Aggregation pheromones

Pheromone-mediated aggregations may facilitate reproduction or feeding and provide protection from natural enemies or environmental conditions. The so-called Allee effect that denotes a positive relationship between fitness and population size or density can generally explain aggregative behavior patterns.

The best-documented example of aggregation pher-omones causing communal feeding comes from bark beetles (Scolytidae). These beetles attack healthy trees and can surmount antiherbivore defense of their host tree (i.e., toxic chemicals such as phenolic compounds or physical barriers such as resins). Tunneling and inoculation with fungi, which the pheromone producing sex carries in its specialized mycangia, help to circumvent these defense mechanisms. Single beetles, however, have no chance to attack a living tree. Only mass attacks of thousands of beetles attracted by aggregation pheromones can successfully colonize the tree and render the tree suitable for feeding of both larvae and adult beetles. Female bark beetles of the species Dendroctonus brevicomis are initially attracted to their host plant (Pinus ponderosa) by compounds that are normally involved in the tree's herbivore defense, such as the monoterpene myrcene. Females boring into the tree release the sex pheromone (+)-exo-brevicomin, which synergistically acts together with the myrcene to attract male beetles. Arriving males enter the burrows and start to liberate their own pheromone (—)-frontalin. The mixture of the tree-derived myrcene with both (+)-exo-brevicomin and (—)-frontalin conjoins to give a highly attractive aggregation pheromone that attracts both sexes in high numbers. This complex mode of attraction may have originated from an ancestral mate-finding system.

Spatial aggregations may also function as a defense against predation by diluting the risk of being eaten (the likelihood of being attacked is decreased for each individual), by confusing the predator (collective evasion behavior, for example, shoaling in fish), or by swamping the predator with prey (exceeding the predator's feeding capacity). An example for chemically mediated defensive aggregations can be found in the California spiny lobster (Panulirus interruptus). Den-inhabiting lobsters are vulnerable as individuals but not in aggregations, because cohabiting animals collectively wave their antennae to fend off predatory fishes, thereby reducing the penetrability of a den to these predators. Compounds, which are emitted by both sexes, are responsible for the formation of these aggregations.

Furthermore, pheromone-mediated aggregations can help endure harsh climatic conditions by reducing the water loss (e.g., ladybirds, cockroaches, house mites, ticks, and barnacles) or increasing the thermal tolerance (e.g., communal hibernation in garter snakes, thermoregulation in army ant bivouacs).

Pheromones involved in marking home ranges and territories

Scent marks play an important role in the territorial behavior of terrestrial vertebrates including lizards and salamanders. The territory of an animal can be defined as an area that is consistently defended against conspecifics to secure resources like food or den sites. In contrast, the term 'home range' refers to the area within which an animal concentrates its activity, but which is not necessarily defended against others, and which can overlap with neighboring home ranges. Territories and home ranges may be held by an individual, a mated pair, or a group, and are often marked by glandular secretions, feces, and/or urine, which are placed at conspicuous sites. Scent marks are placed in lines along or near the edge of the territory (boundary marking; many carnivores, such as wolf (Canis lupus)), throughout the territory, or toward its center (hinterland marking; e.g., honey badger (Mellivora capensis); Figure 6). Both types of marking represent a continuum with a strong intra- and interspecific variation depending on many ecological factors such as habitat structure, population density, home range size, and social structure. In general, males tend to mark more than females and dominant males mark more than subordinate individuals.

The obvious benefit of scent marks is that an animal can leave relatively long-lasting messages that can be 'read' afterward by conspecifics, even at night or in dense vegetation. On the other hand, scent marking can incur considerable costs because a certain amount of time has to be allocated to marking and scent marks may be eavesdropped by predators or parasites.

Despite their widespread use, the actual function of scent marks is still unclear. Among others, the following main hypotheses have been proposed: (1) Scent marks deter potential intruders (scent fence hypothesis). (2) Intruders can recognize the holder of the territory by matching his odors with the scent marks spread around the territory (scent matching hypothesis). (3) Scent marks act as 'railway signals' that facilitate the orientation of the resident within its territory and/or mediate spatiotemporal separation of neighbors (spatiotemporal separation hypothesis). (4) Scent marks (urine in particular) contain information about the reproductive status and estrous condition of a female and are used to attract males for mating (reproductive advertisement hypothesis). Many species also scent-overmark, that is, they deposit scents that partially overlap or completely cover the mark of another individual. Even though the function of the scent marks is not perfectly understood, it seems very likely that they represent an honest signal of phenotypic quality that can be used by conspecifics to evaluate competitors or potential mates.

Also, some ant species use odors for territorial marking. Weaver ants of the genus Oecophylla, for example, use

• Token urination positions v Latrines visited by female

Figure 6 Hinterland marking in the honey badger Mellivora capensis (Mustelidae). (a) Female honey badger. (b) Spatial distribution of token urination sites (i.e., few drops of urine) and latrines (i.e., common defecation and scent mark sites that are visited by a variety of different individuals of both sexes) within the home range of an adult female (201 km2). The dotted line represents the line subjectively drawn after completing the mapping process to close the polygon. (a) Photograph by Colleen and Keith Begg, with permission. (b) Reprinted from Animal Behaviour, Vol. 66, Begg CM, Begg KS, du Toit JT, and Mills MGL, Scent marking behavior of the honey badger, Mellivora capensis (Mustalidae) in the southern Kalahari, pp. 917-929, Copyright (2003), with permission from Elsevier.

• Token urination positions v Latrines visited by female

Figure 6 Hinterland marking in the honey badger Mellivora capensis (Mustelidae). (a) Female honey badger. (b) Spatial distribution of token urination sites (i.e., few drops of urine) and latrines (i.e., common defecation and scent mark sites that are visited by a variety of different individuals of both sexes) within the home range of an adult female (201 km2). The dotted line represents the line subjectively drawn after completing the mapping process to close the polygon. (a) Photograph by Colleen and Keith Begg, with permission. (b) Reprinted from Animal Behaviour, Vol. 66, Begg CM, Begg KS, du Toit JT, and Mills MGL, Scent marking behavior of the honey badger, Mellivora capensis (Mustalidae) in the southern Kalahari, pp. 917-929, Copyright (2003), with permission from Elsevier.

pheromones to mark an area as belonging to a given colony and being subject to defense. When entering a new terrain, workers of this arboricolous ant species recruit nestmates by laying a trail, which attracts nest-mates in large numbers. In a second step, recruited workers mark the territory with a brown fluid that is extruded from the anus and which hardens into a convex solid. Conspecific workers from an alien colony respond to these spots initially with hostility and aversion, then by recruiting nestmates. Thus, the anal substance functions as a true territorial pheromone.

Other ant species, such as the leaf-cutting ant Atta cephalotes, use pheromones to mark their nest entrances in order to assist foragers in the final stages of homing. Such nest exit pheromones are considerably more persistent than the ephemeral recruitment pheromones and show a high degree of colony specificity. In contrast to home range and territorial marking, colony-specific nest marking has been described in many ant species and seems to be a widespread phenomenon.

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