Social pheromones

In complex animal societies such as social insects or mammals, semiochemicals play a crucial role in the maintenance of social homeostasis, that is, regulation of the population density, behavior, and physiology of the society members. The pheromones involved can have priming effects, which predominantly coordinate reproduction and the division of labor (see section titled 'Primer pheromones'), or act as releasers. The most important function of social pheromones that affect the behavior of receptive conspecifics is the recognition of kin or fellow group members. The ability to distinguish groupmates from non-groupmates is central for according favors to sufficiently related individuals only, and for avoiding close kin as mates. The chemical signatures involved in these recognition processes are often very complex and can be determined by genetic factors, the environment (e.g., diet, nest odor, maternal labels), or a combination of both. In nearly all animal species investigated to date, the discrimination of familiar from unfamiliar odors seems to involve learning signatures and then matching this template against the signature of other animals. Two general principles can be distinguished, as discussed here.

The first mechanism, which is termed direct familiarization, involves learning the characteristics of surrounding individuals and recognizing these animals later. Regardless of whether these cues are genetically or environmentally determined, individuals might recognize as a relative any conspecific encountered in a location that predictably contains only kin (e.g., a nest or burrow). Such direct familiarization accounts for the kin-differential behavior among newly emerged juveniles in Columbian ground squirrels (Spermophilus columbianus), to take one case. Another example is territorial mammals that often respond more aggressively to invasion by non-neighbors than territorial neighbors (i.e., dear enemy effect). This concept is exemplified in the Eurasian beaver (Castorfiber) that responds stronger to experimental scent marks from strangers (i.e., destroying scent mounds and overmarking them) than to those of neighbors. Further examples of animals that learn to discriminate familiar from unfamiliar social odors can be found in birds (Pachyptila desolata), reptiles (Diposaurus dorsalis), rodents (Microtus pennysylva-nicus), bats (Pipistellus pipistrellus), and primates (Lemur fulmus).

The second mechanism, which animals use to recognize other kin, is the so-called indirect familiarization. This principle, which is also called phenotypic matching, is implemented in recognition systems using either the characteristics of nestmates or the own-odor phenotype (i.e., armpit effect or self-inspection) as a reference to identify unfamiliar kin. The primitive eusocial paper wasp (Polistes fuscatus), for instance, will accept orphan kin (aunts, nieces, and a minority of cousins from nearby nests which have been destroyed), but keep away non-kin, which may rob the nest. The cues involved are hydrocarbon secretions from the sternal glands of both workers and queens, which are secreted onto the paper comb. These colony-specific and heritable hydrocarbons are learned by hatching pupae, thereby allowing them later to distinguish close relatives from non-kin. Similar recognition mechanisms are found not only in other species of social insects (wasps, bees, ants, termites), but also in vertebrates such as fish (Salvelinus alpinus), amphibians (Bufo americanus), reptiles (Iguana iguana), rodents (Mus musculus), and many mammalian species including humans. Familiarization and phenotype matching often occurs during a sensitive period at early life stages and the social pheromones involved may act as both primers and releasers.

Self-matching as a mechanism to determine the degree of relatedness offers some advantages over using parents or nestmates as a reference: it can also operate when individuals encounter unfamiliar kin for the first time, it can mediate kin recognition when multiple paternities occur within litters, and it is more robust against brood parasites. However, due to the experimental difficulty of denying an individual's experience with its own pheno-type while still allowing normal development, few cases of this type of phenotype matching are known. These involve the bluegill sunfish (Lepomis macrochirus), golden hamsters (Mesocricetus auratus), peacocks (Pavo cristatus), and chacma baboons (Papio cynocephalus).

Finally, recognition may operate without any previous learning experience and be purely genetically based. Here, the presence of a recognition allele which codes for an olfactory or visual signal can be recognized in another individual, no matter if kin or not. The only case known in animals for such a green beard phenomenon, as it is sometimes called, is that of the red imported fire ant Solenopsis invicta. These polygynous ants use a system of three linked genes coding for an odor cue (green beard) that can be recognized in others and causes all workers that bear the allele to kill all queens that do not bear it.

Another purely genetically based recognition system is the so-called quorum sensing, which is used by many

Figure 7 Examples for bacterial pheromones involved in quorum sensing of (a) Gram-positive and (b) Gram-negative bacteria. The respective species are given in brackets. Most intensively studied are the N-acylhomoserine lactones such as that produced by Vibrio fischeri. Adapted from http://www.nottingham.ac.uk/quorum/molecules.htm with permission from Paul Williams.

(a) Gram +

(b) Gram -

1

o H

OO

3-Oxo-C6-homoserine lactone (Vibrio fischeri )

Butyrolactone (Streptomyces griseus)

H Tyr Ile Asn N lpLeu H Phe H 0

2-Heptyl-3-hydroxyl-4-quinalone (Pseudomonas aeruginosa)

Cyclic thiolactone (Staphylococcus aureus)

bacteria to sense the population of bacteria around them (Figure 7). Even though the type of pheromone used strongly depends on the bacterial species, the principle of quorum sensing applies rather generally. The concentration of a specific signaling compound, which is released by the bacteria into the environment, increases with increasing population sizes. Monitoring its concentration provides bacteria with the capacity to determine the number of bacterial cells of the same species in the vicinity and adjust their gene regulation accordingly. Benefits of this quorum-sensing system include access to complex nutrients and environmental niches, collective defense against other microorganisms, or the enhanced ability to overcome eukaryotic host defenses. The chemicals involved are N-acylhomoserine lactones in Gram-negative and 7-butyrolactones or peptides in Gram-positive bacteria.

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