Alarm Pheromone Composition and Behavioral Activity in Fungus-Growing Ants

Pheromone representation in the ant antennal lobe changes with age

While the neural basis of age-related decline has been extensively studied,1,2,3 less is known about changes in neural function during the pre-senescent stages of adulthood. Adult neural plasticity is likely a key factor in social insect age polyethism, where individuals perform different tasks as they age and divide labor in an age-dependent manner.4,5,6,7,8,9 Primarily, workers transition from nursing to foraging tasks,5,10 become more aggressive, and more readily display alarm behavior11,12,13,14,15,16 as they get older. While it is unknown how these behavioral dynamics are neurally regulated, they could partially be generated by altered salience of behaviorally relevant stimuli.4,6,7 Here, we investigated how odor coding in the antennal lobe (AL) changes with age in the context of alarm pheromone communication in the clonal raider ant (Ooceraea biroi).17 Similar to other social insects,11,12,16 older ants responded more rapidly to alarm pheromones, the chemical signals for danger. Using whole-AL calcium imaging,18 we then mapped odor representations for five general odorants and two alarm pheromones in young and old ants. Alarm pheromones were represented sparsely at all ages. However, alarm pheromone responses within individual glomeruli changed with age, either increasing or decreasing. Only two glomeruli became sensitized to alarm pheromones with age, while at the same time becoming desensitized to general odorants. Our results suggest that the heightened response to alarm pheromones in older ants occurs via increased sensitivity in these two core glomeruli, illustrating the importance of sensory modulation in social insect division of labor and age-associated behavioral plasticity.

Pheromone representation in the ant antennal lobe changes with age

While the neural basis of age-related decline has been extensively studied (1-3), less is known about changes in neural function during the pre-senescent stages of adulthood. Adult neural plasticity is likely a key factor in social insect age polyethism, where individuals perform different tasks as they age and divide labor in an age-dependent manner (4-9). Primarily, workers transition from nursing to foraging tasks (5, 10), become more aggressive, and more readily display alarm behavior (11-16) as they get older. While it is unknown how these behavioral dynamics are neurally regulated, they could partially be generated by altered salience of behaviorally relevant stimuli (4, 6, 7). Here, we investigated how odor coding in the antennal lobe (AL) changes with age in the context of alarm pheromone communication in the clonal raider ant (Ooceraea biroi) (17). Similar to other social insects (11, 12, 16), older ants responded more rapidly to alarm pheromones, the chemical signals for danger. Using whole-AL calcium imaging (18), we then mapped odor representations for five general odorants and two alarm pheromones in young and old ants. Alarm pheromones were represented sparsely at all ages. However, alarm pheromone responses within individual glomeruli changed with age, either increasing or decreasing. Only two glomeruli became sensitized to alarm pheromones with age, while at the same time becoming desensitized to general odorants. Our results suggest that the heightened response to alarm pheromones in older ants occurs via increased sensitivity in these two core glomeruli, illustrating the importance of sensory modulation in social insect division of labor and age-associated behavioral plasticity.

Alarm communication predates eusociality in termites

Termites (Blattodea: Isoptera) have evolved specialized defensive strategies for colony protection. Alarm communication enables workers to escape threats while soldiers are recruited to the source of disturbance. Here, we study the vibroacoustic and chemical alarm communication in the wood roach Cryptocercus and in 20 termite species including seven of the nine termite families, all life-types, and all feeding and nesting habits. Our multidisciplinary approach shows that vibratory alarm signals represent an ethological synapomorphy of termites and Cryptocercus. In contrast, chemical alarms have evolved independently in several cockroach groups and at least twice in termites. Vibroacoustic alarm signaling patterns are the most complex in Neoisoptera, in which they are often combined with chemical signals. The alarm characters correlate to phylogenetic position, food type and hardness, foraging area size, and nesting habits. Overall, species of Neoisoptera have developed the most sophisticated communication system amongst termites, potentially contributing to their ecological success.

Interpretive review: Semiochemicals in domestic pigs and dogs

This interpretive review includes discussion of the available scientific literature with interpretations by the authors. The broad field of semiochemicals can be confusing to scientists and consumers. This review attempts to summarize the known scientific studies for pig and dog semiochemicals while at the same time attempting to refine our use of terminology. The specific objectives of this interpretive review are to summarize and interpret much of the key scientific literature (but not the lay literature) on semiochemicals in pigs and dogs to include (1) definitions of semiochemicals and related molecules including pheromones, (2) to briefly summarize olfactory organs, and (3) and to examine the scientific literature for semiochemical mechanisms and applications in dogs and pigs (two domesticated species with known olfactory acuity). Dogs and pigs have olfactory features that are similar in that they both lack certain olfactory organs (Grueneberg ganglion and Septal Organ) and they have a small vomeronasal organ (VNO) without some major receptors that are found in other species. The primary olfactory organs for both pigs and dogs are the main olfactory epithelium and perhaps the trigeminal nerve. Several examples of pheromones activating the brain via the MOE or Trigeminal nerve rather than the VNO challenge the concept that the VNO is the site of pheromone sensing. We believe it is not appropriate to label something a pheromone when evidence is not available to show that it is a pheromone. We offer definitions for the terms semiochemicals, pheromones, interomones and others and then determine if the evidence is sufficient to call certain semiochemicals a pheromone. Here we review mixed, largely negative, scientific reports of the efficacy of some products labeled as "pheromones" that are more appropriately called semiochemicals. Interomones can have a more powerful effect on dog behavior and physiology than semiochemicals marketed as pheromones. Because marketing of semiochemicals is far ahead of the science, bringing some logic and uniformity to the field will benefit animals and hopefully cause less consumer confusion. Semiochemicals have the potential to offer powerful solutions to behavioral problems using more naturally occurring molecules.

Rustling ants: Vibrational communication performed by two Camponotus species in Borneo

Interactions between ants and plants are classic examples of cooperation between individuals of different species. Usually, plants provide shelter or food for ants and in turn are defended against herbivores by their insect allies. To coordinate attacks, ants use multi-modal alarm signals consisting of vibrational and chemical components. This can also be observed in Borneo, where two Camponotus species inhabit the ocreas (diverging, tubular leaf sheaths) of the rattan palm Korthalsia robusta. When ants are disturbed, they beat or scratch mandibles and abdomens on the plant surface resulting in loud rustling sounds. To describe the characteristics of these signals, we recorded them with a Laser-Doppler-vibrometer in the field. Analyses of temporal patterns and dominant frequency revealed that the signals of the two species differ fundamentally. To assess transmission characteristics of the rattan palm, we conducted experiments under controlled lab-conditions. We show that the ocrea is an adequate structure for converting airborne sound into substrate vibrations, acting as a mediator between these two modalities. We hypothesize that the ants' vibratory signal has multiple functions, with the substrate-borne component used as an alarm signal for conspecifics, and the airborne component acting as vibro-acoustic aposematism against predators or herbivores to protect the host plant.

Behavioral performance and division of labor influence brain mosaicism in the leafcutter ant Atta cephalotes

Brain evolution is hypothesized to be driven by behavioral selection on neuroarchitecture. We developed a novel metric of relative neuroanatomical investments involved in performing tasks varying in sensorimotor and processing demands across polymorphic task-specialized workers of the leafcutter ant Atta cephalotes and quantified brain size and structure to examine their correlation with our computational approximations. Investment in multisensory and motor integration for task performance was estimated to be greatest for media workers, whose highly diverse repertoire includes leaf-quality discrimination and leaf-harvesting tasks that likely involve demanding sensory and motor processes. Confocal imaging revealed that absolute brain volume increased with worker size and functionally specialized compartmental scaling differed among workers. The mushroom bodies, centers of sensory integration and learning and memory, and the antennal lobes, olfactory input sites, were larger in medias than in minims (gardeners) and significantly larger than in majors ("soldiers"), both of which had lower scores for involvement of olfactory processing in the performance of their characteristic tasks. Minims had a proportionally larger central complex compared to other workers. These results support the hypothesis that variation in task performance influences selection for mosaic brain structure, the independent evolution of proportions of the brain composed of different neuropils.

Insect alarm pheromones in response to predators: Ecological trade-offs and molecular mechanisms

Insect alarm pheromones are chemical substances that are synthesized and released in response to predators to reduce predation risk. Alarm pheromones can also be perceived by predators, who take advantage of alarm cues to locate prey. While selection favors evolution of alarm pheromone signals that are not easily detectable by predators, predator evolution selects for better prey detection ability. Here, we review the diversity of alarm signals, and consider the behavioral and ecological conditions under which they have evolved. We show that components of alarm pheromones are similar across many insects, although aphids exhibit different behavioral responses to alarm cues compared to social insects. The effects of alarm pheromones on prey behavior depend on factors such as the concentration of pheromones and the density of conspecifics. We also discuss the molecular mechanisms of alarm pheromone perception underlying the evolutionary arms race between predators and prey, and the function of olfactory proteins and receptors in particular. Our review provides a novel synthesis of the diversity and function of insect alarm pheromones, while suggesting avenues that might better allow researchers to exploit population-level responses to alarm signaling for the sustainable management of pests and vector-borne pathogens.

Mate attraction, chemical defense, and competition avoidance in the parasitoid wasp Leptopilina pacifica

A major hypothesis for the evolution of chemical signals is that pheromones arise from non-communicative precursor compounds. However, data supporting this hypothesis are rare, primarily because the original functions of the antecedent compounds often have been lost. A notable exception, however, is the parasitoid wasp species Leptopilina heterotoma, whose compound (−)-iridomyrmecin is used as a defensive secretion, a cue for females to avoid competition with con- and hetero-specific females, and as the primary component of the females’ sex pheromone. To better understand the evolution of sex pheromones from defensive compounds, we examined the chemical ecology of L. pacifica, the sister species of L. heterotoma. Here, we show that L. pacifica also produces a defensive secretion containing a species-specific mixture of mostly iridoid compounds. However, the composition of the secretion is more complex than in L. heterotoma, and iridomyrmecin is only a minor component. Moreover, in contrast to L. heterotoma, conspecific female competitors were not avoided by female subjects, and a role of the iridoids in the female sex pheromone of L. pacifica can be excluded, as only the females’ cuticular hydrocarbons (CHCs) resulted in the elicitation of courtship by males. Although closely related, the two sister species show substantial differences in the use of the defensive secretion for communicative purposes. Variation in pheromone usage in this genus still presents a conundrum, highlighting the need for additional studies to understand the selective forces shaping the evolution of pheromone composition.

Age-dependent release of and response to alarm pheromone in a ponerine ant

Social insect societies are characterized by division of labour and communication within the colony. The most frequent mode of communication is by chemical signals. In general, pheromones elicit specific responses in the receiver, although reactions may vary depending on the receiving individual's physiological or motivational state. For example, it has been shown that pheromones can elicit different responses in morphological worker castes. However, comparably little is known about such effects in worker castes of monomorphic species. Here, we comprehensively studied a monomorphic species showing age polyethism, the thelytokous ant Platythyrea punctata Our analyses revealed that the species' alarm pheromone consists of (S)-(-)-citronellal and (S)-(-)-actinidine, and is produced in the mandibular glands. Ants responded with increased movement activity and increasing ant density towards the pheromone source in whole-colony bioassays, confirming the alarming effect of these compounds. We found age classes to differ in their absolute pheromone content, in the propensity to release alarm pheromone upon disturbance and in their reaction towards the pheromone. Absolute amounts of pheromone content may differ simply because the biosynthesis of the pheromone begins only after adult eclosion. Nonetheless, our results indicate that this clonal species exhibits age-related polyethism in the emission of as well as in the response to its alarm pheromone.

Volatiles from the Mandibular Gland Reservoir Content of Colobopsis explodens Laciny and Zettel, 2018, Worker Ants (Hymenoptera: Formicidae)

Forty-five volatile organic compounds (VOCs) were identified or annotated in the mandibular gland reservoir content (MGRC) of the Southeast Asian ant Colobopsis explodens Laciny and Zettel, 2018 (Hymenoptera: Formicidae), using headspace solid-phase microextraction (HS-SPME) coupled to gas chromatography-mass spectrometry (GC-MS) and liquid extraction combined with GC-MS. In extension of previous reports on VOCs of C. explodens, members of different compound classes, such as alkanes, aliphatic and aromatic carboxylic acids, and phenolics, were detected. The ketone 2-heptanone and the biochemically related phenolics benzene-1,3,5-triol (phloroglucinol, PG), 1-(2,4,6-trihydroxyphenyl)ethanone (monoacetylphloroglucinol, MAPG), 5,7-dihydroxy-2-methylchromen-4-one (noreugenin), and 1-(3-Acetyl-2,4,6-trihydroxyphenyl)ethanone (2,4-diacetylphloroglucinol, DAPG) dominated the GC-MS chromatograms. The identities of the main phenolics MAPG and noreugenin were further verified by liquid chromatography-high resolution-tandem mass spectrometry (LC-HRMS/MS). A comparative study of MGRC samples originating from three distinct field expeditions revealed differences in the VOC profiles, but the presence and relative abundances of the dominating constituents were largely consistent in all samples. Our study considerably extends the knowledge about the number and type of VOCs occurring in the MGRC of C. explodens. Based on the type of the detected compounds, we propose that the likely irritant and antibiotic phenolic constituents play a role in defense against arthropod opponents or in protection against microbial pathogens.

Comparative chemical analysis of army ant mandibular gland volatiles (Formicidae: Dorylinae)

Army ants are keystone species in many tropical ecosystems. Yet, little is known about the chemical compounds involved in army ant communication. In the present study, we analyzed the volatile mandibular gland secretions—triggers of ant alarm responses—of six Neotropical army ant species of the genus Eciton (outgroup: Nomamyrmex esenbeckii). Using solid-phase microextraction, we identified 12 chemical compounds, primarily ketones with associated alcohols, one ester and skatole. Most compounds were shared among species, but their relative composition was significantly different. By comparing chemical distances of mandibular gland secretions to species divergence times, we showed that the secretions’ compositions are not strictly determined by phylogeny. By identifying chemical bouquets of seven army ant species, our study provides a valuable comparative resource for future studies aiming to unveil the chemicals’ precise role in army ant alarm communication.

Verification of Argentine ant defensive compounds and their behavioral effects on heterospecific competitors and conspecific nestmates

The invasive Argentine ant (Linepithema humile) has become established worldwide in regions with Mediterranean or subtropical climates. The species typically disrupts the balance of natural ecosystems by competitively displacing some native ant species via strong exploitation and interference competition. Here we report that Argentine ants utilize glandular secretions for inter and intra-specific communications during aggressive interactions with a heterospecific competitor, California harvester ant (Pogonomyrmex californicus). Chemical analyses indicated that Argentine ants deploy glandular secretions containing two major volatile iridoids, dolichodial and iridomyrmecin, on the competitor's cuticular surface during aggressive interactions. Bioassays indicated that the glandular secretions function as a defensive allomone, causing high levels of irritation in the heterospecific. Furthermore, the same glandular secretions elicited alarm and attraction of conspecific nestmates, potentially enabling more rapid/coordinated defense by the Argentine ants. Two major volatile constituents of the glandular secretion, dolichodial and iridomyrmecin, were sufficient to elicit these responses in conspecifics (as a mixture or individual compounds). The current study suggests that invasive Argentine ants' superior exploitation and interference competition may rely on the species' effective semiochemical parsimony.

Gland Origin and Electroantennogram Activity of Volatile Compounds in Ghost Ants, Tapinoma melanocephalum (Hymenoptera: Formicidae) and Behavioral Response to (Z)-9-Nonadecene

Volatile compounds in Tapinoma melanocephalum (Fabricius) (Hymenoptera: Formicidae, Dolichoderinae) workers were analyzed using gas chromatography-mass spectrometry and electroantennogram responses of workers to these compounds were investigated using coupled gas chromatography-electroantennographic detection. Among 18 detected compounds, six elicited electroantennogram response, including 6-methyl-5-hepten-2-one, 4 iridodials, and (Z)-9-nonadecene. (Z)-9-Nonadecene and iridodials were identified the first time in T. melanocephalum. (Z)-9-Nonadecene is a major component in mandibular glands and iridodials are produced in pygidial glands. In contrast to previous report, actinidine was not found in pygidial glands. Behavioral response of workers to the synthetic (Z)-9-nonadecene was investigated. (Z)-9-nonadecene is an attractant to T. melanocephalum. It also affected their locomotion patterns.