Differential modulation of courtship behavior and subsequent aggression by octopamine, dopamine and serotonin in male crickets

Observations on neurophysiological pattern and behavioural traits as death-feigning mechanism in Eucryptorrhynchus scrobiculatus (Coleoptera: Curculionidae)

In adaptation to surrounding environmental stimuli, most insects exhibit defense behaviour (death feigning) to improve survival rates in the wild. However, the underlying mechanism of death feigning remains largely unknown. Here, we tested the neurophysiological pattern and behavioural traits of the death-feigning mechanism in the forestry pest Eucryptorrhynchus scrobiculatus. Using neuroanatomy, LC-MS/MS target metabolomics detection technology and qRT-PCR, we investigated the effects of neurochemicals and metabolic pathways in experimental weevils. Excision and drug tests were conducted to verify the key regulatory body parts involved in regulating the central nervous system in death feigning. Our results reconstructed the death-feigning mechanism of E. scrobiculatus: when the effective stimuli point of arousal weevils received mechanical stimulation, the thoracoabdominal ganglion transmitted signals into the brain through the ventral nerve cord, and then the brain regulated dopamine (DA) and serotonin (5-HT) metabolic pathways, reducing the expression of dopamine (dar2) and octopamine (oar1, oab2) receptor genes, finally inducing death feigning. Our study suggests that the variation of neurotransmitters in the brain is an important indicator of the physiological response of death feigning, and the results provide ecological and theoretical information for future investigations to reveal key behaviour and target genes for pest control.

Intrasexual aggression reduces mating success in field crickets

Aggressive behaviour is thought to have significant consequences for fitness, sexual selection and the evolution of social interactions, but studies measuring its expression across successive encounters-both intra- and intersexual-are limited. We used the field cricket Teleogryllus oceanicus to evaluate factors affecting repeatability of male aggression and its association with mating success. We quantified focal male aggression expressed towards partners and received from partners in three successive, paired trials, each involving a different male partner. We then measured a proxy of focal male fitness in mating trials with females. The likelihood and extent of aggressive behaviour varied across trials, but repeatability was negligible, and we found no evidence that patterns of focal aggression resulted from interacting partner identity or prior experience. Males who consistently experienced aggression in previous trials showed decreased male mating 'efficiency'-determined by the number of females a male encountered before successfully mating, but the effect was weak and we found no other evidence that intrasexual aggression was associated with later mating success. During mating trials, however, we observed unexpected male aggression towards females, and this was associated with markedly decreased male mating efficiency and success. Our findings suggest that nonadaptive aggressive spillover in intersexual mating contexts could be an important but underappreciated factor influencing the evolution of intrasexual aggression.

Bite force, body size, and octopamine mediate mating interactions in the house cricket (Acheta domesticus)

Mating interactions are rife with conflict because the evolutionary interests of males and females seldom coincide. Intersexual conflict affects sexual selection, yet the proximate factors underlying male coercive ability and female resistance are poorly understood. Male combat outcomes are often influenced by bite force, with superior biters being more likely to achieve victory over poorer biters in a range of species, including crickets. If good performers also achieve mating success through sexual coercion, then bite force might play a role in intersexual conflict as well. We tested the capacity of bite force to influence mating interactions in house crickets both directly by measuring bite forces of males and females and by altering male bite capacity through neuropharmacological manipulation. In addition, the invertebrate neurotransmitter octopamine both mediates aggression and underlies motivation to bite in male house crickets. By blocking octopamine receptors through the application of an antagonist, epinastine, we tested the effects of reduced bite force on male mating success. Our results show that male bite capacity, in combination with body size, influences both the likelihood and the outcomes of mating interactions, whereas treatment of males with epinastine eliminates motivation to mate. Our results suggest a functional role for bite force in affecting both sexual conflict and sexual selection and expand our knowledge of the influence of biogenic amines on reproductive behaviour.

Sex peptide regulates female receptivity through serotoninergic neurons in Drosophila

The courtship ritual is a dynamic interplay between males and females. Courtship successfully leading to copulation is determined by the intention of both parties which is conveyed by complex action sequences. In Drosophila, the neural mechanisms controlling the female's willingness to mate, or sexual receptivity, have only recently become the focus of investigations. Here, we report that pre-mating sexual receptivity in females requires activity within a subset of serotonergic projection neurons (SPNs), which positively regulate courtship success. Of interest, a male-derived sex peptide, SP, which was transferred to females during copulation acted to inhibit the activity of SPN and suppressed receptivity. Downstream of 5-HT, subsets of 5-HT7 receptor neurons played critical roles in SP-induced suppression of sexual receptivity. Together, our study reveals a complex serotonin signaling system in the central brain of Drosophila which manages the female's desire to mate.

Injected serotonin decreases foraging aggression in black widow spiders (Latrodectus hesperus), but dopamine has no effect

A fundamental goal of animal behavior research is to discover the proximate mechanisms driving individual behavioral differences. Biogenic amines are known to mediate various aspects of behavior across many species, including aggression, one of the most commonly measured behavioral traits in animals. Arthropods provide an excellent system to manipulate biogenic amines and quantify subsequent behavioral changes. Here, we investigated the role of dopamine (DA) and serotonin (5-HT) on foraging aggression in western black widow spiders (Latrodectus hesperus), as measured by the number of attacks on a simulated prey animal in the web. We injected spiders with DA or 5-HT and then quantified subsequent changes in behavior over 48 h. Based on previous work on insects and spiders, we hypothesized that increasing DA levels would increase aggression, while increasing 5-HT would decrease aggression. We found that injection of 5-HT did decrease black widow foraging aggression, but DA had no effect. This could indicate that the relationship between DA and aggression is complex, or that DA may not play as important a role in driving aggressive behavior as previously thought, at least in black widow spiders. Aggressive behavior is likely also influenced by other factors, such as inter-individual differences in genetics, metabolic rates, environment, and other neurohormonal controls.

Octopamine affects courtship call structure in male Acheta domesticus crickets

Secondary sexual displays vary considerably in both type and structure both within and across animal species. Although such variation is of keen interest to evolutionary biologists, the functional factors driving variation in male displays are poorly understood. In crickets, acoustic calls are produced by muscular contractions via stridulation of file and scraper wing components. We tested the effect of varying octopamine, an important biogenic amine neurohormone in invertebrates, on call production in male Acheta domesticus house crickets by blocking the octopamine receptors that influence skeletal muscle function with epinastine, a synthetic octopamine antagonist. We then measured male courtship calls and analyzed the call structure to quantify the differences in call structure based on the changes in carrier frequency, and whether chirps or ticks are a more prevalently expressed frequency in treated vs untreated males. Males treated with epinastine exhibited clear differences in call structure compared to untreated controls, such that epinastine-treated males were more likely to produce simpler calls and to exhibit their carrier frequencies as ticks rather than chirps. Thus, we were able to directly modify male courtship calling performance during mating interactions by altering the neuropharmacological milieu, demonstrating the potential role of biogenic amines in contributing to the diversity of call types in nature.

Octopamine modulates insect mating and Oviposition

The neuro-mechanisms that regulate insect reproduction are not fully understood. Biogenic amines, including octopamine, are neuromodulators that have been shown to modulate insect reproduction in various ways, e.g., promote or inhibit insect mating or oviposition. In this study, we examined the role of octopamine in regulating the reproduction behaviors of a devastating underground insect pest, the dark black chafer (Holotrichia parallela). We first measured the abundance of octopamine in different neural tissues of the adult chafer pre- and post-mating, demonstrating that octopamine decreased in the abdominal ganglia of females but increased in males post-mating. We then fed the adult H. parallela with a concentration gradient of octopamine to test the effects on insect reproductive behaviors. Compared with its antagonist mianserin, octopamine at the concentration of 2 µg/mL resulted in the highest increase in males' preference for sex pheromone and females' oviposition, whereas the mianserin-treatment increased the survival rate and prolonged the lifespan of H. parallela. In addition, we did not observe significant differences in egg hatchability between octopamine and mianserin-treated H. parallela. Our results demonstrated that octopamine promotes H. parallela mating and oviposition with a clear low dosage effect, illustrated how neural substrates modulate insect behaviors, and provided insights for applying octopamine in pest management.

Octopaminergic system orchestrates combat and mating behaviors: A potential regulator of alternative male mating tactics in an armed beetle

Male-male combats over females and territories are widespread across animal taxa. The winner of a combat gains resources, while the loser suffers significant costs (e.g. time, energy and injury) without gaining resources. Many animals have evolved behavioral flexibility, depending on their nutritional condition and experience, to avoid combat in order to reduce such costs. In these cases, male aggression often correlates with mating behavior changes, that is, the deployment of alternative reproductive tactics. Therefore, uncovering the physiological mechanism that orchestrates combat and mating behaviors is essential to understand the evolution of alternative mating tactics. However, so far, our knowledge is limited to specific behaviors (i.e., fighting or mating) of specific model species. In this study, we used an armed beetle (Gnatocerus cornutus) and hypothesized that one of the key neuromodulators of invertebrate aggression, octopamine (OA), would control male combat and other mating behaviors. Using receptor agonists (chlordimeform and benzimidazole), we showed that the octopaminergic (OAergic) system down-regulated the combat and courtship behaviors, while it up-regulated locomotor activity and sperm size. This suggests that the OAergic system orchestrates a suite of fighting and mating behaviors, thereby implying that correlated behavioral responses to OAergic signaling may have driven the evolution of alternative mating tactics in this beetle.

She's got nerve: roles of octopamine in insect female reproduction

The biogenic monoamine octopamine (OA) is a crucial regulator of invertebrate physiology and behavior. Since its discovery in the 1950s in octopus salivary glands, OA has been implicated in many biological processes among diverse invertebrate lineages. It can act as a neurotransmitter, neuromodulator and neurohormone in a variety of biological contexts, and can mediate processes including feeding, sleep, locomotion, flight, learning, memory, and aggression. Here, we focus on the roles of OA in female reproduction in insects. OA is produced in the octopaminergic neurons that innervate the female reproductive tract (RT). It exerts its effects by binding to receptors throughout the RT to generate tissue- and region-specific outcomes. OA signaling regulates oogenesis, ovulation, sperm storage, and reproductive behaviors in response to the female's internal state and external conditions. Mating profoundly changes a female's physiology and behavior. The female's OA signaling system interacts with, and is modified by, male molecules transferred during mating to elicit a subset of the post-mating changes. Since the role of OA in female reproduction is best characterized in the fruit fly Drosophila melanogaster, we focus our discussion on this species but include discussion of OA in other insect species whenever relevant. We conclude by proposing areas for future research to further the understanding of OA's involvement in female reproduction in insects.

Long-Term Dietary Restriction Leads to Development of Alternative Fighting Strategies

In competition for food, mates and territory, most animal species display aggressive behavior through visual threats and/or physical attacks. Such naturally-complex social behaviors have been shaped by evolution. Environmental pressure, such as the one imposed by dietary regimes, forces animals to adapt to specific conditions and ultimately to develop alternative behavioral strategies. The quality of the food resource during contests influence animals' aggression levels. However, little is known regarding the effects of a long-term dietary restriction-based environmental pressure on the development of alternative fighting strategies. To address this, we employed two lines of the wild-type Drosophila melanogaster Canton-S (CS) which originated from the same population but raised under two distinct diets for years. One diet contained both proteins and sugar, while the second one was sugar-free. We set up male-male aggression assays using both CS lines and found differences in aggression levels and the fighting strategies employed to establish dominance relationships. CS males raised on a sugar-containing diet started fights with a physical attack and employed a high number of lunges for establishing dominance but displayed few wing threats throughout the fight. In contrast, the sugar-free-raised males favored wing threats as an initial aggressive demonstration and used fewer lunges to establish dominance, but displayed a higher number of wing threats. This study demonstrates that fruit flies that have been raised under different dietary conditions have adapted their patterns of aggressive behavior and developed distinct fighting strategies: one favoring physical attacks, while the other one favoring visual threats.

Pre-adult aggression and its long-term behavioural consequences in crickets

Social experience, particularly aggression, is considered a major determinant of consistent inter-individual behavioural differences between animals of the same species and sex. We investigated the influence of pre-adult aggressive experience on future behaviour in male, last instar nymphs of the cricket Gryllus bimaculatus. We found that aggressive interactions between male nymphs are far less fierce than for adults in terms of duration and escalation. This appears to reflect immaturity of the sensory apparatus for releasing aggression, rather than the motor system controlling it. First, a comparison of the behavioural responses of nymphs and adults to mechanical antennal stimulation using freshly excised, untreated and hexane-washed antennae taken from nymphs and adults, indicate that nymphs neither respond to nor produce sex-specific cuticular semiochemicals important for releasing aggressive behaviour in adults. Second, treatment with the octopamine agonist chlordimeform could at least partially compensate for this deficit. In further contrast to adults, which become hyper-aggressive after victory, but submissive after defeat, such winner and loser effects are not apparent in nymphs. Aggressive competition between nymphs thus appears to have no consequence for future behaviour in crickets. Male nymphs are often attacked by adult males, but not by adult females. Furthermore, observations of nymphs raised in the presence, or absence of adult males, revealed that social subjugation by adult males leads to reduced aggressiveness and depressed exploratory behaviour when the nymphs become adult. We conclude that social subjugation by adults during pre-adult development of nymphs is a major determinant of consistent inter-individual behavioural differences in adult crickets.

Serotonin in Animal Cognition and Behavior

Serotonin (5-hydroxytryptamine, 5-HT) is acknowledged as a major neuromodulator of nervous systems in both invertebrates and vertebrates. It has been proposed for several decades that it impacts animal cognition and behavior. In spite of a completely distinct organization of the 5-HT systems across the animal kingdom, several lines of evidence suggest that the influences of 5-HT on behavior and cognition are evolutionary conserved. In this review, we have selected some behaviors classically evoked when addressing the roles of 5-HT on nervous system functions. In particular, we focus on the motor activity, arousal, sleep and circadian rhythm, feeding, social interactions and aggressiveness, anxiety, mood, learning and memory, or impulsive/compulsive dimension and behavioral flexibility. The roles of 5-HT, illustrated in both invertebrates and vertebrates, show that it is more able to potentiate or mitigate the neuronal responses necessary for the fine-tuning of most behaviors, rather than to trigger or halt a specific behavior. 5-HT is, therefore, the prototypical neuromodulator fundamentally involved in the adaptation of all organisms across the animal kingdom.

Octopamine neuron dependent aggression requires dVGLUT from dual-transmitting neurons

Neuromodulators such as monoamines are often expressed in neurons that also release at least one fast-acting neurotransmitter. The release of a combination of transmitters provides both "classical" and "modulatory" signals that could produce diverse and/or complementary effects in associated circuits. Here, we establish that the majority of Drosophila octopamine (OA) neurons are also glutamatergic and identify the individual contributions of each neurotransmitter on sex-specific behaviors. Males without OA display low levels of aggression and high levels of inter-male courtship. Males deficient for dVGLUT solely in OA-glutamate neurons (OGNs) also exhibit a reduction in aggression, but without a concurrent increase in inter-male courtship. Within OGNs, a portion of VMAT and dVGLUT puncta differ in localization suggesting spatial differences in OA signaling. Our findings establish a previously undetermined role for dVGLUT in OA neurons and suggests that glutamate uncouples aggression from OA-dependent courtship-related behavior. These results indicate that dual neurotransmission can increase the efficacy of individual neurotransmitters while maintaining unique functions within a multi-functional social behavior neuronal network.

The stalk-eyed fly as a model for aggression - is there a conserved role for 5-HT between vertebrates and invertebrates?

Serotonin (5-HT) has largely been accepted to be inhibitory to vertebrate aggression, whereas an opposing stimulatory role has been proposed for invertebrates. Herein, we argue that critical gaps in our understanding of the nuanced role of 5-HT in invertebrate systems drove this conclusion prematurely, and that emerging data suggest a previously unrecognized level of phylogenetic conservation with respect to neurochemical mechanisms regulating the expression of aggressive behaviors. This is especially apparent when considering the interplay among factors governing 5-HT activity, many of which share functional homology across taxa. We discuss recent findings using insect models, with an emphasis on the stalk-eyed fly, to demonstrate how particular 5-HT receptor subtypes mediate the intensity of aggression with respect to discrete stages of the interaction (initiation, escalation and termination), which mirrors the complex behavioral regulation currently recognized in vertebrates. Further similarities emerge when considering the contribution of neuropeptides, which interact with 5-HT to ultimately determine contest progression and outcome. Relative to knowledge in vertebrates, much less is known about the function of 5-HT receptors and neuropeptides in invertebrate aggression, particularly with respect to sex, species and context, prompting the need for further studies. Our Commentary highlights the need to consider multiple factors when determining potential taxonomic differences, and raises the possibility of more similarities than differences between vertebrates and invertebrates with regard to the modulatory effect of 5-HT on aggression.