Resource defense and juvenile hormone: the "challenge hypothesis" extended to insects

Social interactions upregulate hemolymph tryptophan and tyrosine levels in the male lobster cockroach

In the present study, we found that tryptophan (TRP) and tyrosine (TYR) levels are increased in hemolymph of male Nauphoeta cinerea after social contact with either male or female conspecifics. Hemolymph was collected from individual males before and after the social interactions, and samples were analyzed by HPLC-ECD; analyte identities were confirmed by UPLC/MS. After a male-male first encounter fight, hemolymph TRP and TYR levels were significantly increased in dominants compared with the levels before the encounter. Conversely, TRP and TYR in subordinates were maintained at levels similar to those before the encounter. While after-fight TRP and TYR levels were significantly higher in dominants than subordinates, no significant differences were found in the contestants before the fight. Moreover, contact with an isolated male antenna was sufficient to stimulate attack behavior and increase hemolymph TRP and TYR titers to levels similar to those seen in dominants. After a male-female interaction, two distinct outcomes could be observed. Either hemolymph TRP and TYR levels were increased in successfully mated males, or TRP and TYR levels were unchanged in males that only exhibited premating wing-raising behavior but failed in mating. After contacting the antenna of a socially naïve male with an isolated female antenna, three patterns of behavior and related amino acid response were observed: 1) only premating wing-raising behavior with significant increase of TRP and TYR levels, 2) only attack behavior with significant increase of TRP and TYR levels, and 3) mixed wing-raising and attack behaviors with no significant changes in TRP and TYR levels. The present results show a robust response of hemolymph TRP and TYR to social contact. In light of previously characterized responses in pheromone and juvenile hormone levels, these amine responses suggest that the physiological response of N. cinerea to social contact is multi-dimensional.

Whither the challenge hypothesis?

Almost fifty years ago the advent of assay methods to measure circulating levels of hormones revolutionized endocrinology in relation to investigations of free-living and captive animals. This new field "environmental endocrinology" revealed that endocrine profiles in animals in their natural habitat were not only different from captive animals, but often deviated from predictions. It quickly became apparent that the organization and analysis of data from the field should be sorted by life history stages such as for reproductive processes, migration, molt etc. and spaced in time according to natural duration of those processes. Presentation of data by calendar date alone gives much simpler, even misleading, patterns. Stage-organized analyses revealed species-specific patterns of hormone secretion and dramatic inter-individual differences. The "Challenge Hypothesis" sparked exploration of these results, which diverged from expectations of hormone-behavior interactions. The hypothesis led to specific predictions about how the hypothalamo-pituitary-gonad axis, and particularly circulating patterns of testosterone, might respond to social challenges such as simulated territorial intrusions. Initially, a group of studies on free-living and captive birds played a key role in the formulation of the hypothesis. Over the decades since, the effects of social challenge and environmental context on hormonal responses have been tested in all vertebrate taxa, including humans, as well as in insects. Although it is now clear that the Challenge Hypothesis in its original form is simplistic, field and laboratory tests of the hypothesis have led to other concepts that have become seminal to the development of environmental endocrinology as a field. In this special issue these developments are addressed and examples from many different taxa enrich the emerging concepts, paving the way for investigations using recent technologies for genetic and transcriptome analyses.

The challenge hypothesis in insects

The 'challenge hypothesis' provides a predictive framework for how the social environment influences within-species variation in hormone titers. High testosterone levels are beneficial during reproduction and competition, but they also impose costs because they may suppress traits like parental care and immunity. As a result, the challenge hypothesis predicts that individuals will change their testosterone levels to match the current social environment. Although the vast majority of work on the challenge hypothesis has focused on androgens in vertebrates, there is growing evidence that insect hormones, especially juvenile hormone (JH), may respond to social stimuli in ways that parallel androgens in vertebrates. Many insects rapidly upregulate JH titers during social competition with rivals. Some insects also modulate JH titers based on contest outcomes, with winners upregulating JH and losers downregulating JH. This review will integrate work on social modulation of hormone titers in vertebrates and insects. First, we provide background on insect hormones and describe the functional parallels between androgens and JH. Second, we review evidence that insects rapidly change JH titers in response to social competition. Finally, we highlight opportunities for future work on social modulation of hormones in insects. Overall, the challenge hypothesis provides a useful conceptual framework for hypothesis-driven research in insect endocrinology. Comparing vertebrates and insects provides insight into how selection has shaped patterns of hormone responsiveness as well as the generality of hypotheses originally developed for vertebrates.

Juvenile hormone interacts with multiple factors to modulate aggression and dominance in groups of orphan bumble bee (Bombus terrestris) workers

Juvenile hormone (JH) is a key regulator of insect development and reproduction. Given that JH commonly affects adult insect fertility, it has been hypothesized to also regulate behaviors such as dominance and aggression that are associated with reproduction. We tested this hypothesis in the bumble bee Bombus terrestris for which JH has been shown to be the major gonadotropin. We used the allatoxin Precocene-I (P-I) to reduce hemolymph JH titers and replacement therapy with the natural JH to revert this effect. In small orphan groups of workers with similar body size but mixed treatment, P-I treated bees showed lower aggressiveness, oogenesis, and dominance rank compared with control and replacement therapy treated bees. In similar groups in which all bees were treated similarly, there was a clear dominance hierarchy, even in P-I and replacement therapy treatment groups in which the bees showed similar levels of ovarian activation. In a similar experiment in which bees differed in body size, larger bees were more likely to be dominant despite their similar JH treatment and ovarian state. In the last experiment, we show that JH manipulation does not affect dominance rank in groups that had already established a stable dominance hierarchy. These findings solve previous ambiguities concerning whether or not JH affects dominance in bumble bees. JH positively affects dominance, but bees with similar levels of JH can nevertheless establish dominance hierarchies. Thus, multiple factors including JH, body size, and previous experience affect dominance and aggression in social bumble bees.

Winner and loser effects in lobster cockroach contests for social dominance

In the context of animal aggression, the winner/loser effect is a cross-taxa phenomenon. In the present study, the effect of social contest experience on winning and losing subsequent encounters was investigated in the furious male lobster cockroach, Nauphoeta cinerea. Dominant and subordinate individuals were generated as the result of an encounter between two socially naïve males (SNMs); the winner and loser were designated as 1st encounter dominants and 1st encounter subordinates, respectively. With these dominants and subordinates, three experiments were conducted: (I) the original pair met in a re-encounter, (II) the 1st encounter dominants and subordinates were paired with an inexperienced SNM, (III) the 1st encounter dominants and subordinates were paired with an experienced individual of the same rank. Each experiment was conducted at 1 week, 2 weeks, 3 weeks, 4 weeks and 5 weeks after the 1st encounter fight. Juvenile hormone (JH) III titer was monitored in all individuals before and after each subsequent encounter. Our results showed that, in the original pairing and in the pairing with SNMs, the probability that a 1st encounter dominant (or subordinate) would win (or lose) the subsequent encounter fit well with the 95% confidence interval of the theoretical criteria proposed by Begin et al. (1969), indicating the existence of the winning/losing effect. However, this effect was inconsistent along the five-week observation period. For all 1st encounter dominants, at each week after the 1st encounter, the before subsequent encounter JH III titers distribution was significantly different from that on the 1st encounter day; the distributions of before subsequent encounter JH III titers could be further clustered into two groups, the higher JH III group and the lower JH III group, which were significantly correlated with subsequent winning and losing, respectively. For the 1st encounter subordinates, the distributions of before subsequent encounter JH III titers were not significantly different from that of SNMs, but the titer distributions were significantly shifted to a higher level compared to the 1st encounter day. Compared with before subsequent encounter, the after subsequent encounter hemolymph JH III level was significantly increased in winners and significantly decreased in losers. From these data, we propose that instability of the winner and loser effects may occur due to physiological costs and recovery; this instability may partly explain why the social hierarchy is unstable in this cockroach species.

Using knowledge from human research to improve understanding of contest theory and contest dynamics

Our understanding of animal contests and the factors that affect contest dynamics and decisions stems from a long and prosperous collaboration between empiricists and theoreticians. Over the last two decades, however, theoretical predictions regarding the factors that affect individual decisions before, during and after a contest are becoming increasingly difficult to test empirically. Extremely large sample sizes are necessary to experimentally test the nuanced theoretical assumptions surrounding how information is used by animals during a contest, how context changes the information used, and how individuals change behaviour as a result of both the information available and the context in which the information is acquired. In this review, we discuss how the investigation of contests in humans through the collaboration of biologists and psychologists may advance contest theory and dynamics in general. We argue that a long and productive history exploring human behaviour and psychology combined with technological advancements provide a unique opportunity to manipulate human perception during contests and collect unbiased data, allowing more targeted examinations of particular aspects of contest theory (e.g. winner/loser effects, information use as a function of age). We hope that our perspective provides the impetus for many future collaborations between biologists and psychologists.

Sperm and sex peptide stimulate aggression in female Drosophila

Female aggression towards other females is associated with reproduction in many taxa, and traditionally thought to be related to the protection or provisioning of offspring, such as through increased resource acquisition. However, the underlying reproductive factors causing aggressive behaviour in females remain unknown. Here we show that female aggression in the fruit fly Drosophila melanogaster is strongly stimulated by the receipt of sperm at mating, and in part by an associated seminal fluid protein, the sex peptide. We further show that the post-mating increase in female aggression is decoupled from the costs of egg production and from post-mating decreases in sexual receptivity. Our results suggest that male ejaculates can have a surprisingly direct influence on aggression in recipient females. Male ejaculate traits thus influence the female social competitive environment with potentially far-reaching ecological and evolutionary consequences.

Socially selected ornaments influence hormone titers of signalers and receivers

Decades of behavioral endocrinology research have shown that hormones and behavior have a bidirectional relationship; hormones both influence and respond to social behavior. In contrast, hormones are often thought to have a unidirectional relationship with ornaments. Hormones influence ornament development, but little empirical work has tested how ornaments influence hormones throughout life. Here, we experimentally alter a visual signal of fighting ability in Polistes dominulus paper wasps and measure the behavioral and hormonal consequences of signal alteration in signalers and receivers. We find wasps that signal inaccurately high fighting ability receive more aggression than controls and receiving aggression reduces juvenile hormone (JH) titers. As a result, immediately after contests, inaccurate signalers have lower JH titers than controls. Ornaments also directly influence rival JH titers. Three hours after contests, wasps who interacted with rivals signaling high fighting ability have higher JH titers than wasps who interacted with rivals signaling low fighting ability. Therefore, ornaments influence hormone titers of both signalers and receivers. We demonstrate that relationships between hormones and ornaments are flexible and bidirectional rather than static and unidirectional. Dynamic relationships among ornaments, behavior, and physiology may be an important, but overlooked factor in the evolution of honest communication.

Integrating resource defence theory with a neural nonapeptide pathway to explain territory-based mating systems

The ultimate-level factors that drive the evolution of mating systems have been well studied, but an evolutionarily conserved neural mechanism involved in shaping behaviour and social organization across species has remained elusive. Here, we review studies that have investigated the role of neural arginine vasopressin (AVP), vasotocin (AVT), and their receptor V1a in mediating variation in territorial behaviour. First, we discuss how aggression and territoriality are a function of population density in an inverted-U relationship according to resource defence theory, and how territoriality influences some mating systems. Next, we find that neural AVP, AVT, and V1a expression, especially in one particular neural circuit involving the lateral septum of the forebrain, are associated with territorial behaviour in males of diverse species, most likely due to their role in enhancing social cognition. Then we review studies that examined multiple species and find that neural AVP, AVT, and V1a expression is associated with territory size in mammals and fishes. Because territoriality plays an important role in shaping mating systems in many species, we present the idea that neural AVP, AVT, and V1a expression that is selected to mediate territory size may also influence the evolution of different mating systems. Future research that interprets proximate-level neuro-molecular mechanisms in the context of ultimate-level ecological theory may provide deep insight into the brain-behaviour relationships that underlie the diversity of social organization and mating systems seen across the animal kingdom.

No scope for social modulation of steroid levels in a year-round territorial damselfish

Both latitude and mating system have been proposed to shape relationships between steroid hormone levels and social behavior. Recently it has been postulated that species with long lasting non-seasonal territorial behavior have low androgen responsiveness. Tropical damselfishes are an ideal family to test this proposition because they show a large variety in mating systems. Here we contribute to the comparative dataset by measuring the response in steroid levels after social modulation in the banded sergeant, Abudefduf septemfasciatus, a species with non-seasonal territoriality. In highly territorial and brooding males, we found low androgen and cortisol levels that did not increase after experimental intraspecific simulated territorial intrusions (STI tests). No relationship was found between the variation in steroid hormone levels and territorial responses to naturally occurring territorial intrusions. Although steroid levels were low, male A. septemfasciatus were highly territorial both to STI challenges and to fishes that passed the territory. They often chased intruders for several meters away from the territory. This indicates that during nest defence in a non-seasonal territorial damselfish species, territorial behaviors are shown independent of variation in androgen and cortisol levels.

The evolution of honest communication: integrating social and physiological costs of ornamentation

Much research on animal communication has addressed how costs such as social costs or physiological costs favor the accuracy of signals. Previous work has largely considered these costs separately, but we may be missing essential connections by studying costs in isolation. After all, social interactions produce rapid changes in hormone titers which can then affect individual behavior and physiology. As a result, social costs are likely to have widespread physiological consequences. Here, I present a new perspective on the factors that maintain honest signals by describing how the interplay between social costs and physiological costs may maintain an accurate link between an animal's abilities and ornament elaboration. I outline three specific mechanisms by which the interaction between social behavior and hormones could favor honest signals and present specific predictions for each of the three models. Then, I review how ornaments alter agonistic behavior, agonistic behavior influences hormones, and how these hormonal effects influence fitness. I also describe the few previous studies that have directly tested how ornaments influence hormones. Finally, opportunities for future work are discussed. Considering the interaction between social behavior and physiology may address some challenges associated with both social and physiological models of costs. Understanding the dynamic feedbacks between physiology and social costs has potential to transform our understanding of the stability of animals' communication systems.

Changes in testosterone mediate the effect of winning on subsequent aggressive behaviour

Testosterone concentrations rise rapidly in the context of competitive interactions and remain elevated in winners relative to losers. Theoretical models suggest that this divergent neuroendocrine response serves to mediate future dominance behaviours. Although research in animal models provides compelling support for this model, evidence for its applicability to human social behaviour is limited. In the current study, men and women were randomly assigned to experience a series of victories or defeats, after which aggressive behaviour was assessed using a well-validated behavioural measure. Winning produced elevated testosterone concentrations relative to losing in men, but not women. More importantly, testosterone reactivity to competition mediated the effect of winning on subsequent aggressive behaviour in men, but not women. We discuss limitations of the current study (e.g., the status manipulation may have affected other variables not measured in the study including competitiveness and physical activity expended), as well as discuss a potential neural mechanism underlying the effect of testosterone reactivity on aggressive behaviour.

Mating enhances the probability of winning aggressive encounters in male lobster cockroaches

In the present study, we report that contact with isolated female antenna significantly increases both the pheromone 3-hydroxy-2-butanone (3H-2B) release and the hemolymph JH III level in all examined aggressive posture-adopting (AP) and NP (non-AP-adopting) socially naïve males, with significantly faster concomitant pre-mating wing-raising behavior in AP as compared to NP males. 3H-2B release and JH III level were significantly increased after mating in both AP and NP males. A positive correlation was observed between mating experience and dominant status. Furthermore, mated-AP males initiated fights more rapidly and fought for a significantly longer duration than mated-NP males; retention with the paired female for 24h did not affect this increase. JH III level and 3H-2B release were significantly increased in dominant males as compared to subordinates. These results suggest that prior mating experience in invertebrates may enhance aggression in subsequent male-male encounters, with accompanying physiological (hormone and pheromone) responses.

Threat perception and familiarity moderate the androgen response to competition in women

Social interactions elicit androgen responses whose function has been posited to be the adjustment of androgen-dependent behaviors to social context. The activation of this androgen response is known to be mediated and moderated by psychological factors. In this study we tested the hypothesis that the testosterone (T) changes after a competition are not simply related to its outcome, but rather to the way the subject evaluates the event. In particular we tested two evaluative dimensions of a social interaction: familiarity with the opponent and the subjective evaluation of the outcome as threat or challenge. Challenge/threat occurs in goal relevant situations and represent different motivational states arising from the individuals' subjective evaluation of the interplay between the task demands and coping resources possessed. For challenge the coping resources exceed the task demands, while threat represents a state where coping resources are insufficient to meet the task demands. In this experiment women competed in pairs, against a same sex opponent using the number tracking test as a competitive task. Losers appraised the competition outcome as more threatening than winners, and displayed higher post-competition T levels than winners. No differences were found either for cortisol (C) or for dehydroepiandrosterone. Threat, familiarity with the opponent and T response were associated only in the loser condition. Moderation analysis suggests that for the women that lost the competition the effect of threat on T is moderated by familiarity with the opponent.

Androgen-mediated nurturing and aggressive behaviors during paternal care in bluegill sunfish (Lepomis macrochirus)

Male parental care in vertebrates often involves both defensive and nurturing behaviors. Whether androgens differentially mediate these two types of behaviors, or a trade-off exists between them, has been studied by behavioral endocrinologists for years but predominantly in species with biparental care. In such systems, potential detrimental effects of elevated androgens on parental care behaviors are often compensated for by changes in behavior of the unmanipulated parent. In contrast, for species where only one parent provides care, manipulation of androgen levels may more clearly determine if there are differential effects of androgens on these two types of behaviors and whether the proposed trade-off between defensive and nurturing behavior exists. Here, we manipulated androgen levels in two ways in bluegill sunfish (Lepomis macrochirus), a species where males provide sole parental care for the young. At the onset of the care period, males were implanted with 11-ketotestosterone, a major teleost androgen, the androgen receptor antagonist flutamide, or a blank implant. A separate control group experienced no manipulation. Males were then observed over several days and tested for their aggressiveness towards an experimentally-presented brood predator and for nurturing behavior (fanning of the eggs, removal of dead or fungal-infected eggs). Males implanted with 11-ketotestosterone displayed 64% more aggressive behaviors and 71% fewer nurturing behaviors than control groups. In contrast, males implanted with flutamide displayed 7% fewer aggressive behaviors and 126% more nurturing behaviors than control males. Taken together, these results show that aggression and nurturing behaviors are mediated by androgens and suggest that there is a trade-off between the two behaviors during parental care in this species.

Winning fights induces hyperaggression via the action of the biogenic amine octopamine in crickets

Winning an agonistic interaction against a conspecific is known to heighten aggressiveness, but the underlying events and mechanism are poorly understood. We quantified the effect of experiencing successive wins on aggression in adult male crickets (Gryllus bimaculatus) by staging knockout tournaments and investigated its dependence on biogenic amines by treatment with amine receptor antagonists. For an inter-fight interval of 5 min, fights between winners escalated to higher levels of aggression and lasted significantly longer than the preceding round. This winner effect is transient, and no longer evident for an inter-fight interval of 20 min, indicating that it does not result from selecting individuals that were hyper-aggressive from the outset. A winner effect was also evident in crickets that experienced wins without physical exertion, or that engaged in fights that were interrupted before a win was experienced. Finally, the winner effect was abolished by prior treatment with epinastine, a highly selective octopamine receptor blocker, but not by propranolol, a ß-adrenergic receptor antagonist, nor by yohimbine, an insect tyramine receptor blocker nor by fluphenazine an insect dopamine-receptor blocker. Taken together our study in the cricket indicates that the physical exertion of fighting, together with some rewarding aspect of the actual winning experience, leads to a transient increase in aggressive motivation via activation of the octopaminergic system, the invertebrate equivalent to the adrenergic system of vertebrates.

Genes, hormones, and circuits: an integrative approach to study the evolution of social behavior

Tremendous progress has been made in our understanding of the ultimate and proximate mechanisms underlying social behavior, yet an integrative evolutionary analysis of its underpinnings has been difficult. In this review, we propose that modern genomic approaches can facilitate such studies by integrating four approaches to brain and behavior studies: (1) animals face many challenges and opportunities that are ecologically and socially equivalent across species; (2) they respond with species-specific, yet quantifiable and comparable approach and avoidance behaviors; (3) these behaviors in turn are regulated by gene modules and neurochemical codes; and (4) these behaviors are governed by brain circuits such as the mesolimbic reward system and the social behavior network. For each approach, we discuss genomic and other studies that have shed light on various aspects of social behavior and its underpinnings and suggest promising avenues for future research into the evolution of neuroethological systems.

Short-term phenotypic plasticity in long-chain cuticular hydrocarbons

Cuticular hydrocarbons provide arthropods with the chemical equivalent of the visually extravagant plumage of birds. Their long chain length, together with the number and variety of positions in which methyl branches and double bonds occur, provide cuticular hydrocarbons with an extraordinary level of information content. Here, we demonstrate phenotypic plasticity in an individual's cuticular hydrocarbon profile. Using solid-phase microextraction, a chemical technique that enables multiple sampling of the same individual, we monitor short-term changes in cuticular hydrocarbon profiles of individual crickets, Teleogryllus oceanicus, in response to a social challenge. We experimentally manipulate the dominance status of males and find that dominant males, on losing fights with other dominant males, change their hydrocarbon profile to more closely resemble that of a subordinate. This result demonstrates that cuticular hydrocarbons can be far more responsive to changes in social dominance than previously realized.

Juvenile hormone III produced in male accessory glands of the longhorned beetle, Apriona germari, is transferred to female ovaries during copulation

We report on juvenile hormone (JH) biosynthesis in vitro by male accessory glands (MAGs) in the longhorned beetle, Aprionona germari, accompanied by the transfer of JH from males to females during copulation. JH was extracted from the MAGs and separated by reversed-phase high-performance liquid chromatography. JH III was identified as the major JH by gas chromatography-mass spectrometry. A radiochemical assay and a non-radioactive method were used to measure the in vitro rate of JH biosynthesis by the MAGs. After 4 h of incubation with 3H-methionine in the medium, the radioactivity in the MAGs substantially increased. In a separate assay, incubation of the MAGs with non-radioactive methionine for 4 h resulted in a 39% increase in JH III. Seven-day-old males were injected with medium 199 containing 3H-methionine and 24 h later they were mated with virgin females. Hemolymph and the MAGs were collected from the mated males and hemolymph, ovaries and eggs were collected from the mated females for assaying radioactive JH. The radioactivity incorporated into JH in the MAGs was transferred to the females during copulation and later transferred into their eggs. Assayed 1 h after copulation, JH III level in the MAGs decreased 42% and the content of JH III in the male hemolymph did not change, whereas the content of JH III in the female hemolymph and ovaries both increased.

Juvenile hormone and the ontogeny of cockroach aggression

Our previous study [Kou et al., 2008. Juvenile hormone levels are increased in winners of cockroach fights. Horm. Behav. 52, 252-260] showed that the basic principle of the challenge hypothesis (hormone levels can respond to social stimuli to modulate aggression in vertebrates] could be applied to juvenile hormone (JH) levels and aggression in the lobster cockroach Nauphoeta cinerea. In that study, 80- to 85-day-old socially naïve males were used, as fighting is much more easily initiated in these older animals than in younger males, and JH III levels in the dominant were found to be significantly increased after an encounter compared to before the encounter and were significantly higher than those in the subordinates. In N. cinerea, newly emerged males usually show no aggressiveness towards each other and aggression is only initiated after several days of close contact. To investigate the development of aggression from an early age, in the present study, newly emerged males were paired to investigate the relationship between JH levels and aggression. The results showed that injection of JH III significantly increased the probability of the young males being fight winners. In each age group in which aggression was initiated, the dominants had significantly higher JH levels than either the subordinates or the same aged non-fighters. JH injection of subordinates on the day of rank establishment had no effect on the probability of rank switch. These results indicate that, (i) in newly emerged male pairs, JH plays a decisive role in rank establishment and the fact that dominant status is significantly associated with a higher JH titer and subordinate status with a lower JH titer is consistent with the basic principle of the challenge hypothesis, and (ii) after rank establishment, the lack of effect of JH treatment on rank change is consistent with the idea of "social inertia" in vertebrates.

Aggressive behavior and change in salivary testosterone concentrations predict willingness to engage in a competitive task

The current study investigated relationships among aggressive behavior, change in salivary testosterone concentrations, and willingness to engage in a competitive task. Thirty-eight male participants provided saliva samples before and after performing the Point Subtraction Aggression Paradigm (a laboratory measure that provides opportunity for aggressive and defensive behavior while working for reward; all three involve pressing specific response keys). Baseline testosterone concentrations were not associated with aggressive responding. However, aggressive responding (but not point reward or point protection responding) predicted the pre- to post-PSAP change in testosterone: Those with the highest aggressive responding had the largest percent increase in testosterone concentrations. Together, aggressive responding and change in testosterone predicted willingness to compete following the PSAP. Controlling for aggression, men who showed a rise in testosterone were more likely to choose to compete again (p=0.03) and controlling for testosterone change, men who showed the highest level of aggressive responding were more likely to choose the non-competitive task (p=0.02). These results indicate that situation-specific aggressive behavior and testosterone responsiveness are functionally relevant predictors of future social behavior.

Juvenile hormone titers and caste differentiation in the damp-wood termite Hodotermopsis sjostedti (Isoptera, Termopsidae)

Termites are social insects, presenting morphologically distinct castes, performing specific tasks in the colony. The developmental processes underlying caste differentiation are mainly controlled by juvenile hormone (JH). Although many fragmentary data support this fact, there was no comparative work on JH titers during the caste differentiation processes. In this study, JH titer variation was investigated using a liquid chromatography-mass spectrometry (LC-MS) quantification method in all castes of the Japanese damp-wood termite Hodotermopsis sjostedti, especially focusing on the soldier caste differentiation pathway, which was induced by treatment with a JH analog. Hemolymph JH titers fluctuated between 20 and 720pg/microl. A peak of JH was observed during molting events for the pseudergate stationary molt and presoldier differentiation, but this peak was absent prior to the imaginal molt. Soldier caste differentiation was generally associated with high JH titers and nymph to alate differentiation with low JH titers. However, JH titer rose in females during alate maturation, probably in relation to vitellogenesis. In comparison, JH titer was surprisingly low in neotenics. On the basis of these results in both natural and artificial conditions, the current model for JH action on termite caste differentiation is discussed and re-appraised.

Social and nonsocial stimuli and juvenile hormone titer in a male burying beetle, Nicrophorus orbicollis

We investigated the interaction of social and nonsocial stimuli on juvenile hormone (JH) titer in male burying beetles (Nicrophorus orbicollis). The initial JH response to discovery of a carcass was substantial (10-15-fold increase over controls) and rapid (<1h), and occurred whether or not a female was present. By 3h after discovery, JH titers were declining, the decline being more pronounced when a female was not present. We also tested the effect of larval stimulation on JH titer in care-giving males by removing a male's brood and replacing it with a brood of first or third instar larvae. Males initially providing care for begging first instar larvae continued to maintain high titers of JH when the replacement broods were first but not third instars. Males caring for third instar larvae (normally low JH titers) maintained low levels of JH regardless of the developmental stage of the replacement brood. This suggests that once males begin to care for nutritionally independent third instar larvae, JH titers remain low regardless of subsequent larval stimulation. Burying beetles are socially and hormonally complex organisms in which stimuli from a breeding resource, mating partners, rivals and young interact to alter the JH profile of breeding adults.

Antenna contact and agonism in the male lobster cockroach, Nauphoeta cinerea

On any given day, about 35% of 80- to 85-day-old socially naïve male (SNM) lobster cockroaches (Nauphoeta cinerea) spontaneously adopted an aggressive posture (AP) without encountering another male [spontaneous AP (SAP)]. Although SAP SNMs showed significantly higher release of the pheromone 3-hydroxy-2-butanone (3H-2B) than non-SAP SNMs, there was no significant difference in hemolymph juvenile hormone (JH) III titer. When different body parts were tested for induction of the attack behavior, the antenna was found to be the most effective. After 1 min of contact with an antenna from another SAP SNM, attack behavior was induced in 100% of SAP and 76.2% of non-SAP SNMs, and the JH III titer was significantly increased in all responders. Among the non-SAP SNMs, the JH III titer before antenna contact was significantly lower in the non-responders than in the responders, and, although the JH III increase induced by 1 min antenna contact was similar between responders and non-responders, the final JH III titer of the non-responders was significantly lower. A similar attack response, JH III titer change, and 3H-2B release were seen when the individual's own antenna was used. After 5 min of contact with an antenna from another SAP SNM, attack behavior was induced in 100% of SAP and 82% of non-SAP SNMs; in the former, 3H-2B release was similar before and after antenna contact, but the JH III titer was significantly increased after antenna contact, while, in the latter, both 3H-2B release and JH III titer were significantly increased after antenna contact. Among the non-SAP SNMs, JH III titer in the non-responders was not elevated after 5 min antenna contact, and was significantly lower than that in the responders. A pentane-washed antenna did not induce attack behavior or increase the hemolymph JH III titer, and a pentane-washed antenna coated with 3H-2B also failed to induce attack behavior. These results indicate that N. cinerea male-male agonistic interactions, to which the vertebrate challenge hypothesis can be applied, are due to contact pheromone on the antenna, resulting in the concomitant expression of attack behavior and an increase in 3H-2B release and JH III titer.

Can the "challenge hypothesis" be applied to insects?

The challenge hypothesis, conceived for testosterone and vertebrates, has recently been applied to juvenile hormone (JH) and insects. Scott [Scott, M.P., 2006a. Resource defense and juvenile hormone: the "challenge hypothesis" extended to insects. Horm. Behav. 49, 276-281] found that JH in the burying beetle Nicrophorus orbicollis increased in response to a social challenge in the presence of a breeding resource, while there was no such JH response in the purported brood parasite Nicrophorus pustulatus. Two important implications of the challenge hypothesis that need to be tested are whether JH affects dominance and why there are interspecific differences in the JH response to a social challenge. The effect of JH on dominance in burying beetles was examined by topical application of JH III to one of two competing females at 24 h and again at 1 h prior to presentation of a breeding resource (mouse carcass). JH supplementation had no effect on dominance in intraspecific interactions in N. orbicollis, as measured by possession of the carcass on Day 1, 3 or 7. Similarly, JH did not alter carcass ownership during competition between N. orbicollis and N. pustulatus nor did it affect reproductive success. Irrespective of JH supplementation, N. pustulatus became increasingly dominant as the trials progressed, rarely occupying the carcass on Day 1, but excluding N. orbicollis in nearly half the trials by Day 7. These findings, and a brief review of the burying beetle literature, suggest that the challenge hypothesis, as applied to testosterone and vertebrates, does not yet have an analogous model for JH and insects.

The role of juvenile hormone in competition and cooperation by burying beetles

Few studies have addressed the physiological mechanisms that modulate aggression in insects. In some social insects, there is a correlation of JH and aggression in colony defense and in the establishment of dominance, but only a few studies demonstrate a causal relationship. Burying beetles aggressively defend a breeding resource, a carcass, and juvenile hormone (JH) hemolymph titers increase rapidly upon the discovery of a carcass. In this study, I show that treatment with the JH analog, methoprene, in the absence of a carcass increases the probability of injuries from aggressive interactions, but treatment to one member of a pair of competing Nicrophorus orbicollis females does not increase the probability that she will win control of the resource. In addition, higher JH levels are not associated with greater competitive ability in communally breeding Nicrophorus tomentosus females. Treatment of one female N. tomentosus does not increase her share of the communal brood. Methoprene seems to make a less competitive female more persistent and less willing to concede, which, although maintaining her share of reproduction, results in her exclusion from the brood chamber.

Effect of juvenile hormone on vitellogenin gene expression in the fat body of burying beetles, Nicrophorus orbicollis

In the burying beetles, Nicrophorus orbicollis, it is not clear the extent to which synthesis of Vitellogenin (Vg) is under hormonal control. Juvenile hormone (JH) and ovarian development increase rapidly upon discovery of a carcass, the necessary reproductive resource, but in its absence, treatment with JH does not accelerate ovarian development. Here we investigated the effect of manipulating JH titers on Vg gene expression in the fat bodies of newly eclosed and sexually mature females. Topical applications of the JH analogue methoprene at doses of 200, 300, and 400 microg of methoprene/g beetle to newly eclosed females resulted in up to a 5-fold increase in fat body Vg mRNA levels compared to acetone-treated control females. However, none of these increases were statistically significant. We also measured hemolymph Vg in females subjected to prolonged treatment with an intermediate dose of 300 microg of methoprene/g beetle and found no statistically significant increase in hemolymph Vg following treatment. There was, however, a significantly positive correlation between Vg mRNA and hemolymph Vg. Conversely, although injecting sexually mature females with fluvastatin (an inhibitor of JH biosynthesis in the corpora allata) at a dose of 40 microg/ beetle resulted in significantly decreased JH titers in treated females compared to saline-injected controls, Vg transcription rates were not affected. We suggest that in burying beetles, JH alone is not sufficient to upregulate the expression of the Vg genes and that other factors may be involved in regulating their vitellogenic cycle as well.