Energetic cost of communication

Social behavior in South American electric fishes: Linking neuroendocrine regulation, signal plasticity, and reproductive strategies

Cross-species analysis can provide valuable insights into the neural and hormonal mechanisms underlying behavior. South American weakly electric fishes (order Gymnotiformes) are ideal models due to their well-characterized electric signals, which convey important social information. These signals are quantifiable and traceable to specific brain and peripheral substrates. This review focuses on social electric signaling in two nocturnal gymnotiform species found syntopically in Uruguay: Gymnotus omarorum and Brachyhypopomus gauderio. We examine the influence of sex, social context, and neuromodulators on signal flexibility across day/night and seasonal cycles. Common features include a nocturnal increase in basal electric rate mediated by melatonin, enhancing awareness and social engagement; androgen-mediated seasonal protection of electric signals against high summer temperatures; and the production of social electric signals, such as chirps and interruptions, during social interactions, modulated by vasotocinergic and serotonergic systems. Key differences lie in neuromodulator involvement and signal plasticity: B. gauderio exhibits greater signal flexibility, with sex- and context-dependent waveform changes and a broader repertoire of transient social signals used in dyadic interactions, supported by distinct neural mechanisms. These differences likely reflect species-specific reproductive strategies and their associated costs, such as predation pressure. This review underscores the value of studying electric behavior to understand the integration of internal states with environmental and social cues, offering insights into mechanisms underlying behavioral responses to natural challenges.

Dialects in North American elk bugle calls: comparisons between source and translocated elk populations

Identifying the factors shaping dialects can reveal selective pressures and cultural influences on acoustic signals. Translocations of species have been considered the “gold standard” for identifying population differences in vocalizations, including dialects. The North American elk (Cervus canadensis) is a species that has experienced numerous translocations, but no published studies exist on dialects in elk or any other ungulate species. Adult male North American elk utter an iconic vocalization during the breeding season, which is termed the bugle call due to the power, duration, and pitch of the sound. We investigated if dialect differences existed between three populations: a source population in Wyoming (N = 10 bulls, 132 calls) and translocated populations in Colorado (N = 13 bulls, 92 calls) and Pennsylvania (N = 20 bulls, 160 calls). Dialect differences existed between the populations in 9 of 10 measured variables, in both the frequency domain and time domain. Habitat factors (acoustic adaptation hypothesis) affecting sound transmission were inconsistently related to dialects in the populations. Genetic factors may be related to dialects, as the historically bottlenecked Pennsylvania population, with low heterozygosity and low allelic richness, exhibited the least variability in acoustic measures. Cultural transmission and vocal learning may also contribute to dialect development in this highly vocal, social ungulate species.

Can Sex-Specific Metabolic Rates Provide Insight Into Patterns of Metabolic Scaling?

Females and males can exhibit striking differences in body size, relative trait size, physiology and behavior. As a consequence the sexes can have very different rates of whole-body energy use, or converge on similar rates through different physiological mechanisms. Yet many studies that measure the relationship between metabolic rate and body size only pay attention to a single sex (more often males), or do not distinguish between sexes. We present four reasons why explicit attention to energy-use between the sexes can yield insight into the physiological mechanisms that shape broader patterns of metabolic scaling in nature. First, the sexes often differ considerably in their relative investment in reproduction which shapes much of life-history and rates of energy use. Second, males and females share a majority of their genome but may experience different selective pressures. Sex-specific energy profiles can reveal how the energetic needs of individuals are met despite the challenge of within-species genetic constraints. Third, sexual selection often pushes growth and behavior to physiological extremes. Exaggerated sexually selected traits are often most prominent in one sex, can comprise up to 50% of body mass and thus provide opportunities to uncover energetic constraints of trait growth and maintenance. Finally, sex-differences in behavior such as mating-displays, long-distance dispersal and courtship can lead to drastically different energy allocation among the sexes; the physiology to support this behavior can shape patterns of metabolic scaling. The mechanisms underlying metabolic scaling in females, males and hermaphroditic animals can provide opportunities to develop testable predictions that enhance our understanding of energetic scaling patterns in nature.

Territorial Displays of the Ctenophorus decresii Complex: A Story of Local Adaptations

Closely related species make for interesting model systems to study the evolution of signaling behavior because they share evolutionary history but have also diverged to the point of reproductive isolation. This means that while they may have some behavioral traits in common, courtesy of a common ancestor, they are also likely to show local adaptations. The Ctenophorus decresii complex is such a system, and comprises six closely related agamid lizard species from Australia: C. decresii, C. fionni, C. mirrityana, C. modestus, C. tjanjalka, and C. vadnappa. In this study, we analyze the motion displays of five members of the C. decresii complex in the context of their respective habitats by comparing signal structure, habitat characteristics and signal contrast between all species. Motor pattern use and the temporal sequence of motor patterns did not differ greatly, but the motion speed distributions generated during the displays were different for all species. There was also variation in the extent to which signals contrasted with plant motion, with C. vadnappa performing better than the other species at all habitats. Overall, this study provides evidence that members of the C. decresii complex exhibit local adaptations in signaling behavior to their respective habitat, but they also maintain some morphological and behavioral traits in common, which is likely a consequence from the ancestral state.

Diel Patterns of Pheromone Release By Male Sea Lamprey

Costs to producing sexual signals can create selective pressures on males to invest signaling effort in particular contexts. When the benefits of signaling vary consistently across time, males can optimize signal investment to specific temporal contexts using biological rhythms. Sea lamprey, Petromyzon marinus, have a semelparous life history, are primarily nocturnal, and rely on pheromone communication for reproduction; however, whether male investment in pheromone transport and release matches increases in spawning activity remains unknown. By measuring 1) 3keto-petromyzonol sulfate (3kPZS, a main pheromone component) and its biosynthetic precursor petromyzonol sulfate (PZS) in holding water and tissue samples at 6 points over the course of 24 hours, and 2) 3kPZS release over the course of several days, we demonstrate that 3kPZS release exhibits a consistent diel pattern across several days with elevated pheromone release just prior to sunset and at night. Trends in hepatic concentrations and circulatory transport of PZS and 3kPZS were consistent with patterns of 3kPZS release and suggest the possibility of direct upregulation in pheromone transport and release rather than observed release patterns being solely a byproduct of increased behavioral activity. Our results suggest males evolved a signaling strategy that synchronizes elevated pheromone release with nocturnal increases in sea lamprey behavior. This may be imperative to ensure that male signaling effort is not wasted in a species having a single, reproductive event.

Leptinergic Regulation of Vertebrate Communication Signals

Animal communication signals are regulated by multiple hormonal axes that ensure appropriate signal targeting, timing, and information content. The regulatory roles of steroid hormones and many peptide hormones are well understood and documented across a wide range of vertebrate taxa. Two recent studies have reported a novel function for leptin, a peptide hormone central to energy balance regulation: regulating communication signals of weakly electric fish and singing mice. With only limited evidence available at this time, a key question is just how widespread leptinergic regulation of communication signals is within and across taxa. A second important question is what features of communication signals are subject to leptinergic regulation. Here we consider the functional significance of leptinergic regulation of animal communication signals in the context of both direct and indirect signal metabolic costs. Direct costs arise from metabolic investment in signal production, while indirect costs arise from the predation and social conflict consequences of the signal's information content. We propose a preliminary conceptual framework for predicting which species will exhibit leptinergic regulation of their communication signals and which signal features leptin will regulate. This framework suggests a number of directly testable predictions within and across taxa. Accounting for additional factors such as life history and the potential co-regulation of communication signals by leptin and glucocorticoids will likely require modification or elaboration of this model.

Exploring the Evolution of Perception: An Agent-Based Approach

Perception is central to the survival of an individual for many reasons, especially as it affects the ability to gather resources. Consequently, costs associated with perception are partially shaped by resource availability. Understanding the interplay of environmental factors (such as the density and distribution of resources) with species-specific factors (such as growth rate, mutation, and metabolic costs) allows the exploration of possible trajectories by which perception may evolve. Here, we used an agent-based foraging model with a context-dependent movement strategy in which each agent switches between undirected and directed movement based on its perception of resources. This switching behavior is central to our goal of exploring how environmental and species-specific factors determine the evolution and maintenance of perception in an ecological system. We observed a non-linear response in the evolved perceptual ranges as a function of parameters in our model. Overall, we identified two groups of parameters, one of which promotes evolution of perception and another group that restricts it. We found that resource density, basal energy cost, perceptual cost and mutation rate were the best predictors of the resultant perceptual range distribution, but detailed exploration indicated that individual parameters affect different parts of the distribution in different ways.

The energetics of social signaling during roost location in Spix's disc-winged bats

Long-term social aggregations are maintained by multiple mechanisms, including the use of acoustic signals, which may nonetheless entail significant energetic costs. To date, however, no studies have gauged whether there are significant energetic costs to social call production in bats, which heavily rely on acoustic communication for a diversity of social tasks. We measured energetic expenditure during acoustic signaling in Spix's disc-winged bats (Thyroptera tricolor), a species that commonly uses contact calls to locate the ephemeral furled leaves that they use for roosting. To determine the cost of sound production, we measured oxygen consumption using intermittent-flow respirometry methods, with and without social signaling. Our results show that the emission of contact calls significantly increases oxygen consumption; vocal individuals spent, on average, 12.42 kJ more during social signaling trials than they spent during silent trials. We also found that as resting metabolic rate increased in males, there was a decreasing probability that they would emit response calls. These results provide support to the 'allocation model', which predicts that only individuals with lower self-maintenance costs can afford to spend energy in additional activities. Our results provide a step forward in our understanding of how physiology modulates behavior, specifically how the costs of call production and resting metabolic rate may explain the differences in vocal behavior among individuals.

The weakly electric fish, Apteronotus albifrons, actively avoids experimentally induced hypoxia

Anthropogenic environmental degradation has led to an increase in the frequency and prevalence of aquatic hypoxia (low dissolved oxygen concentration, DO), which may affect habitat quality for water-breathing fishes. The weakly electric black ghost knifefish, Apteronotus albifrons, is typically found in well-oxygenated freshwater habitats in South America. Using a shuttle-box design, we exposed juvenile A. albifrons to a stepwise decline in DO from normoxia (> 95% air saturation) to extreme hypoxia (10% air saturation) in one compartment and chronic normoxia in the other. On average, A. albifrons actively avoided the hypoxic compartment below 22% air saturation. Hypoxia avoidance was correlated with upregulated swimming activity. Following avoidance, fish regularly ventured back briefly into deep hypoxia. Hypoxia did not affect the frequency of their electric organ discharges. Our results show that A. albifrons is able to sense hypoxia at non-lethal levels and uses active avoidance to mitigate its adverse effects.

Development of the electric organ in embryos and larvae of the knifefish, Brachyhypopomus gauderio

South American Gymnotiform knifefish possess electric organs that generate electric fields for electro-location and electro-communication. Electric organs in fish can be derived from either myogenic cells (myogenic electric organ/mEO) or neurogenic cells (neurogenic electric organ/nEO). To date, the embryonic development of EOs has remained obscure. Here we characterize the development of the mEO in the Gymnotiform bluntnose knifefish, Brachyhypopomus gauderio. We find that EO primordial cells arise during embryonic stages in the ventral edge of the tail myotome, translocate into the ventral fin and develop into syncytial electrocytes at early larval stages. We also describe a pair of thick nerve cords that flank the dorsal aorta, the location and characteristic morphology of which are reminiscent of the nEO in Apteronotid species, suggesting a common evolutionary origin of these tissues. Taken together, our findings reveal the embryonic origins of the mEO and provide a basis for elucidating the mechanisms of evolutionary diversification of electric charge generation by myogenic and neurogenic EOs.

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Developmental staging of the electric bluntnose knifefish, Brachyhypopomus gauderio embryos and larvae.

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The primordia of the myogenic electric organ originate in the ventral somite, migrate in the ventral fin and develop to the electric organ.

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Evolutionary conservation between the nerve codes in the B. gauderio and the neurogenic electric organs in the Apteronotidae.

Effects of hypoxia on aerobic metabolism and active electrosensory acquisition in the African weakly electric fish Marcusenius victoriae

Environmental hypoxia has effected numerous and well-documented anatomical, physiological and behavioural adaptations in fishes. Comparatively little is known about hypoxia's impacts on sensing because it is difficult to quantify sensory acquisition in vivo. Weakly electric fishes, however, rely heavily on an easily-measurable sensory modality-active electric sensing-whereby individuals emit and detect electric organ discharges (EODs). In this study, hypoxia tolerance of a mormyrid weakly electric fish, Marcusenius victoriae, was assessed by examining both its metabolic and EOD rates using a critical threshold (p_crit ) paradigm. The routine metabolic rate was 1.42 mg O₂ h^-1 , and the associated critical oxygen tension was 14.34 mmHg. Routine EOD rate was 5.68 Hz with an associated critical tension of 15.14 mmHg. These metabolic indicators of hypoxia tolerance measured in this study were consistent with those in previous studies on M. victoriae and other weakly electric fishes. Furthermore, our results suggest that some aerobic processes may be reduced in favour of maintaining the EOD rate under extreme hypoxia. These findings underscore the importance of the active electrosensory modality to these hypoxia-tolerant fish.

Convergent patterns of evolution of mitochondrial oxidative phosphorylation (OXPHOS) genes in electric fishes

The ability to generate and detect electric fields has evolved in several groups of fishes as a means of communication, navigation and, occasionally, predation. The energetic burden required can account for up to 20% of electric fishes' daily energy expenditure. Despite this, molecular adaptations that enable electric fishes to meet the metabolic demands of bioelectrogenesis remain unknown. Here, we investigate the molecular evolution of the mitochondrial oxidative phosphorylation (OXPHOS) complexes in the two most diverse clades of weakly electric fishes-South American Gymnotiformes and African Mormyroidea, using codon-based likelihood approaches. Our analyses reveal that although mitochondrial OXPHOS genes are generally subject to strong purifying selection, this constraint is significantly reduced in electric compared to non-electric fishes, particularly for complexes IV and V. Moreover, analyses of concatenated mitochondrial genes show strong evidence for positive selection in complex I genes on the two branches associated with the independent evolutionary origins of electrogenesis. These results suggest that adaptive evolution of proton translocation in the OXPHOS cellular machinery may be associated with the evolution of bioelectrogenesis. Overall, we find striking evidence for remarkably similar effects of electrogenesis on the molecular evolution of mitochondrial OXPHOS genes in two independently derived clades of electrogenic fishes. This article is part of the theme issue 'Linking the mitochondrial genotype to phenotype: a complex endeavour'.

A Teleost Fish Model to Understand Hormonal Mechanisms of Non-breeding Territorial Behavior

Aggressive behaviors occurring dissociated from the breeding season encourage the search of non-gonadal underlying regulatory mechanisms. Brain estrogen has been shown to be a key modulator of this behavior in bird and mammal species, and it remains to be understood if this is a common mechanism across vertebrates. This review focuses on the contributions of Gymnotus omarorum, the first teleost species in which estrogenic modulation of non-breeding aggression has been demonstrated. Gymnotus omarorum displays year-long aggression, which has been well characterized in the non-breeding season. In the natural habitat, territory size is independent of sex and determined by body size. During the breeding season, on the other hand, territory size no longer correlates to body size, but rather to circulating estrogens and gonadosomatic index in females, and 11-ketotestosterone in males. The hormonal mechanisms underlying non-breeding aggression have been explored in dyadic encounters in lab settings. Males and females display robust aggressive contests, whose outcome depends only on body size asymmetry. This agonistic behavior is independent of gonadal hormones and fast acting androgens. Nevertheless, it is dependent on fast acting estrogenic action, as acute aromatase blockers affect aggression engagement, intensity, and outcome. Transcriptomic profiling in the preoptic area region shows non-breeding individuals express aromatase and other steroidogenic enzyme transcripts. This teleost model reveals there is a role of brain estrogen in the control of non-breeding aggression which seems to be common among distant vertebrate species.

The effect of normoxia exposure on hypoxia tolerance and sensory sampling in a swamp-dwelling mormyrid fish

Effects of energetic limitations on the performance of sensory systems are generally difficult to quantify. Weakly electric fishes provide an ideal model system to quantify the effects of metabolic stressors on sensory information acquisition, because they use an active-sensing strategy that permits easy measurement of the sensing effort. These fishes discharge an electric signal and sense perturbations of the resulting electric field. We used the mormyrid Petrocephalus degeni to quantify the relationship between routine metabolic rate and the rate of sensory sampling (rate of electric organ discharge, EOD) while under progressive hypoxia by quantifying the critical oxygen tension (P_C-MR) and the critical electric organ discharge threshold (P_C-EOD). P_C-MR was significantly higher in fish acclimated to normoxia for over 40 days compared to animals tested within 1-5 days of capture from a hypoxic swamp, which suggests high costs of maintaining hypoxia tolerance; however, there was no acclimation effect on P_C-EOD. All P. degeni reached their P_C-EOD prior to their P_C-MR. However, below the respective critical tension value, EOD rate decreased more gradually than the metabolic rate suggesting that the fish were increasing the proportion of their energy budget allocated to acquiring sensory information as dissolved-oxygen levels dropped. Trade-offs between sensory sampling and other physiological functions are also suggested by the increase in routine EOD rate with long-term normoxia acclimation, in contrast to metabolic rate, which showed no significant changes. These results highlight the relationship between sensory sampling and metabolic rate in response to progressive hypoxia and the plasticity of hypoxia tolerance.

Electroreception, electrogenesis and electric signal evolution

Electroreception, the capacity to detect external underwater electric fields with specialised receptors, is a phylogenetically widespread sensory modality in fishes and amphibians. In passive electroreception, a capacity possessed by c. 16% of fish species, an animal uses low-frequency-tuned ampullary electroreceptors to detect microvolt-range bioelectric fields from prey, without the need to generate its own electric field. In active electroreception (electrolocation), which occurs only in the teleost lineages Mormyroidea and Gymnotiformes, an animal senses its surroundings by generating a weak (< 1 V) electric-organ discharge (EOD) and detecting distortions in the EOD-associated field using high-frequency-tuned tuberous electroreceptors. Tuberous electroreceptors also detect the EODs of neighbouring fishes, facilitating electrocommunication. Several other groups of elasmobranchs and teleosts generate weak (< 10 V) or strong (> 50 V) EODs that facilitate communication or predation, but not electrolocation. Approximately 1.5% of fish species possess electric organs. This review has two aims. First, to synthesise our knowledge of the functional biology and phylogenetic distribution of electroreception and electrogenesis in fishes, with a focus on freshwater taxa and with emphasis on the proximate (morphological, physiological and genetic) bases of EOD and electroreceptor diversity. Second, to describe the diversity, biogeography, ecology and electric signal diversity of the mormyroids and gymnotiforms and to explore the ultimate (evolutionary) bases of signal and receptor diversity in their convergent electrogenic-electrosensory systems. Four sets of potential drivers or moderators of signal diversity are discussed. First, selective forces of an abiotic (environmental) nature for optimal electrolocation and communication performance of the EOD. Second, selective forces of a biotic nature targeting the communication function of the EOD, including sexual selection, reproductive interference from syntopic heterospecifics and selection from eavesdropping predators. Third, non-adaptive drift and, finally, phylogenetic inertia, which may arise from stabilising selection for optimal signal-receptor matching.

The hidden cost of sexually selected traits: the metabolic expense of maintaining a sexually selected weapon

Sexually selected weapons are among the most exaggerated traits in nature. Sexual selection theory frequently assumes a high cost of this exaggeration; yet, those costs are rarely measured. We know very little about the energetic resources required to maintain these traits at rest and the difference in energetic costs for the largest individuals relative to the smallest individuals. Knowledge in this area is crucial; resting metabolic rate can account for 30-40% of daily energy expenditure in wild animals. Here, we capitalized on the phenomenon of autotomy to take a unique look at weapon maintenance costs. Using Leptoscelis tricolor (Hemiptera: Coreidae), we measured CO2 production rates before and after a weapon was shed. Males in this insect species use enlarged hind femora as weapons in male-male combat, and yet can shed them readily, without regeneration, upon entrapment. We found that metabolic rate decreased by an average of 23.5% in males after leg loss and by 7.9% in females. Notably, larger males had less of a drop in metabolic rate per gram of weapon lost. Our findings suggest that sexually selected weapons contribute to a large portion of resting metabolic rate in males, but these costs do not scale in direct proportion to size; larger males can have larger weapons for a reduced metabolic cost. These energetic maintenance costs may be integral to the evolution of the allometries of sexually selected weapons, and yet they remain largely unexplored.

The costs of a big brain: extreme encephalization results in higher energetic demand and reduced hypoxia tolerance in weakly electric African fishes

A large brain can offer several cognitive advantages. However, brain tissue has an especially high metabolic rate. Thus, evolving an enlarged brain requires either a decrease in other energetic requirements, or an increase in overall energy consumption. Previous studies have found conflicting evidence for these hypotheses, leaving the metabolic costs and constraints in the evolution of increased encephalization unclear. Mormyrid electric fishes have extreme encephalization comparable to that of primates. Here, we show that brain size varies widely among mormyrid species, and that there is little evidence for a trade-off with organ size, but instead a correlation between brain size and resting oxygen consumption rate. Additionally, we show that increased brain size correlates with decreased hypoxia tolerance. Our data thus provide a non-mammalian example of extreme encephalization that is accommodated by an increase in overall energy consumption. Previous studies have found energetic trade-offs with variation in brain size in taxa that have not experienced extreme encephalization comparable with that of primates and mormyrids. Therefore, we suggest that energetic trade-offs can only explain the evolution of moderate increases in brain size, and that the energetic requirements of extreme encephalization may necessitate increased overall energy investment.

Energetics of Sensing and Communication in Electric Fish: A Blessing and a Curse in the Anthropocene?

Weakly electric freshwater fish use self-generated electric fields to image their worlds and communicate in the darkness of night and turbid waters. This active sensory/communication modality evolved independently in the freshwaters of South America and Africa, where hundreds of electric fish species are broadly and abundantly distributed. The adaptive advantages of the sensory capacity to forage and communicate in visually-unfavorable environments and outside the detection of visually-guided predators likely contributed to the broad success of these clades across a variety of Afrotropical and neotropical habitats. Here we consider the potentially high and limiting metabolic costs of the active sensory and communication signals that define the gymnotiform weakly electric fish of South America. Recent evidence from two well-studied species suggests that the metabolic costs of electrogenesis can be quite high, sometimes exceeding one-fourth of these fishes' daily energy budget. Supporting such an energetically expensive system has shaped a number of cellular, endocrine, and behavioral adaptations to restrain the metabolic costs of electrogenesis in general or in response to metabolic stress. Despite a suite of adaptations supporting electrogenesis, these weakly electric fish are vulnerable to metabolic stresses such as hypoxia and food restriction. In these conditions, fish reduce signal amplitude presumably as a function of absolute energy shortfall or as a proactive means to conserve energy. In either case, reducing signal amplitude compromises both sensory and communication performance. Such outcomes suggest that the higher metabolic cost of active sensing and communication in weakly electric fish compared with the sensory and communication systems in other neotropical fish might mean that weakly electric fish are disproportionately susceptible to harm from anthropogenic disturbances of neotropical aquatic habitats. Fully evaluating this possibility, however, will require broad comparative studies of metabolic energetics across the diverse clades of gymnotiform electric fish and in comparison to other nonelectric neotropical fishes.

Food deprivation reduces and leptin increases the amplitude of an active sensory and communication signal in a weakly electric fish

Energetic demands of social communication signals can constrain signal duration, repetition, and magnitude. The metabolic costs of communication signals are further magnified when they are coupled to active sensory systems that require constant signal generation. Under such circumstances, metabolic stress incurs additional risk because energy shortfalls could degrade sensory system performance as well as the social functions of the communication signal. The weakly electric fish Eigenmannia virescens generates electric organ discharges (EODs) that serve as both active sensory and communication signals. These EODs are maintained at steady frequencies of 200-600Hz throughout the lifespan, and thus represent a substantial metabolic investment. We investigated the effects of metabolic stress (food deprivation) on EOD amplitude (EODa) and EOD frequency (EODf) in E. virescens and found that only EODa decreases during food deprivation and recovers after restoration of feeding. Cortisol did not alter EODa under any conditions, and plasma cortisol levels were not changed by food deprivation. Both melanocortin hormones and social challenges caused transient EODa increases in both food-deprived and well-fed fish. Intramuscular injections of leptin increased EODa in food-deprived fish but not well-fed fish, identifying leptin as a novel regulator of EODa and suggesting that leptin mediates EODa responses to metabolic stress. The sensitivity of EODa to dietary energy availability likely arises because of the extreme energetic costs of EOD production in E. virescens and also could reflect reproductive strategies of iteroparous species that reduce social signaling and reproduction during periods of stress to later resume reproductive efforts when conditions improve.

Vocal performance affects metabolic rate in dolphins: implications for animals communicating in noisy environments

Many animals produce louder, longer or more repetitious vocalizations to compensate for increases in environmental noise. Biological costs of increased vocal effort in response to noise, including energetic costs, remain empirically undefined in many taxa, particularly in marine mammals that rely on sound for fundamental biological functions in increasingly noisy habitats. For this investigation, we tested the hypothesis that an increase in vocal effort would result in an energetic cost to the signaler by experimentally measuring oxygen consumption during rest and a 2 min vocal period in dolphins that were trained to vary vocal loudness across trials. Vocal effort was quantified as the total acoustic energy of sounds produced. Metabolic rates during the vocal period were, on average, 1.2 and 1.5 times resting metabolic rate (RMR) in dolphin A and B, respectively. As vocal effort increased, we found that there was a significant increase in metabolic rate over RMR during the 2 min following sound production in both dolphins, and in total oxygen consumption (metabolic cost of sound production plus recovery costs) in the dolphin that showed a wider range of vocal effort across trials. Increases in vocal effort, as a consequence of increases in vocal amplitude, repetition rate and/or duration, are consistent with behavioral responses to noise in free-ranging animals. Here, we empirically demonstrate for the first time in a marine mammal, that these vocal modifications can have an energetic impact at the individual level and, importantly, these data provide a mechanistic foundation for evaluating biological consequences of vocal modification in noise-polluted habitats.

Negligible energetic cost of sonar jamming in a bat–moth interaction

Energetic cost can constrain how frequently animals exhibit behaviors. The energetic cost of acoustic signaling for communication has been the subject of numerous studies; however, the cost of acoustic signaling for predator defense has not been addressed. We studied the energetic cost and efficiency of sound production for the clicks produced by the moth Bertholdia trigona (Grote, 1879) (Grote’s bertholdia) to jam the sonar of predatory bats. This moth is an excellent model species because of its extraordinary ability to produce sound—it clicks at the highest known rate of any moth, up to 4500 clicks·s–1. We measured the metabolic cost of clicking, resting, and flying from moths suspended in a respirometry chamber. Clicking was provoked by playing back an echolocation attack sequence. The cost of sound production for B. trigona was low (66% of resting metabolic rate) and the acoustic efficiency, or the percentage of metabolic power that is converted into sound, was moderately high (0.30% ± 0.15%) compared with other species. We discuss mechanisms that allow B. trigona to achieve their extraordinary clicking rates and high acoustic efficiency. Clicking for jamming bat sonar incurs negligible energetic cost to moths despite being the most effective known anti-bat defense. These results have implications for both the ecology of predator–prey interactions and the evolution of jamming signals.

Metabolic efficiency in courtship favors males with intermediate mass in the Australian redback spider, Latrodectus hasselti

Recent studies have suggested that metabolic efficiency may be an important factor in male mating success when females require vigorous and/or prolonged courtship. In capital breeding animals in which a male's resource pool is fixed at adulthood the relationship between energy expenditure and courtship performance may be especially important, as males are expected to utilize their finite resources efficiently when soliciting mates. Males may benefit from being efficient, i.e., achieving a sufficiently high level of courtship signaling at low energetic cost, if it enables them to acquire mates before their limited energy reserves are depleted. We investigated the relationship between metabolic efficiency and courtship vibrational signaling in the Australian redback spider, Latrodectus hasselti, a semelparous capital breeder where males invest heavily in courtship to secure a mating. We assessed metabolic rate in a sample of males and measured two courtship components (duty cycle and amplitude) that reflected the energy content of web-borne vibrations. We then calculated two indices of metabolic efficiency for these courtship properties. There was a quadratic relationship between mass and duty cycle such that the highest duty cycle signals were performed by males having intermediate mass. Furthermore, intermediate-mass males were also the most metabolically efficient. Prolonged courtship is necessary in L. hasselti for successful mating, and the results of this study suggest that intermediate-mass males are superior courters because they utilize their finite resource pool most efficiently to produce high energy vibrational signals.

The energetics of electric organ discharge generation in gymnotiform weakly electric fish

Gymnotiform weakly electric fish produce an electric signal to sense their environment and communicate with conspecifics. Although the generation of such relatively large electric signals over an entire lifetime is expected to be energetically costly, supporting evidence to date is equivocal. In this article, we first provide a theoretical analysis of the energy budget underlying signal production. Our analysis suggests that wave-type and pulse-type species invest a similar fraction of metabolic resources into electric signal generation, supporting previous evidence of a trade-off between signal amplitude and frequency. We then consider a comparative and evolutionary framework in which to interpret and guide future studies. We suggest that species differences in signal generation and plasticity, when considered in an energetics context, will not only help to evaluate the role of energetic constraints in the evolution of signal diversity but also lead to important general insights into the energetics of bioelectric signal generation.

The mitonuclear compatibility hypothesis of sexual selection

Why females assess ornaments when choosing mates remains a central question in evolutionary biology. We hypothesize that the imperative for a choosing female to find a mate with nuclear oxidative phosphorylation (OXPHOS) genes that are compatible with her mitochondrial OXPHOS genes drives the evolution of ornaments. Indicator traits are proposed to signal the efficiency of OXPHOS function thus enabling females to select mates with nuclear genes that are compatible with maternal mitochondrial genes in the formation of OXPHOS complexes. Species-typical pattern of ornamentation is proposed to serve as a marker of mitochondrial type ensuring that females assess prospective mates with a shared mitochondrial background. The mitonuclear compatibility hypothesis predicts that the production of ornaments will be closely linked to OXPHOS pathways, and that sexual selection for compatible mates will be strongest when genes for nuclear components of OXPHOS complexes are Z-linked. The implications of this hypothesis are that sexual selection may serve as a driver for the evolution of more efficient cellular respiration.

Cellular mechanisms of developmental and sex differences in the rapid hormonal modulation of a social communication signal

Some gymnotiform electric fish species rapidly modify their electric signal waveforms by altering the action potential (AP) waveforms of their electrocytes, the excitable cells that produce the electric organ discharge (EOD). The fish Brachyhypopomus gauderio alters EOD amplitude and pulse duration as a social signal in accordance with the prevailing social conditions, under the dual regulation of melanocortins and androgens. We show here that B. gauderio uses two distinct cellular mechanisms to change signal amplitude, and its use of these two mechanisms varies with age and sex of the signaler. EOD amplitude and waveform are regulated by the coordinated timing and shaping of two APs generated from two opposing excitable membranes in each electrocyte. The two membranes fire in sequence within 100 μs of each other with the second AP being broader than the first. We have shown previously that mature males increase EOD amplitude and duration when melanocortin peptide hormones act directly on electrocytes to selectively broaden the second AP and increase the delay between the two APs by approximately 25 μs. Here we show that females selectively broaden only the second AP as males do, but increase amplitude of both APs with no change in delay between them, a previously unreported second mechanism of EOD amplitude change in B. gauderio. Juvenile fish broaden both APs and increase the delay between the APs. Cellular mechanisms of EOD plasticity are therefore shaped during development, presumably by sex steroids, becoming sexually dimorphic at maturity.

The metabolic cost of communicative sound production in bottlenose dolphins (Tursiops truncatus)

Bottlenose dolphins (Tursiops truncatus) produce various communicative sounds that are important for social behavior, maintaining group cohesion and coordinating foraging. For example, whistle production increases during disturbances, such as separations of mother-calf pairs and vessel approaches. It is clear that acoustic communication is important to the survival of these marine mammals, yet the metabolic cost of producing whistles and other socials sounds and the energetic consequences of modifying these sounds in response to both natural and anthropogenic disturbance are unknown. We used flow-through respirometry to determine whether the metabolic cost of sound production could be quantified in two captive dolphins producing social sounds (whistles and squawks). On average, we found that metabolic rates measured during 2 min periods of sound production were 1.2 times resting values. Up to 7 min were required for metabolism to return to resting values following vocal periods. The total metabolic cost (over resting values) of the 2 min vocal period plus the required recovery period (163.3 to 2995.9 ml O2 or 3279.6 to 60,166.7 J) varied by individual as well as by mean duration of sounds produced within the vocal period. Observed variation in received cumulative sound energy levels of vocalizations was not related to total metabolic costs. Furthermore, our empirical findings did not agree with previous theoretical estimates of the metabolic cost of whistles. This study provides the first empirical data on the metabolic cost of sound production in dolphins, which can be used to estimate metabolic costs of vocal responses to environmental perturbations in wild dolphins.

Differential expression of genes and proteins between electric organ and skeletal muscle in the mormyrid electric fish Brienomyrus brachyistius

Electric organs (EOs) have evolved independently in vertebrates six times from skeletal muscle (SM). The transcriptional changes accompanying this developmental transformation are not presently well understood. Mormyrids and gymnotiforms are two highly convergent groups of weakly electric fish that have independently evolved EOs: while much is known about development and gene expression in gymnotiforms, very little is known about development and gene expression in mormyrids. This lack of data limits prospects for comparative work. We report here on the characterization of 28 differentially expressed genes between SM and EO tissues in the mormyrid Brienomyrus brachyistius, which were identified using suppressive subtractive hybridization (SSH). Forward and reverse SSH was performed on tissue samples of EO and SM resulting in one cDNA library enriched with mRNAs expressed in EO, and a second library representing mRNAs unique to SM. Nineteen expressed sequence tags (ESTs) were identified in EO and nine were identified in SM using BLAST searching of Danio rerio sequences available in NCBI databases. We confirmed differential expression of all 28 ESTs using RT-PCR. In EO, these ESTs represent four classes of proteins: (1) ion pumps, including the α- and β-subunits of Na(+)/K(+)-ATPase, and a plasma membrane Ca(2+)-ATPase; (2) Ca(2+)-binding protein S100, several parvalbumin paralogs, calcyclin-binding protein and neurogranin; (3) sarcomeric proteins troponin I, myosin heavy chain and actin-related protein complex subunit 3 (Arcp3); and (4) the transcription factors enhancer of rudimentary homolog (ERH) and myocyte enhancer factor 2A (MEF2A). Immunohistochemistry and western blotting were used to demonstrate the translation of seven proteins (myosin heavy chain, Na(+)/K(+)-ATPase, plasma membrane Ca(2+)-ATPase, MEF2, troponin and parvalbumin) and their cellular localization in EO and SM. Our findings suggest that mormyrids express several paralogs of muscle-specific genes and the proteins they encode in EOs, unlike gymnotiforms, which may post-transcriptionally repress several sarcomeric proteins. In spite of the similarity in the physiology and function of EOs in mormyrids and gymnotiforms, this study indicates that the mechanisms of development in the two groups may be considerably different.

Is sociality required for the evolution of communicative complexity? Evidence weighed against alternative hypotheses in diverse taxonomic groups

Complex social communication is expected to evolve whenever animals engage in many and varied social interactions; that is, sociality should promote communicative complexity. Yet, informal comparisons among phylogenetically independent taxonomic groups seem to cast doubt on the putative role of social factors in the evolution of complex communication. Here, we provide a formal test of the sociality hypothesis alongside alternative explanations for the evolution of communicative complexity. We compiled data documenting variations in signal complexity among closely related species for several case study groups--ants, frogs, lizards and birds--and used new phylogenetic methods to investigate the factors underlying communication evolution. Social factors were only implicated in the evolution of complex visual signals in lizards. Ecology, and to some degree allometry, were most likely explanations for complexity in the vocal signals of frogs (ecology) and birds (ecology and allometry). There was some evidence for adaptive evolution in the pheromone complexity of ants, although no compelling selection pressure was identified. For most taxa, phylogenetic null models were consistently ranked above adaptive models and, for some taxa, signal complexity seems to have accumulated in species via incremental or random changes over long periods of evolutionary time. Becoming social presumably leads to the origin of social communication in animals, but its subsequent influence on the trajectory of signal evolution has been neither clear-cut nor general among taxonomic groups.

Food restriction promotes signaling effort in response to social challenge in a short-lived electric fish

Vertebrates exposed to stressful conditions release glucocorticoids to sustain energy expenditure. In most species elevated glucocorticoids inhibit reproduction. However individuals with limited remaining reproductive opportunities cannot afford to forgo reproduction and should resist glucocorticoid-mediated inhibition of reproductive behavior. The electric fish Brachyhypopomus gauderio has a single breeding season in its lifetime, thus we expect males to resist glucocorticoid-mediated inhibition of their sexual advertisement signals. We studied stress resistance in male B. gauderio (i) by examining the effect of exogenous cortisol administration on the signal waveform and (ii) by investigating the effect of food limitation on androgen and cortisol levels, the amplitude of the electric signal waveform, the responsiveness of the electric signal waveform to social challenge, and the amount of feeding activity. Exogenous cortisol administration did reduce signal amplitude and pulse duration, but endogenous cortisol levels did not rise with food limitation or social challenge. Despite food limitation, males responded to social challenges by further increasing androgen levels and enhancing the amplitude and duration of their electric signal waveforms. Food-restricted males increased androgen levels and signal pulse duration more than males fed ad libitum. Socially challenged fish increased food consumption, probably to compensate for their elevated energy expenditure. Previous studies showed that socially challenged males of this species simultaneously elevate testosterone and cortisol in proportion to signal amplitude. Thus, B. gauderio appears to protect its cortisol-sensitive electric advertisement signal by increasing food intake, limiting cortisol release, and offsetting signal reduction from cortisol with signal-enhancing androgens.

Developmental plasticity in metabolic rates reinforces morphological plasticity in response to social cues of sexual selection

Developmental plasticity allows individuals to match their phenotype to the competitive environment they are most likely to encounter. Although there are numerous studies that demonstrate adaptive shifts in life-history and metric traits, we still have a poor understanding of whether internal physiological processes demonstrate plasticity and whether this plasticity supports life-history and metric traits. Here we use the Australian redback spider (Latrodectus hasselti), a species that demonstrates adaptive developmental shifts in response to the availability of females and the density of males, to examine the relationship between the routine metabolic rate (RMR) and the expression of size, body-condition and development rate. We reared immature males in three diet treatments, and in social environments that varied the presence/absence of females and the density of males and measured their RMR, weight, size and developmental rate at maturity. We show that although RMR decreased with decreasing resource abundance, RMR was positively correlated with the density of rivals. Moreover, RMR was not correlated with size or body-condition at maturity. Our results demonstrate that plasticity in the RMR supports plasticity in metric and life-history traits to create an integrated phenotype that matches the competitive environment.

Signal modulation as a mechanism for handicap disposal

Signal honesty may be compromised when heightened competition provides incentive for signal exaggeration. Some degree of honesty might be maintained by intrinsic handicap costs on signalling or through imposition of extrinsic costs, such as social punishment of low quality cheaters. Thus, theory predicts a delicate balance between signal enhancement and signal reliability that varies with degree of social competition, handicap cost, and social cost. We investigated whether male sexual signals of the electric fish Brachyhypopomus gauderio would become less reliable predictors of body length when competition provides incentives for males to boost electric signal amplitude. As expected, social competition under natural field conditions and in controlled lab experiments drove males to enhance their signals. However, signal enhancement improved the reliability of the information conveyed by the signal, as revealed in the tightening of the relationship between signal amplitude and body length. Signal augmentation in male B. gauderio was independent of body length, and thus appeared not to be curtailed through punishment of low quality (small) individuals. Rather, all individuals boosted their signals under high competition, but those whose signals were farthest from the predicted value under low competition boosted signal amplitude the most. By elimination, intrinsic handicap cost of signal production, rather than extrinsic social cost, appears to be the basis for the unexpected reinforcement of electric signal honesty under social competition. Signal modulation may provide its greatest advantage to the signaller as a mechanism for handicap disposal under low competition rather than as a mechanism for exaggeration of quality under high competition.