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

Evolution of neural circuitry and cognition

Neural circuits govern the interface between the external environment, internal cues and outwardly directed behaviours. To process multiple environmental stimuli and integrate these with internal state requires considerable neural computation. Expansion in neural network size, most readily represented by whole brain size, has historically been linked to behavioural complexity, or the predominance of cognitive behaviours. Yet, it is largely unclear which aspects of circuit variation impact variation in performance. A key question in the field of evolutionary neurobiology is therefore how neural circuits evolve to allow improved behavioural performance or innovation. We discuss this question by first exploring how volumetric changes in brain areas reflect actual neural circuit change. We explore three major axes of neural circuit evolution-replication, restructuring and reconditioning of cells and circuits-and discuss how these could relate to broader phenotypes and behavioural variation. This discussion touches on the relevant uses and limitations of volumetrics, while advocating a more circuit-based view of cognition. We then use this framework to showcase an example from the insect brain, the multi-sensory integration and internal processing that is shared between the mushroom bodies and central complex. We end by identifying future trends in this research area, which promise to advance the field of evolutionary neurobiology.

How hibernation in frogs drives brain and reproductive evolution in opposite directions

Environmental seasonality can promote the evolution of larger brains through cognitive and behavioral flexibility but can also hamper it when temporary food shortage is buffered by stored energy. Multiple hypotheses linking brain evolution with resource acquisition and allocation have been proposed for warm-blooded organisms, but it remains unclear how these extend to cold-blooded taxa whose metabolism is tightly linked to ambient temperature. Here, we integrated these hypotheses across frogs and toads in the context of varying brumation (hibernation) durations and their environmental correlates. We showed that protracted brumation covaried negatively with brain size but positively with reproductive investment, likely in response to brumation-dependent changes in the socio-ecological context and associated selection on different tissues. Our results provide novel insights into resource allocation strategies and possible constraints in trait diversification, which may have important implications for the adaptability of species under sustained environmental change.

Ketamine disrupts locomotion and electrolocation in a novel model of schizophrenia, Gnathonemus petersii fish

The present study aimed to examine a weakly electric fish Gnathonemus petersii (G. petersii) as a candidate model organism of glutamatergic theory of schizophrenia. The idea of G. petersii elevating the modeling of schizophrenia symptoms is based on the fish's electrolocation and electrocommunication abilities. Fish were exposed to the NMDA antagonist ketamine in two distinct series differing in the dose of ketamine. The main finding revealed ketamine-induced disruption of the relationship between electric signaling and behavior indicating impairment of fish navigation. Moreover, lower doses of ketamine significantly increased locomotion and erratic movement and higher doses of ketamine reduced the number of electric organ discharges indicating successful induction of positive schizophrenia-like symptoms and disruption of fish navigation. Additionally, a low dose of haloperidol was used to test the normalization of the positive symptoms to suggest a predictive validity of the model. However, although successfully induced, positive symptoms were not normalized using the low dose of haloperidol; hence, more doses of the typical antipsychotic haloperidol and probably also of a representative of atypical antipsychotic drugs need to be examined to confirm the predictive validity of the model.

Convergent mosaic brain evolution is associated with the evolution of novel electrosensory systems in teleost fishes

Brain region size generally scales allometrically with brain size, but mosaic shifts in brain region size independent of brain size have been found in several lineages and may be related to the evolution of behavioral novelty. African weakly electric fishes (Mormyroidea) evolved a mosaically enlarged cerebellum and hindbrain, yet the relationship to their behaviorally novel electrosensory system remains unclear. We addressed this by studying South American weakly electric fishes (Gymnotiformes) and weakly electric catfishes (Synodontis spp.), which evolved varying aspects of electrosensory systems, independent of mormyroids. If the mormyroid mosaic increases are related to evolving an electrosensory system, we should find similar mosaic shifts in gymnotiforms and Synodontis. Using micro-computed tomography scans, we quantified brain region scaling for multiple electrogenic, electroreceptive, and non-electrosensing species. We found mosaic increases in cerebellum in all three electrogenic lineages relative to non-electric lineages and mosaic increases in torus semicircularis and hindbrain associated with the evolution of electrogenesis and electroreceptor type. These results show that evolving novel electrosensory systems is repeatedly and independently associated with changes in the sizes of individual major brain regions independent of brain size, suggesting that selection can impact structural brain composition to favor specific regions involved in novel behaviors.

Larger animals tend to have larger brains and smaller animals tend to have smaller ones. However, some species do not fit the pattern that would be expected based on their body size. This variation between species can also apply to individual brain regions. This may be due to evolutionary forces shaping the brain when favouring particular behaviours. However, it is difficult to directly link changes in species behaviour and variations in brain structure.

One way to understand the impact of evolutionary adaptations is to study species that have developed new behaviours and compare them to related ones that lack such a behaviour. An opportunity to do this lies in the ability of several species of fish to produce and sense electric fields in water. While this system is not found in most fish, it has evolved multiple times independently in distantly-related lineages. Schumacher and Carlson examined whether differences in the size of brains and individual regions between species were associated with the evolution of electric field generation and sensing.

Micro-computed tomography, or μCT, scans of the brains of multiple fish species revealed that the species that can produce electricity – also known as ‘electrogenic’ species’ – have more similar brain structures to each other than to their close relatives that lack this ability. The brain regions involved in producing and detecting electrical charges were larger in these electrogenic fish. This similarity was apparent despite variations in how total brain size has evolved with body size across species.

These results demonstrate how evolutionary forces acting on particular behaviours can lead to predictable changes in brain structure. Understanding how and why brains evolve will allow researchers to better predict how species’ brains and behaviours may adapt as human activities alter their environments.

A Spark in the Dark: Uncovering Natural Activity Patterns of Mormyrid Weakly Electric Fish

To understand animal ecology, observation of wildlife in the natural habitat is essential, but particularly challenging in the underwater realm. Weakly electric fishes provide an excellent opportunity to overcome some of these challenges because they generate electric organ discharges (EODs) to sense their environment and to communicate, which can be detected non-invasively. We tracked the EOD and swimming activity of two species of mormyrid weakly electric fishes (Marcusenius victoriae and Petrocephalus degeni) over diel cycles in the laboratory, and we recorded EODs and environmental dissolved oxygen (DO) concentration and temperature over several months in a naturally hypoxic habitat in Uganda. Under laboratory conditions, both species showed increases of activity and exploration behavior that were closely synchronized to the onset of the dark phase. In the wild, fish preferred structurally complex habitats during the day, but dispersed toward open areas at night, presumably to forage and interact. Nocturnal increase of movement range coincided with diel declines in DO concentration to extremely low levels. The fact that fish showed pronounced nocturnal activity patterns in the laboratory and in the open areas of their habitat, but not under floating vegetation, indicates that light intensity exerts a direct effect on their activity. We hypothesize that being dark-active and tolerant to hypoxia increases the resistance of these fish against predators. This study establishes a new technology to record EODs in the field and provides a window into the largely unknown behavior of mormyrids in their natural habitat.

Hypoxia Tolerant Species: The Wisdom of Nature Translated into Targets for Stroke Therapy

Human neurons rapidly die after ischemia and current therapies for stroke management are limited to restoration of blood flow to prevent further brain damage. Thrombolytics and mechanical thrombectomy are the available reperfusion treatments, but most of the patients remain untreated. Neuroprotective therapies focused on treating the pathogenic cascade of the disease have widely failed. However, many animal species demonstrate that neurons can survive the lack of oxygen for extended periods of time. Here, we reviewed the physiological and molecular pathways inherent to tolerant species that have been described to contribute to hypoxia tolerance. Among them, Foxo3 and Eif5A were reported to mediate anoxic survival in Drosophila and Caenorhabditis elegans, respectively, and those results were confirmed in experimental models of stroke. In humans however, the multiple mechanisms involved in brain cell death after a stroke causes translation difficulties to arise making necessary a timely and coordinated control of the pathological changes. We propose here that, if we were able to plagiarize such natural hypoxia tolerance through drugs combined in a pharmacological cocktail it would open new therapeutic opportunities for stroke and likely, for other hypoxic conditions.

Exceptionally Steep Brain-Body Evolutionary Allometry Underlies the Unique Encephalization of Osteoglossiformes

Brain-body static allometry, which is the relationship between brain size and body size within species, is thought to reflect developmental and genetic constraints. Existing evidence suggests that the evolution of large brain size without accompanying changes in body size (that is, encephalization) may occur when this constraint is relaxed. Teleost fish species are generally characterized by having close-fitting brain-body static allometries, leading to strong allometric constraints and small relative brain sizes. However, one order of teleost, Osteoglossiformes, underwent extreme encephalization, and its mechanistic bases are unknown. Here, I used a dataset and phylogeny encompassing 859 teleost species to demonstrate that the encephalization of Osteoglossiformes occurred through an increase in the slope of evolutionary (among-species) brain-body allometry. The slope is virtually isometric (1.03 ± 0.09 SE), making it one of the steepest evolutionary brain-body allometric slopes reported to date, and it deviates significantly from the evolutionary brain-body allometric slopes of other clades of teleost. Examination of the relationship between static allometric parameters (intercepts and slopes) and evolutionary allometry revealed that the dramatic steepening of the evolutionary allometric slope in Osteoglossiformes was a combined result of evolution in the slopes and intercepts of static allometry. These results suggest that the evolution of static allometry, which likely has been driven by evolutionary changes in the rate and timing of brain development, has facilitated the unique encephalization of Osteoglossiformes.

Gill surface area provides a clue for the respiratory basis of brain size in the blacktip shark (Carcharhinus limbatus)

Brain size varies dramatically, both within and across species, and this variation is often believed to be the result of trade-offs between the cognitive benefits of having a large brain for a given body size and the energetic cost of sustaining neural tissue. One potential consequence of having a large brain is that organisms must also meet the associated high energetic demands. Thus, a key question is whether metabolic rate correlates with brain size. However, using metabolic rate to measure energetic demand yields a relatively instantaneous and dynamic measure of energy turnover, which is incompatible with the longer evolutionary timescale of changes in brain size within and across species. Morphological traits associated with oxygen consumption, specifically gill surface area, have been shown to be correlates of oxygen demand and energy use, and thus may serve as integrated correlates of these processes, allowing us to assess whether evolutionary changes in brain size correlate with changes in longer-term oxygen demand and energy use. We tested how brain size relates to gill surface area in the blacktip shark Carcharhinus limbatus. First, we examined whether the allometric slope of brain mass (i.e., the rate that brain mass changes with body mass) is lower than the allometric slope of gill surface area across ontogeny. Second, we tested whether gill surface area explains variation in brain mass, after accounting for the effects of body mass on brain mass. We found that brain mass and gill surface area both had positive allometric slopes, with larger individuals having both larger brains and larger gill surface areas compared to smaller individuals. However, the allometric slope of brain mass was lower than the allometric slope of gill surface area, consistent with our prediction that the allometric slope of gill surface area could pose an upper limit to the allometric slope of brain mass. Finally, after accounting for body mass, individuals with larger brains tended to have larger gill surface areas. Together, our results provide clues as to how fishes may evolve and maintain large brains despite their high energetic cost, suggesting that C. limbatus individuals with a large gill surface area for their body mass may be able to support a higher energetic turnover, and, in turn, a larger brain for their body mass.

Neuromodulation or energy failure? Metabolic limitations silence network output in the hypoxic amphibian brainstem

Hypoxia tolerance in the vertebrate brain often involves chemical modulators that arrest neuronal activity to conserve energy. However, in intact networks, it can be difficult to determine whether hypoxia triggers modulators to stop activity in a protective manner or whether activity stops because rates of ATP synthesis are insufficient to support network function. Here, we assessed the extent to which neuromodulation or metabolic limitations arrest activity in the respiratory network of bullfrogs-a circuit that survives moderate periods of oxygen deprivation, presumably, by activating an inhibitory noradrenergic pathway. We confirmed that hypoxia and norepinephrine (NE) reduce network output, consistent with the view that hypoxia may cause the release of NE to inhibit activity. However, these responses differed qualitatively; hypoxia, but not NE, elicited a large motor burst and silenced the network. The stereotyped response to hypoxia persisted in the presence of both NE and an adrenergic receptor blocker that eliminates sensitivity to NE, indicating that noradrenergic signaling does not cause the arrest. Pharmacological inhibition of glycolysis and mitochondrial respiration recapitulated all features of hypoxia on network activity, implying that reduced ATP synthesis underlies the effects of hypoxia. Finally, activating modulatory mechanisms that dampen neuronal excitability when ATP levels fall, KATP channels and AMP-dependent protein kinase, did not resemble the hypoxic response. These results suggest that energy failure-rather than inhibitory modulation-silences the respiratory network during hypoxia and emphasize the need to account for metabolic limitations before concluding that modulators arrest activity as an adaptation for energy conservation in the nervous system.

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.

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.

Intraspecific Energetic Trade-Offs and Costs of Encephalization Vary from Interspecific Relationships in Three Species of Mormyrid Electric Fishes

The evolution of increased encephalization comes with an energetic cost. Across species, this cost may be paid for by an increase in metabolic rate or by energetic trade-offs between the brain and other energy-expensive tissues. However, it remains unclear whether these solutions to deal with the energetic requirements of an enlarged brain are related to direct physiological constraints or other evolved co-adaptations. We studied the highly encephalized mormyrid fishes, which have extensive species diversity in relative brain size. We previously found a correlation between resting metabolic rate and relative brain size across species; however, it is unknown how this interspecific relationship evolved. To address this issue, we measured intraspecific variation in relative brain size, the sizes of other organs, metabolic rate, and hypoxia tolerance to determine if intraspecific relationships between brain size and organismal energetics are similar to interspecific relationships. We found that 3 species of mormyrids with varying degrees of encephalization had no intraspecific relationships between relative brain size and relative metabolic rate or relative sizes of other organs, and only 1 species had a relationship between relative brain size and hypoxia tolerance. These species-specific differences suggest that the interspecific relationship between metabolic rate and relative brain size is not the result of direct physiological constraints or strong stabilizing selection, but is instead due to other species level co-adaptations. We conclude that variation within species must be considered when determining the energetic costs and trade-offs underlying the evolution of extreme encephalization.

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.

Extreme Enlargement of the Cerebellum in a Clade of Teleost Fishes that Evolved a Novel Active Sensory System

Brains, and the distinct regions that make up brains, vary widely in size across vertebrates [1, 2]. Two prominent hypotheses have been proposed to explain brain region scaling evolution. The mosaic hypothesis proposes that changes in the relative sizes of particular brain regions are the result of selection acting independently on those regions [2, 3]. The concerted hypothesis proposes that the brain evolves as a coordinated structure due to developmental constraints [4]. These hypotheses have been widely debated [3-7], and recent studies suggest a combination of the two best describes vertebrate brain region scaling [8-10]. However, no study has addressed how the mosaic and concerted models relate to the evolution of novel behavioral phenotypes. We addressed this question using African mormyroid fishes. The mormyroids have evolved a novel active electrosensory system and are well known for having extreme encephalization [11] and a large cerebellum [2, 12], which is cited as a possible example of mosaic evolution [2]. We found that compared to outgroups without active electrosensing, mormyroids experienced mosaic increases in the sizes of the cerebellum and hindbrain, and mosaic decreases in the sizes of the telencephalon, optic tectum, and olfactory bulb. However, the evolution of extreme encephalization within mormyroids was associated with concerted changes in the sizes of all brain regions. This suggests that mosaic evolutionary change in the regional composition of the brain is most likely to occur alongside the evolution of novel behavioral functions, but not with the evolution of extreme encephalization.

Oxygen, evolution and redox signalling in the human brain; quantum in the quotidian

Rising atmospheric oxygen (O₂ ) levels provided a selective pressure for the evolution of O₂ -dependent micro-organisms that began with the autotrophic eukaryotes. Since these primordial times, the respiring mammalian cell has become entirely dependent on the constancy of electron flow, with molecular O₂ serving as the terminal electron acceptor in mitochondrial oxidative phosphorylation. Indeed, the ability to 'sense' O₂ and maintain homeostasis is considered one of the most important roles of the central nervous system (CNS) and probably represented a major driving force in the evolution of the human brain. Today, modern humans have evolved with an oversized brain committed to a continually active state and, as a consequence, paradoxically vulnerable to failure if the O₂ supply is interrupted. However, our pre-occupation with O₂ , the elixir of life, obscures the fact that it is a gas with a Janus face, capable of sustaining life in physiologically controlled amounts yet paradoxically deadly to the CNS when in excess. A closer look at its quantum structure reveals precisely why; the triplet ground state diatomic O₂ molecule is paramagnetic and exists in air as a free radical, constrained from reacting aggressively with the brain's organic molecules due to its 'spin restriction', a thermodynamic quirk of evolutionary fate. By further exploring O₂ 's free radical 'quantum quirkiness', including emergent (quantum) physiological phenomena, our understanding of precisely how the human brain senses O₂ deprivation (hypoxia) and the elaborate redox-signalling defence mechanisms that defend O₂ homeostasis has the potential to offer unique insights into the pathophysiology and treatment of human brain disease.

RETRACTED ARTICLE: The quantum physiology of oxygen; from electrons to the evolution of redox signaling in the human brain

Rising atmospheric oxygen (O₂) levels provided a selective pressure for the evolution of O₂-dependent micro-organisms that began with the autotrophic eukaryotes. Since these primordial times, the respiring mammalian cell has become entirely dependent on the constancy of electron flow with molecular O₂ serving as the terminal electron acceptor in mitochondrial oxidative phosphorylation. Indeed, the ability to “sense” O₂ and maintain homeostasis is considered one of the most important roles of the central nervous system (CNS) and likely represented a major driving force in the evolution of the human brain. Today, modern humans have evolved with an oversized brain committed to a continually active state and as a consequence, paradoxically vulnerable to failure if the O₂ supply is interrupted. However, our pre-occupation with O₂, the elixir of life, obscures the fact that it is a gas with a Janus Face, capable of sustaining life in physiologically controlled amounts yet paradoxically deadly to the CNS when in excess. A closer look at its quantum structure reveals precisely why; the triplet ground state diatomic O₂ molecule is paramagnetic and exists in air as a free radical, constrained from reacting aggressively with the brain’s organic molecules due to its “spin restriction”, a thermodynamic quirk of evolutionary fate. By further exploring O₂’s free radical “quantum quirkiness” including emergent quantum physiological phenomena, our understanding of precisely how the human brain senses O₂ deprivation (hypoxia) and the elaborate redox-signaling defense mechanisms that defend O₂ homeostasis has the potential to offer unique insights into the pathophysiology and treatment of human brain disease.

Effects of hypoxia on swimming and sensing in a weakly electric fish

Low dissolved oxygen (hypoxia) can severely limit fish performance, especially aerobically expensive behaviours including swimming and acquisition of sensory information. Fishes can reduce oxygen requirements by altering these behaviours under hypoxia, but the underlying mechanisms can be difficult to quantify. We used a weakly electric fish as a model system to explore potential effects of hypoxia on swim performance and sensory information acquisition, which enabled us to non-invasively record electric signalling activity used for active acquisition of sensory information during swimming. To quantify potential effects of hypoxia, we measured critical swim speed (U_crit) and concurrent electric signalling activity under high- and low-dissolved oxygen concentrations in a hypoxia-tolerant African mormyrid fish, Marcusenius victoriae Fish were maintained under normoxia for 6 months prior to experimental treatments, and then acclimated for 8 weeks to normoxia or hypoxia and tested under both conditions (acute: 4 h exposure). Acute hypoxia exposure resulted in a significant reduction in both U_crit and electric signalling activity in fish not acclimated to hypoxia. However, individuals acclimated to chronic hypoxia were characterized by a higher U_crit under both hypoxia and normoxia than fish acclimated to normoxia. Following a 6 month re-introduction to normoxia, hypoxia-acclimated individuals still showed increased performance under acute hypoxic test conditions, but not under normoxia. Our results highlight the detrimental effects of hypoxia on aerobic swim performance and sensory information acquisition, and the ability of fish to heighten aerobic performance through acclimation processes that can still influence performance even months after initial exposure.

Testing ontogenetic patterns of sexual size dimorphism against expectations of the expensive tissue hypothesis, an intraspecific example using oyster toadfish (Opsanus tau)

Trade‐offs associated with sexual size dimorphism (SSD) are well documented across the Tree of Life. However, studies of SSD often do not consider potential investment trade‐offs between metabolically expensive structures under sexual selection and other morphological modules. Based on the expectations of the expensive tissue hypothesis, investment in one metabolically expensive structure should come at the direct cost of investment in another. Here, we examine allometric trends in the ontogeny of oyster toadfish (Opsanus tau) to test whether investment in structures known to have been influenced by strong sexual selection conform to these expectations. Despite recovering clear changes in the ontogeny of a sexually selected trait between males and females, we find no evidence for predicted ontogenetic trade‐offs with metabolically expensive organs. Our results are part of a growing body of work demonstrating that increased investment in one structure does not necessarily drive a wholesale loss of mass in one or more organs.

No evidence for the expensive-tissue hypothesis in Fejervarya limnocharis

Because the brain is one of the energetically most expensive organs of animals, trade-offs have been hypothesized to exert constraints on brain size evolution. The expensive-tissue hypothesis predicts that the cost of a large brain should be compensated by decreasing size of other metabolically costly tissues, such as the gut. Here, we analyzed the relationships between relative brain size and the size of other metabolically costly tissues (i.e., gut, heart, lung, kidney, liver, spleen or limb muscles) among four Fejervarya limnocharis populations to test the predictions of the expensive-tissue hypothesis. We did not find that relative brain size was negatively correlated with relative gut length after controlling for body size, which was inconsistent with the prediction of the expensive-tissue hypothesis. We also did not find negative correlations between relative brain mass and relative size of the other energetically expensive organs. Our findings suggest that the cost of large brains in F. limnocharis cannot be compensated by decreasing size in other metabolically costly tissues.

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.

Hypoxia acclimation increases novelty response strength during fast-starts in the African mormyrid, Marcusenius victoriae

Many fishes perform quick and sudden swimming maneuvers known as fast-starts to escape when threatened. In pulse-type weakly electric fishes these responses are accompanied by transient increases in the rate of electric signal production known as novelty responses. While novelty responses may increase an individual's information about their surroundings, they are aerobically powered and may come at a high energetic cost when compared to fast-starts, which rely primarily on anaerobic muscle. The juxtaposition between two key aspects of fast-starts in these fishes - the aerobic novelty response and the anaerobic swimming performance - makes them an interesting model for studying effects of hypoxia on escape performance and sensory information acquisition. We acclimated the hypoxia-tolerant African mormyrid Marcusenius victoriae to either high or low dissolved oxygen (DO) levels for 8weeks, after which fast-starts and novelty responses were quantified under both high (normoxic) and low-DO (hypoxic) test conditions. Hypoxia-acclimated fish exhibited higher maximum curvature than normoxia-acclimated fish. Displacement of normoxia-acclimated fish was not reduced under acute hypoxic test conditions. Novelty responses were given upon each startle, whether or not the fish performed a fast-start; however, novelty responses associated with fast-starts were significantly stronger than those without, suggesting a functional link between fast-start initiation and the motor control of the novelty response. Overall, hypoxia-acclimated individuals produced significantly stronger novelty responses during fast-starts. We suggest that increased novelty response strength in hypoxia-acclimated fish corresponds to an increased rate of sensory sampling, which may compensate for potential negative effects of hypoxia on higher-level processing.

Why are there so many explanations for primate brain evolution?

The question as to why primates have evolved unusually large brains has received much attention, with many alternative proposals all supported by evidence. We review the main hypotheses, the assumptions they make and the evidence for and against them. Taking as our starting point the fact that every hypothesis has sound empirical evidence to support it, we argue that the hypotheses are best interpreted in terms of a framework of evolutionary causes (selection factors), consequences (evolutionary windows of opportunity) and constraints (usually physiological limitations requiring resolution if large brains are to evolve). Explanations for brain evolution in birds and mammals generally, and primates in particular, have to be seen against the backdrop of the challenges involved with the evolution of coordinated, cohesive, bonded social groups that require novel social behaviours for their resolution, together with the specialized cognition and neural substrates that underpin this. A crucial, but frequently overlooked, issue is that fact that the evolution of large brains required energetic, physiological and time budget constraints to be overcome. In some cases, this was reflected in the evolution of 'smart foraging' and technical intelligence, but in many cases required the evolution of behavioural competences (such as coalition formation) that required novel cognitive skills. These may all have been supported by a domain-general form of cognition that can be used in many different contexts.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.