The Journal of Experimental Biology

Oxygen is essential for all terrestrial animals, but there is dramatic variability in how well different animals and even different cell types can adapt to reduced oxygen availability. We used a meta-analysis of the literature, with a focus on mouse studies, to identify pathways that might act to protect animals in low oxygen environments. We identified 108 genes whose mRNA levels change under hypoxia, and 55 genes critical for mounting a response to hypoxia. With this data, we developed a list of conserved genes, and we tested three C.elegans genes previously uncharacterized in hypoxia, mxl-3, yap-1, and ador-1, and found that loss of function altered egg-laying during and after hypoxia. Our method provides a more targeted approach of how to screen for hypoxic phenotypes and study in more genetically tractable organisms to show mechanisms.

Extraocular motoneurons are the final neuronal relay implicated in gaze motor control and are known to be subdivided in functional subgroups, differently implicated in ocular motion dynamics. However, the maturation of these functional populations of extraocular motoneurons, in relation with the development of gaze-stabilizing reflexes remains largely unexplored. In amphibian tadpoles, the angular vestibulo-ocular reflex (VOR) appears later than other visuo-vestibular ocular reflexes and matures until the metamorphosis climax. Two types of Abducens motoneurons have been described to participate to the angular VOR in larval frog: spontaneous motor units, exhibiting a robust resting activity and silent motor units recruited only during head motion. The aim of this study was to investigate the maturation of these two types of Abducens motor units in relation with the development of the angular VOR by evaluating their discharge dynamic in response to head rotation in semi-intact preparations of larval Xenopus laevis. During larval life, the discharge modulation during sinusoidal head rotations increases significantly for silent units only, demonstrating a better sensitivity of this Abducens motoneuron sub-population to horizontal semicircular canal activation. In addition, this functional maturation was accompanied by an increase of the myelination in the lateral rectus motor nerve, promoting a faster conductivity in late larval stages than in early one. These findings showed that the development of the angular VOR is supported by a selective maturation of extraocular motoneurons subpopulations, specifically implicated in the improvement of the ocular kinematic during the reflex.

Ankle stiffness is decreased during movement compared to posture; however, the etiology of this decrease remains unknown. Determining what gives rise to this decrease is critical for understanding how humans successfully interact with their physical world and how that ability is compromised by functional impairments. While the triceps surae and Achilles tendon primarily dictate ankle stiffness, the relative contributions across posture and movement remain unknown. Therefore, our study sought to quantify the relative contributions of the muscle and tendon to ankle stiffness and how those contributions differ between posture and movement. We used our technique, which combines B-mode ultrasound imaging with joint-level perturbations, to quantify ankle, muscle, and tendon stiffness simultaneously. Since ankle, muscle, and tendon stiffness all scale with torque, participants matched torque between posture and movement tasks. During posture, the Achilles tendon is the dominant contributor to ankle stiffness. However, during movement, the triceps surae and Achilles tendon contribute more equally to ankle stiffness, which can be attributed to a significant decrease in muscle stiffness during movement. Here, we provide the first empirical data on how state-dependent properties of the triceps surae and Achilles tendon contribute to ankle stiffness in conditions relevant to locomotion.

In most muscles, contraction is initiated by neural activation. Some groups of insects break this rule, flapping at frequencies far exceeding the neural drive to their flight muscles. These insects muscles (termed asynchronous) produce force in response to stretch, enabling flight at faster frequencies than would be possible through the slow calcium-dependent processes associated with neural activation. The first flapping insects lacked stretch-activated physiology, which then evolved on top of neural activation dynamics before likely being reduced again in some groups including moths. Stretch and neural activation can co-exist, but it remains unclear if stretch-activation alone is sufficient to generate asynchronous flapping in insect flight muscle. Building on prior closed-loop muscle physiology platforms, we develop a new way to perform a gain-of-function muscle physiology experiment called the physiological dynamic clamp. Inspired by dynamic clamp experiments in neuroscience, we couple isolated intact flight muscle from a hawkmoth, Manduca sexta, to simulated stretch-activation in virtual reality. Tuning virtual reality parameters allows us to manipulate the degree of stretch-activation in silico while retaining all other physiological properties of the muscle. With artificially enhanced stretch activation, we find that hawkmoth muscle can support stretch-activated work at typical wingbeat frequencies. When simultaneously stimulated at wingbeat frequency, interference between stretch and neural activation results in variable work production. However, this interference disappears when the two activation timescales are close to each other resulting in entrainment to the neural drive. Matching time scales suggests an evolutionary path for smoothly transitioning to stretch-activated, asynchronous flight and back again.

Environmental temperature strongly influences muscle contractile mechanics and locomotor performance in ectotherms, yet animals routinely develop across a range of temperatures while maintaining effective movement. We tested the hypothesis that developmental temperature induces compensatory changes in the intrinsic mechanical properties of the muscles that power the fast-start escape response in larval zebrafish (Danio rerio). Larvae were reared at 25{degrees}C, 28{degrees}C, or 32{degrees}C, and contractile properties of intact tail myotomal muscles were measured across experimental temperatures. Acute changes in experimental temperature strongly affected twitch kinetics, particularly relaxation rate (Q10 = 2.1), resulting in substantial changes in twitch duration. In contrast, rearing temperature produced adaptive changes that opposed these acute thermal effects. At a common experimental temperature, muscles from cold-reared larvae exhibited faster intrinsic relaxation and greater force production during shortening at a physiologically relevant velocity, whereas warm-reared larvae showed slower relaxation and reduced shortening force. As a result, twitch kinetics were largely normalized when measurements were made at each group's rearing temperature, reducing the apparent thermal sensitivity of relaxation rate (Q10 = 1.1). To identify molecular correlates of these functional adaptations, we performed label-free quantitative LCMS proteomic analysis. Cold rearing increased the abundance of Sarco/Endoplasmic Reticulum Calcium-ATPase (SERCA) proteins, driven primarily by elevated atp2a1 expression, while warm rearing reduced the abundance of the major parvalbumin isoforms pvalb1 and pvalb2. These changes implicate remodeling of intracellular calcium handling as a mechanism underlying thermal compensation of muscle function. Together, our results demonstrate that developmental temperature modifies the intrinsic mechanical properties of larval zebrafish muscle in ways that counteract the direct effects of environmental temperature, thereby preserving the timing and power-generating capacity required for fast-start escape performance.

Neural circuits produce reliable activity even after environmental disturbances. This occurs because neurons respond to perturbations in a compensatory manner, a process termed homeostatic plasticity. Bullfrogs undergo prolonged periods underwater during winter, when lung ventilation and its neural control system ceases activity, but air-breathing resumes unscathed when environmental temperatures increase weeks to months later. Compensatory neural mechanisms that contribute involve upregulation of excitatory synaptic transmission on motoneurons driven by inactivity, but whether inactivity or acclimation to low temperatures drive other forms of compensation is not known. The GABAA receptor contribution to respiratory rhythm generation is downregulated following overwintering, which promotes network excitability. Therefore, we disentangled the contributions of cold temperature acclimation and inactivity experienced during overwintering on reduced GABAergic signaling. Here, we show that cold temperature, and not inactivity, reduces GABAA signaling in the respiratory rhythm generating network, without influencing GABAA transmission onto motoneurons. Therefore, cold temperature acclimation drives reduced GABAergic signaling selectively in inter-neuronal rhythm generating circuits, while excitatory motoneurons synapses are strengthened by inactivity in the overwintering environment. Most work interprets compensatory plasticity as activity-dependent during activity perturbations, but we reveal that different aspects of a disruptive environment elicit distinct forms of plasticity across a motor network.

To better understand how animals cope with increasingly variable and challenging environments, there is a need to study how prolonged exposure to elevated glucocorticoid hormones (i.e. one mediator of the stress response) affects their physiology. While glucocorticoid elevation is known to increase oxidative stress and accelerate cellular ageing, there is evidence that king penguins (Aptenodytes patagonicus) can prevent oxidative stress during acute stress exposure, suggesting that species may differ in their sensitivity to glucocorticoids downstream negative effects. As king penguins thrive in a seemingly harsh environment, we hypothesized that they may be able to limit the deleterious effects usually associated with chronic glucocorticoid elevation, either through resistance (i.e. prevention of downstream negative effect) or resilience (i.e. rapid recovery following transient negative effect). To test this hypothesis, we experimentally elevated corticosterone levels in incubating king penguins and quantified treatment effects on a suite of physiological traits at multiple time points across incubation and early chick-rearing, up to ca. 2 months after implantation. Corticosterone-treated individuals showed a prolonged increase in corticosterone and decrease in body condition, confirming our treatment likely mimicked sustained stress exposure. Heterophil-to-lymphocyte ratio was only increased transiently, and there was no clear evidence that treatment influenced oxidative stress or telomere length maintenance. Plasma energy metabolites were mainly affected early after implantation, with rapid recovery over time. Overall, our results suggest that adult king penguins show at least moderate resistance and resilience to chronic corticosterone elevation, especially in preventing cellular integrity loss, though at-sea physiological effects remain to be determined.

Many animals navigate their world largely by seeing and feeling it. To disentangle these visual and mechanosensory contributions, we developed a virtual reality assay targeting the optomotor response in adult wild-type zebrafish swimming against flow. By projecting dynamic visual patterns onto the walls of a variable-speed flow tank, we decoupled wide-field optic flow from hydrodynamic velocity. We then tested fish responses to abrupt visual perturbations while they held station in the unsteady wake behind a bluff body. These perturbations reliably elicited compensatory optomotor responses, with fish aligning to the direction of the moving stimulus. Notably, this behavior was absent in uniform flows, suggesting that fish prioritize visual input when predictive lateral line signaling is compromised. We propose that this sensory shift serves to optimize swimming energetics in turbulent wakes. Extending this framework, we further show that zebrafish swimming against flow, whether alone or in groups, exhibit heightened escape responses to looming visual stimuli. Together, our findings reveal that fish sensory strategies are not fixed but dynamically tuned to hydrodynamic context: favoring visual cues in turbulent environments and lateral line input in uniform flows. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=84 SRC="FIGDIR/small/715425v1_ufig1.gif" ALT="Figure 1"> View larger version (18K): org.highwire.dtl.DTLVardef@1d7ba00org.highwire.dtl.DTLVardef@1f456f1org.highwire.dtl.DTLVardef@7826c4org.highwire.dtl.DTLVardef@391a68_HPS_FORMAT_FIGEXP M_FIG C_FIG

Tree shrews (Tupaia belangeri) are increasingly used in comparative neuroscience, yet their respiratory physiology remains poorly characterized. We quantified spontaneous breathing and respiratory rhythm variability in awake adult tree shrews (n = 10; 5 males, 5 females) using whole-body plethysmography. Respiratory frequency decreased by approximately 16% with acclimatization to the recording chamber, while respiratory timing, body-mass-normalized respiratory amplitude, inspiratory flow, and minute ventilation remained relatively stable. After acclimatization, mean respiratory parameters were similar between sexes, but short-term breath-to-breath variability (SD1) was greater in males than females, whereas SD2 was comparable. These findings establish baseline respiratory characteristics in awake tree shrews and identify sex-dependent differences in short-term respiratory rhythm stability.

Animals use long-range signals, such as hormones and neural signals, to coordinate the actions of distant organs. There is no precise, quantitative framework that explains the problems these control systems must solve and thus predicts their behavior under varied conditions. We consider this problem in the context of blood glucose regulation by the hormone insulin, the failure of which produces diabetes. We show that existing mathematical models of glucose regulation admit equivalent control strategies with no hormones at all, and thus cannot explain the need for hormonal regulation. We therefore introduce a minimal model of inter-organ variations in local glucose, and show that control strategies based on local glucose measurements face severe trade-offs between different control objectives. In contrast, we show that hormonal control signals from the pancreas can overcome these limitations. By exposing the benefits of hormonal control, our work paves the way to a detailed understanding of physiological design principles, with possible implications for the engineering of an artificial pancreas.

Movement is integral to animal life, and most animal movement is actuated by the same engine: skeletal muscle. Muscle input is typically mediated by skeletal elements, resulting in musculoskeletal systems that are "geared": at any instant, the muscle force and velocity are related to the output force and velocity only via a proportionality constant G, the "mechanical advantage". The functional analysis of such "simple machines" has traditionally centred around this instantaneous interpretation, such that a small vs large G is thought to reflect a fast vs forceful system, respectively. But evidence is mounting that a complete analysis ought to also consider the mechanical energy output of a complete contraction. Here, we approach this task systematically, and use the theory of physiological similarity to study how gearing affects the flow of mechanical energy in a minimalist model of a musculoskeletal system. Gearing influences the flow of mechanical energy in two key ways: it can curtail muscle work output, because it determines the ratio between the characteristic muscle work and kinetic energy capacity; and it defines how each unit of muscle work is partitioned into different system energies, i. e. into kinetic vs. "parasitic" energy such as heat. As a consequence of both effects, delivering maximum work in minimum time and with maximum transmission efficiency generally requires a mechanical advantage of intermediate magnitude. This optimality condition can be expressed in terms of two dimensionless numbers, which reflect the key geometric, physiological, and physical properties of the interrogated musculoskeletal system, and the environment in which the contraction takes place. Illustrative application to exemplar musculoskeletal systems predicts plausible mechanical advantages in disparate biomechanical scenarios; yields a speculative explanation for why gearing is typically used to attenuate the instantaneous force output (Gopt < 1); and predicts how G needs to vary systematically with animal size to optimise the delivery of mechanical energy, in superficial agreement with empirical observations. A many-to-one-mapping from musculoskeletal geometry to mechanical performance is identified, such that differences in G alone do not provide a reliable indicator for specialisation for force vs speed--neither instantaneously, nor in terms of mechanical energy output. The energy framework presented here can be used to estimate an optimal mechanical advantage across variable muscle physiology, anatomy, mechanical environment and animal size, and so facilitates investigation of the extent to which selection has made efficient use of gearing as degree of freedom in musculoskeletal "design".

Collective motion emerges from local interactions among individuals, yet whether interaction rules inferred from trajectory data correspond to the mechanisms actually used by animals remains unresolved. Here, we address this question using an autonomous closed-loop robotic fish implementing a data-driven model of social interactions reconstructed from the schooling fish Hemigrammus rhodostomus. The robot continuously updated its behavior from real-time tracking of freely swimming fish while reproducing spontaneous locomotion, wall avoidance, and anisotropic attraction and alignment. We compared entirely biological groups, biohybrid groups containing one robotic fish, and numerical simulations using identical behavioral descriptors across isolated individuals, pairs, and groups of five fish. The robotic fish successfully integrated into natural schools and reproduced the principal signatures of collective coordination, providing the first direct causal validation of interaction rules reconstructed from behavioral trajectories. Biohybrid experiments showed that a robot responding only to its single most influential neighbor was sufficient to sustain natural collective coordination. By contrast, numerical simulations reproduced the behavior of biological groups most accurately when each fish interacted with its two most influential neighbors. This discrepancy identifies the contribution of hydrodynamic interactions, which remain available to living fish but are absent from the robotic controller, demonstrating that physical and behavioral interactions jointly shape collective organization. These findings establish closed-loop biohybrid robotics as a powerful framework for experimentally testing the mechanisms underlying collective animal behavior. SignificanceInferring the behavioral mechanisms underlying collective animal behavior from trajectory data alone cannot establish causality. We combined a data-driven model of fish social interactions with an autonomous closed-loop robotic fish that continuously interacted with freely swimming conspecifics. This biohybrid approach provides the first direct causal validation of interaction rules reconstructed from behavioral trajectories. Comparing biological groups, biohybrid groups, and numerical simulations further reveals that hydrodynamic interactions complement social interactions in shaping collective organization. While living fish require information from their two most influential neighbors to reproduce natural schools, a robotic fish lacking hydrodynamic feedback achieves comparable coordination by responding to only its single most influential neighbor, demonstrating the power of closed-loop biohybrid robotics for testing mechanisms of collective behavior.

Humans often collaborate under asymmetric information, for example when two people carry a table and only one knows the destination. They coordinate without speech using cues from movement kinematics, interaction forces, and object states. Characterizing this sensorimotor communication is difficult because these signals both execute the task and convey information, whose meaning is context-dependent. Here, we investigated a virtual table-carrying task where one partner knew the target while the other inferred it from visuo-haptic feedback. Participants flexibly adapted kinematic and haptic cues across contexts to convey intention. We introduce an explainable machine-learning framework that decodes intent from ongoing multimodal signals and quantifies where individual features are informative. Incorporating the decoded signals into a drift-diffusion model accurately predicted the uninformed partner's target choices and decision times. Together, our framework explains how humans communicate through action and offers principles for collaborative robots to infer and express intent through physical interaction.

Cruising speed is a key factor affecting prey-search efficiency and migration range in continuously swimming animals. Tunas and lamnid sharks (e.g., white sharks) have convergently evolved traits for high-speed cruising, including the ability to maintain slow-twitch, aerobic red muscle (RM) warmer than ambient water, known as RM endothermy. Despite their well-known high cruising speeds, kinematic features underlying their elevated speeds remain unclear. Swim speed is the product of tailbeat frequency (TBF; Hz) and stride length (SL, the absolute distance traveled per tailbeat; m). RM endothermy is expected to elevate TBF by enhancing muscle contraction performance. Furthermore, within RM-endothermic fishes, tunas and lamnid sharks may exhibit distinct kinematic features because of differences in caudal fin morphology and tailbeat amplitude. Here, we compiled kinematic parameters from 20 fish species, including five RM-endothermic species, measured in the wild using animal-borne sensors. Comparative analyses showed that, for a given body mass and water temperature, RM-endothermic fishes exhibited 1.9 times higher cruising speed and TBF than ectothermic fishes, while SL remained similar. Within RM-endothermic fishes, tunas exhibited 2.3 times higher TBF than similar-sized lamnid sharks, whereas lamnid sharks showed 1.7 times longer SL than similar-sized tunas. These results indicate that RM endothermy is generally associated with higher TBF, while significant kinematic differences remain between tunas and lamnid sharks. This divergence may be partly explained by the greater caudal fin area and tailbeat amplitude in lamnid sharks. It may also reflect contrasting skeletal types of teleosts and elasmobranchs, which potentially influence body stiffness and swimming kinematics.

When sensory modalities yield conflicting information, animals must rapidly reassess stimuli to select their actions. We induced auditory-visual conflict in free-flying echolocating Egyptian fruit bats, by fitting animals with prisms that shifted the perceived visual location of a landing perch while echoes returned from its veridical location. Bats that course-corrected within a single goal-directed flight did so by decoupling sonar gaze from steering, to enable rapid reweighting of visual and auditory cues. We designed artificial agents that used Bayesian inference to construct estimates of goal locations in their environment. When competing estimates directed active-sensing behaviors distinctly from steering, agents course-corrected more rapidly. Consistent with this idea, when bats were fit with prisms and earplugs that attenuated auditory localization cues, they were unable to course-correct. Removing prisms produced no systematic after-effects. Our framework suggests that instead of correcting their behavior after failure, animals could efficiently employ active sensing to resolve sensory conflict before failure occurs. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=178 SRC="FIGDIR/small/743555v1_ufig1.gif" ALT="Figure 1"> View larger version (41K): org.highwire.dtl.DTLVardef@16262edorg.highwire.dtl.DTLVardef@4cd82aorg.highwire.dtl.DTLVardef@103e6fforg.highwire.dtl.DTLVardef@13292bf_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOCover figure.C_FLOATNO Bat wearing helmet with clear-glasses and tracking markers. Photograph (C) 2026 Nikita M. Finger / Moss Laboratory. C_FIG

Shorebirds provide an excellent model for investigating the relationship between bill morphology and food acquisition. Food acquisition comprises three successive behavioural stages: foraging (locomotion and prey capture), feeding (food handling and transport), and swallowing. During food transport, these birds use two non-lingual mechanisms, surface-tension transport (ST) and ballistic transport (BT), whose characteristics depends on the kinematics of head and beak movements and the physical properties of the food. We investigated the behavioural flexibility of these transport mechanisms in captive and free-ranging individuals of two species with contrasting beak morphologies: the Pied avocet (Recurvirostra avosetta) and the Black-winged stilt (Himantopus himantopus). Although these species have beaks of a similar size, avocets are distinguished by their upward-curved beaks, whereas stilts have a rather straight beak. We examined the effects of food water content and the presence or absence of water in the beak on food transport by quantifying maximum gape, maximum head displacement, and maximum head velocity. Transport kinematics were jointly influenced by food properties and water availability in the beak. Moist food improved transport performance in both species, whereas dry food required compensatory increases in gape amplitude and head movements, demonstrating that neither ST nor BT constitutes a fixed behavioural sequence. Species also differed consistently in their transport strategies: Black-winged stilts relied on slower, larger-amplitude head movements, whereas Pied Avocets exhibited faster, more precise movements, particularly when water was present. These findings demonstrate that ST and BT share common biomechanical foundations while being governed by rapid kinematic adjustments to changing environmental conditions that probably correspond to flexible motor control. This behavioural flexibility in food transport is therefore likely to enhance feeding performance and ecological resilience in the heterogeneous habitats occupied by shorebirds, suggesting that context-dependent modulation of transport behaviour represents an important adaptive feature of these shorebirds.

Caenorhabditis elegans is a microscopic nematode used extensively as a model organism in studies of neuromuscular function and neurodegenerative disorders. A mutation in mir-1 affects signalling at the neuromuscular junction. We investigate the effect of this mutation on the propulsive power exerted by nematodes as they grow in size with age. We compare the motility of wild-type and mir-1(gk276) mutant nematodes in a Newtonian fluid using a two-component, two dimensional (2C-2D) Digital Microscopic Particle Image Velocimetry ({micro}-PIV) technique. Beating amplitudes of the head and tail, the wavelength of undulatory waves and the swimming speed scale linearly with size in both the wild-type and mutant strains. The beating frequency is independent of size or position along the body. Differences in the magnitudes of these kinematic parameters between the two strains, however, grow systematically with age. The swimming speed scales linearly with the wave speed of the neuromuscular undulation in both nematode strains with a conserved ratio. The magnitude of mean power and mean local fluid circulation in the mutant is significantly lower compared to those of the wild-type animals of the same age. This indicates that a mutation in mir-1 adversely affects motility in C. elegans.

Motor systems controlling locomotion must generate repetitive rhythmic activity, while also still retaining the ability to generate a diverse range of outputs. How motor systems monitor, regulate, and promote diversity of their own outputs is not well understood. Here, we perform single-step, variable-order and hidden-state Markov modelling (HSMM) on spontaneous fictive locomotor activity in the isolated Drosophila larval nervous system to examine how a motor system balances constraint and promotion of diversity amongst competing motor programs. We show that spontaneous fictive activity is structured by interacting mechanisms operating at multiple levels of sequence organisation. Analysis of one-step transition rules revealed a bias in activity towards activity states underlying exploration that in turn, promote transition to diverse outputs. In contrast, higher-order Markov, N-gram, and HSMM analysis indicated a memory biased towards revisiting recently executed motor programs. These mechanisms together suggest that the Drosophila larval locomotor system maintains a dynamic repertoire of possible motor outputs by monitoring recent activity and biasing future transitions accordingly. In this sense, fictive rhythmogenesis reflects a diversity-generating process: the larval locomotor network does not simply repeat a fixed motor programme or randomly transition from one state to another, but rather continually regulates access to rhythmic states based on recent experience. Together, these findings suggest that fictive locomotor dynamics are consistent with adaptive winner-takes-all competition between central pattern generating (CPG) modules that balance constraint and promotion of motor program diversity.

The framework of homeostatic plasticity posits that neurons regulate cellular properties through feedback homeostasis to maintain activity during changes in the environment. However, when disturbances occur in wild animals they are often caused by environmental variables that induce their own acclimation effects, making it difficult to discern if activity-sensitive feedback plays a role in ecological settings. We addressed this problem using a natural activity perturbation, where frogs hibernate in cold water, leaving brainstem motor circuits that generate breathing inactive for long periods. We show here that motor inactivity, amid complex environmental variables in the hibernation environment, represents a key signal for increasing AMPA-glutamate receptors (AMPARs) on motoneurons. Homeostatic upregulation of AMPARs do not regulate neural activity per se but instead correspond with enhanced evoked transmission selectively at cool temperatures. The results show how homeostatic synaptic plasticity may allow animals to restart motor behavior after chronic inactivity encountered in the natural environment. More broadly, these results introduce the concept of homeostatic plasticity as a mechanism to shape thermal tolerance ranges of neural performance in ecological settings.

An animals response to chemosensory cues depends on the animals prior experience, internal state, or life stage. However, the molecular mechanisms that regulate sensory valence (i.e., whether a chemosensory cue is attractive or repulsive) remain poorly understood. We investigated the mechanisms that specify sensory valence using the responses of the free-living nematode Caenorhabditis elegans to carbon dioxide (CO2). C. elegans exhibits highly flexible responses to CO2: well-fed animals are repelled by CO2, while both starved animals and well-fed animals raised under high CO2 conditions are attracted to CO2. Here, we show that CO2 attraction in animals raised at high CO2 requires a cGMP signaling pathway that involves the cGMP-dependent protein kinase EGL-4. This pathway does not regulate CO2 response in starved animals, indicating that the role of EGL-4 in mediating CO2 attraction depends on satiety state. Cultivation under high CO2 conditions leads to increased cGMP levels in the CO2-detecting BAG neurons, consistent with a specific requirement for EGL-4 in high-CO2-cultivated animals. We also show that EGL-4 regulates CO2 valence by altering neuropeptide expression in BAG. Our results indicate that sensory valence is established in a context-dependent manner at the level of the primary sensory neuron.