The Journal of Experimental Biology

Most animals move intermittently. Of these, animals that fly and swim move in two stages: during active propulsion, when they beat their wings or swing their tails, and right after, when they glide or coast through the air or water. The kinematics of active movements are typically well studied, but the coasting period afterwards is less understood. We used larval zebrafish as a model of intermittent locomotion. Larval zebrafish swim in discrete bouts lasting ~200 ms, coast for short durations and distances, and then are largely stationary for ~800 ms. We find that larval zebrafish do not coast like a passive object. While they coast for distances that are correlated with the prior swim's strength, as expected from physics, they seem to be able to modulate the mechanics of their coasting: the time constant is not a constant, but varies from coast-to-coast. Furthermore, the time constant is correlated with the duration of the interval between swim bouts. This suggests that larval zebrafish can either shorten or extend their coasting duration, and that coasting inhibits the initiation of the next swim. Finally, the time constant changes quickly, while other swimming kinematics (e.g., distance swum) are autocorrelated over the course of at least several seconds. Altogether, this suggests that fish, and perhaps other animals that move intermittently, actively modulate their post-propulsion movement, possibly to gather sensory feedback in the absence of blurry visual feedback associated with fast locomotion.

Background: Insufficient oxygen (hypoxia) or complete lack of oxygen (anoxia) causes cellular dysfunction and death in terrestrial animals. As a result, animals have developed homeostatic strategies, including changes in movement strategies to escape and survive stressful environments. Caenorhabditis elegans respond to changes in oxygen by increasing the frequency of reversals (backward movement) and increasing locomotor speed; these behaviors enable them to escape unfavorable environments and move to more suitable ones. The molecular and neural mechanisms underlying behavioral responses to low-oxygen environments are incomplete. Neuropeptides regulate C. elegans behaviors by modifying neural circuit function in response to hypoxia. Loss of function in C. elegans neuropeptide nlp-40 and its receptor aex-2 increased survival to anoxia [1]. However, the cells in which NLP-40 and AEX-2 function for anoxia, as well as the mechanisms underlying responses to anoxia, remain elusive. We hypothesized that NLP-40 and AEX-2 regulate anoxia by altering escape behaviors. Results: We found that neuropeptide NLP-40 functions in cholinergic neurons and AEX-2 functions in cholinergic or GLR-1 interneurons or motor neurons to regulate susceptibility to anoxia. We found that loss of either nlp-40 or aex-2 decreased locomotor activity and reversal frequency early during 48 hours of anoxia, suggesting that nlp-40 and aex-2 are required for the proper execution of these behaviors. We found that neuronal knockdown of nlp-40 recapitulated the behaviors observed in nlp-40 mutant animals and increased survival; however, restoring aex-2 function in glr-1 interneurons or motor neurons was sufficient to rescue behaviors to wild-type and restore vulnerability to anoxia. Conclusions: Overall, our results suggest that NLP-40 and AEX-2 likely function in cholinergic or glr-1 interneurons or motor neurons to regulate escape behaviors during anoxia. Escape behaviors are part of the homeostatic response and are typically protective in the wild; however, in our paradigm, we show that loss of either nlp-40 or aex-2 reduces escape responses and confers protection against 48 hours of anoxia. Collectively, these results highlight a new role for NLP-40 and AEX-2 in anoxia and shed light on mechanisms underlying the adaptive responses to anoxia.

Background: Nervous system regeneration requires creation of new neurons and their integration into new and existing networks. How new neurons find their targets and reconnect in the environment of regenerating tissue remains a major gap in knowledge. The planarian Schmidtea mediterranea is well suited to the study of neural regeneration due to the remarkable ability of planarians to regenerate their entire body, including the nervous system. However, we have not yet been able to explore synaptogenesis in regeneration using planarians due to a lack of tools to study synapses. Here we investigate potential players in planarian synapse, focusing on the conserved syntaxin protein family, which functions in neurotransmitter vesicle fusion at the synapse. Results: We found that reduction of either syntaxin-1 (stx1) or syntaxin-binding protein 1-1 (stxbp1-1) decreased locomotion and impeded brain regeneration, specifically in synapse-rich regions. Conclusions: Our observations support STX-1 and STXBP1-1 as bona fide synapse components, providing a starting point for studies neuronal connectivity during homeostatic remodeling and regeneration.

Simulated microgravity provides a controlled perturbation for testing how loss of mechanical load affects human tissues. After 24 hours in rotating-wall vessels, neural, cardiac and tonsil organoids developed tissue-specific transcriptional responses that overlapped published spaceflight datasets, accompanied by higher transcriptomic-age estimates in neural and cardiac models, altered cardiac-organoid contraction dynamics and modified tonsil-organoid antiviral gene-expression signatures. These findings establish a multi-organoid framework for studying acute responses to gravitational unloading.

Known for their rapid water surface locomotion, whirligig beetles (Gyrinidae) are also fast divers. They can transition from the surface to underwater swimming in under 100 ms. Diving requires them to generate sufficient hydrodynamic torque to break the water surface and reorient the body. However, the mechanism by which whirligigs create the necessary pitch-down torque is unknown. Here, we show that their diving is achieved by bending the abdomen. The inertial hydrodynamic pressure on the flexed abdomen generates the torque necessary for surface breaking and body rotation. The torque is maximized at an abdominal angle of 45{degrees}, whereas for effective diving, whirligigs display a preferential abdominal angle of around 28{degrees}, closely aligning with the angle that maximizes the torque-to-drag ratio. This abdominal angle is optimized by balancing torque generation with the drag penalty. Our findings inspire a mechanism for surface-to-underwater transition, offering design principles for multimodal robots capable of locomotion across the air-water interface.

Increasing experimental evidence suggests that biological systems cancel predictable components of sensory signals while maintaining sensitivity to externally induced state changes. This strategy provides task-specific sensor responses for posture, locomotion, and gaze control. A prime example is found in interneurons that respond to visual image shifts resulting from the relative motion between an animal's eyes and its visual surroundings. Such optic flow-processing interneurons, found across phyla and are particularly well characterized in Dipteran and other flying insects. We studied optic flow-processing interneurons in the Monarch butterfly whose large and highly contrasted wings sweep through the visual field with every wing-beat cycle, potentially obscuring interneuron output signals. Our results show baseline spiking activity increases when animals flap their wings, and individual spikes are phase-locked to the wing-beat cycle, even in the dark, when no visual motion input is available. A qualitative estimate of the interneurons' response to directional wing motion through its receptive field is not sufficient to explain the recorded activity patterns. Our results suggest that an additional internal signal suppresses responses to wing-induced visual motion to support effective vision-based stabilization reflexes. These findings support the principle that self-generated signals are suppressed while sensitivity to external modulation is preserved.

Crank length affects the geometry of the bicycle-rider system and influences the motion of the lower limb throughout the pedal cycle. Conventional crank selection has largely relied on rider stature and mechanical power, yet mechanical power and cycling economy remain remarkably insensitive to crank length across a broad range. Yet, elite cyclists and triathletes increasingly adopt shorter cranks, particularly in disciplines where aerodynamic drag strongly affects cycling performance. However, the mechanisms that connect crank length to mechanical power and aerodynamic drag remain unclear. Here we show that crank length couples rider anthropometry, hip kinematics, and aerodynamic drag. Population anthropometry of 6,068 individuals reveals that stature alone poorly predicts personalized crank length, while geometric analysis identifies a nearly linear relationship between crank length and minimum lower hip angle: a 10-mm reduction in crank length increases hip-angle margin by approximately 1.42 degrees with little mechanical penalty. This geometric freedom permits more flexion of the torso and a more aerodynamic posture; the resulting aerodynamic advantage grows rapidly with cycling speed because aerodynamic power scales with velocity cubed. Data from 150 elite cyclists and triathletes support this shift toward shorter cranks, with triathletes adopting shorter relative crank lengths than cyclists. Our results establish crank length as a personalized design variable that connects lower-limb geometry to aerodynamics. These insights enable personalized crank selection, integrated rider-bicycle design, and performance optimization for the increased aerodynamic demands of high-speed cycling.

Spiders arrest their fall by abseiling on dragline silk, varying silk release rate to avoid excessive strains that could break the thread. To understand how this is achieved, we develop a minimal dynamical model of abseiling and formulate a time-optimal control problem subject to physical limits on silk strain and dragline spinning rate. We find that the optimal trajectory includes an interval over which the strain is constant, resulting in uniform deceleration consistent with published experiments. To test the generality of this strategy, we then turn to a distinct extreme motor behavior seen in jumping spiders: using a dragline anchored at the takeoff site to correct their orientation while jumping at speeds exceeding 100 body lengths per second. We show that the same constant-strain strategy suffices to counteract the initial body rotation and ensure an upright landing without significant loss of range. Together, our theoretical framework suggests how strain regulation may serve as a strategy for controlling extreme movements such as abseiling seen in many spiders and may be specialized to tasks such as aerial braking and reorientation in jumping spiders.

The spatial visual acuity of an eye is determined by both the spatial sampling density of the retina and the angular sensitivity of individual photoreceptors. In insect compound eyes, these are respectively characterised by the interommatidial angle, {Delta}{varphi}, and photoreceptor acceptance angle, {Delta}{rho}. During movement, however, finite photoreceptor integration times cause retinal image motion to broaden the effective acceptance angle, potentially degrading spatial resolution beyond its anatomical limit. This problem is especially acute in flying insects, which experience large rotational velocities during locomotion and must actively stabilise gaze to limit retinal image motion. Butterflies present a particularly demanding case because their flight is characterised by large, wingbeat-coupled thoracic oscillations. Here, we quantify how gaze stabilisation limits motion blur in freely flying Monarch butterflies Danaus plexippus by combining free-flight kinematic reconstruction of the head and thorax with separate intracellular measurements of photoreceptor acceptance angles and integration times from quiescent butterflies. We found that, without head compensation, thoracic oscillations would broaden the effective photoreceptor acceptance angle from its quiescent value of 2.17{degrees} to 9.38{degrees} during take-off. Compensatory head movements substantially reduced this motion-induced broadening, limiting the effective acceptance angle to 5.28{degrees}. Nevertheless, this 2.4-fold broadening of effective photoreceptor acceptance angle represents a substantial degradation of spatial acuity in flight, highlighting motion blur as an important constraint on the visual ecology of flying butterflies.

To correct lower limb length discrepancies, orthopedic surgeons often perform limb lengthening procedures. These procedures involve directly lengthening a primary bone (e.g., femur), which also stretches the muscles and tendons spanning the elongation site. Limb lengthening procedures improve lower-limb length symmetry, often at the expense of chronic joint-level decrements. For example, knee joint range of motion and strength does not always return to pre-surgery values following femur lengthening. These joint-level decrements governed by altered muscle force-length characteristics, but it is unestablished how human muscles remodel during surgical limb lengthening. Here, we characterized bilateral human vastus lateralis fascicle force-length profiles following femur lengthening. At 2.2 {+/-} 1.7 years (Avg {+/-} SE) post-initial surgery, the lengthened limb's vastus lateralis fascicles were longer, effectively stiffer, and weaker than the contralateral limb. The lengthened limb's vastus lateralis fascicles were 38 mm longer (Avg) than that of the contralateral limb, even though the lengthened limb's femur was only 31 mm longer (Avg). The onset of vastus lateralis fascicle passive force production began at 22% shorter (Avg) operating lengths in lengthened versus contralateral limb muscles. Additionally, vastus lateralis peak active force production was 34% less (Avg) in the lengthened versus contralateral limbs, and in-series quadriceps tendon stiffness was not stiffer in lengthened limbs. These data suggest that active muscle tissues lengthen more than passive muscle tissues following limb lengthening, effectively creating stiffer muscles. Years after limb lengthening, patient muscles exhibit numerous differences in their force-length properties versus that of natural growth, likely contributing to impaired joint-function.

Coleoid cephalopods (cuttlefish, octopus, and squid) are marine mollusks with elaborate nervous systems that support a diverse repertoire of complex behaviors. These include the neural control of the color, pattern, and texture of the skin, facilitating both adaptive camouflage and innate patterning that may reflect internal state. The development of transgenic cephalopods expressing fluorescent proteins, optogenetic actuators, and reporters of neural activity would contribute a new and important technology to cephalopod biology. The generation of transgenic cephalopods, however, has remained a major challenge. Here, we report the development of stable transgenic dwarf cuttlefish (Ascarosepion bandense) expressing ubiquitous nuclear-localized mScarlet, a red fluorescent protein. We evaluated multiple strategies for transgenesis, and established cuttlefish lines using both CRISPR and the transposons Sleeping Beauty and Minos. The stable expression of transgenes enabled live imaging of cell dynamics during embryonic development. The Minos transposon emerged as the most efficient transgenesis strategy and is adaptable to promoters and transgenes of choice. These strategies now enable the generation of diverse genetic tools for mechanistic studies of cephalopod biology.

We present a network model that enables a navigating agent to shape the spatial and temporal structure of its paths through its environment. The model is inspired by the insect fan-shaped body. In it, a recurrent neural network receives an input that encodes an agent's velocity vector. Meanwhile, network activity determines the setpoint of a control loop that influences this velocity vector through motor outputs. The network thus guides future movement using memory of past movement. We illustrate how the network's dynamics determine the trajectory features it extracts to produce the setpoint. We then relate the joint dynamics of network activity and the agent's velocity vector to the structure of the resulting paths. This network can produce paths suitable for different forms of exploration. It can also stabilize movements initiated by lower-level systems. Our analyses illustrate how recurrent neural dynamics and memory can shape behavior when embedded in closed loops.

Early embryos must mount stress defenses without compromising the developmental programs that drive morphogenesis, but whether genetic background alters this balance remains unclear. We compared cellularizing Drosophila melanogaster embryos from inbred and outcrossed backgrounds under control conditions and after acute or chronic heat-stress. Both backgrounds developed successfully and showed modest transcriptomic differences at 18{degrees}C. Heat stress at 32{degrees}C, however, sharply separated their phenotypic and transcriptional responses. Inbred embryos showed poor larval hatching after acute and chronic heat, and this deficit persisted through adult eclosion. Their transcriptomes exhibited a greater stress-response burden, with enrichment of proteostasis, DNA repair, mitochondrial maintenance, RNA processing, and metabolic remodeling programs. Outcrossed embryos retained substantially greater developmental survival and higher abundance of transcripts associated with development, adhesion, epithelial organization, signaling, transport, sensory responses, and morphogenesis. Background-dependent divergence was greatest during acute heat. Under chronic heat, background-dependent differences were fewer, with most overlapping those seen during acute heat and a smaller set detected only under chronic heat. Transcript-origin analysis further showed that acute outcrossed-biased genes were predominantly maternally derived and showed no enrichment for purely zygotic transcripts, whereas chronic outcrossed-biased genes contained a larger zygotic component. In summary, the less viable inbred background mounted the broader molecular stress signature, whereas the more viable outcrossed background retained greater abundance of development-associated transcripts. These findings identify genetic background as a major determinant of how embryonic heat responses are distributed between molecular stress defense and developmental gene expression.

Standing balance relies on the continuous detection of body motion across a wide range of amplitudes and frequencies. According to Weber's law, sensory discrimination should degrade in proportion to stimulus amplitude, yet vestibular self-motion perception deviates from this prediction. Whether this deviation extends to the vestibular control of posture remains unknown. To test this hypothesis, we delivered amplitude-increasing electrical vestibular stimulation (EVS, 0-2 mA) at four frequencies (0.1, 0.2, 0.5, and 1 Hz) to 10 healthy young adults, and quantified balance discrimination thresholds (DTs) from the gain and inter-trial variability of the lateral ground reaction force, using signal detection theory. Balance control DTs increased sublinearly with EVS amplitude at all frequencies, deviating from Weber's law. Both balance-correcting response gain and variability accounted for this deviation: gain decreased more slowly than the strictly inverse relationship that would have alone produced strict adherence to Weber's law, while variability exhibited an initial increase followed by saturation at higher EVS amplitudes, a signal-dependent structure necessary to reproduce the observed DT pattern. EVS frequency modulated the shape of this deviation but affected neither overall DT nor the signal-to-noise ratio of the balance-correcting response, which instead remained similar across frequencies. This study revealed a particular property of postural control discrimination by extending the psychophysical and neural deviations from Weber's law previously reported for vestibular perception. Overall, these results provide a better understanding of the balance control discrimination process and provide potential additional assessment criteria for vestibular impairments.

Social genetic effects can substantially influence phenotypes in animals and plants and the study of them is of interest. Group housing in animals for example, in pigs provide an ideal framework for studying interactions among individuals. In animals, the genetic of social effects has been studied by simplifying the underlying interactions for example, by assuming that all the individuals in the group contribute equally to the social environment. This homogeneous behaviour is not observed in real conditions. Here, we address this issue showing that identification of interacting individuals is feasible in pig behaviour during growth, and we propose a model that leads to more accurate modelling of social genetics. Our approach infers the strength of interaction via time interval between visits of animals to automatic feeders. This measure of strength was then used in a reaction norm model to evaluate the genetic determinism of social effects. We also build a framework to transform variance components estimated for a trait continuously measured, such as feed intake per visit, into variance of total or aggregated trait e.g. during the growth period. Including social effects improves the fitting of the models. Visit-based social interaction was highest at short time intervals, decreasing rapidly and becoming negligible after 80 seconds. Variance components for the aggregated trait the total feed intake support the contribution of the social interactions. Our approach allows more precise understanding of social effects through individual feeding events and a measure of interaction strength, and further translates these into in an aggregated phenotype of interest.

The turquoise killifish (Nothobranchius furzeri) is an emerging vertebrate model for development and ageing due to its exceptionally rapid maturation and short lifespan. However, how microRNA (miRNA) regulation coordinates gene expression across its compressed developmental trajectory remains poorly understood. Here, we integrated miRNA and mRNA expression from the same unexposed animals across three developmental stages: newly hatched fry (4 weeks total age), juveniles (~8 weeks total age), and aged adults (~8 months of age). Developmental stage was the dominant source of variation in both datasets, with miRNA and mRNA profiles independently resolving a consistent progression from newly hatched fry to juveniles and adults. Transcriptomic changes reflected ongoing tissue maturation, including increased expression of crystallin genes associated with lens development, while more than 2,000 genes were differentially expressed during the juvenile transition. Integration of 398 developmentally dynamic miRNAs with predicted mRNA targets identified specific miRNAs whose developmental changes were significantly enriched for opposing changes in their target genes. These included conserved developmental regulators from the miR-200/141, miR-29, let-7, and miR-34 families, which increased toward adulthood as their predicted target cohorts decreased. Stage-controlled analyses further identified miR-34a, miR-29a, miR-29b, miR-15a, and a novel candidate whose target relationships persisted independently of the overall developmental trajectory. Notably, miR-29 showed particularly strong induction during maturation, consistent with its conserved roles in extracellular-matrix remodeling, differentiation, and vertebrate ageing. Together, these findings reveal coordinated remodeling of conserved miRNA-mRNA regulatory programs across the rapid maturation of N. furzeri and establish a framework for investigating post-transcriptional regulation during vertebrate development in a species with limited miRNA annotation.

The transcriptomic interaction between altered gravity and innate immunity remains largely unexplored. In this study, we established a novel dual-stress model combining 3g hypergravity with lipopolysaccharide (LPS) immune challenge in 5 days post-fertilization (dpf) zebrafish (Danio rerio) larvae. RNA sequencing revealed that exposure to hypergravity alone induced no differentially expressed genes (DEGs), indicating a remarkable transcriptional resilience. In contrast, the addition of an immune challenge under hypergravity conditions triggered a substantial transcriptomic response involving 2,706 DEGs. Functional enrichment analyses identified significant alterations in pathways related to immune function, development, and circadian regulation. Targeted qPCR analyses further validated the expression patterns of selected genes. These findings suggest that while zebrafish larvae can effectively tolerate hypergravity alone, the combination of gravitational and immune stressors elicits profound molecular responses. This study provides new insights into the interplay between altered gravity and innate immunity using an animal model, contributing to future strategies aimed at safeguarding astronaut health and supporting space-based aquaculture systems.

Vocal communication depends on the biomechanical integrity of the vocal folds, yet how impairment alters vocal behavior remains poorly understood. We used the California mouse (Peromyscus californicus), a highly vocal species that produces songs through airflow-induced vocal fold vibration, to experimentally examine how defined vocal fold injury alters vocal production. Longitudinal recordings were combined with histological assessment to track changes in vocal function after impairment. Injury produced acute edema of the lamina propria and thyroarytenoid muscle. Despite stable levels of vocal activity, the vocal phenotype shifted characteristically following injury. Thus, vocal behavior can persist despite substantial alteration of the biomechanical tissues underlying sound production, demonstrating that maintenance of behavioral output does not necessarily indicate preservation of the physiological system that generates it. More broadly, these findings establish an experimental link between vocal fold injury and vocal phenotype and provide a framework for understanding how tissue damage and remodeling can reshape acoustic communication even when vocal behavior is maintained. Because repeated vocalization can impose substantial mechanical demands on vocal folds, this framework may have broad relevance for understanding how vocal use, injury, and tissue remodeling interact to shape vocal communication.

Within natural environments, different sensory stimuli can drive mutually-exclusive behaviors such as fighting, feeding, or courting. When experiencing competing cues, how do animals decide to continue executing the current behavior or to switch to a new behavior? To address this question, we developed a closed-loop behavioral paradigm that presents actively courting male Drosophila with a fictive heat stimulus, forcing the male to choose between two competing behaviors: continue courting (stay) or escape the heat (switch). By modeling the sensorimotor transformations underlying this decision process, we discovered that male flies continually evaluate social cues, including female speed and her distance, in order to make a stay/switch decision. We investigated the neural correlates of stay/switch decision-making via brain-wide calcium imaging in head-fixed male flies presented with both fictive female visual cues and heat stimuli. We built models that predicted stay/switch decisions from neural data, which revealed that activity in a small number of brain regions, including the anterior optic tubercle, predicted decision outcomes up to 4 seconds prior to stimulus onset, whereas after stimulus onset, distributed activity across the brain was correlated with the behavioral decision. Together, these results demonstrate that Drosophila continually evaluate social feedback to inform decisions about how to respond to environmental cues, and that brain activity changes prior to presentation of the environmental cue can inform upcoming decisions. Overall, this study provides new insight into the processes underlying naturalistic decision-making in Drosophila.

Cephalopod skin exhibits a wide range of dynamic visual displays, including traveling waves, light/dark flashes, and flickering, all generated by muscle-regulated expansion and retraction of pigment-filled elastic sacs known as chromatophores. While central neural control is essential for high-level body-wide pattern coordination, experimental evidence from excised and denervated skin reveals that intricate spatiotemporal patterns can also emerge spontaneously in the skin. This raises questions about the respective roles of central versus peripheral control. To determine the simplest control necessary to generate such patterns, we developed ChromatoNet, a biophysical model of chromatophore arrays that includes mechanical interactions on 1- and 2-dimensional lattices with the excitability properties of radial muscles. With minimal external input, our modified Morris--Lecar model reproduces a rich repertoire of skin dynamics, including spontaneous flickering as well as traveling and spiral waves. Both spontaneous and stimulus-evoked waves emerge at an intrinsic resonant frequency, determined by model parameters. Propagation speeds remain stable across conditions and are consistent with the range observed in denervated skin preparations. The model generates diverse output states in response to local couplings and the time-dependence and shape of the input stimulation, with little top-down direction. The results provide a proof-of-principle that chromatophore/muscle dynamics with minimal central neural driving can produce rich dynamic patterns.