Current Biology

While the external cues that generate behavioral responses have been extensively studied, behavioral changes in response to internal stimuli are much less understood. C. elegans hermaphrodites are self-fertile until they exhaust their supply of self-sperm, after which time they can only produce more progeny via mating with a male animal. Fertile hermaphrodites are less likely to be mated by males than sperm-exhausted hermaphrodites. We report that sperm-dependent hermaphrodite escape is facilitated by an aversive response to physical contact (touch stimulus) by males, specifically male turns around the hermaphrodite nose and male contact at the vulva. Using germline masculinizing and feminizing mutants, we found that sperm are both necessary and sufficient to induce contact-dependent mating evasion. Surprisingly, loss of the entire somatic gonad resulted in ectopic evasion behavior, via an oppositional monoaminergic signal. Hermaphrodites lacking the HSN neuron are constitutively receptive to male mating touch both at the vulva and at the nose, suggesting that HSN transmits fertility status to the nervous system. HSN also modulates other downstream circuits, such as the nose touch sensory neurons ASH and FLP, which are required for evasion of male contact at the nose. Taken together, we identified a signaling cascade wherein presence vs. absence of sperm is transmitted via monoaminergic signaling from the somatic gonad to the nervous system. This demonstrates how changes in internal state, such as fertility status, act to modulate neural circuits and alter the valence of sensory input.

Sex differences are pervasive across the animal kingdom1,2, shaped by reproductive strategies, ecological roles, and physiological demands. Neurobiological studies have largely focused on sex differences in reproduction and aggression3,4, leaving understudied how sex shapes other survival-relevant behaviours such as responses to predatory threats. Previous work in vertebrates uncovered sex differences in individual defence responses deployed by rats5 and in social modulation of freezing responses in zebrafish6. The origin of these differences and their potential underlying neuronal circuitry remain however unknown. Here we characterise defensive responses to a visual threat of female and male fruit flies tested alone and in groups. Individually, defensive responses are comparable across sex, with similar freezing levels and moderately increased escape responses in males. In groups, however, we uncovered strong sex differences in freezing as males show consistently shorter freezing bouts, reflecting a stronger social buffering effect than in females. Using a generalised linear mixed model, we demonstrate that the shorter freezes of male flies, when tested in groups, are not explained by well-known locomotor differences between sexes7,8. Although males move more and faster, generating stronger social motion cues that mediate the "safety in numbers" effect in female flies9,10, their response to the surrounding motion is not increased. We further identify the nervous system as the source of this sex-dimorphic behaviour by feminising males through pan-neuronal ectopic expression of transformer, a key regulator of the cell-autonomous sex-differentiation pathway. Finally, we ruled out the visual projection neurons, lobula columnar neurons 11 (LC11), detectors of social motion10, as the source of the neuronal-sex differences observed. This study identifies a neuronal origin of sex differences in socially modulated behaviour that extends beyond sex-specific behavioural modules or locomotor function, important to understand differential impact on survival upon threat across sexes.

Cryptic female choice - female-mediated bias in fertilization after mating - is well established in animals and can also occur in plants when multiple pollens compete on the same pistil. However, whether interactions between pollen and pistil tissues after pollen deposition contribute to this process remains unknown. Here, we experimentally test whether such interactions mediate cryptic female choice in the angiosperm Brassica rapa. We quantified fertilization success of pollen donors competing on the same pistil using paternity analyses, and in parallel, made semi-in vivo assays to measure pollen tubes trajectories emerging from the excised styles and growing toward unfertilized ovules for each donor-recipient pair. We show that pollen tube growth towards ovules predicts higher fertilization success under pollen competition. Thus, we document a previously unobserved mechanism of cryptic female choice based on physical interactions between male and female components of reproduction. In addition, different recipient plants favour different pollen donors, consistent with non-directional female choice. Plants with longer styles bias paternities more strongly towards the most successful pollen donor. Overall, our study demonstrates that interactions between pollen tubes and pistil tissues after pollen germination enable plants to bias paternity toward particular donors.

Animals inhabiting aquatic or terrestrial habitats experience different constraints on their physiology and locomotion, and are exposed to fundamentally different sensory environments. Across evolutionary timescales, most species have adapted to live exclusively either in water or on land. Newts are among the vertebrates that defy this rule and split their adult lives between freshwater ponds and terrestrial habitats. In these amphibians, transitions across environments cause remarkable phenotypic plasticity in their body morphology. But whether and how the nervous system and behavior also adapt to these environmental changes remains poorly explored. Here, we establish the Iberian ribbed newt Pleurodeles waltl as a new model to study the neurobiology of environmental plasticity in a vertebrate. We first show that experimental transitions between aquatic and terrestrial laboratory settings recapitulate morphological changes observed in the wild. Furthermore, aquatic and terrestrial newts display plasticity in sensory and motor behaviors, including differences in walking gait and odor responsiveness. In the olfactory system, the transition from water to land involves a profound remodeling of the nasal epithelium, including reversible transcriptomic changes in secretory and support cells, and an increase of neurogenesis. Together, our findings reveal how plasticity of specific cell types in the nervous system supports behavioral adaptations across environments. More broadly, this work establishes newts as a model to study the functional constraints and convergent adaptations that may have shaped the evolution of vertebrate nervous systems in water and on land.

Many animals use the Earth's magnetic field to orient and navigate over long distances, yet the sensory mechanism that underlies this ability remains unresolved. A central question is whether the compass depends on light: a cryptochrome-based radical-pair mechanism, best characterised in birds, requires a photon to generate the magnetically sensitive species, whereas magnetic particle-based and inductive mechanisms do not. In mammals, which lack the light-sensitive cryptochromes that the avian model requires, this question is unresolved. Here we tested whether migratory bats, Pipistrellus nathusii, require light for migratory orientation, studying wild migrants at a major flyway under laboratory-controlled light conditions, including verified total darkness. Bats were acclimated to starlight-level illumination or to total darkness before release into a circular arena in which take-off direction was recorded, and take-off latency was taken in every trial as a control for behavioural state. Bats released under starlight-level illumination, and bats released in total darkness after starlight acclimation, both oriented in the south-westerly direction typical of free-flying bats at this flyway; their mean headings lay 11 degrees apart and did not differ in direction, in angular concentration, or in take-off latency. Bats released in total darkness after prolonged darkness produced the same mean heading but did not reach significance under a Rayleigh test, and did not differ from either other group. To our knowledge, this is the first demonstration that a migratory mammal expresses its seasonally appropriate migratory heading in total darkness. Bats orienting in darkness took off five to eight minutes after their last photon exposure, seven to eight orders of magnitude beyond the microsecond lifetime of the only cryptochrome radical pair demonstrated to exist and to be magnetically sensitive. Because a compass reading is informative only at the moment of the directional decision, a signal generated before a bat entered darkness could not encode its heading at take-off; the magnetically sensitive species must therefore form without photons. The radical pairs capable of doing so have not been shown to exist, even in vitro. A migratory mammal thus expresses its seasonal heading without concurrent light, in a lineage lacking the photoreceptor on which the light-dependent compass in birds relies.

Predators routinely fail to capture prey despite being larger, faster, and more powerful. Half a century ago, theory predicted that prey could evade biomechanically superior predators using brinkmanship, delaying evasive maneuvers until the last possible instant, but this hypothesis has never been tested in the wild. Here, using more than 2,000 hours of high-speed stereo-camera recordings of predator and prey coral reef fishes, we show that prey successfully evade 97% of attacks. However, contrary to the standard explanation that prey evade through superior maneuverability, we find that prey evade by exploiting their predator's sensory-motor delay - the interval between a stimulus reaching the predator and the onset of the predator's motor response. Although predators partially compensate for this by forecasting prey position during interception steering, prey consistently time evasive maneuvers within a brief window just tens of milliseconds before collision, within which no viable predator trajectory can intercept them. Prey do not, however, turn at the time that maximizes miss distance, but slightly later, a bias that makes escape robust to variability in their own response timing. These findings reveal how temporal delays in perception and action place fundamental limits on the outcome of predator-prey interactions, and show that prey can exploit delays in a way that approaches the theoretical limit of escape performance.

Mammalian circadian rhythms are primarily entrained by light, but nonphotic cues can also reorganize behavioral timing through mechanisms that remain poorly understood. Nocturnal foot shocks delivered to rodents while they forage away from the safety of their nesting area have been shown to entrain circadian behavioral rhythms and shift foraging and feeding to the daytime. To identify the neural circuits underlying this nonphotic fear entrainment, we optogenetically stimulated tachykinin 1-expressing neurons in the parabrachial nucleus (Tac1PBN) during the subjective night while the animals foraged outside of their nest, which recapitulated the total activity-rest phase switch in circadian behaviors induced by foot shocks. Furthermore, selective stimulation of Tac1PBN projections to the central amygdala (CeA) produced a significant but reduced phase shift compared to direct stimulation of Tac1PBN cell bodies. When Bmal1, a core clock gene, was conditionally deleted from the CeA, mice failed to fear-entrain, implicating the CeA molecular clock as a necessary component for fear entrainment. Together, these experiments demonstrate that activation of a defined neuronal population outside of the suprachiasmatic nucleus (SCN) can reorganize circadian behavior by engaging a non-SCN circadian oscillator network that requires an intact CeA molecular clock.

Learning and memory depend on plasticity within neural circuits, but the extent to which the underlying architecture of memory networks is stereotyped or structured remains unclear. Random connectivity may maximise stimulus discrimination, whereas structured connectivity could preferentially route behaviourally relevant information. Here, we compare five adult Drosophila melanogaster Mushroom Body (MB) hemispheres across three connectomes and establish a unified cross-connectome framework for uniglomerular projection neurons (uPNs), Kenyon Cells (KCs), Mushroom Body Output Neurons (MBONs) and Lateral Horn Centrifugal Neurons (LHCENTs). Despite variation in KC abundance and fine-scale connectivity, we identify reproducible organisation across the uPN[->]KC and KC[->]MBON pathways. Food-associated uPN channels consistently contact a greater proportion of KCs than expected by chance, and this bias is preferentially routed through KCs towards approach-promoting MBONs. Sensory input also remains stereotyped across sister MBONs from the same hemisphere despite variation in their individual KC partners. Finally, recurrent LHCENT feedback (predicted-inhibitory) preferentially targets KCs receiving these food-associated inputs. Together, these findings show that a memory network can accommodate local variability while preserving structured pathways that prioritise ethologically relevant sensory information.

The K-Pg extinction ended the world of non-avian dinosaurs 66 Mya and ushered in the Age of Mammals. During the [~]10 My of recovery, Schizophora, an immensely successful group of flies, appeared, flourished and diversified. This large radiation produced over half of all dipteran families ([~]78/150), including the family for Drosophila, the model genetic powerhouse. In the context of this evolutionary radiation, we investigate the loss of two of three highly conserved, neurosensory transmembrane channel (TMC) genes. Here, we show that these genes were separately lost across multiple diverging schizophoran lineages of the early Paleogene, suggesting a powerful environmental driver was involved. We also show that unlinked genes encoding the calcium- and integrin-binding (CIB) subunits of the missing TMC complexes were lost during the same time frame. Because the lost genes encode complexes involved in thermal nociception, we propose the external driver was likely the Paleocene- Eocene thermal maximum (PETM), a 200 ky interval of elevated global temperatures occurring 56 Mya, slightly before the earliest schizophoran fossil from 53 Mya. These results suggest that gene loss may have been adaptive for most schizophoran lineages to emerge past the hothouse Earth of the PETM.

Freshwater biodiversity is disproportionally high relative to the limited availability of freshwater habitats. This pattern is exemplified by freshwater fishes. Over 50% of freshwater fish species are concentrated in a single clade, Ostariophysi, including the 5000 species of minnows, carps, and loaches, the 4500 species of catfishes, and the over 2000 species of tetras, pirahnas, and characins. However, the relationships and ages of ostariophysans remain uncertain. We show that the initial diversification of ostariophysans involved only two freshwater invasions and was driven by the strikingly rapid origination of major crown clades, including Neotropical electric fishes, lutefishes, and multiple major living clades of catfishes, carps and minnows, and tetras and characins, within five million years of the Cretaceous-Paleogene mass extinction. This result is congruent with the record of well-preserved body fossils of ostariophysans, but contrasts with the controversial assignment of isolated teeth and bones from the Cretaceous to nested lineages of this set of freshwater fish radiations. Although we confirm that Alepocephaliformes, an obscure marine lineage, is the living sister to Ostariophysi, our results demonstrate that the former clade only recently invaded the deep ocean, a transition that involved the loss of structures essential for enhanced auditory capabilities in ostariophysans and the related herrings and anchovies. These results establish a surprisingly young age for the major lineages of living freshwater fishes.

One of the most puzzling aspects of perception is postdiction: later information can change how a previous stimulus is perceived [1,2]. This indicates that perception is not a livestream of the external world, but rather a post-hoc reconstruction. How does the brain revise perception? It has been proposed that recurrent processing in the sensory hierarchy may sustain earlier information long enough for later information to impact it [3]. We hypothesised that this allows feedback from higher-order multisensory regions to alter the representation of earlier visual stimuli in the primary visual cortex (V1) [4]. Here, we tested this hypothesis using two postdictive illusions [5], where the presence or absence of a tone determines whether an illusory flash is perceived, or an actually presented flash is perceptually suppressed, respectively. This allowed us to isolate neural signals reflecting illusory percepts from sensory inputs in retinotopic space. We collected 7T layer-specific fMRI data while participants (N=23) experienced the illusions. We found that postdictive illusions induced similar activity patterns as real percepts, specifically in the deep layers of V1, suggesting postdictive feedback. Informational connectivity analyses suggested that the source of this feedback was the Superior Temporal Cortex (STC). We conclude that the postdictive montage of perception is reflected in V1, implemented through a delaying feedback loop with higher-order sensory cortex. This extends the role V1 plays in perceptual inference: beyond receiving sensory inputs reflecting the present and predictions of the future, it is involved in the post-hoc revision of perception.

Yolk-cytoplasm segregation is among the earliest spatial organization events in the developing embryo of many oviparous animals. The segregation process is intimately linked to early embryonic cleavage and pattern formation, and exhibits a wide range of spatial and temporal diversity. However, the underlying cytoskeletal mechanism remains largely unknown, except for a small number of species. Using quantitative live imaging, we investigated yolk segregation in the Drosophila embryo during the syncytial nuclear cycles 11-14. We find that the yolk vesicles move progressively inward in spatial and temporal coordination with the inward expanding microtubule networks that are nucleated from centrosomes positioned at the cortex, whereas cortical actin meshwork remains spatially restricted. Using the gnu RNAi embryo to decouple nuclear migration and division from cytoskeletal dynamics, we establish causality with targeted pharmacological disruption and find that microtubule dynamics is required for yolk segregation, while depolymerization of actin has no discernible effect. In support of a mechanism of growth-propelled passive displacement, microtubule plus end comets come in apparent contact with yolk vesicles, and injected, inert microbeads are displaced towards the embryo center presumably by the same pushing force. These findings identify microtubule polymerization as a predominant driver of yolk-cytoplasm segregation in Drosophila and suggest that diverse cytoskeletal mechanisms evolved to accomplish this crucial reorganization process

Animals use multiple sensory systems to identify signs of danger, including dead conspecifics. Fish can identify decaying conspecifics from chemosensory cues, which prompt a long-lasting avoidance; the neural basis of this persistent chemosensory-driven state is not understood. Here, we describe a persistent alarm state in larval zebrafish, where brief exposure to the polyamine decay product, cadaverine, initiates a minutes-long increase in heart rate and suppression of visually-evoked swimming. Cadaverine activates both olfactory and trigeminal neurons, with persistently-active cells in the forebrain and dorsolateral hindbrain. However, we find that cadaverine's persistent effects on hindbrain activity, heart rate, and locomotor suppression are not mediated by olfaction. Instead, they are mediated through trigeminal chemosensation of the basic pH of concentrated cadaverine in solution. Knockout of TrpA1 channels, which are expressed in trigeminal neurons and associated with chemical nociception across species, prevents the persistent suppression of movement from brief exposure to cadaverine. pH-responsive trigeminal neurons project into the dorsolateral hindbrain to mediate persistent activity and long-lasting locomotor suppression through local inhibition. These results demonstrate that non-olfactory chemosensation mediates persistent behavioral responses to death and decay in zebrafish, illustrating how diverse chemosensory systems detect ethologically-relevant cues to drive adaptive behavior.

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.

Halteres, the miniaturized hindwings of flies, function as biological gyroscopes that encode angular velocity during flight. Although their established role is to detect flight perturbations and trigger compensatory manoeuvers, here we show that halteres also function as a gyrocompass, maintaining an accurate estimate of heading during rapid turns. To overcome the incompatibility between rapid body rotations and two-photon imaging, we exploited the physics of haltere mechano-transduction to reproduce the inertial forces acting on the halteres during rapid body rotations using micron-scale oscillations of the thorax. Our experiments demonstrate that flies update their internal compass during rapid turns by integrating inertial measurements with predictions derived from efference copy signals. Because body saccades exceed the temporal bandwidth of visual motion processing, this inertial computation allows flies to maintain an accurate internal compass throughout these rapid manoeuvres. This capability may enable flies to compute critical parameters such as wind direction and ground speed by comparing measurements made before and after each saccade. Our findings suggest that an ancestral flight-stabilization system was evolutionarily co-opted into a biological gyrocompass, a new function that may have contributed to the adaptive radiation of flies.

Animals navigate through dynamic environments in search of resources. Dopaminergic systems are known to control movement and assign motivational value to cues, actions and outcomes, but how they guide ongoing locomotion towards a goal remains unclear. Here, we demonstrate that Drosophila mushroom body innervating dopaminergic neurons encode steering signals in world-centred (allocentric) and body-centred (egocentric) coordinates during goal-directed navigation. Thirsty flies track water vapour through upwind directed turning and locomotion, that persists for several seconds after the vapour disappears. We identified a population of rewarding dopaminergic neurons to be specifically required for maintaining directed tracking following vapour offset. Recordings of these dopaminergic neurons during behaviour revealed delayed bilateral activity that followed upwind-directed turning in allocentric coordinates. Moreover, lateralized ipsilateral activity in a subset of these dopaminergic neurons encode whether the fly turns left or right in egocentric coordinates. Finally, these dopaminergic neurons also exhibited increased ipsilateral activity when turning in the direction where water vapour was previously encountered. Therefore, activity of mushroom body dopaminergic neurons represents steering and direction-specific actions that together maintain goal-directed tracking in turbulent environments.

Animals encounter a variety of food items and must determine their future value and preservability. Because animals in laboratory settings are typically provided with uniform food items, there is a dearth of knowledge on how animals assess natural complex food. Here, we dissect how wild agoutis-a critical seed disperser of the Neotropics-decide whether to consume or bury food in small caches, i.e. scatter-hoarding. We find that agoutis preferentially cache nuts over fresh fruits and, when foraging, undergo a sharp transition from eating to scatter-hoarding. Remarkably, agoutis scatter-hoarded empty nuts with intact shells but not filled nuts with broken shells. This decision was dependent on an oro-manual food assessment, an ecological information-seeking behavior. They used their lips and incisors - a somatosensory fovea - to navigate the surface of the nut optimally accumulating decision-critical sensory information. Together, these findings provide a mechanistic account of preservability assessment in a wild rodent, revealing how information-seeking behaviors support long-term value optimization in natural environments.

Mammals primarily use glucose as an energy source, but they can also utilize other isocaloric monosaccharides, such as galactose and fructose. Since the gustatory system cannot distinguish between different sugars in ingested foods, it remains unclear whether and how mammals control the intake of different monosaccharides. Here we perform in vivo calcium imaging of the nodose ganglion in mice and find that intestinal glucose and fructose activate distinct populations of vagal sensory neurons. While glucose suppresses food intake in fed mice, fructose does not, resulting in increased calorie intake. However, after vagotomy, both glucose and fructose suppressed food intake, suggesting that vagal circuits activated by fructose facilitate feeding. This specialized vagal sensory pathway for fructose may partly explain the excess calorie intake induced by a fructose-rich diet.

Landing is an especially demanding stage of flight. Flying animals must rapidly transform multimodal sensory information into the coordinated movement of multiple limbs in order to transition from flight to a stable standing posture, often on surfaces that vary in stability, texture, and orientation. Failure to do so can be costly, leading to injury or death. Here, we examine the role of mechanosensory feedback in guiding landing in the fruit fly, Drosophila melanogaster. We describe a novel behavioral assay in which we deliver localized mechanical contact to individual legs of tethered, flying flies while simultaneously quantifying their three-dimensional behavioral kinematics. We find that mechanical contact of the distal legs reliably induces flight cessation, and initial leg contact triggers distinct patterns of inter-leg coordination. Recruitment of subsequent leg contacts predicts landing likelihood, suggesting that coordinated feedback from multiple legs reinforces the landing transition. Next, we combine synapse-resolution connectomes with optogenetic manipulations to identify the mechanosensory pathways underlying these behaviors. Finally, we describe a population of inhibitory ascending neurons that integrates mechanosensory input from the legs and facilitates the transition from visually-mediated landing initiation to touch-mediated flight cessation. Overall, our results reveal an important set of neural circuits that convert local tactile feedback into a coordinated, rapid behavioral state transition.

With the newly uncovered access to a genetic line labeling all campaniform sensilla in Drosophila melanogaster and the many insights gathered from larger insects, we analyzed the distribution and patterning of this class of proprioceptors throughout the fly nervous system and studied CS function in motor control. We demonstrate via two-photon calcium imaging microscopy that campaniform sensilla activation induces activity in many leg muscles, showing that these proprioceptive stimuli can influence motor neuron activity. We then dissected how the lack of these proprioceptive stimuli influence walking behavior, leg kinematics, and interleg coordination in freely-walking flies using transient optogenetic inhibition and video tracking with high spatiotemporal resolution. We show that CS inhibition robustly affects Drosophila melanogasters ability to reach their typical walking speeds. Detailed analysis of leg kinematic suggests that shorter stance amplitudes characterized by their long-lasting duration were behind the speed deficits. Dissecting this phenotype also revealed the inability to maintain coordinated interleg stepping patterns as well as postural control in absence of proprioceptive force and load feedback.