Journal of Experimental Biology

Polarization vision subserves diverse biological functions, such as navigation, communication and target-motion detection. Regarding the detection of biological targets, studies on semi-terrestrial crabs suggest that polarization and intensity contrast are processed in separate visual channels. Information about the polarization contrast of targets would be extracted independently of intensity contrast, and the two signals combined downstream in the visual system. However, understanding how the information about these visual attributes is processed and integrated has been limited, as it is technically challenging to present visual stimuli in which both the polarization and the intensity contrast of a stimulus are controlled. Here we developed a monitor screen that allows us to present stimuli in which both contrasts can be controlled. Thus, to study how polarization and intensity information is processed to increase target detection, we presented moving stimuli with controlled polarization and intensity contrast while recording the cardiac response of the semi-terrestrial crab Neohelice granulata as a sensitive readout of its visual perception. Our results suggest that Neohelice possesses similar sensitivity to vertically and horizontally polarized light; thus, previously reported responses of the animals to polarized stimuli in which figure and background have the same intensity are likely accounted for by the comparison of two polarization channels. In addition, we determined that a moving polarization-only stimulus has a salience equivalent to that of an intensity-only stimulus with a Michelson contrast of 0.51. Finally, we studied how polarization and intensity contrast information is integrated, and found that polarization contrast increases the salience of an intensity-contrast-based target mostly when its intensity contrast is low, i.e. when information about intensity contrast is more ambiguous.

We investigated the hearing mechanism of Manduca sexta caterpillars to determine whether their behavioural responses to acoustic stimulation are caused by sound-induced substrate vibration or by direct detection of airborne sound. Behavioural response thresholds to direct substrate vibration and airborne sound were measured using pure-tone stimuli at 150 Hz and 2000 Hz. A frequency of 150 Hz was selected based on previous behavioural measurements showing the lowest sound detection thresholds between approximately 100 and 200 Hz, while 2000 Hz was selected based on laser Doppler vibrometry measurements that revealed a mechanical resonance near this frequency in a thoracic hair. At both test frequencies, the direct substrate vibration detection thresholds were more than 100 times greater, on average, than the sound-induced substrate vibrations measured at the behavioural sound detection thresholds. Thus, the behavioural responses to airborne sound cannot be explained by sound-induced substrate vibration. During airborne sound stimulation, caterpillars exhibited graded behavioural responses with increasing sound pressure level, progressing from freezing to twitching and finally to jump-startle. Together, these findings provide direct experimental evidence that the behavioural responses of M. sexta caterpillars to acoustic stimulation cannot be explained by sound-induced substrate vibration and instead support direct detection of airborne sound. Understanding how caterpillars respond to airborne sound may also provide biological inspiration for the development of bio-inspired acoustic sensors.

Foraging hematophagous flies such as the stable fly, Stomoxys calcitrans, are shown or expected to sense multiple characteristics of visual host cues. These include relative intensity, spectral composition, and the angle and degree of polarization. Here, we investigated the role of each of these visual cue characteristics on stable fly attraction. In laboratory two-choice bioassays, we offered groups of female or male stable flies a choice between paired photostimuli that differed in their visual characteristics, recording the flies' landing responses on stimuli as the response criterion. Bioassay flies preferentially selected lower-intensity visual stimuli even when paired stimuli markedly differed in spectral composition and polarization characteristics (a high degree of horizontal or vertical polarization). When paired stimuli had identical relative intensity, flies failed to discriminate between the stimulus that presented ultraviolet, blue, green, and red wavebands and a corresponding paired stimulus lacking one of these wavebands. Our results support the hypothesis that foraging stable flies may respond to colour and polarization characteristics of visual stimuli as modulations of perceived intensity. Acquired knowledge on the mechanisms that underlie stable fly visual attraction may inform the design of vision-based traps so that they become optimally attractive to stable flies and other biting flies.

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.

Flight feather structures - shaft, barbs, and barbules - have transformed through evolution. Although size, shape and orientation of these structures likely influence the aerodynamic performance of the feathers, their impact remains unclear. Here we investigated the feather structures influence on flow and aerodynamic performance, using computational fluid dynamic modelling, by modifying the morphology of a section of a Jackdaw (Corvus monedula) flight feather. We show that the flow over the leading vane is a key factor, and that changing the barb-to-shaft angle from its natural state as well as the trailing vane barb angle, reduces performance, which could explain evolutionary changes from early Paraves species. All in all, our results suggest that current feather composition may be close to a local maximum for both aerodynamic efficiency and stability/flow predictability, suggesting evolution of feather structures have been driven by aerodynamic performance, which should have implications for designing bio-inspired technologies.

In endotherms, body temperature is regulated by balancing the rates of metabolic heat production and dissipation. Birds and mammals maintain high and relatively constant body temperatures, but there is growing evidence that body temperature and heterothermy vary considerably between individuals of a population. Based on the interdependence of metabolic rate and body temperature, variation in body temperature could be expected to be related to metabolic performance. In this study, we aimed to determine if body temperature is a variable and repeatable trait in captive zebra finches (Taeniopygia guttata) and if body temperature could be related to different metrics of performance. We measured body temperature for 14 days in in caged male zebra finches to assess within- and among-individual variation and repeatability, as well as during measurements of three metabolic traits; basal (BMR), summit cold-induced (Msum) and maximum exercise-induced (MMR) metabolic rate. Median Tb (R = 0.48), and peak Tb (R = 0.53) body temperature were repeatable and positively correlated, suggesting that the birds show consistent differences in body temperature. We found that birds with higher daytime body temperatures showed higher thermoregulatory performance in a cold challenge and had higher body temperature during forced exercise. However, daytime body temperature was not related to MMR or BMR. Our study is one of few that have measured the repeatability of body temperature in birds. Our results show that individuals may have distinct body temperature phenotypes which could be related to metabolic and thermoregulatory performance. However, additional research is needed to disentangle the interrelated effects of metabolic capacity, body temperature, and heat dissipation on performance and how this might change with thermal conditions and activity levels.

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.

Flying insects process motion across the visual scene to regulate their ground speed, correct course deviations, and behaviorally relevant stimuli such as avoiding predators or finding hosts. The yellow-fever mosquito Aedes aegypti provides a useful model for studying these visually guided behaviors, as both sexes use vision to track moving features. Approaching a moving target presents mosquitoes with a demanding visual problem: they must extract target motion from the changing visual scene while simultaneously controlling their own flight and responding to potentially dangerous movements. To understand this, we first studied free-flight responses to static and moving vertical targets in a wind tunnel system capable of projecting dynamic visual stimuli, and found that males and females are sexually dimorphic with respect to their responses to target motion. When the feature was static, both males and females advanced toward it during flight; however, when it oscillated laterally at 2 Hz, only females continued to pursue it. In a subsequent yaw-unrestrained magnetic tethering paradigm, we showed that this divergence cannot be simply explained by sex-specific differences in visuomotor tuning dynamics, while micro-CT reconstructions revealed only modest sexual differences in eye morphology and broadly similar patterns of estimated spatial acuity. Together, these results indicate that males retain the biomechanical capacity to track oscillating targets but actively suppress approach behavior during free flight, whereas females pursue salient features independently of target motion. This pattern aligns with sex-divergent visual ecologies because the risk of approaching a large moving feature too closely likely outweighs the benefits for males seeking mates, whereas for host-seeking females the reproductive necessity of blood feeding overrides this avoidance.

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 can sense the Earth's magnetic field and use it for orientation. However, the mechanism underlying magnetoreception has not been identified, partially due to the lack of reproducible and genetically tractable model systems. Here, we establish a directional assay for planaria to test their responses to different magnetic fields. Girardia dorotocephala showed a preferred magnetic directionality in the horizontal plane in an Earth-strength magnetic field in the absence of visible light, which shifted if the azimuth was rotated and was absent when the magnetic field was cancelled. This directional preference differed between labs. Furthermore, the directional response was affected by a short but strong magnetic pulse, resulting in a random distribution. The effect of the pulse was temporary, with directional preferences recovering over several days. Taken together, these data are consistent with a magnetoreception mechanism in planarians that functions independently of light and relies on permanently magnetised material, such as single-domain magnetite. As planarians are genetically tractable and well-established, they are a promising invertebrate model organism that might offer insights into the mechanisms underlying magnetoreception in animals.

Planarians exhibit a variety of behaviors in response to external stimuli and have emerged as a promising animal model for studying the neural regulation of behavior. Although their feeding behavior has been well documented, robust experimental methods for quantifying their food intake remain limited. In this study, we developed a novel methodology for the quantitative assessment of feeding termed the "Complete-or- Partial (CoP) assay." This assay evaluates feeding responses as a binary outcome (complete consumption versus partial ingestion) in a volume-dependent manner at the individual level. Using this method, we examined the effects of food quality on feeding responses by comparing two distinct diets: a "basal diet" containing the minimum concentration of liver homogenate necessary to induce ingestion and support growth, and a "high-density diet" with a ten-fold higher concentration. The CoP assay enabled the robust estimation of food intake volume and successfully detected a significant overconsumption of the high-density diet compared to the basal diet, demonstrating that planarians regulate their food intake in a quality-dependent manner. Furthermore, individuals exhibiting partial ingestion did not immediately feed again even when presented with the same diet freshly prepared, indicating that they had reached satiety. Thus, we propose that the CoP assay offers a robust and scalable platform for evaluating both food intake volume and satiety status, providing a powerful tool to facilitate future molecular analyses of feeding regulation in planarians.

Thermal tolerance limits are an important determinant of species vulnerability to climate change. However, most of our knowledge of thermal tolerance pertains to resting animals, which could yield unrealistic predictions about how air temperature will affect animals when they are active. To gain a better understanding of thermoregulatory contstraints in active birds, we induced zebra finches in captivity to exercise continuously in air temperatures ranging from cool to warm (12- 36 C) while we simultaneously measured their metabolic rate, body temperature, and evaporative water loss. During activity, birds overheated in air temperatures well below their resting thermal tolerance limit, and aerobic scope was reduced by 22% at the warmest temperature. Although zebra finches showed a 8-fold increase in evaporative water loss during exercise in the warmest temperature, this was not sufficiently high to compensate for the substantial increase in heat produced during activity. This could explain why birds became hyperthermic and showed reductions in metabolic rate in the higher air temperatures. Thus, our results provide a mechanistic explanation for how heat could limit aerobic capacity and scope in active birds. These findings support growing evidence that increased air temperature can lead to sub-lethal fitness costs in free-living birds.

Whilst forms of non-associative learning such as habituation are widely reported in single-celled organisms, whether associative learning is possible in systems without a nervous system is contested. In this study, we implemented a conditioning paradigm in the acellular slime mould Physarum polycephalum through repeated pairings of nutrients with an aversive blue light stimulus. We first established that blue light is aversive under our conditioning protocol, slowing migration and producing broader, less directed growth, confirming its well documented effect on behaviour. Subjects were then trained in up to four training sessions by placing an oat stimulus inside a region with blue light before testing with the light presented alone. Trained subjects were more likely to reach the blue light region compared to untrained controls, with shorter latency, suggesting that Physarum weakens its avoidance of blue light after the region has repeatedly coincided with a nutrient. However, engagement did not strengthen with additional pairings, a pattern that admits multiple competing explanations in which associative and non-associative processes plausibly contribute to more transient associative-like effects. We conclude that a history of co-presentation is sufficient to relax an aversive response in Physarum but cannot, within the present study, attribute that change to associative learning.

In vertebrates, top-down visual attention is a cognitive process where internal goals modulate the tuning of peripheral sensory systems. This leads to increased perceived contrast to both goal-relevant objects and areas of the visual field that are attended. Such a system would also be beneficial to bees, enabling them to detect and recognise the most profitable flowers in their environment. We tested whether bumblebees possess a top-down attentional system resembling that seen in vertebrates. We trained two groups of bees to collect rewards under high contrast targets. To potentially induce a difference in attention while searching for the targets, one group received a higher concentration of sucrose rewards compared to the other. During tests, the targets were presented with a series of lower contrasts to measure the contrast sensitivity curves of the bees induced by the different learnt reward levels. We predicted a stronger effect of any attention-like process on contrast sensitivity in the high reward group. We also repeated this experiment with the neonicotinoid pesticide imidacloprid dissolved in the sucrose rewards to test whether this affects bee attention. Across all test contrasts, higher rewards significantly increased bee accuracy when locating targets, lowered contrast thresholds and reduced the latency to make first choices. Imidacloprid reduced bee accuracy but did not influence first choice latency. These results suggest that learnt floral rewards can influence bee behavioural contrast sensitivity in a manner resembling vertebrate top-down attention and that imidacloprid may modulate this through effects on their nervous system.

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.

This study investigates the impact temperature-induced vertebral anomalies have on the C-start escape response of Astyanax mexicanus, a model species in evolutionary developmental biology. Employing three temperature treatments to induce varying degrees of skeletal anomalies, we assessed their effects on key swimming performance metrics including, C-start time, curvature coefficient, head displacement distance, and displacement velocity. Through the use of linear mixed models and generalized linear mixed models, our results reveal that specific anomalies such as vertebral fusions and anomalous haemal and neural spines affected the curving ability of C-start escape responses. However, these did not negatively impact other performance parameters, with velocity, distance, and response time showing no significant impacts from any anomaly types, when assessed individually. This suggests a complex interplay between structural deformities and compensatory physiological mechanisms that maintain functional performance. Other variables measured had a stronger and significant impact on swimming performance, including standard length, vertebral number, and temperature treatment, which influenced escape speed, curving ability, and overall locomotor performance. Our findings challenge conventional perceptions about the debilitating impact of vertebral anomalies, indicating that many affected fish can still effectively perform escape maneuvers critical for survival.

Species exhibit variation in their circadian activity rhythms, including being active during the day (diurnal), the night (nocturnal), or the twilight periods (crepuscular). While the influence of environmental factors such as light on entraining and controlling activity patterns is well known, it is unclear how social and abiotic factors affect circadian behaviors. The daily activity patterns of Lake Tanganyikan cichlid fish are diverse, and may be associated with species diversification. Intriguingly, some shell-dwelling cichlid species, which anecdotal observation suggests are diurnal, displayed strong nocturnal activity when assayed in a "common garden" lab setup. Here, by integrating field and lab-based studies, we provide three lines of evidence that social and environmental cues mask underlying circadian rhythms in shell-dwelling cichlids. First, we demonstrate that shell-dwelling cichlids are diurnal in their natural habitat, but convert to nocturnal activity when assayed alone in a common garden. Second, we identify that in the presence of a shell and conspecifics the highly social N. multifasciatus becomes diurnal. In contrast, the circadian activity pattern of a closely related, but sub-social species, N. brevis, is masked only by a shell, and unaffected by the presence of conspecifics. Third, we demonstrate that the masking effect of shells and conspecifics is circadian and continues in the absence of light. Fourth, we identify that the response to these cues is influenced by sex, with greater effects in female fish, and by pedigree, with stronger effects in individuals bred and raised in captivity compared to those captured in the wild. Together, these experiments reveal new relationships between circadian rhythms and sociality in these fishes, offering broader insights into the ecological and evolutionary drivers of these behaviours.

Developing in suboptimal temperatures can have widespread negative effects in organisms, but early exposure to thermal challenges can also trigger increased investment in thermoregulatory capacity. Within birds, the effects of cold exposure during incubation may differ between precocial and altricial species due to differences in the ontogeny of thermoregulation. To date, most of the evidence showing that embryonic cold exposure shapes future thermoregulatory capacity comes from a few precocial species in laboratory settings. Less is known about sensitivity to low-temperature incubation in altricial species which only become fully endothermic after hatching. In this study, we tested the effects of cold temperatures during embryonic development by temporarily cooling the nest boxes of free-living tree swallows (Tachycineta bicolor) during late incubation and subsequently exposing nestlings to an acute cold challenge. Cold exposure during early development made nestlings better at maintaining their body temperatures during the acute challenge. Developmentally cold-exposed birds also had higher thyroxine (T4) levels when sampled at lower ambient temperatures, whereas control birds showed the opposite relationship. Among nestlings that failed to maintain their body temperature during the acute cold challenge, developmentally cold-exposed nestlings secreted more corticosterone during the acute cold challenge than control nestlings. Despite these changes, cold-exposed birds did not differ from controls in cold-induced metabolic rates, body mass, or bacteria killing ability. These results are, to our knowledge, the first to show that developmental cold exposure increases the capacity to maintain body temperature during future cold challenges in a songbird. These findings suggest that experiencing even a mild decline in ambient temperatures during embryonic development could prime birds to cope more effectively with harsh or variable environmental conditions later in life.

Mosquitoes use their sensitive hearing in a well-described courtship and mating behavior, but their ability to discriminate other stimuli relevant to their survival and reproduction via this sensory channel remains underexplored. Here, we characterize a rapid climb response in free-flying mosquitoes elicited by loud low-frequency sounds (tested at 180Hz and 300Hz) but not by tones of equivalent acoustic loudness reflecting conspecific female (450Hz) and male (700Hz) wingbeat frequencies. Presentation of the same sounds in tethered flight elicited modulation of both wingstroke amplitude and frequency components similar in magnitude to those observed in response to a looming visual stimulus. These tones are spectrally consistent with recorded audio of a dragonfly takeoff, playback of which is sufficient to release the behavior. Taken together, these data are consistent with the hypothesis that mosquitoes use their hearing to detect the acoustic signature of aerial predators such as dragonflies.

Humans tend to walk at speeds that minimize energy expenditure and time duration. While most walking experiments examine individuals in relative isolation, everyday locomotion frequently occurs in social contexts. We investigated whether walking speed is modulated in response to a social interaction during a cooperative task. Participants completed 96 randomized walking trials where they approached and retrieved boxes varying in distance (2.5-10 m) and mass (0-6.8 kg). Boxes either rested on the ground or were handed off by an experimenter, signaling prosocial effort benefitting the participant. Approach speed was measured with inertial measurement units placed at the feet and fit to a saturating exponential function of walking distance using a nonlinear mixed-effects regression model. Based on the energy-time optimization framework, we hypothesized that participants would approach more quickly when larger boxes were held at farther distances, to reduce energy and time costs of the experimenter, despite exerting more effort themselves. Participants walked 8.5% faster (1.29 m s-1 versus 1.19 m s-1; p = 3.85 x 10-6) when the largest box was held out by the experimenter versus left on the ground. However, the manner in which the box was held had no influence on approach speeds (p = 0.31). Exploratory analyses identified modest trends between experiment responses and individual characteristics, but none reached statistical significance. The findings suggest that locomotor decisions reflect not only an individuals own energy and time costs but also the perceived costs borne by others. This study demonstrates that social context can meaningfully influence walking behavior. Summary StatementHumans walk more quickly in response to the perceived energy and time others spend helping them, even at the cost of exerting more energy themselves.