Sperm cooperation can improve fertilization success in species with intense sperm competition, but the mechanisms that assemble cooperative groups remain unclear. We tested whether extracellular zinc, an abundant component of the epididymal and seminal environment, promotes sperm clustering in the deer mouse, Peromyscus maniculatus. Zinc increased the proportion of sperm in groups, and these groups persisted after washout, suggesting that zinc induces relatively stable sperm-sperm interactions. Chelation reduced clustering, indicating that these interactions remain reversible. Deer mouse sperm also accumulated on zinc-conjugated beads, whereas sperm from the non-clustering house mouse, Mus musculus, did not, consistent with a species-specific, zinc-dependent adhesive property. Together, these findings identify extracellular zinc as a candidate signal that promotes cooperative sperm clustering and suggest that extracellular zinc may provide a physiological cue that enables cooperative sperm behavior in species under strong sperm competition.
The ability to change one's current heading, i.e. to turn, is essential for all walking animals. While several studies have addressed how leg movement or inter-leg coordination may change during turning, relatively little is known about how turning-related changes at the level of single-leg stepping (low level) scale with turn magnitude at the level of body trajectories (high level). Here, we used spontaneous and visually induced turns of unrestrained walking stick insects to test (i) how high-level parameters of unrestrained turning scale with low-level parameters of leg movement, and (ii) the effect of visual guidance on turning parameters. To this end, we used a step change in stationary landmark position in an open-field arena to constrain timing and magnitude of target-directed turns. These visually guided turns were compared with spontaneous turns in an all-white condition without guiding landmarks. We show that visually induced turns were walked at a larger forward velocity and had fewer short steps than spontaneous turns. The scaling of turning responses was dominated by an increase in turning duration (factor 1.87) rather than turning speed (factor 1.32). Increased rotational velocity correlated with reduced forward velocity, though with flexible timing of both effects. These changes were accompanied by larger shifts in step direction, as well as an increased asymmetry of step types between inner and outer legs, suggesting a mix of distinct turning strategies, that depend on overall turn angle. Future models on six-legged locomotion should thus consider the incorporation of more than one mechanism to govern turning.
Dietary macronutrient balance is a major determinant of infection outcomes, yet the mechanisms linking host nutrition to pathogen success remain poorly understood. In the desert locust, Schistocerca gregaria, protein-biased diets enhance several markers of immune function but paradoxically increase susceptibility to infection by the entomopathogenic fungus Metarhizium robertsii. We hypothesized that dietary protein may benefit the pathogen by increasing access to host nutritional resources, and predicted that protein-biased diets would increase hemolymph concentrations of total protein and branched-chain amino acids, nutrients known to support Metarhizium growth, and that infection would cause declines in these compounds. We also hypothesized that infected locusts would behaviorally compensate by shifting nutrient intake toward a more resistant, carbohydrate-biased intake target. To test these predictions, we examined how dietary protein-to-carbohydrate balance influenced hemolymph nutrient reserves and nutrient intake behavior following fungal infection. Protein-biased diets increased total hemolymph protein and concentrations of 8 out of 12 amino acids measured, but did not affect hemolymph glucose or trehalose concentrations. Infection caused declines in glucose, protein, and the three branched chain amino acids and tyrosine. Infected locusts did not alter nutrient intake behavior. Together, these findings are consistent with the hypothesis that protein-biased diets accelerate mortality during fungal infection by creating a more favorable nutritional environment for the pathogen.
Territorial competition has costs such as wounds received, energy spent, and elevated chronic stress. How does Betta splendens, a species that has been artificially selected for enhanced territorial aggression, maintain high levels of metabolically costly displays (gill flaring) over repeated days of territorial competition? Here we conducted two experiments. The first exposed male Betta splendens to a size-matched unresolvable competition (a mirror) daily for three weeks. We found that Betta splendens indeed maintain gill flaring throughout the competition schedule and exhibited no baseline stress impacts (measured via a scototaxis assay). We then exposed Betta splendens to one, two, or three weeks of competition and quantified cortisol levels, neurogenesis, and neural response in brain regions associated with social processing. We again found that Betta splendens showed consistently elevated gill flaring and again found no impacts on stress (measured via waterborne cortisol). Competition-exposed fish displayed increased neural response in the caudal supracommissural nucleus of the ventral telencephalon (Vs), a region that is homologous to the medial amygdala/bed nucleus of the stria terminalis and a central mediator of social decision-making. Neural response in this same region positively correlated with individual variation in gill flaring. Competition did not influence neurogenesis (doublecortin expression). Lastly, a measure of reproductive investment (gonadosomatic index) did not correlate to behavior but did correlate to cortisol levels, suggesting a cohesive physiological state related to reproduction in this species. Taken together, this work details the mechanisms that are relevant to the maintenance of costly aggressive behavior.
Navigation behaviour of aquatic animals has often been studied in unidirectional flow. Yet many aquatic habitats have highly variable flow directions, particularly those affected by waves. We use field measurements of natural flows and underwater video to survey how the nudibranch Hermissenda crassicornis navigates in wave-affected flow conditions. We compare navigation tracks relative to feeding events in low and high prey densities and low and high flow direction variability. Regardless of flow direction variability or prey density we found evidence of chemical and flow cue-based navigation towards prey. Evidence of flow-based navigation was only significant when flow direction was less variable and calculated over timescales of wave-affected flow rather than tidal flow. Slugs oriented parallel to flow (either upstream or downstream) before feeding, possibly orienting to flow oscillations from wave-action when prey is temporarily upstream (and thus using a form of chemical-gated rheotaxis). In higher flow direction variability, slug track metrics still suggested a navigational strategy was used to find prey, but the apparent absence of any orientation to flow suggested the slugs use a different strategy (possibly chemotaxis) in conditions that could produce highly mixed chemical cues. Finally, when slugs were distant from feeding, they moved cross-stream which may increase chances of intercepting prey chemical cues. Collectively, these findings possibly provide further evidence of context-dependent switching of gastropod navigational strategies. In addition, they also suggest that chemical-gated rheotaxis is a viable navigation strategy in habitats with either unidirectional or wave-affected flow.
Colour vision arises from the comparison of spectral input across at least two differently tuned photoreceptors. In most vertebrates, colour discrimination is mediated by two to four differently tuned cone photoreceptors, providing sensitivity from ultraviolet (< 400 nm) to red (∼600-700 nm) wavelengths. However, microspectrophotometry (MSP) indicated that the white-spotted boxfish, Ostracion meleagris, collected from Hawaiian reefs could possess five spectrally distinct cones. To explore the potential for increased cone diversity, we used a multidisciplinary approach to investigate colour vision in O. meleagris and its sister species, O. cubicus, from the Great Barrier Reef. Contrary to the MSP data, retinal transcriptomics and fluorescence in situ hybridisation found no evidence for five spectral sensitivities in either species. Instead, single cones (PR3) expressed sws2b, while the two members of the double cones (PR1 and PR2) expressed rh2a and rh2c opsins, respectively. Amino-acid modelling revealed putative visual pigments with uniformly spaced peak spectral sensitivities at 420 nm (SWS2B-based), 461-474 nm (RH2C) and 523-528 nm (RH2A). We compared the colour vision performance of O. cubicus with that of a well-studied trichromatic reef fish, the Picasso triggerfish (Rhinecanthus aculeatus). Compared to R. aculeatus, O. cubicus exhibited significantly lower detection thresholds for blue and achromatic grey, but not for green stimuli. Together, we find no support for five distinct spectral channels in the colour vision system in Australian boxfishes. Instead, these boxfish likely have trichromatic colour vision optimised for contrast detection, highlighting the importance of integrating multiple lines of investigation for accurately characterising animal colour perception.
Long distance migration in birds requires dynamic remodelling of flight muscle induced by behavioural, endocrine and metabolic adjustments. Here, we studied redheaded buntings in the nonmigratory and migratory state to assess changes in plasma hormones, muscle fibre diameter, lipid droplet accumulation and gene expression in the pectoralis and supracoracoideus muscles. We investigated genes that are involved in (1) muscle growth and atrophy, and (2) fatty acid transport and metabolism. Long-day exposure induced nighttime migratory restlessness, increased body mass, fat and muscle scores, and enlarged testes, compared with short days. Plasma insulin and testosterone levels were elevated in the migratory group, whereas T3 levels remained unchanged. Only the pectoralis muscle showed significant hypertrophy and high lipid accumulation under long days, whereas the supracoracoideus remained unchanged. We found elevated mRNA levels of fabp3 (fatty acid binding protein) and ar (androgen receptors) in both muscles, but scd1 (stearoyl-coA desaturase 1) only in the pectoralis. In contrast, igf1 (insulin-like growth factor 1) mRNA levels decreased in pectoral muscle in migratory state. These findings suggest that although fabp3 may facilitate fatty acid transport to both muscles, scd1 contributes to lipogenesis in pectoral muscle only. This suggests that pectoral muscles play a more active role than supracoracoideus muscle in uptake, modification and storage of lipids during migration. These findings demonstrate muscle-specific physiological remodelling underlying the migratory phenotype in buntings. Network analysis further revealed greater integration of phenotype, histology, hormone and molecular traits during migration, suggesting enhanced coordination of physiological systems during migration.
The xenohormesis hypothesis states that consuming stress-exposed food organisms can increase the consumer's tolerance to similar stresses. We tested this hypothesis using the freshwater model herbivore Daphnia magna feeding on the microalga Nannochloropsis limnetica grown at either 25°C (heat stressed), 20°C (presumed optimum) or 12°C (cold stressed). Daphnia magna maintained at either 20°C or 28°C and consuming heat-stressed algae had a significantly higher short-term heat tolerance. Furthermore, D. magna kept at either 12°C or 28°C (but not at 20°C) achieved a significantly higher lifespan when feeding on heat-stressed algae compared with D. magna feeding on non-stressed or cold-stressed algae, with fecundity being higher on cold-stressed food. This was accompanied by higher antioxidant capacity in D. magna fed heat-stressed algae, but no differences in lipid peroxidation were observed. Fatty acid analysis revealed that D. magna fed heat-stressed algae contained significantly lower levels of the essential polyunsaturated fatty acid (PUFA) eicosapentaenoic acid (EPA), which is thought to be particularly susceptible to lipid peroxidation, than their counterparts feeding on algae grown at 20°C. We conclude that the lower EPA level in D. magna fed heat-stressed algae has caused the higher heat tolerance in D. magna by altering PUFA-mediated membrane properties, i.e. increasing membrane rigidity. Our findings suggest that the consumption of heat-stressed algae can provide heat tolerance and lifespan benefits in zooplankton, but at the cost of reduced fecundity. Further studies are needed to better understand the influence of heat waves on trophic interaction at the phytoplankton-zooplankton interface.
Frequency and intensity of animal vocalisations scale with body size resulting in smaller animals generally producing weaker and higher frequency sounds than larger animals. Echolocating bats are very small mammals that not only vocalise to communicate but also produce high frequency calls to perform echo guided foraging in a variety of environments. Thus, in addition to body size, vocal scaling in bats may likely be driven by foraging specialisations and environmental context. To test this hypothesis, we used a microphone array to record the echolocation behaviour of seven wild bat species (6-32 grams) orienting through a narrow tunnel during nightly cave emergence. Under these recording conditions, the bats faced the same sensory scene while solving the same natural behavioural task. As predicted from mammalian vocal scaling, the centroid frequency of calls decreased with body size for both constant frequency (CF) and frequency modulating (FM) bats. However, the frequency scaling slope of FM bats was less negative than predicted, suggesting convergence of call frequency across body size and foraging specialisations. Contrary to general scaling principles, call intensity and call intervals also converged across body size meaning that smaller bats called louder, and larger bats called more often than predicted by their size. This leads us to conclude that environmental context is a very important driver of bat echolocation behaviour and that this deviation from normal scaling principles must be considered when performing comparative sensory studies across bats of different sizes, foraging specialisations and sensory scenes.
Emergent technologies and integrative approaches have transformed our ability to understand organismal behavior, physiology, and evolutionary ecology. Baleen whales (Mysticeti) have evolved the largest body sizes by filter-feeding on dense aggregations of prey in diverse ocean ecosystems. Although the wide range of foraging modes and prey capture abilities among mysticetes has been well-documented, the functional morphology underlying these relationships remains poorly understood. Gray whale (Eschrichtius robustus) morphology and ecological niche contrast starkly from closely related lunge-feeding rorqual species (Balaenopteridae), with gray whales foraging closer to the coastline using longer skulls and flatter mandibles in comparison to most rorquals. We used video and motion-sensing biologging tags to quantify the kinematics of gray whales feeding on infaunal ghost shrimp (Neotrypnea californeinsis) in the shallow waters of the northern Puget Sound, Washington, USA. Tag-based kinematics were integrated with 3D models and morphological dimensions of the skull to calculate the skull area in contact with the seafloor during each feeding roll, resulting in an average minimum benthic contact area of 4.9 +/- 1.1 m2. Daily rates of feeding events (mean 292 +/- 153) were comparable to those of lunge-feeding rorqual species despite using an average of 47.3% (weighted by deployment length) of the available high-tide feeding window. We posit that the paddle-like mandibles of gray whales reflect an adaptation for infaunal prey capture that may facilitate benthic suction-filter feeding in shallow coastal habitats, potentially opening a unique coastal foraging niche among mysticetes.
The interactions between thermal environments and morphology provide critical insights into how organisms adapt to ecological pressures. Leg and bill morphology in birds are commonly shaped by their thermal regime, with thermoregulatory use of the legs predicted by high temperature extremes and bill size limited by cold temperature extremes. This general pattern has a well-documented exception, however, where the bills of birds in saline habitats appear adapted to high rather than low temperatures. Here we present a test of the thermoregulatory use of the bill for high temperatures in three Ammospiza sparrow taxa, which face dual challenges of heat stress and freshwater limitation during the breeding season while representing a gradient of specialization to tidal saltmarshes. We quantified heat dissipation across ambient temperatures (10-35°C) using thermal imaging and examined inter- and intraspecific variation in bill-mediated heat loss and its relationship to bill size. We found that larger bills facilitated greater heat loss, with Seaside Sparrows, the most tidally adapted taxon, exhibiting the highest heat dissipation rates and thermal efficiency. Notably, Seaside Sparrows also displayed nonlinear heat dissipation patterns, with physiological control enabling rapid heat loss up to a thermal breakpoint (21.5°C) before plateauing. In contrast, Saltmarsh and Nelson's sparrows exhibited linear, passive heat dissipation patterns typical of other songbirds. These findings highlight the dual contributions of morphological and physiological adaptations to thermoregulation in a freshwater-limited ecosystem, report the most fine-tuned physiological control of bill vasculature yet in a passerine, and offer insights into the evolutionary pressures driving avian thermoregulatory strategies.
Understanding how phenotypic plasticity arises in sensory traits is a key challenge in evolutionary and developmental biology. Eusocial insects offer a useful model for exploring it, because their castes perform distinct tasks and occupy different light environments. We examined morphological variation and scaling in the visual systems of female castes of Protopolybia sedula, a neotropical wasp with simultaneous polygyny. Although queens are larger than workers, the two castes showed similar eye area, ommatidial diameter and ommatidia number. However, queens showed greater variability in ommatidial diameter, while workers varied more in eye area and ommatidia number. Allometric patterns also diverged: queen eye area scaled hypoallometrically and ommatidial diameter hyperallometrically with body size, whereas workers displayed isometric scaling for both traits. Analysis of ommatidia size and number indicates workers allocate more resources to increasing ommatidia number, while queens emphasise enlarging ommatidia. These differences highlight distinct developmental and scaling strategies that generate caste-specific visual adaptations.
Bird feathers serve numerous essential functions, with water repellency being particularly critical, as many other feather functions depend on remaining dry. Despite its importance, significant gaps remain in our understanding of the mechanisms underlying feather water repellency. This property arises primarily from feather microstructure. Feather morphology varies widely among bird species. However, the functional consequences of this variation for feather wettability remain poorly understood. Advances in computer-aided 3D modeling enable us to investigate such structure-function relationships with unprecedented precision. In this study, we created 3D feather models from micro-computed tomography scans of a seaside sparrow feather and systematically altered its morphology, specifically barbule density, to observe how feathers interact with water droplets using computational fluid dynamics simulations. We quantified the spatio-temporal distribution of droplet behavior, measured spreading radii during interactions, and assessed final contact angles for feathers with varying barbule densities. We validated our simulation results with barbule-density estimates and static contact-angle measurements on contour feathers from four New World sparrow species, including the seaside sparrow, spanning a gradient in water exposure. Together, the computational and experimental results demonstrate that higher barbule density reduces water repellency. Collectively, these findings highlight how feather microstructure directly governs feather wettability.
Bees use vibrations across behavioural contexts, including a specialised foraging routine in which they vibrate flowers to release pollen (floral vibrations). Floral vibrations often have complex spectral properties, combining fundamental frequencies and their harmonics, yet their functional consequences remain unknown. We experimentally replicated bee buzzes with identical fundamental frequency (300 Hz) but different harmonic content (0, 2 or 4 harmonics) to determine their effect on pollen release, a key functional outcome for both the bee and the flower. We normalised signals to the same average power by holding root mean squared (RMS) acceleration constant and applied three RMS levels to test for interactions between power and spectral composition. We applied vibrations to flowers and quantified both the input signal (at the shaker) and the vibration transmitted at a distal anther, and measured pollen release. Harmonic content did not explain pollen release at any RMS level. In contrast, RMS acceleration strongly predicted pollen removal. The RMS measured at the distal anther was only weakly correlated with the input RMS, suggesting that the flower strongly modifies the vibration signal. Both input RMS and distal anther RMS explained pollen release, irrespective of spectral composition. Our results suggest that pollen release is determined by vibration amplitude (RMS acceleration) rather than spectral composition and is jointly governed by the bee-generated input and the mechanical filtering properties of the flower.
Streaked shearwaters (Calonectris leucomelas) return to their breeding colonies, which are established in elevated locations on islands, by increasing their flight altitude immediately before landing. We quantitatively analysed the characteristics of this energetically demanding ascending flight and examined how body mass influences flapping behaviour using high temporal resolution biologging data from eight individuals. We compared the proportion of time spent flapping, the period of dorsal-ventral axis acceleration cycle (the inverse of wingbeat frequency) and acceleration amplitude between ascending and sea-surface flight. The proportion of time spent flapping was considerably larger during ascending flight (94.6±6.5%) than during sea-surface flight (40.8±13.0%). The mean dorsal-ventral acceleration cycle was markedly shorter (0.233±0.008 s versus 0.247±0.008 s) and the mean amplitude was larger (8.85±0.82 m s-2 versus 7.20±0.49 m s-2) during ascending flight than during sea-surface flight. During ascent, birds increased wingbeat frequency, accompanied by greater body acceleration amplitude, indicating increased mechanical power output required for climbing flight. The increases in wingbeat frequency and amplitude were larger for heavier birds when transitioning from sea-surface to ascending flight, revealing that body mass strongly influenced flapping performance under high energy demand scenarios. Individual variation in body mass strongly affected the biomechanics and energy requirements of flight, providing insights into size-related trade-offs in the flight behaviour of island-breeding seabirds.
Hibernation is a seasonally regulated process dramatically reducing metabolic rate and subsequently core body temperature (Tb). Many hibernators repeatedly cycle between torpor and arousal (T-A cycling) throughout the hibernation season. How these cycles are regulated is still unresolved. The sensing of metabolites accumulating/depleting during torpor may regulate T-A cycling. Here, we report the development of a microdialysis protocol to sample at a high temporal resolution from the third ventricle (3V) of the hypothalamus, from an individual hibernating golden hamster (Mesocricetus auratus). Over multiple days and T-A cycles we saw no negative impact on hibernation physiology or dynamics during the microdialysis. We collected samples at 2 h resolution over two successive T-A cycles and measured metabolites by targeted and untargeted metabolomics. Finally, we developed an analysis pipeline to identify accumulating or depleting metabolites during torpor, providing a proof of concept that cerebrospinal fluid microdialysis from the 3V of a hibernating golden hamster can be used in future studies to understand regulation of T-A cycling.
