Prey Capture Success Research Articles

A test of a generalized, optimal habitat selection model for drift‐feeding fishes: brook charr

AbstractOptimality theory is useful for modeling habitat use in animals, although many models have not been rigorously tested. We evaluated the ability of the Grossman et al. net‐energy‐intake (NEI) optimal foraging model for drift‐feeding stream fishes, to predict the holding velocity (microhabitat) of native brook charr (BC; Salvelinus fontinalis) inhabiting a southern Appalachian stream. This model postulates that fish choose holding velocities that maximize their NEI, and is broadly applicable to stream fishes, many of which are drift‐feeders. We used laboratory prey capture success versus water velocity data to quantify the relationship between prey capture success, water velocity, and holding velocity of BC (N = 21, mean standard length = 147 mm ± 14 sd). Parameterization of the Grossman et al. model yielded an optimal holding velocity prediction of 18.5 cm·s−1. Based on fitting criteria from previous work, the model prediction was successful because it fell within the 95% confidence interval (13.5–20.5 cm·s−1) of mean holding velocities (17.0 cm·s−1, N = 26) of BC in Lynn Camp Prong, Tennessee. Consequently, NEI maximization likely is a causal factor affecting BC habitat choice. Our result argues strongly for the general usefulness of this model.

Past thermal conditions affect hunting behaviour in larval antlions.

Some sit-and-wait predators, such as antlion larvae, construct traps to capture passing prey. The location of these traps depends on many abiotic and biotic factors, including temperature and the presence of conspecifics, which probably stimulate behaviours that minimize the costs and maximize the benefits of trap building. Here, we exposed second instar antlion larvae to elevated temperatures of 25°C (mild treatment) or 31°C (harsh treatment) for one month and then transferred them to common conditions (20°C) to examine the effects of previous thermal treatment on aggregation tendency and trap size. We predicted that antlions that experienced harsh conditions would subsequently increase the neighbouring distance and trap diameter to reduce competition with conspecifics and improve prey capture success, compensating for past conditions. In contrast with these predictions, antlions exposed to harsh conditions displayed a trend in the opposite direction, towards the decreased neighbouring distance. Furthermore, some of these antlions also built smaller traps. We discuss possible reasons for our results. The effects of previous thermal exposure have rarely been considered in terms of trap construction in antlions. Described effects may possibly apply to other sit-and-wait predators and are significant considering that many of these predators are long-lived.

Tail Autotomy Alters Prey Capture Performance and Kinematics, but not Success, in Banded Geckos.

Tails are versatile structures with diverse forms and functions across vertebrates. They are involved in almost all behaviors critical to survival including locomotion, feeding, and predator avoidance. Although the tail's role in locomotion and stability has been widely studied, its role in prey capture is relatively unknown. Lizards are an ideal system to examine the tail's impact on prey capture as most are capable of autotomizing, or dropping, their tail in response to predation and intraspecific competition. Tail autotomy can lower reproduction, decrease locomotor performance, impart instability during jumping, and decrease social status. Desert banded geckos (Coleonyx variegatus) frequently capture evasive prey in nature and appear to use their tail during strikes. However, it is unclear if these tail movements are important for the strike itself, or if they simply draw attention to that part of the body. We used high-speed 3D videography to quantify prey capture performance and kinematics of C. variegatus striking at crickets before and after total caudal autotomy. Trials were conducted within 2 h of autotomy and then repeatedly over a 2-week period. Overall, prey capture success was unaffected by caudal autotomy. However, maximum strike velocity decreased significantly after autotomy, highlighting the importance of the tail during prey capture. Strike kinematics were altered after autotomy in several ways, including geckos adopting a more sprawled posture. Maximum pectoral girdle and mid-back height were significantly lower during post-autotomy strikes, whereas maximum pelvic girdle height was unaffected. However, individual variation was considerable. This downward pitching of the body after tail loss suggests that the tail is necessary for counterbalancing the anterior portion of the body and resisting the rotational inertia incurred after pushing off with the hindlimbs. Utilizing autotomy to test tail function in prey capture can provide valuable insight into how the tail is used in terrestrial predation across a wide variety of species and ecological niches.

Gusty winds, flappy wings

Birds must fly in all manner of windy weather. At first glance, this may not seem like such an impressive feat; after all, flying is what birds are born to do. But if you consider that they remain suspended in the air while being continually buffeted by sudden gusts of wind, which do not send them tumbling, the trick appears more remarkable. Humans caught on a ship in inclement weather have a hard time remaining upright even with handrails to grab onto, so it is quite striking how birds deal with wind in such a graceful style. Stability also ensures safe landings and successful prey capture, and provides flying animals with a steady view of the world. Spurred on to learn more, a team of researchers from the Royal Veterinary College and the University of Bristol, UK, set out to study how birds remain stable when flying through unexpected gusts of wind.The team trained a barn owl to glide through a 17 m laboratory corridor equipped with high-speed cameras that tracked the bird's movement. To deliver a perturbing gust to the flying bird, they put a fan at the end of the corridor placed beneath the flight path to deliver an unexpected blast to the underside of the bird. The owl experienced three increasing gusts of 3.1, 4.5 and 5.2 m s–1 – comparable to gentle, yet destabilizing, breezes – as it flew through the corridor. Then they measured how the owl coped with the gusts by tracking its wing movements, in addition to comparing the movement of the flying bird's torso with the movement predicted from computer simulations, to calculate how well the gusts were tackled.Employing a near-immediate response when gliding into the unexpected gust, the bird pivoted its wings around its shoulders, forming a V-shape when viewed from the front, with its head and torso located at the bottom, while its wings formed each ascending limb of the V. This initial manoeuvre deflected 32% of the lift generated by the gust within 80 ms of its impact, ensuring that the bird's torso and head remained on an unperturbed course. The team found that the bird's quick response was aided by the build and shape of the wing: the mass distribution along its length ensures that the wing's pivoting motion imparts no secondary upward or downward movement to the torso, keeping the head firmly in place during the unexpected impact. Then, after the initial wing rotation, the bird tipped the front edge of the wing downward, which decreased the lift force generated by the gust, reducing the impact of the gust by an additional 48%, thereby allowing the bird to continue smoothly on its way.Analysing the manoeuvre, the team found that the speed of the response was the key to maintaining stability, suggesting that the bird's central nervous system may not be important for tackling the initial impact of a gust. This means that passive mechanisms, such as the shape and weight of the bird wing, absorb wind gusts, much like a suspension system in a car ensures a smooth ride through rough terrain. This novel discovery is also exciting news for engineers trying to build small aircraft. Understanding how shape and form endow flying stability may greatly simplify the task of flying through unpredictable weather conditions without needing to depend on complex sensors to gauge stability. Taking a hint from the barn owl: flapping wings can make for unflappably steady flight.

Exploring the Impacts of Methylmercury‐Induced Behavioral Alterations in Larval Yellow Perch in Lake Michigan Using an Individual‐Based Model

AbstractThe inclusion of sublethal behavioral effects in population models has predicted lower estimates of cohort abundance and survival. Methylmercury, a persistent contaminant in the Great Lakes, has been shown to alter foraging and predator avoidance behaviors of larval Yellow Perch Perca flavescens, which may have indirect effects on recruitment. Poor recruitment of Yellow Perch in Lake Michigan has been associated with changes in trophic structure and adverse habitat conditions, but the potential effects of contaminants have not been fully explored. To test this, we adapted existing larval fish individual‐based models to incorporate laboratory‐derived methylmercury behavioral impairments on a larval Yellow Perch cohort in Lake Michigan. Overall, swimming speed reductions following methylmercury exposure did not drastically affect cohort survival. In contrast, the impairment to prey capture success resulted in an additional 8, 45, and 28% starvation of the cohort in simulations with tissue concentrations of 0.21, 0.95, and 3.14 μg/g total mercury whole‐embryo wet weight, respectively. While our experimental methylmercury concentrations were higher than those typically found in Lake Michigan, our findings suggest that contaminants could be an additional factor impacting recruitment of Yellow Perch in systems highly contaminated with mercury.

Longer development provides first-feeding fish time to escape hydrodynamic constraints.

What is the functional effect of prolonged development? By controlling for size, we quantify first-feeding performance and hydrodynamics of zebrafish and guppy offspring (5 ± 0.5 mm in length), which differ fivefold in developmental time and twofold in ontogenetic state. By manipulating water viscosity, we control the hydrodynamic regime, measured as Reynolds number. We predicted that if feeding performance were strictly the result of hydrodynamics, and not development, feeding performance would scale with Reynolds number. We find that guppy offspring successfully feed at much greater distances to prey (1.0 vs. 0.2 mm) and with higher capture success (90 vs. 20%) compared with zebrafish larvae, and that feeding performance was not a result of Reynolds number alone. Flow visualization shows that zebrafish larvae produce a bow wave ~0.2 mm in length, and that the flow field produced during suction does not extend beyond this bow wave. Due to well-developed oral jaw protrusion, the similar-sized suction field generated by guppy offspring extends beyond the horizon of their bow wave, leading to successful prey capture from greater distances. These findings suggest that prolonged development and increased ontogenetic state provides first-feeding fish time to escape the pervasive hydrodynamic constraints (bow wave) of being small.

Oxygen consumption of drift-feeding rainbow trout: the energetic tradeoff between locomotion and feeding in flow.

To forage in fast, turbulent flow environments where prey is abundant, fishes must deal with the high associated costs of locomotion. Prevailing theory suggests that many species exploit hydrodynamic refuges to minimize the cost of locomotion while foraging. Here, we challenge this theory based on direct oxygen consumption measurements of drift-feeding trout (Oncorhynchus mykiss) foraging in the freestream and from behind a flow refuge at velocities up to 100 cm s-1 We demonstrate that refuging is not energetically beneficial when foraging in fast flows because of a high attack cost and low prey capture success associated with leaving a station-holding refuge to intercept prey. By integrating optimum foraging theory with empirical data from respirometry and video tracking, we developed a mathematical model to predict when drift-feeding fishes should exploit or avoid refuges based on prey density, size and flow velocity. Our optimum foraging and refuging model provides new mechanistic insights into locomotor costs, habitat use and prey choice of fish foraging in current-swept habitats.

Temperature-Dependent Biomarkers of Estrogenic Exposure in a Piscivore Freshwater Fish.

The biological effects of endocrine-active compounds and increasing water temperatures as a result of climate change have been studied extensively and independently, but there is a dearth of research to examine the combined effect of these factors on exposed organisms. Recent data suggest that estrogenic exposure and rising ambient temperatures independently impact predator-prey relationships. However, establishing these connections in natural settings is complex. These obstacles can be circumvented if biomarkers of estrogenic exposure in resident fish can predict changes in predator-prey relationships. To test the effects of estrone and temperature, the piscivore bluegill sunfish (Lepomis macrochirus) was exposed for 30days to estrone at concentrations (90 ± 17.6ng/L [mean ± standard deviation] and 414 ± 146ng/L) previously shown to reduce prey-capture success. Exposures were conducted at four temperatures (15°C, 18°C, 21°C, 24°C) to simulate breeding season ambient temperatures across the natural range of this species. A suite of morphological and physiological biomarkers previously linked to estrogenic exposures were examined. Biomarkers of estrone exposure were more commonly and severely impacted in male fish than in female fish. Notably, the gonadosomatic index was lower and gonads were less mature in exposed males. Additionally, temperature modulated the effects of estrone similarly in males and females with fish exposed at higher temperatures typically exhibiting a decreased morphological index. This study provides evidence that alterations in hepatic function and gonadal function may cause shifts in metabolism and energy allocation that may lead to declining prey capture performance.

Visual determinants of prey chasing behavior in a mudflat crab.

The crab Neohelice granulata inhabits mudflats where it is preyed upon by gulls and, conversely, preys on smaller crabs. Therefore, on seeing moving stimuli, this crab can behave as prey or predator. The crab escape response to visual stimuli has been extensively investigated from the behavioral to the neuronal level. The predatory response (PR), however, has not yet been explored. Here, we show that this response can be reliably elicited and investigated in a laboratory arena. By using dummies of three different sizes moved on the ground at three different velocities over multiple trials, we identified important stimulation conditions that boost the occurrence of PR and its chances of ending in successful prey capture. PR probability was sustained during the first 10 trials of our experiments but then declined. PR was elicited with high probability by the medium size dummy, less effectively by the small dummy, and hardly brought about by the large dummy, which mostly elicited avoidance responses. A GLMM analysis indicated that the dummy size and the tracking line distance were two strong determinants for eliciting PR. The rate of successful captures, however, mainly depended on the dummy velocity. Our results suggest that crabs are capable of assessing the distance to the dummy and its absolute size. The PR characterized here, in connection with the substantial knowledge of the visual processing associated with the escape response, provides excellent opportunities for comparative analyses of the organization of two distinct visually guided behaviors in a single animal.

Temperature and Estrogen Alter Predator-Prey Interactions between Fish Species.

SynopsisA variety of environmental estrogens are commonly detected in human-impacted waterways. Although much is known about the effects of these environmental estrogens on the reproductive physiology and behavior of individuals within species, comparatively less is known about how these compounds alter the outcomes of interactions between species. Furthermore, few studies have considered how the effects of contaminants are modulated by natural variation in abiotic factors, such as temperature. To help fill this knowledge gap, we conducted a factorial experiment to examine the independent and combined effects of estrone (E1) and temperature on the outcome of predator–prey interactions between two common North American freshwater fishes, fathead minnows (Pimephales promelas) and bluegill sunfish (Lepomis macrochirus). Larval fathead minnows and adult sunfish were exposed to either a low (mean±standard deviation, 90.1 ± 18 ng/L; n = 16) or high (414 ± 147 ng/L; n = 15) concentration of E1 or to a solvent control for 30 days at one of four natural seasonal temperatures (15°C, 18°C, 21°C, and 24°C) before predation trials were performed. Exposure to E1 was associated with a significant increase in larval predation mortality that was independent of temperature. Across all temperature treatments, approximately 74% of control minnows survived; this survivorship significantly exceeded that of minnows exposed to either concentration of E1 (49% and 53% for minnows exposed to the low and high concentrations, respectively). However, exposure to E1 also impaired the prey-capture success of sunfish, partially mitigating predation pressure on exposed minnows. Overall prey-capture success by sunfish showed an inverted U-shaped distribution with temperature, with maximal prey consumption occurring at 21°C. This study illustrates the vulnerability of organismal interactions to estrogenic pollutants and highlights the need to include food web interactions in assessments of risk.

Factors influencing prey capture success and profitability in Australasian gannets (Morus serrator).

ABSTRACTKnowledge of the factors influencing foraging efficiency in top predators can provide insights into the effects of environmental variability on their populations. Seabirds are important marine predators foraging in a highly temporally and spatially variable environment. While numerous studies have focussed on search time and its effects on foraging energetics in seabirds, relatively little is known about the factors influencing capture success and prey profitability in these predators. In the present study, animal-borne cameras were used to investigate the chase durations, capture success, handling durations and profitability of prey consumed by Australasian gannets (Morus serrator) (n=95) from two breeding colonies in south-eastern Australia exposed to different oceanographic conditions. Capture success was generally lower when individuals foraged alone. However, foraging in multi-species groups and in high prey densities increased chase time, while larger prey elicited longer handling times. While prey type influenced profitability, high prey density and foraging in multi-species groups was found to lower prey profitability due to increased time expenditure. While previous studies have found group foraging reduces search time, the increased profitability explains why some animals may favour solitary foraging. Therefore, future studies should combine search time and the currently found factors.

The dive performance of immature king penguins following their annual molt suggests physiological constraints.

Like all birds, penguins undergo periodic molt, during which they replace old feathers. However, unlike other birds, penguins replace their entire plumage within a short period while fasting ashore. During molt, king penguins (Aptenodytes patagonicus) lose half of their initial body mass, most importantly their insulating subcutaneous fat and half of their pectoral muscle mass. The latter might challenge their capacity to generate and sustain a sufficient mechanical power output to swim to distant food sources and propel themselves to great depth for successful prey capture. To investigate the effects of the annual molt fast on their dive/foraging performance, we studied various dive/foraging parameters and peripheral temperature patterns in immature king penguins across two molt cycles, after birds had spent their first and second year at sea, using implanted data-loggers. We found that the dive/foraging performance of immature king penguins was significantly reduced during post-molt foraging trips. Dive and bottom duration for a given depth were shorter during post-molt and post-dive surface interval duration was longer, reducing overall dive efficiency and underwater foraging time. We attribute this decline to the severe physiological changes that birds undergo during their annual molt. Peripheral temperature patterns differed greatly between pre- and post-molt trips, indicating the loss of the insulating subcutaneous fat layer during molt. Peripheral perfusion, as inferred from peripheral temperature, was restricted to short periods at night during pre-molt but occurred throughout extended periods during post-molt, reflecting the need to rapidly deposit an insulating fat layer during the latter period.

Is cooperation in prey capture flexible in the Indian social spider Stegodyphus sarasinorum?

Among social spiders, cooperation is a key characteristic behaviour. Cooperation in prey capture increases the probability of successful prey capture and to some extent reduces the individual costs associated with foraging. We assessed spider cooperation in prey capture under natural conditions in relation to the number of spiders in the colony and the type and size of the prey captured by the social spider Stegodyphus sarasinorum Karsch, 1892 (Araneae: Eresidae). First, we determined natural prey in the spider webs and found that beetles (Coleoptera) were the most frequent prey followed by grasshoppers (Orthoptera). These two prey types were then used to study the cooperative hunting behaviour of this spider. We investigated prey capture frequency, recruitment and immobilization time when spiders are more active in the mornings and less active around midday. The study revealed that the immobilization time and recruitment time were shorter when hunting beetles, the smaller sized prey, while larger numbers of spiders were recruited in response to grasshoppers, the larger prey. The study concluded that cooperative behaviour in S. sarasinorum depends on the size of prey present.

Persistent and transgenerational effects of raw and ozonated oil sands process-affected water exposure on a model vertebrate, the zebrafish

Exposure to oil sands process-affected water (OSPW), a by-product of Canadian oil sands mining operations, can cause both acute and chronic adverse effects in aquatic life. Ozonation effectively degrades naphthenic acids in OSPW, mitigating some of the toxicological effects of exposure. In this study we examined the effect of developmental exposure to raw and ozonated OSPW had on the breeding success, prey capture, and alarm cue response in fish months/years after exposure and the transgenerational effect exposure had on gene expression, global DNA methylation, and larval basal activity. Exposure to raw and ozonated OSPW had no effect on breeding success, and global DNA methylation. Exposure altered the expression of vtg and nkx2.5 in the unexposed F1 generation. Exposure to both raw and ozonated OSPW had a transgenerational impact on larval activity levels, anxiety behaviors, and maximum swim speed compared to the control population. Prey capture success was unaffected, however, the variability in the behavioral responses to the introduction of prey was decreased. Fish developmentally exposed to either treatment were less active before exposure and did not have an anxiety response to the alarm cue hypoxanthine-3-n-oxide. Though ozonation was able to mitigate some of the effects of OSPW exposure, further studies are needed to understand the transgenerational effects and the implications of exposure on complex fish behaviors.

Predation and Risk Behaviors of Free-Roaming Owned Cats in Auckland, New Zealand via the Use of Animal-Borne Cameras.

Free-roaming cats are at increased risk of injuring themselves as well as other domestic and fauna species, yet relatively little is known about the frequency at which risk and predation behaviors occur in a typical day. In this study, cat risk, and predation behavioral information was collected using animal-borne video cameras and global positioning system (GPS) units that were attached to break-free cat collars. The observation period was one to three consecutive days for 37 convenience sampled free-roaming owned cats in Auckland, New Zealand. Video footage was manually reviewed and all predation and risk behavior events were recorded. These included stalking, pursuing, and seizing prey as well as altercations with other cats, ingesting harmful substances, and venturing into hazardous locations such as roads and storm drains. During the observation period, 23 of the 37 cats (62.2%) engaged in a total of 121 predation events. Of these, 40 resulted in successful prey capture with 18 of the 40 captures involving New Zealand native fauna species. Invertebrates were the most common taxa preyed upon (n = 55; 46%), followed by skinks (n = 8; 7%). No mammalian, avian or amphibian prey were captured and no cat took prey back to their residence. A total of 326 risk behaviors were observed for 32 out of the 37 cats (86.5%) with the most common being cats venturing onto the road (n = 132; 41%). Younger cats (aged 1–6 six years) engaged in significantly more predation and risk behaviors than older cats (aged 7 years and above). Sex, breed, number of cats in a household, and geographic location were not found to be predictors of cats' participation in predation or risk behaviors. Given the high frequency of predation and risk behaviors in free-roaming owned cats, it may be beneficial to educate owners about strategies to minimize risk such as housing them indoors, containing them to their properties or monitoring their time spent outdoors.

Mechanics of foraging success and optimal microhabitat selection in Alaskan Arctic grayling (Thymallus arcticus)

Most fishes residing in temperate streams in the Northern Hemisphere are drift-feeders. Despite this fact, little is known about the mechanisms of drift-feeding itself. We used Alaskan Arctic grayling (Thymallus arcticus), an abundant boreal drift-feeder, to examine the effects of water velocity on several aspects of drift-feeding behavior and test predictions of the Grossman et al. (2002) net energy intake model for microhabitat choice. Water velocity had a negative effect on prey capture, a positive effect on holding velocity, and little effect on reactive distance. We also found that dominance was a better predictor of prey capture success than size rank, although neither of these variables influenced holding velocity or reactive distance. The Grossman et al. (2002) model successfully predicted holding velocities of grayling in one Alaskan stream, but not another. Model failure might have occurred due to higher turbulence, increased predation, or interspecific competition with Dolly Varden (Salvelinus malma). These results help inform the study of habitat selection in drift-feeding fishes as well as management and conservation of Arctic grayling.

Foraging behaviour and optimal microhabitat selection in Yukon River Basin nonanadromous Dolly Varden Charr (Salvelinus malma)

AbstractSpecies conservation requires understanding the mechanistic processes of habitat selection and their effects on fitness. Nonetheless, there are few fitness‐based habitat selection models for aquatic organisms. We examined multiple aspects of foraging behaviour of nonanadromous Dolly Varden Charr (Salvelinus malma) in Panguingue Creek, Alaska, USA and applied these data to test a fitness‐based microhabitat selection model. Velocity negatively affected prey capture success, positively affected holding velocity, and had no effect on reactive distance. Dominance was a better predictor of prey capture success than length difference between competitors, but there was no relationship between these variables and holding velocity or reactive distance. We used the velocity–prey capture success relationship to parameterise the microhabitat habitat selection model and compared the predicted optimal holding velocity to the 95% confidence interval (24.9–29.3 cm/s) of holding velocities occupied by Dolly Varden (N = 29) in Panguingue Creek. The prediction of 24.0 cm/s fell just slightly (0.9 cm/s) outside the lower limit of the confidence interval; the model barely failed to predict holding velocity for this species in Panguingue Creek. Although this discrepancy fell within measurement error, model failure also may have been due to influence of high turbulence on fish holding velocities in the creek, low sample sizes imposed by permitting limitations, or field logistical issues. The relationship between velocity and prey capture success is an important aspect of drift feeder habitat selection. Our optimal holding velocity prediction for Dolly Varden should aid in the management and conservation of this species.

Guiding lights: Foraging responses of juvenile nocturnal orb‐web spiders to the presence of artificial light at night

AbstractThe reach of artificial light at night (ALAN) is growing rapidly around the globe, including the increasing use of energy‐efficient LED lights. Many studies document the physiological costs of light at night, but far fewer have focused on the potential benefits for nocturnal insectivores and the likely ecological consequences of shifts in predator–prey relationships. We investigated the effects of ALAN on the foraging behaviour and prey capture success in juvenile Australian garden orb‐web spiders (Eriophora biapicata). Laboratory experiments demonstrated that juvenile spiders were attracted to LED lights when choosing foraging sites, but prey availability was a stronger cue for remaining in a foraging site. Field experiments revealed a significant increase in prey capture rates for webs placed near LED lights. This suggests that any physiological costs of light at night may be offset by the foraging benefits, perhaps partially explaining recently observed increases in the size, fecundity and abundance of some orb‐web spider species in urban environments. Our results highlight the potential long‐term consequences of night lighting in urban ecosystems, through the impact of orb‐web spiders on insect populations.

Hydrodynamics of frontal striking in aquatic snakes: drag, added mass, and the possible consequences for prey capture success

Transient locomotion under water is highly constrained by drag and added mass, yet some aquatic snakes catch their prey using a fast forward acceleration, with the mouth opened. These aquatic snakes show a convergence of their head shape in comparison with closely related species that do not forage under water. As both drag and added mass are related to some extent to the shape of the moving object, we explored how shape impacts the hydrodynamic forces applied to the head of a snake during a prey capture event. We compared two 3D-printed heads representing typical shapes of aquatically-foraging and non-aquatically-foraging snakes, and frontal strike kinematics based on in vivo observations. By using direct force measurements, we calculated the drag and added mass coefficient of the two models. Our results show that both drag and added mass are reduced in aquatic snakes. The drag coefficient of the aquatic model is 0.24, which is almost two times smaller than the non-aquatic model. The added mass coefficient of the aquatic model is 0.15 versus 0.24 for the non-aquatic model, showing that the convergence of head shape in aquatically foraging snakes is associated with a hydrodynamic advantage during frontal striking. The vorticity field measurements with particle image velocimetry show that a less intense recirculation bubble behind the jaw of the aquatic model, compared to the non-aquatic model, might be the basis of this advantage.

Testing Darwin's Hypothesis about the Wonderful Venus Flytrap: Marginal Spikes Form a "Horrid Prison" for Moderate-Sized Insect Prey.

Botanical carnivory is a novel feeding strategy associated with numerous physiological and morphological adaptations. However, the benefits of these novel carnivorous traits are rarely tested. We used field observations, lab experiments, and a seminatural experiment to test prey capture function of the marginal spikes on snap traps of the Venus flytrap (Dionaea muscipula). Our field and laboratory results suggested inefficient capture success: fewer than one in four prey encounters led to prey capture. Removing the marginal spikes decreased the rate of prey capture success for moderate-sized cricket prey by 90%, but this effect disappeared for larger prey. The nonlinear benefit of spikes suggests that they provide a better cage for capturing more abundant insects of moderate and small sizes, but they may also provide a foothold for rare large prey to escape. Our observations support Darwin's hypothesis that the marginal spikes form a "horrid prison" that increases prey capture success for moderate-sized prey, but the decreasing benefit for larger prey is unexpected and previously undocumented. Thus, we find surprising complexity in the adaptive landscape for one of the most wonderful evolutionary innovations among all plants. These findings enrich understanding of the evolution and diversification of novel trap morphology in carnivorous plants.