Stationary Prey Research Articles

African penguin Spheniscus demersus foraging on juvenile fish in jellyfish tentacles

Technological advances have enabled the observation of foraging behaviour in wild marine animals. We can observe where they go, how deep they dive, how much energy they expend, and with the use of animal-borne cameras we can capture specific foraging behaviours. Here we describe a newly observed foraging behaviour in African Penguins Spheniscus demersus in which they target fish located in jellyfish tentacles. As animal-borne cameras have only been deployed on African Penguins since 2015 it is unclear whether this behaviour is novel or previously unobserved. The behaviour appears to be opportunistic and beneficial to the African Penguins as it enables them to catch stationary prey. As the availability of the African Penguin’s traditional schooling fish prey changes in the southern Benguela Current ecosystem due to climate change and overfishing, opportunistic foraging strategies like this could enable African Penguins to supplement foraging success at a low energetic cost.

The Lévy flight foraging hypothesis: comparison between stationary distributions and anomalous diffusion *

We consider a stationary prey in a given region of space and we aim at detecting optimal foraging strategies. On the one hand, when the prey is uniformly distributed, the best possible strategy for the forager is to be stationary and uniformly distributed in the same region. On the other hand, in several biological settings, foragers cannot be completely stationary, therefore we investigate the best seeking strategy for Lévy foragers in terms of the corresponding Lévy exponent. In this case, we show that the best strategy depends on the region size in which the prey is located: large regions exhibit optimal seeking strategies close to Gaussian random walks, while small regions favor Lévy foragers with small fractional exponent. We also consider optimal strategies in view of the Fourier transform of the distribution of a stationary prey. When this distribution is supported in a suitable volume, then the foraging efficiency functional is monotone increasing with respect to the Lévy exponent and accordingly the optimal strategy is given by the Gaussian dispersal. If instead the Fourier transform of the distribution of a stationary prey is supported in the complement of a suitable volume, then the foraging efficiency functional is monotone decreasing with respect to the Lévy exponent and therefore the optimal strategy is given by a null fractional exponent (which in turn corresponds, from a biological standpoint, to a strategy of ‘ambush’ type). We will devote a rigorous quantitative analysis also to emphasize some specific differences between the one-dimensional and the higher-dimensional cases.

Dazzled by shine: gloss as an antipredator strategy in fast moving prey.

Previous studies on stationary prey have found mixed results for the role of a glossy appearance in predator avoidance-some have found that glossiness can act as warning coloration or improve camouflage, whereas others detected no survival benefit. An alternative untested hypothesis is that glossiness could provide protection in the form of dynamic dazzle. Fast moving animals that are glossy produce flashes of light that increase in frequency at higher speeds, which could make it harder for predators to track and accurately locate prey. We tested this hypothesis by presenting praying mantids with glossy or matte targets moving at slow and fast speed. Mantids were less likely to strike glossy targets, independently of speed. Additionally, mantids were less likely to track glossy targets and more likely to hit the target with one out of the two legs that struck rather than both raptorial legs, but only when targets were moving fast. These results support the hypothesis that a glossy appearance may have a function as an antipredator strategy by reducing the ability of predators to track and accurately target fast moving prey.

Visual Attack on the Moving Prey by Cuttlefish.

Visual attack for prey capture in cuttlefish involves three well characterized sequential stages: attention, positioning, and seizure. This visually guided behavior requires accurate sensorimotor integration of information on the target’s direction and tentacular strike control. While the behavior of cuttlefish visual attack on a stationary prey has been described qualitatively, the kinematics of visual attack on a moving target has not been analyzed quantitatively. A servomotor system controlling the movement of a shrimp prey and a high resolution imaging system recording the behavior of the cuttlefish predator, together with the newly developed DeepLabCut image processing system, were used to examine the tactics used by cuttlefish during a visual attack on moving prey. The results showed that cuttlefish visually tracked a moving prey target using mainly body movement, and that they maintained a similar speed to that of the moving prey right before making their tentacular strike. When cuttlefish shot out their tentacles for prey capture, they were able to either predict the target location based on the prey’s speed and compensate for the inherent sensorimotor delay or adjust the trajectory of their tentacular strike according to the prey’s direction of movement in order to account for any changes in prey position. These observations suggest that cuttlefish use the various visual tactics available to them flexibly in order to capture moving prey, and that they are able to extract direction and speed information from moving prey in order to allow an accurate visual attack.

An escape theory model for directionally moving prey and an experimental test in juvenile Chinook salmon.

Prey evaluate risk and make decisions based on the balance between the costs of predation and those of engaging in antipredator behaviour. Economic escape theory has been valuable in understanding the responses of stationary prey under predation risk; however, current models are not applicable for directionally moving prey.Here we present an extension of existing escape theory that predicts how much predation risk is perceived by directionally moving prey. Perceived risk is measured by the extent antipredator behaviour causes a change in travel speed (the distance to a destination divided by the total time to reach that destination). Cryptic or cautious antipredator behaviour slows travel speed, while prey may also speed up to reduce predator–prey overlap. Next, we applied the sensitization hypothesis to our model, which predicts that prey with more predator experience should engage in more antipredator behaviour, which leads to a larger change in travel speed under predation risk. We then compared the qualitative predictions of our model to the results of a behavioural assay with juvenile Chinook salmon Oncorhynchus tshawytscha that varied in their past predator experience.We timed salmon swimming downstream through a mesh enclosure in the river with and without predator cues present to measure their reaction to a predator. Hatchery salmon had the least predator experience, followed by wild salmon captured upstream (wild‐upstream) and wild‐salmon captured downstream (wild‐downstream).Both wild salmon groups slowed down in response to predator cues, whereas hatchery salmon did not change travel speed. The magnitude of reaction to predator cues by salmon group followed the gradient of previous predator experience, supporting the sensitization hypothesis.Moving animals are conspicuous and vulnerable to predators. Here we provide a novel conceptual framework for understanding how directionally moving prey perceive risk and make antipredator decisions. Our study extends the scope of economic escape theory and improves general understanding of non‐lethal effects of predators on moving prey.

Binocular Encoding in the Damselfly Pre-motor Target Tracking System.

Akin to all damselflies, Calopteryx (family Calopterygidae), commonly known as jewel wings or demoiselles, possess dichoptic (separated) eyes with overlapping visual fields of view. In contrast, many dragonfly species possess holoptic (dorsally fused) eyes with limited binocular overlap. We have here compared the neuronal correlates of target tracking between damselfly and dragonfly sister lineages and linked these changes in visual overlap to pre-motor neural adaptations. Although dragonflies attack prey dorsally, we show that demoiselles attack prey frontally. We identify demoiselle target-selective descending neurons (TSDNs) with matching frontal visual receptive fields, anatomically and functionally homologous to the dorsally positioned dragonfly TSDNs. By manipulating visual input using eyepatches and prisms, we show that moving target information at the pre-motor level depends on binocular summation in demoiselles. Consequently, demoiselles encode directional information in a binocularly fused frame of reference such that information of a target moving toward the midline in the left eye is fused with information of the target moving away from the midline in the right eye. This contrasts with dragonfly TSDNs, where receptive fields possess a sharp midline boundary, confining responses to a single visual hemifield in a sagittal frame of reference (i.e., relative to the midline). Our results indicate that, although TSDNs are conserved across Odonata, their neural inputs, and thus the upstream organization of the target tracking system, differ significantly and match divergence in eye design and predatory strategies. VIDEO ABSTRACT.

Corvids exhibit dynamic risk assessment during escape

It is widely accepted that stationary prey are able to carefully assess the risk levels associated with an approaching predator to make informative decisions on when to escape. However, little is known about subsequent decision-making process. We set out to compare whether escape durations of three species of corvids differ depending on how a human observer (in the role of a predator) behaves after the escape has begun. When birds were being followed during escape, escape durations were the longest, escape trajectory was modified the most during escape, and a larger proportion of individuals changed from terrestrial to aerial escape strategy compared to observations where birds were not followed. Mean horizontal escape angle of ca 120° was also a potential indication that monitoring the threat is taken into account when deciding on the escape trajectory. While there were some differences between the behaviour of these three closely related species, the general patterns supported the notion that birds dynamically assess risk during escape to find an optimal balance between getting caught and spending too much time and energy on escaping. Further research using different predator-prey combinations or making comparisons between habitats could help understand the generality of our results.

Foraging behaviour of endemic Dull-blue Flycatcher Eumyias sordidus Volden 1870, (Passeriformes: Muscicapidae) in Tropical montane cloud forests of Sri Lanka.

Foraging behavior of endemic Sri Lanka Dull-blue Flycatcher (Eumyias sordidus) was studied in the tropical montane cloud forests of Horton Plains National park (HPNP), situated in the highland plateau of the Nuwara Eliya District from July 2015 to July 2017. Three main natural habitats were identified as Cloud Forest (CF), Cloud Forest Die-back (CFD) and Grassland (GL) associated with HPNP. Focal sampling method was used to study the foraging behaviour of E. sordidus. Foraging individuals were observed directly or through a binocular from 0600h to 1800h, on three consecutive days per month. Line transects and opportunistic observations were used to obtain foraging data. Foraging plant types were identified by using field guides. Total of 1694 foraging attempts were recorded during the study period. Maximum number of foraging observations (60.14%) were recorded from CF and least number of foraging observations (5.69%) were recorded from GL Habitat. Observed foraging height was 4.18±2.77m (Mean±Standard Deviation) above ground and distance to the canopy above the bird was 3.01±1.76m (M±SD). They preferred the trees with 6.04±2.75m (M±SD) height, 0.73±0.41m (M±SD) average diameter-at-breast height and 1.71±0.71m (M±SD) average trunk height in the montane forest habitat. E.sordidus diet consisted of prey (97.03%) and fruits (2.97%). Prey were captured in Air (29.70%), ground (25.49%), leaves (15.59%), moss (13.37%), twigs (6.44%), trunks (4.21%) and flowers (2.23%), substrates. Rubus ellipticus and Rubus rugosus were the main feeding plants of E. sordidus. Aerial foraging manoeuvre was identified as sallying (hawking). Four types (outward-sally-gleaning, upward-sally-gleaning, outward-striking and upward striking) of sally-gleaning capture techniques were observed. Return-to-perch frequency was 26%. Ground foraging manoeuvres were identified as ground-sally-gleaning, stand-ground-gleaning and hopping-ground-gleaning. When the stationary prey was taken from the substrate while perching, simple-perch-gleaning and landing-and-gleaning capture techniques were observed. The birds perched and picked at fruits without sallying (76%) and occasionally some individuals ate only a portion of the fruit without removing it from the plant (24%). Finding of the present study indicated that E. sordidus utilized prey items along with fruits of two plant species belongs to family Rosaceae which is indigenous to the montane cloud forests. The physical techniques of different prey capture methods and standardized nomenclature for these techniques were suggested from this study. Keywords: Foraging behaviour, Endemic birds, Eumyias sordidus, Montane Cloud Forest

The effect of food properties on grasping and manipulation in the aquatic frog Xenopus laevis.

The ability to grasp an object is fundamental from an evolutionary perspective. Involved in many daily activities, grasping has been extensively studied in primates and other mammals. Yet other groups of tetrapods, including anurans, have also evolved significant forelimb prehensile capacities that are often thought to have originated in an arboreal context. In addition, grasping is also observed in aquatic species. But how aquatic frogs use their forelimbs to capture and manipulate prey remains largely unknown. The aim of this study is to explore how the grasping and manipulation of food items in aquatic frogs is impacted by food properties such as size and mobility. To do so, we uses the aquatic frog Xenopus laevis and quantified the use of the hands and fingers while processing mobile and stationary prey of different sizes (small, intermediate and large). Our results show that X. laevis is able to individualize the digits and that the mobility and the length of the prey significantly influence the kind of grasping pattern used. Grasping abilities are thus not specific to terrestrial or arboreal species. These results illustrate how prey properties impact grasping and manipulation strategies in an aquatic frog and shed further light on the ecological contexts that may have given rise to the origin of grasping in frogs.

Visual cues used in directing predatory strikes by the jumping spider Yllenus arenarius (Araneae, Salticidae)

Salticids are known for their complex predatory behaviour, which is based on the analysis of visual information from their prey, but the role of cues used in different predatory tasks is poorly known. We investigated which cues are used to identify the preferred target on the prey's body, examining the reactions of the euryphagous salticid Yllenus arenarius to various virtual prey presented on a miniature screen. We manipulated the number of head-indicating details (ranging from prey with four details, including a head spot, antennae, legs and wings, to prey lacking any details), the position of these details in relation to motion direction (in the leading versus in the trailing part of the body), the local motion of legs and the presence of horizontal motion. When all cues pointed to the same body end the spiders identified the preferred target almost unerringly regardless of the number of details. Movement alone, movement combined with a different number of details in the leading part, local motion of legs and head spot alone on motionless prey elicited the same reactions. When the cues provided contradictory information (motion direction and details pointing to opposite body ends) the spiders struck the trailing end more often the more details were placed there, and they visually inspected body ends of their prey before attack. These results indicate that the spiders used the direction of the prey's motion and the complexity of head-indicating details when making decisions related to strike targeting. These findings elucidate the role of motion and the complexity of details forming ‘false heads’, an antipredator adaptation assumed to redirect predatory strikes on prey from various animal groups. We demonstrate that in stationary prey even very simple patterns efficiently redirect predatory strikes. We provide the first experimental evidence of the effectiveness of ‘false head’ complexity in moving prey.

Sonar sound groups and increased terminal buzz duration reflect task complexity in hunting bats.

More difficult tasks are generally regarded as such because they demand greater attention. Echolocators provide rare insight into this relationship because biosonar signals can be monitored. Here we show that bats produce longer terminal buzzes and more sonar sound groups during their approach to prey under presumably more difficult conditions. Specifically, we found Daubenton’s bats, Myotis daubentonii, produced longer buzzes when aerial-hawking versus water-trawling prey, but that bats taking revolving air- and water-borne prey produced more sonar sound groups than did the bats when taking stationary prey. Buzz duration and sonar sound groups have been suggested to be independent means by which bats attend to would-be targets and other objects of interest. We suggest that for attacking bats both should be considered as indicators of task difficulty and that the buzz is, essentially, an extended sonar sound group.

Foraging mechanisms of siscowet lake trout (Salvelinus namaycush siscowet) on pelagic prey

Abstract The reaction distance, angle of attack, and foraging success were determined for siscowet lake trout ( Salvelinus namaycush siscowet ) during laboratory trials under lighting conditions that approximated downwelling spectral irradiance and intensity (9.00 × 10 8 –1.06 × 10 14 photons m − 2 s − 1 ) at daytime depths. Siscowet reaction distance in response to golden shiners ( Notemigonus crysoleucas ) was directly correlated with increasing light intensity until saturation at 1.86 × 10 11 photons m − 2 s − 1 , above which reaction distance was constant within the range of tested light intensities. At the lowest tested light intensity, sensory detection was sufficient to locate prey at 25 ± 2 cm, while increasing light intensities increased reaction distance up to 59 ± 2 cm at 1.06 × 10 14 photons m − 2 s − 1 . Larger prey elicited higher reaction distances than smaller prey at all light intensities while moving prey elicited higher reaction distances than stationary prey at the higher light intensities (6.00 × 10 9 to 1.06 × 10 14 photons m − 2 s − 1 ). The capture and consumption of prey similarly increased with increasing light intensity while time to capture decreased with increasing light intensity. The majority of orientations toward prey occurred within 120° of the longitudinal axis of the siscowet's eyes, although reaction distances among 30° increments along the entire axis were not significantly different. The developed predictive model will help determine reaction distances for siscowet in various photic environments and will help identify the mechanisms and behavior that allow for low light intensity foraging within freshwater systems.

When hawks attack: animal-borne video studies of goshawk pursuit and prey-evasion strategies.

Video filmed by a camera mounted on the head of a Northern Goshawk (Accipiter gentilis) was used to study how the raptor used visual guidance to pursue prey and land on perches. A combination of novel image analysis methods and numerical simulations of mathematical pursuit models was used to determine the goshawk's pursuit strategy. The goshawk flew to intercept targets by fixing the prey at a constant visual angle, using classical pursuit for stationary prey, lures or perches, and usually using constant absolute target direction (CATD) for moving prey. Visual fixation was better maintained along the horizontal than vertical direction. In some cases, we observed oscillations in the visual fix on the prey, suggesting that the goshawk used finite-feedback steering. Video filmed from the ground gave similar results. In most cases, it showed goshawks intercepting prey using a trajectory consistent with CATD, then turning rapidly to attack by classical pursuit; in a few cases, it showed them using curving non-CATD trajectories. Analysis of the prey's evasive tactics indicated that only sharp sideways turns caused the goshawk to lose visual fixation on the prey, supporting a sensory basis for the surprising frequency and effectiveness of this tactic found by previous studies. The dynamics of the prey's looming image also suggested that the goshawk used a tau-based interception strategy. We interpret these results in the context of a concise review of pursuit-evasion in biology, and conjecture that some prey deimatic 'startle' displays may exploit tau-based interception.

Sensory Biology: Listening in the Dark for Echoes from Silent and Stationary Prey

New research shows how bats use echolocation unexpectedly to detect silent and stationary prey in darkness. Bats may use acoustic search images to identify potential prey when prey-generated noises, visual and olfactory cues are absent.

The neurobiology and behavior of the American water shrew (Sorex palustris)

American water shrews (Sorex palustris) are aggressive predators that dive into streams and ponds to find prey at night. They do not use eyesight for capturing fish or for discriminating shapes. Instead they make use of vibrissae to detect and attack water movements generated by active prey and to detect the form of stationary prey. Tactile investigations are supplemented with underwater sniffing. This remarkable behavior consists of exhalation of air bubbles that spread onto objects and are then re-inhaled. Recordings for ultrasound both above and below water provide no evidence for echolocation or sonar, and presentation of electric fields and anatomical investigations provide no evidence for electroreception. Counts of myelinated fibers show by far the largest volume of sensory information comes from the trigeminal nerve compared to optic and cochlear nerves. This is in turn reflected in the organization of the water shrew's neocortex, which contains two large somatosensory areas and much smaller visual and auditory areas. The shrew's small brain with few cortical areas may allow exceptional speed in processing sensory information and producing motor output. Water shrews can accurately attack the source of a water disturbance in only 50ms, perhaps outpacing any other mammalian predator.

Echolocation intensity and directionality of perching and flying fringe-lipped bats, Trachops cirrhosus (Phyllostomidae)

The Neotropical frog-eating bat, Trachops cirrhosus, primarily hunts stationary prey, either by gleaning on the wing, or in a sit-and-wait mode hanging from a perch. It listens passively for prey-generated sounds, but uses echolocation in all stages of the hunt. Like other bats in the family Phyllostomidae, T. cirrhosus has a conspicuous nose leaf, hypothesized to direct and focus echolocation calls emitted from the nostrils. T. cirrhosus is highly flexible in its cognitive abilities and its use of sensory strategies for prey detection. Additionally, T. cirrhosus has been observed to echolocate both with closed and open mouth. We hypothesize that its flexibility extends to echolocation call design. We investigated the effect of hunting mode, perching or flying, as well as the effect of mouth opening, on the acoustic parameters and directionality of the echolocation call. We used a multi-microphone array, a high-speed video camera, and a microphone-diode-video system to directly visualize the echolocation sound beam synchronized with the bat's behavior. We found that T. cirrhosus emits a highly directional sound beam with half amplitude angle (HAM) of 12–18° and DI (directionality index) of ~17 dB, among the most directional bat sonar beams measured to date. The directionality was high both when flying and when perching. The emitted intensity was low, around 88 dB SPL at 10 cm from the mouth, when hanging, but higher, around 100 dB SPL at 10 cm, when flying or just before take-off. Our data suggests that the limited search volume of T. cirrhosus sonar beam defined by the high directionality and the rather low intensity of its echolocation calls is adapted to the highly cluttered hunting habitat and to the perch hunting mode.

Dolphin Bait-Balling Behaviors in Relation to Prey Ball Escape Behaviors

Delphinids frequently coordinate behaviors to contain or capture schooling fishes, but we know little about how these behaviors relate to prey escape behaviors, and corresponding costs and benefits. In this study, we described prey ball escape behaviors and investigated how prey ball behaviors related to dusky dolphin, Lagenorhynchus obscurus, prey herding and capturing behaviors using above-water and underwater video methods. Prey balls exhibited horizontal and vertical movements, both of which would have increased feeding costs for dolphins compared with feeding on stationary prey at the surface. The only prey ball behavior that we observed to precede escape was ‘funneling’ or the brief formation of a prey ball shape where the height was at least twice the width. Funneling was observed most often for large prey balls, immediately before they descended. When prey balls ascended, there was an insignificant trend for dolphins to do a greater proportion of herding passes that did not include a prey capture (p = 0.094), and dolphins did a larger proportion of prey-capture attempts on the sides of prey balls (p = 0.017) and a smaller proportion through the edges of prey balls (p = 0.037). When prey balls ascended, dolphins also did a smaller proportion of attempts with a dorsal body orientation (p = 0.037); there were insignificant trends for them to do a smaller proportion with a side orientation (p = 0.100) but a larger proportion with a ventral body orientation (p = 0.081). Thus, funneling is an effective behavior by which large prey balls are able to vertically escape, and dolphins may counter vertical prey escape behaviors by doing herding passes that do not include a prey-attempt, and changing location or body orientation of prey-capture behaviors.

The role of early sensory experience in the prey catching responses of Salamandra salamandra to stationary prey.

Two groups of newly metamorphosed Salamandra salamandra (L.) were raised with living and with stationary prey, respectively. One year later, the group which had experience exclusively with stationary prey was significantly better in responding to stationary prey than the group which only had experience with moving prey. This was the case both under light and dark conditions. The experiments show that prey-catching behavior in salamanders can be considerably modified by experience.

The trigger for barn owl (Tyto alba) attack is the onset of stopping or progressing of the prey

Aiming at the question of whether barn owls favor to strike a moving or a stationary prey, we scrutinized the timing of launching 50 attacks of five barn owls (10 attacks/owl) in a captive environment. Attacks on stationary voles outnumbered attacks on moving voles, but this could merely reflect the higher portion of time during which voles were stationary. Notably, the majority of attacks (85-100%) were launched in less than 2s (measured at accuracy of 0.04 s) after the voles ceased to progress or initiated progression. Therefore, the trigger for launching an attack is the transition of the prey from locomotion to still posture or vice versa.

Dusky Dolphins Influence Prey Accessibility for Seabirds in Admiralty Bay, New Zealand

Although seabirds frequently aggregate with feeding delphinids, the benefits to seabirds of feeding with dolphins have been rarely reported. We examined how dusky dolphins (Lagenorhynchus obscurus) influenced prey accessibility for seabirds in Admiralty Bay, New Zealand. Interactions of dusky dolphins and seabirds were characterized during 335 feeding bouts of dusky dolphins (52 video-recorded underwater). Dolphins increased prey accessibility for seabirds because they swam under the bottom half of prey balls for 59% of passes that were within 2 m of the prey ball. During feeding bouts by dolphins, 51% of prey balls ascended, whereas only 13% descended. Dolphins also influenced prey mobility; only 24% of stationary feeding bouts became mobile after dolphins began feeding, and 17% subsequently became stationary again. Significantly more Australasian gannets (Morus serrator) were near mobile than stationary prey balls after feeding, but not during feeding bouts. This suggests that feeding gannets increase mobility of prey balls, but that feeding dolphins counteract this effect. Seabirds also used dusky dolphins to locate prey. Numbers of gannets, shearwaters (Puffinus), and gulls (Larus) increased during the first 2 min of dolphin feeding, even when other seabirds were not present. Gannets fed with dolphins for 40% of gannet feeding observations and shearwaters fed with dolphins for 24% of shearwater feeding observations.