Assessment of rattlesnake dangerousness by California ground squirrels: exploitation of cues from rattling sounds

Sensory integration of danger and safety cues may explain the fear of a quiet coyote

Sensory integration theory predicts natural selection should favour adaptive responses of animals to multiple forms of information, yet empirical tests of this prediction are rare, particularly in free-living mammals. Studying indirect predator cues offers a salient opportunity to inquire about multimodal risk assessment and its potentially interactive effects on prey responses. Here we exposed California ground squirrels from two study sites (that differ in human and domestic dog activity) to acoustic and/or olfactory predator cues to reveal divergent patterns of signal dominance. Olfactory information most strongly predicted space use within the testing arena. That is, individuals, especially those at the human-impacted site, avoided coyote urine, a danger cue that may communicate the proximity of a coyote. By contrast, subjects allocated less time to risk-sensitive behaviours when exposed to acoustic cues. Specifically, although individuals were consistent in their behavioural responses across trials, 'quiet coyotes' (urine without calls) significantly increased the behavioural reactivity of prey, likely because coyotes rarely vocalize when hunting. More broadly, our findings highlight the need to consider the evolution of integrated fear responses and contribute to an emerging understanding of how animals integrate multiple forms of information to trade off between danger and safety cues in a changing world.

Scaling patterns of body plans differ among squirrel ecotypes

Body size is often hypothesized to facilitate or constrain morphological diversity in the cranial, appendicular, and axial skeletons. However, how overall body shape scales with body size (i.e., body shape allometry) and whether these scaling patterns differ between ecological groups remains poorly investigated. Here, we test whether and how the relationships between body shape, body size, and limb lengths differ among species with different locomotor specializations, and describe the underlying morphological components that contribute to body shape evolution among squirrel (Sciuridae) ecotypes. We quantified the body size and shape of 87 squirrel species from osteological specimens held at museum collections. Using phylogenetic comparative methods, we first found that body shape and its underlying morphological components scale allometrically with body size, but these allometric patterns differ among squirrel ecotypes: chipmunks and gliding squirrels exhibited more elongate bodies with increasing body sizes whereas ground squirrels exhibited more robust bodies with increasing body size. Second, we found that only ground squirrels exhibit a relationship between forelimb length and body shape, where more elongate species exhibit relatively shorter forelimbs. Third, we found that the relative length of the ribs and elongation or shortening of the thoracic region contributes the most to body shape evolution across squirrels. Overall, our work contributes to the growing understanding of mammalian body shape evolution and how it is influenced by body size and locomotor ecology, in this case from robust subterranean to gracile gliding squirrels.

Frequency modulation of rattlesnake acoustic display affects acoustic distance perception in humans

The estimation of one's distance to a potential threat is essential for any animal's survival. Rattlesnakes inform about their presence by generating acoustic broadband rattling sounds.¹ Rattlesnakes generate their acoustic signals by clashing a series of keratinous segments onto each other, which are located at the tip of their tails.^1-3 Each tail shake results in a broadband sound pulse that merges into a continuous acoustic signal with fast-repeating tail shakes. This acoustic display is readily recognized by other animals^4,5 and serves as an aposematic threat and warning display, likely to avoid being preyed upon.^1,6 The spectral properties of the rattling sound^1,3 and its dependence on the morphology and size of the rattle have been investigated for decades^7-9 and carry relevant information for different receivers, including ground squirrels that encounter rattlesnakes regularly.^10,11 Combining visual looming stimuli with acoustic measurements, we show that rattlesnakes increase their rattling rate (up to about 40 Hz) with decreasing distance of a potential threat, reminiscent of the acoustic signals of sensors while parking a car. Rattlesnakes then abruptly switch to a higher and less variable rate of 60-100 Hz. In a virtual reality experiment, we show that this behavior systematically affects distance judgments by humans: the abrupt switch in rattling rate generates a sudden, strong percept of decreased distance which, together with the low-frequency rattling, acts as a remarkable interspecies communication signal. VIDEO ABSTRACT.

Visual snake aversion in Octodon degus and C57BL/6 mice

Phobia against spiders or snakes is common in humans, and similar phobia-like behaviors have been observed in non-human animals. Visual images of snakes elicit phobia in humans, but sensory modalities that cause snake aversion in non-human animals are not well examined. In this study, we examined visually induced snake aversion in two rodent species. Using a three-compartment experimental chamber, reactions to images of snakes were compared between the diurnal precocious rodent Octodon degus and nocturnal laboratory mice. The snakes whose images were presented do not live in the original habitats of degus or mice. Snake aversion was assessed by presenting snake vs. no-image, snake vs. flower, snake vs. degu, and snake vs. mouse images. The time spent in a compartment with the snake image and with the non-snake images were measured. Degus avoided images of snakes in every tests. In contrast, mice did not display snake aversion. Degus are diurnal animals, i.e., visual information is important for their survival. Since mice are nocturnal, visual information is less important for survival. Such behavioral differences in the two species may explain the difference in visually induced aversion to snakes. A principal component analysis of the stimulus images suggests that elementary cues, such as color, do not explain the differences in the species' aversion to snakes. Finally, snake aversion in degus suggests that aversion is innate, since the animals were born and raised in a laboratory.

Does Rattling Deter? The Case of Domestic Dogs

Although the rattling of rattlesnakes (Crotalus and Sistrurus) is widely accepted as being aposematic, the hypothesis that rattling deters approach from the snake's potentially dangerous adversaries has not been well tested. In a controlled study using rattling recorded from captive rattlesnakes (C. oreganus helleri) and a variety of comparison sounds or no-sound controls, domestic dogs (Canis familiaris) showed no hesitation to approach camouflaged speakers projecting the recorded rattles. The dogs were equally likely to approach speakers projecting rattling as they were to approach speakers playing control sounds, or speakers that were silent. Furthermore, the dogs spent no less time in front of the speakers projecting the rattles than they did in front of speakers projecting control sounds or no sound. The dogs' reactions may not be representative of other species with whom rattlesnakes come into contact, but the data suggest a need for some circumspection about the role of rattling in the rattlesnake's defensive repertoire. Our results also suggest that dogs may be vulnerable to envenomation because they fail to react to the sound of rattling with avoidance.

Physiological Stress Integrates Resistance to Rattlesnake Venom and the Onset of Risky Foraging in California Ground Squirrels

Using venom for predation often leads to the evolution of resistance in prey. Understanding individual variation in venom resistance is key to unlocking basic mechanisms by which antagonistic coevolution can sustain variation in traits under selection. For prey, the opposing challenges of predator avoidance and resource acquisition often lead to correlated levels of risk and reward, which in turn can favor suites of integrated morphological, physiological and behavioral traits. We investigate the relationship between risk-sensitive behaviors, physiological resistance to rattlesnake venom, and stress in a population of California ground squirrels. For the same individuals, we quantified foraging decisions in the presence of snake predators, fecal corticosterone metabolites (a measure of “stress”), and blood serum inhibition of venom enzymatic activity (a measure of venom resistance). Individual responses to snakes were repeatable for three measures of risk-sensitive behavior, indicating that some individuals were consistently risk-averse whereas others were risk tolerant. Venom resistance was lower in squirrels with higher glucocorticoid levels and poorer body condition. Whereas resistance failed to predict proximity to and interactions with snake predators, individuals with higher glucocorticoid levels and in lower body condition waited the longest to feed when near a snake. We compared alternative structural equation models to evaluate alternative hypotheses for the relationships among stress, venom resistance, and behavior. We found support for stress as a shared physiological correlate that independently lowers venom resistance and leads to squirrels that wait longer to feed in the presence of a snake, whereas we did not find evidence that resistance directly facilitates latency to forage. Our findings suggest that stress may help less-resistant squirrels avoid a deadly snakebite, but also reduces feeding opportunities. The combined lethal and non-lethal effects of stressors in predator–prey interactions simultaneously impact multiple key traits in this system, making environmental stress a potential contributor to geographic variation in trait expression of toxic predators and resistant prey.

Olfactory predator recognition in the brown mouse lemur (Microcebus rufus) in Ranomafana National Park, Madagascar

Predator odors such as urine and feces are known to elicit antipredator behaviors in prey including avoidance, fear, and curiosity. We measured how wild brown mouse lemurs (Microcebus rufus) responded to odors of mammalian, avian, and snake predators as well as nonpredator controls. The first experiment took place under controlled conditions in a laboratory where we recorded the occurrence of four behavioral categories (ignore, curiosity, alert, and fear) in response to a single odor. Subjects exhibited behavioral change significantly more often in response to the predator than to control stimuli, but did not distinguish between familiar and unfamiliar predators. Mammalian predator urine and feces were most likely to elicit behavioral change. The owl was the only predator to never elicit behavioral change, possibly because owls do not provide relevant odor cues. A second experiment employing live traps in the forest found that neither predator nor control odors affected the likelihood of capture. Due to their longevity, odors do not provide accurate information of spatial and temporal risk, and while mouse lemurs may have initially hesitated to enter a trap, in the absence of additional information about risk, they may have eventually ignored the stimuli. This study found that brown mouse lemurs are able to distinguish between predator and nonpredator odors, and that risk assessment may be affected by the experience, as well as predator and sensory stimulus quality.

The effects of temperature on the defensive strikes of rattlesnakes

Movements of ectotherms are constrained by their body temperature owing to the effects of temperature on muscle physiology. As physical performance often affects the outcome of predator-prey interactions, environmental temperature can influence the ability of ectotherms to capture prey and/or defend themselves against predators. However, previous research on the kinematics of ectotherms suggests that some species may use elastic storage mechanisms when attacking or defending, thereby mitigating the effects of sub-optimal temperature. Rattlesnakes (Crotalus spp.) are a speciose group of ectothermic viperid snakes that rely on crypsis, rattling and striking to deter predators. We examined the influence of body temperature on the behavior and kinematics of two rattlesnake species (Crotalus oreganus helleri and Crotalus scutulatus) when defensively striking towards a threatening stimulus. We recorded defensive strikes at body temperatures ranging from 15-35°C. We found that strike speed and speed of mouth gaping during the strike were positively correlated with temperature. We also found a marginal effect of temperature on the probability of striking, latency to strike and strike outcome. Overall, warmer snakes are more likely to strike, strike faster, open their mouth faster and reach maximum gape earlier than colder snakes. However, the effects of temperature were less than would be expected for purely muscle-driven movements. Our results suggest that, although rattlesnakes are at a greater risk of predation at colder body temperatures, their decrease in strike performance may be mitigated to some extent by employing mechanisms in addition to skeletal muscle contraction (e.g. elastic energy storage) to power strikes.

Selective reactions to different killer whale call categories in two delphinid species

The risk of predation is often invoked as an important factor influencing the evolution of social organization in cetaceans, but little direct information is available about how these aquatic mammals respond to predators or other perceived threats. We used controlled playback experiments to examine the behavioral responses of short-finned pilot whales (Globicephala macrorhynchus) off Cape Hatteras, NC, USA, and Risso's dolphins (Grampus griseus) off the coast of Southern California, USA, to the calls of a potential predator, mammal-eating killer whales. We transmitted calls of mammal-eating killer whales, conspecifics and baleen whales to 10 pilot whales and four Risso's dolphins equipped with multi-sensor archival acoustic recording tags (DTAGs). Only playbacks of killer whale calls resulted in significant changes in tagged animal heading. The strong responses observed in both species occurred only following exposure to a subset of killer whale calls, all of which contained multiple non-linear properties. This finding suggests that these structural features of killer whale calls convey information about predatory risk to pilot whales and Risso's dolphins. The observed responses differed between the two species; pilot whales approached the sound source while Risso's dolphins fled following playbacks. These divergent responses likely reflect differences in anti-predator response mediated by the social structure of the two species.

Direct look from a predator shortens the risk-assessment time by prey

Decision making process is an important component of information use by animals and has already been studied in natural situations. Decision making takes time, which is expressed as a cost in evolutionary explanations of decision making abilities of animals. However, the duration of information assessment and decision making process has not been measured in a natural situation. Here, we use responses of wild magpies (Pica pica) to predictably approaching humans to demonstrate that, regardless of whether the bird perceived high (decided to fly away) or low (resumed foraging) threat level, the bird assessed the situation faster when approaching humans looked directly at it than when the humans were not directly looking at it. This indicates that prey is able to extract more information about the predator's intentions and to respond sooner when the predator is continuously ("intently") looking at the prey. The results generally illustrate how an increase of information available to an individual leads to a shorter assessment and decision making process, confirming one of central tenets of psychology of information use in a wild bird species in its natural habitat.

The conservation-welfare nexus in reintroduction programmes: a role for sensory ecology

Since reintroduction programmes involve moving animals from captive or wild environments and releasing them into novel environments, there are sure to be a number of challenges to the welfare of the individuals involved. Behavioural theory can help us develop reintroductions that are better for both the welfare of the individual and the conservation of populations. In addition to modifying captive environments to prepare animals for release to the wild, it is possible to modify the animals’ experience in the post-release environment. For releases to be more successful, they need to better accommodate the ecological and psychological needs of individuals. A better understanding of sensory ecology — how animals acquire and respond to information in their environment — is needed to develop new, more successful management strategies for reintroductions. Sensory ecology integrates ecological and psychological processes, calling for better synergy among researchers with divergent backgrounds in conservation and animal welfare science. This integrative approach leads to new topics of investigation in reintroduction biology, including more careful consideration of post-release stress and the role of social support. Reintroductions are essentially exercises in ‘forced’ dispersal; thus, an especially promising avenue of research is the role of proximate mechanisms governing dispersal and habitat selection decisions. Reintroduction biologists have much to gain from the study of mechanism because mechanisms, unlike function or adaptive value, can be manipulated to enhance conservation and welfare goals.

Tufted capuchin monkeys (Cebus apella) spontaneously use visual but not acoustic information to find hidden food items

Foraging choices in tufted capuchins monkeys are guided by perceptual, cognitive, and motivational factors, but little is known about how these factors might interact. The present study investigates how different types of sensory information affect capuchins' ability to locate hidden food. In two experiments, capuchins were presented with two cups, one baited and one empty. Monkeys were given visual, acoustic, or acoustic-visual information related to the baited cup, the empty cup, or both baited and empty cup. Results show that capuchins spontaneously used visual information to locate food, and that information indicating presence and absence of food led to higher success rates than information indicating only absence of food. In contrast, acoustic information did not lead to success rates above chance levels and failed to enhance performance in combination with visual information. Capuchins spontaneously avoided a visually empty cup, but they did not appear to associate sounds with either the presence or absence of food. Being able to locate food items with the aid of acoustic cues might be a learned process that requires interactive experiences with the task's contingencies.

Frequency information in the vibration-cued escape hatching of red-eyed treefrogs

Incidental acoustic and vibrational cues generated by predators are a potential source of information for prey assessing risk. Substrate vibrations should be excited by most predators, and frequency, amplitude or temporal properties could allow prey to distinguish predator from benign-source vibrations. Red-eyed treefrog embryos detect egg predators using vibrations excited during attacks, hatching rapidly and prematurely to escape. We recorded vibrations in egg clutches during attacks by five species of predators and three common types of benign physical disturbance. We analyzed their frequency distributions to assess if and how frequency properties could be used to discriminate between vibration sources and used vibration playbacks to examine the effects of frequency properties on the escape hatching response. Vibrations produced by predators and benign disturbances generally have broad and overlapping frequency distributions, and all frequencies excited by attacks are also excited by benign disturbances. Decision rules based on the frequency distribution of vibrations alone would therefore result in either high levels of hatching in response to benign vibrations (false alarms) or common failures to hatch in response to predators (missed cues). Nevertheless, embryos hatch in response to predator and not benign disturbances in nature, and our playback results show that vibration frequency information is an important component of their hatching decision. Embryos combine frequency with temporal information to refine their hatching response. Moreover, comparing frequency spectra of predator and benign vibrations suggests that the presence of energy in frequencies outside the range characteristic of attacks might serve as an indicator of benign disturbance.

Innate defensive behaviour and panic-like reactions evoked by rodents during aggressive encounters with Brazilian constrictor snakes in a complex labyrinth: behavioural validation of a new model to study affective and agonistic reactions in a prey versus predator paradigm

Defensive behaviour has been extensively studied, and non-invasive methodologies may be interesting approaches to analyzing the limbic system function as a whole. Using experimental models of animals in the state of anxiety has been fundamental in the search for new anxiolytic and antipanic compounds. The aim of this present work is to examine a new model for the study of affective behaviour, using a complex labyrinth consisting of an arena and galleries forming a maze. Furthermore, it aims to compare the defensive behaviour of Wistar rats, Mongolian gerbils and golden hamsters in a complex labyrinth, as well as the defensive behaviour of Meriones unguiculatus in aggressive encounters with either Epicrates cenchria assisi or Boa constrictor amarali in this same model. Among species presently studied, the Mongolian gerbils showed better performance in the exploration of both arena and galleries of the labyrinth, also demonstrating less latency in finding exits of the galleries. This increases the possibility of survival, as well as optimizes the events of encounter with the predator. The duration of alertness and freezing increased during confrontation with living Epicrates, as well as the duration of exploratory behaviour in the labyrinth. There was an increase in the number of freezing and alertness behaviours, as well as in duration of alertness during confrontations involving E.c. assisi, compared with behavioural reactions elicited by jirds in presence of B.c. amarali. Interestingly, the aggressive behaviour of Mongolian gerbils was more prominent against B.c. amarali compared with the other Boidae snake. E.c. assisi elicited more offensive attacks and exhibited a greater time period of body movement than B.c. amarali, which spent more time in the arena and in defensive immobility than the E.c. assisi. Considering that jirds evoked more fear-like reaction in contact with E.c. assisi, a fixed E.c. assisi kept in a hermetically closed acrylic box was used as control. In these prey/predator encounter-based experiments, there was an increase in the number of alertness and freezing behaviours exhibited by gerbils, and a decrease in the number of crossing elicited by them, when comparing confrontations between the living E.c. assisi and the control. The experiments were performed at 7.0 p.m. In the labyrinth, the snakes showed in confrontation similar performance to that observed in nature (organizing hunting behaviour, offensive/defensive attack, constriction, prey inspection and feeding behaviour), which were essential to the validity of the experiments and gave behavioural validation within the complex labyrinth.

California Ground Squirrel (Spermophilus beecheyi) Defenses Against Rattlesnake Venom Digestive and Hemostatic Toxins

Previous studies have shown that some mammals are able to neutralize venom from snake predators. California ground squirrels (Spermophilus beecheyi) show variation among populations in their ability to bind venom and minimize damage from northern Pacific rattlesnakes (Crotalus oreganus), but the venom toxins targeted by resistance have not been investigated. Four California ground squirrel populations, selected for differences in local density or type of rattlesnake predators, were assayed for their ability to neutralize digestive and hemostatic effects of venom from three rattlesnake species. In Douglas ground squirrels (S. b. douglasii), we found that animals from a location where snakes are common showed greater inhibition of venom metalloprotease and hemolytic activity than animals from a location where snakes are rare. Effects on general proteolysis were not different. Douglas ground squirrels also reduced the metalloprotease activity of venom from sympatric northern Pacific rattlesnakes (C. o. oreganus) more than the activity of venom from allopatric western diamondback rattlesnakes (C. atrox), but enhanced the fibrinolysis of sympatric venom almost 1.8 times above baseline levels. Two Beechey ground squirrel (S. b. beecheyi) populations had similar inhibition of venoms from northern and southern Pacific rattlesnakes (C. o. helleri), despite differences between the populations in the locally prevalent predator. However, the venom toxins inhibited by Beechey squirrels varied among venom from Pacific rattlesnake subspecies, and between these venoms and venom from allopatric western diamondback rattlesnakes. Blood plasma from Beechey squirrels showed highest inhibition of metalloprotease activity of northern Pacific rattlesnake venom, general proteolytic activity and hemolysis of southern Pacific rattlesnake venom, and hemolysis by allopatric western diamondback venom. These results reveal previously cryptic variation in venom activity against resistant prey that suggests reciprocal adaptation at the molecular level.

Effects of group size on the threat-sensitive response to varying concentrations of chemical alarm cues by juvenile convict cichlids

The threat-sensitive predator avoidance model predicts that prey should balance the intensity of antipredator responses against perceived predation risk, resulting in a graded response pattern. Recent studies have demonstrated considerable interspecific variation in the intensity of threat-sensitive response patterns, ranging from strongly graded to relatively nongraded or "hypersensitive" threat-sensitive response patterns. Here, we test for intraspecific plasticity in threat-sensitive responses by varying group size. We exposed juvenile convict cichlids, Archocentrus nigrofasciatus (Günther, 1867), as individuals or in small (groups of three) or large (groups of six) shoals to a series of dilutions of conspecific chemical alarm cues and a distilled water control. Singleton cichlids exhibited significant reductions in time spent moving and in frequency of foraging attempts (relative to distilled water controls) when exposed to a 12.5% dilution of conspecific alarm cue, with no difference in response intensity at higher stimulus concentrations, suggesting a nongraded (hypersensitive) response pattern. Small shoals exhibited a similar response pattern, but at a higher response threshold (25% dilution of stock alarm cue solution). Large shoals, however, exhibited a graded response pattern. These results suggest that group size influences the trade-off between predator avoidance and other fitness related activities, resulting in flexible threat-sensitive response patterns.

The Ontogeny of Contractile Performance and Metabolic Capacity in a High‐Frequency Muscle

High-performance muscles such as the shaker muscles in the tails of western diamond-backed rattlesnakes (Crotalus atrox) are excellent systems for studying the relationship between contractile performance and metabolic capacity. We observed that shaker muscle contraction frequency increases dramatically with growth in small individuals but then declines gradually in large individuals. We tested whether metabolic capacity changed with performance, using shaker muscle contraction frequency as an indicator of performance and maximal activities of citrate synthase and lactate dehydrogenase as indicators of aerobic and anaerobic capacities, respectively. Contraction frequency increased 20-fold in 20-100-g individuals but then declined by approximately 30% in individuals approaching 1,000 g. Mass-independent aerobic capacity was positively correlated with contractile performance, whereas mass-independent anaerobic capacity was slightly but negatively correlated with performance; body mass was not correlated with performance. Rattle mass increased faster than the ability to generate force. Early in ontogeny, shaker muscle performance appears to be limited by aerobic capacity, but later performance becomes limited equally by aerobic capacity and the mechanical constraint of moving a larger mass without proportionally thicker muscles. This high-performance muscle appears to shift during ontogeny from a metabolic constraint to combined metabolic and mechanical constraints.

California ground squirrel (Spermophilus beecheyi) defenses against rattlesnake venom digestive and hemostatic toxins

Previous studies have shown that some mammals are able to neutralize venom from snake predators. California ground squirrels (Spermophilus beecheyi) show variation among populations in their ability to bind venom and minimize damage from northern Pacific rattlesnakes (Crotalus oreganus), but the venom toxins targeted by resistance have not been investigated. Four California ground squirrel populations, selected for differences in local density or type of rattlesnake predators, were assayed for their ability to neutralize digestive and hemostatic effects of venom from three rattlesnake species. In Douglas ground squirrels (S. b. douglasii), we found that animals from a location where snakes are common showed greater inhibition of venom metalloprotease and hemolytic activity than animals from a location where snakes are rare. Effects on general proteolysis were not different. Douglas ground squirrels also reduced the metalloprotease activity of venom from sympatric northern Pacific rattlesnakes (Crotalus oreganus oreganus) more than the activity of venom from allopatric western diamondback rattlesnakes (C. atrox), but enhanced fibrinolysis of sympatric venom almost 1.8 times above baseline levels. Two Beechey ground squirrel (S. b. beecheyi) populations had similar inhibition of venoms from northern and southern Pacific rattlesnakes (C. o. helleri), despite differences between the populations in the locally prevalent predator. However, the venom toxins inhibited by Beechey squirrels did vary among venom from Pacific rattlesnake subspecies, and between these venoms and venom from allopatric western diamondback rattlesnakes. Blood plasma from Beechey squirrels showed highest inhibition of metalloprotease activity of northern Pacific rattlesnake venom, general proteolytic activity and hemolysis of southern Pacific rattlesnake venom, and hemolysis by allopatric western diamondback venom. These results reveal previously cryptic variation in venom activity against resistant prey that suggests reciprocal adaptation at the molecular level.

Snake bioacoustics: toward a richer understanding of the behavioral ecology of snakes

Snakes are frequently described in both popular and technical literature as either deaf or able to perceive only groundborne vibrations. Physiological studies have shown that snakes are actually most sensitive to airborne vibrations. Snakes are able to detect both airborne and groundborne vibrations using their body surface (termed somatic hearing) as well as from their inner ears. The central auditory pathways for these two modes of "hearing" remain unknown. Recent experimental evidence has shown that snakes can respond behaviorally to both airborne and groundborne vibrations. The ability of snakes to contextualize the sounds and respond with consistent predatory or defensive behaviors suggests that auditory stimuli may play a larger role in the behavioral ecology of snakes than was previously realized. Snakes produce sounds in a variety of ways, and there appear to be multiple acoustic Batesian mimicry complexes among snakes. Analyses of the proclivity for sound production and the acoustics of the sounds produced within a habitat or phylogeny specific context may provide insights into the behavioral ecology of snakes. The relatively low information content in the sounds produced by snakes suggests that these sounds are not suitable for intraspecific communication. Nevertheless, given the diversity of habitats in which snakes are found, and their dual auditory pathways, some form of intraspecific acoustic communication may exist in some species.

Defensive burying in rodents: ethology, neurobiology and psychopharmacology

Defensive burying refers to the typical rodent behavior of displacing bedding material with vigorous treading-like movements of their forepaws and shoveling movements of their heads directed towards a variety of noxious stimuli that pose a near and immediate threat, such as a wall-mounted electrified shock-prod. Since its introduction 25 years ago by Pinel and Treit [J. Comp. Physiol. Psychol. 92 (1978) 708], defensive (shock-prod) burying has been the focus of a considerable amount of research effort delineating the methodology/ethology, psychopharmacology and neurobiology of this robust and species-specific active avoidance or coping response. The present review gives a summary of this research with special reference to the behavioral (face and construct) and pharmacological (predictive) validity of the shock-prod burying test as an animal model for human anxiety. Emphasis is also placed on some recent modifications of the paradigm that may increase its utility and reliability as to individual differences in expressed emotional coping responses and sensitivity to pharmacological treatments. Overall, the behavioral and physiological responses displayed in the shock-prod paradigm are expressions of normal and functionally adaptive coping patterns and the extremes of either active (i.e., burying) or passive (i.e., freezing) forms of responding in this test cannot simply be regarded as inappropriate, maladaptive or pathological. For this reason, the shock-prod paradigm is not an animal model for anxiety disorder or for any other psychiatric disease, but instead possesses a high degree of face and construct validity for normal and functionally adaptive human fear and anxious apprehension. However, the apparent good pharmacological validation (predictive validity) of this test reinforces the view that normal and pathological anxiety involves, at least partly, common neurobiological substrates. Therefore, this paradigm is not only suitable for screening potential anxiolytic properties of new drugs, but seems to be especially valuable for unraveling the neural circuitry and neurochemical mechanisms underlying the generation of active and passive coping responses as different expressions of anxiety.

The rattling sound of rattlesnakes (Crotalus viridis) as a communicative resource for ground squirrels (Spermophilus beecheyi) and burrowing owls (Athene cunicularia)

Animal communication involves very dynamic processes that can generate new uses and functions for established communicative activities. In this article, the authors describe how an aposematic signal, the rattling sound of rattlesnakes (Crotalus viridis), has been exploited by 2 ecological associates of rattlesnakes: (a) California ground squirrels (Spermophilus beecheyi) use incidental acoustic cues in rattling sounds to assess the danger posed by the rattling snake, and (b) burrowing owls (Athene cunicularia) defend themselves against mammalian predators by mimicking the sound of rattling. The remarkable similarity between the burrowing owl's defensive hiss and the rattlesnake's rattling reflects both exaptation and adaptation. Such exploitation of the rattling sound has favored alternations in both the structure and the deployment of rattling by rattlesnakes.