Traffic Noise Impacts Glucocorticoid Response, Activity, and Growth in Two Species of Tadpoles

There is a large body of evidence linking increased noise to negative health effects for animals. Anthropogenic noise induces behavioral and physiological reactions across a range of taxa and increased traffic noise affects glucocorticoid (GC) hormones associated with the stress response in amphibians. GCs help to maintain homeostasis while balancing energetic trade-offs between reproduction, growth, and activity. Stressors during early development can impact fitness at later life stages. We measured growth, activity, and GCs in response to high levels of traffic noise in two tadpole species that differ in life history: Acris crepitans and Rana berlandieri. We predicted that earlier exposures to traffic noise will slow down the development and alter the behavior and GC concentrations differently than later exposures. Subjects were initially either exposed to natural levels of traffic noise for 8 days (early exposure) or a white noise control (later exposure), then the treatment was switched. Activity was measured via focal sampling and tadpoles were categorized as active if movement was detected. Tadpoles exposed to white noise initially maintained mass and activity throughout the experiment and early exposure to traffic noise had a greater impact on mass, activity, and GCs. Tadpoles exposed to traffic noise initially lost mass, with A. crepitans regaining mass but not R. berlandieri. When exposed earlier to traffic noise, R. berlandieri increased movement when shifted to the white noise treatment while A. crepitans did not significantly change activity. Acris creptians had higher corticosterone release rates compared to R. berlandieri, and in both species, release rates were higher for tadpoles exposed to noise earlier. The longer-lived R. berlandieri allocated more of their energetic resources into activity, while the shorter-lived A. crepitans allocated energy toward growth. Rana berlandieri and A. crepitans utilized different coping strategies to contend with early exposure to traffic noise, potentially due to differences in life histories. Our findings suggest that these tadpoles employ different coping mechanisms to modulate stress responses in noise-polluted environments, and these mechanisms could influence their fitness later in life. Further study is needed to understand the impact in more sensitive tadpole species.

The ability of damselfish to distinguish between dangerous and harmless sea snakes

In defence of their nests or territories, damselfish (Pomacentridae) attack even large and potentially dangerous intruders. The Indo-Pacific region contains many species of sea snakes, some of which eat damselfish whereas others do not. Can the fishes identify which sea snake taxa pose a threat? We recorded responses of damselfishes to natural encounters with five species of snakes in two shallow bays near Noumea, New Caledonia. Attacks by fishes were performed mostly by demersal territorial species of damselfish, and were non-random with respect to the species, size, sex and colouration of the snakes involved. The most common target of attack was Emydocephalus annulatus, a specialist egg-eater that poses no danger to adult fishes. Individuals of a generalist predator (Aipysurus duboisii) that were melanic (and thus resembled E. annulatus in colour) attracted more attacks than did paler individuals. Larger faster-swimming snake species (Aipysurus laevis, Laticauda saintgironsi) were watched but not attacked, or were actively avoided (Hydrophis major), even though only one of these species (A. laevis) eats pomacentrids. Attacks were more common towards female snakes rather than males, likely reflecting slower swimming speeds in females. In summary, damselfishes distinguish between sea snake species using cues such as size, colour and behaviour, but the fishes sometimes make mistakes.

The Effects of Conspecific Alarm Cues on Larval Cane Toads (Rhinella marina)

Many aquatic organisms detect and avoid damage-released cues from conspecifics, but the chemical basis of such responses, and the effects of prolonged exposure to such cues, remain poorly understood. Injured tadpoles of the cane toad (Rhinella marina) produce chemical cues that induce avoidance by conspecific tadpoles; and chronic exposure to those cues decreases rates of tadpole survival and growth, and reduces body size at metamorphosis. Such effects suggest that we might be able to use the cane toads' alarm cue for biocontrol of invasive populations in Australia. In the present study, we examined behavioral and ecological effects of compounds that are present in cane toad tadpoles and thus, might trigger avoidance of crushed conspecifics. Four chemicals (L-Arg, L-Leu-L-Leu-OH, L-Leu-L-Ile-OH and suberic acid) induced behavioral avoidance in toad tadpoles at some (but not all) dosage levels, so we then exposed toad larvae to these chemicals over the entire period of larval development. Larval survival and size at metamorphosis were decreased by chronic exposure to crushed conspecifics (consistent with earlier studies), but not by exposure to any of the four chemicals. Indeed, L-Arg increased body size at metamorphosis. We conclude that the behavioral response to crushed conspecifics by cane toad tadpoles can be elicited by a variety of chemical cues, but that consistent exposure to these individual chemical cues does not affect tadpole viability or developmental trajectory. The optimal behavioral tactic of a tadpole may be to flee if it encounters even a single chemical cue likely to have come from an injured conspecific (indicative of predation risk), whereas the continuing presence of that single chemical (but no others) provides a less reliable signal of predation risk. Our data are consistent with results from studies on fish, that suggest a role for multiple chemicals in initiating alarm responses to damage-released cues.

Training for Translocation: Predator Conditioning Induces Behavioral Plasticity and Physiological Changes in Captive Eastern Hellbenders (Cryptobranchus alleganiensis alleganiensis) (Cryptobranchidae, Amphibia)

Translocations are stressful, especially when captive animals are naïve to natural stimuli. Captive eastern hellbenders (Cryptobranchus alleganiensis alleganiensis) identify predatory fish as threats, but may be more vulnerable to predation and stress because of inexperience with them. We investigated the use of predator conditioning to prepare hellbenders, behaviorally and physiologically, for the presence of a common predator, largemouth bass (Micropterus salmoides). We reared hellbenders for 30 d with and without continuous exposure to largemouth bass kairomones and heterospecific alarm cues and found conditioned hellbenders became less active compared to unconditioned individuals (p = 0.017). After conditioning, we exposed hellbenders to water, a low concentration of kairomones, or a high concentration of kairomones in a closed respirometer system. We measured activity within respirometer chambers and routine metabolic rate. We found unconditioned hellbenders exposed to low and high concentrations of kairomones were 41% and 119% more active than conditioned animals (p = 0.002 and p < 0.001). Moreover, conditioned individuals had on average 6.5% lower metabolic rates across all three kairomone concentrations compared to unconditioned individuals (p = 0.017). Our data suggest that predator conditioning induces behavioral avoidance tactics and physiological changes that could improve future translocation efforts for hellbenders and other imperiled species.

Predator recognition of chemical cues in crayfish: diet and experience influence the ability to detect predation threats

Aquatic prey often alter their morphology, physiology, and/or behaviour when presented with predatory chemical cues which are heavily influenced by the diet of the predator. We tested the roles that diet and prey familiarity with predators play in the ability of prey to recognize predator threats. Odours from two fish, bass and cichlid fed a vegetarian, protein, heterospecific, and a conspecific diet, were collected and presented to virile crayfish in a choice arena. Our results show that crayfish altered their behaviour in the presence of odours containing conspecific, as opposed to heterospecific diets, but only from familiar predators. A reduced anti-predator response was measured with odours from an unfamiliar predator fed conspecific crayfish. Therefore, crayfish may be able to determine different threat levels based on the different dietary cues from a potential predator, but only when the prey have familiarity with the predators.

Investment into defensive traits by anuran prey (Lithobates pipiens) is mediated by the starvation-predation risk trade-off

Prey can invest in a variety of defensive traits when balancing risk of predation against that of starvation. What remains unknown is the relative costs of different defensive traits and how prey reconcile investment into these traits when energetically limited. We tested the simple allocation model of prey defense, which predicts an additive effect of increasing predation risk and resource availability, resulting in the full deployment of defensive traits under conditions of high risk and resource saturation. We collected morphometric, developmental, and behavioural data in an experiment using dragonfly larvae (predator) and Northern leopard frog tadpoles (prey) subject to variable levels of food availability and predation risk. Larvae exposed to food restriction showed limited response to predation risk; larvae at food saturation altered behaviour, development, and growth in response to predation risk. Responses to risk varied through time, suggesting ontogeny may affect the deployment of particular defensive traits. The observed negative correlation between body size and activity level for food-restricted prey--and the absence of a similar response among adequately-fed prey--suggests that a trade-off exists between behavioural and growth responses when energy budgets are limited. Our research is the first to demonstrate how investment into these defensive traits is mediated along gradients of both predation risk and resource availability over time. The interactions we demonstrate between resource availability and risk level on deployment of inducible defenses provide evidence that both internal condition and extrinsic risk factors play a critical role in the production of inducible defenses over time.

Smelling danger - alarm cue responses in the polychaete Nereis (Hediste) diversicolor (Müller, 1776) to potential fish predation

The harbour ragworm, Nereis (Hediste) diversicolor is a common intertidal marine polychaete that lives in burrows from which it has to partially emerge in order to forage. In doing so, it is exposed to a variety of predators. One way in which predation risk can be minimised is through chemical detection from within the relative safety of the burrows. Using CCTV and motion capture software, we show that H. diversicolor is able to detect chemical cues associated with the presence of juvenile flounder (Platichthys flesus). Number of emergences, emergence duration and distance from burrow entrance are all significantly reduced during exposure to flounder conditioned seawater and flounder mucous spiked seawater above a threshold with no evidence of behavioural habituation. Mucous from bottom-dwelling juvenile plaice (Pleuronectes platessa) and pelagic adult herring (Clupea harengus) elicit similar responses, suggesting that the behavioural reactions are species independent. The data implies that H. diversicolor must have well developed chemosensory mechanisms for predator detection and is consequently able to effectively minimize risk.

Chemical ecology of predator–prey interactions in aquatic ecosystems: a review and prospectusThe present review is one in the special series of reviews on animal–plant interactions

The interaction between predator and prey is an evolutionary arms race, for which early detection by either party is often the key to success. In aquatic ecosystems, olfaction is an essential source of information for many prey and predators and a number of cues have been shown to play a key role in trait-mediated indirect interactions in aquatic communities. Here, we review the nature and role of predator kairomones, chemical alarm cues, disturbance cues, and diet cues on the behaviour, morphology, life history, and survival of aquatic prey, focusing primarily on the discoveries from the last decade. Many advances in the field have been accomplished: testing the survival value of those chemicals, providing field validation of laboratory results, understanding the extent to which chemically mediated learning may benefit the prey, understanding the role of these chemicals in mediating morphological and life-history adaptations, and most importantly, the selection pressures leading to the evolution of chemical alarm cues. Although considerable advances have been made, several key questions remain, the most urgent of which is to understand the chemistry behind these interactions.