Learning to distinguish between predators and non-predators: understanding the critical role of diet cues and predator odours in generalisation

Embryonic exposure to artificial light at night impairs learning abilities and their covariance with behavioural traits in teleost fish

The natural light cycle has profound effects on animals' cognitive systems. Its alteration owing to human activities, such as artificial light at night (ALAN), affects the biodiversity of mammalian and avian species by impairing their cognitive functions. The impact of ALAN on cognition, however, has not been investigated in aquatic species, in spite of the common occurrence of this pollution along water bodies. We exposed eggs of a teleost fish (the zebrafish Danio rerio) to ALAN and, upon hatching, we measured larvae' cognitive abilities with a habituation learning paradigm. Both control and ALAN-exposed larvae showed habituation learning, but the latter learned significantly slower, suggesting that under ALAN conditions, fish require many more events to acquire ecologically relevant information. We also found that individuals' learning performance significantly covaried with two behavioural traits in the control zebrafish, but ALAN disrupted one of these relationships. Additionally, ALAN resulted in an average increase in larval activity. Our results showed that both fish's cognitive abilities and related individual differences are negatively impacted by light pollution, even after a short exposure in the embryonic stage.

Fears from the past? The innate ability of dogs to detect predator scents

Throughout the animal kingdom, antipredator mechanisms are an evolutionary driving force to enable the survival of species classified as prey. Information regarding a predator's location can be determined through chemosensory cues from urine, faeces, visual and/or acoustic signals and anal gland secretions; and in several lab and field-based studies it has been seen that these cues mediate behavioural changes within prey species. These behaviours are often linked to fear and avoidance, which will in turn increase the prey's survival rate. In many studies dogs (Canis lupus familiaris) have been used as a predator species, however, no research has addressed a dog's innate ability to detect predator scents, hence the rationale behind this study. We assessed the innate ability of the untrained domestic dog to detect faecal scents of wild Eurasian brown bear (Ursus arctos arctos) and European lynx (Lynx lynx). The study monitored 82 domestic dogs across the UK and Norway. The dogs were exposed to the two predator faecal scents from Eurasian brown bear and European lynx, a herbivore faecal scent of Eurasian beaver (Castor fiber) and water control. Measurements were taken upon the time spent within a 40 cm radius of each scent and changes in the dog's heart rate when within this 40 cm radius. We found dogs spent a decreased length of time around the predator scents and had an increased heart rate in relation to their basal heart rate. We conclude that dogs can innately sense predator scents of brown bear and lynx and elicit fear towards these odours, as shown through behavioural and physiological changes.

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.

Captive-born collared peccaries learning about their predators: Lessons learnt but not remembered

Captive-born animals frequently lose their anti-predatory abilities due to the absence of encounters with their predators, but these abilities can be regained through specific training. Anti-predator training can, thus, enhance the success of reintroduction programs with predator naïve animals. In addition, a good memory is important to guarantee the effects of the anti-predator training and increase survival rate after release into the wild. In the present study, anti-predator training sessions were applied to 11 captive-born collared peccaries (Pecari tajacu), followed by memory tests at 30, 60 and 90 days after the end of the training sessions. The collared peccaries responded appropriately to training against predators, showing alert, escape and predator avoidance behaviors after anti-predator training; however, the animals maintained these acquired behaviors for only 30 days after the end of the anti-predator training. After 60 days, peccaries responded to the predator in a 'relaxed' manner, exhibiting no anti-predator behaviors. For the trained collared peccaries to be released into the wild, reinforcement in the anti-predator training would be required at least 30 days prior to release.

Coral degradation alters predator odour signatures and influences prey learning and survival

Habitat degradation is a key factor leading to the global loss of biodiversity. This problem is particularly acute in coral reef ecosystems. We investigated whether recognition of predator odours by damselfish was influenced by coral degradation and whether these changes altered survival in the wild. We taught whitespot damselfish to recognize the odour of a predator in the presence of live/healthy coral or dead/degraded coral. Fish were tested for a response to predator odours in environments that matched their conditioning environment or in environments that were mismatched. Next, we taught blue damselfish to recognize the odour of three common reef predators in live and degraded coral environments and then stocked them onto live or degraded patch reefs, where we monitored their subsequent response to predator odour along with their survival. Damselfish learned to recognize predator odours in both coral environments, but the intensity of their antipredator response was much greater when the conditioning and test environments matched. Fish released on degraded coral had about 50% higher survival if they had been trained in the presence of degraded coral rather than live coral. Altering the intensity of antipredator responses could have rather profound consequences on population growth.

Testing social learning of anti-predator responses in juvenile jackdaws: the importance of accounting for levels of agitation

Social learning is often assumed to help young animals respond appropriately to potential threats in the environment. We brought wild, juvenile jackdaws briefly into captivity to test whether short exposures to conspecific vocalizations are sufficient to promote anti-predator learning. Individuals were presented with one of two models-a stuffed fox representing a genuine threat, or a toy elephant simulating a novel predator. Following an initial baseline presentation, juveniles were trained by pairing models with either adult mobbing calls, indicating danger, or contact calls suggesting no danger. In a final test phase with no playbacks, birds appeared to have habituated to the elephant, regardless of training, but responses to the fox remained high throughout, suggesting juveniles already recognized it as a predator before the experiment began. Training with mobbing calls did seem to generate elevated escape responses, but this was likely to be a carry-over effect of the playback in the previous trial. Overall, we found little evidence for social learning. Instead, individuals' responses were mainly driven by their level of agitation immediately preceding each presentation. These results highlight the importance of accounting for agitation in studies of anti-predator learning, and whenever animals are held in captivity for short periods.

Antipredatory behaviour of a mountain lizard towards the chemical cues of its predatory snakes

The ability to early detect a potential predator is essential for survival. The potential of Iberolacerta cyreni lizards to discriminate between chemical cues of their two predatory snakes Coronella austriaca (a non-venomous active-hunter saurophagous specialist) and Vipera latastei (a venomous sit-and-wait generalist) was evaluated herein. A third snake species, Natrix maura, which does not prey on lizards, was used as a pungent control. Thus, the behaviour of I. cyreni was studied regarding four treatments: (1) C. austriaca scent, (2) V. latastei scent, (3) N. maura scent and (4) odourless control. Lizards showed antipredator behaviour (such as slow-motion and tail waving) to C. austriaca and V. latastei chemicals. The antipredatory response was similar for both predators. This ability to react with an intensive behavioural pattern to the chemical cues of their predatory snakes may prevent lizards from being detected, and, if detected, dissuade the predator from beginning a pursuit.

Body size and mortality rates in coral reef fishes: a three-phase relationship

Body size is closely linked to mortality rates in many animals, although the overarching patterns in this relationship have rarely been considered for multiple species. A meta-analysis of published size-specific mortality rates for coral reef fishes revealed an exponential decline in mortality rate with increasing body size, however, within this broad relationship there are three distinct phases. Phase one is characterized by naive fishes recruiting to reefs, which suffer extremely high mortality rates. In this well-studied phase, fishes must learn quickly to survive the many predation risks. After just a few days, the surviving fishes enter phase two, in which small increases in body size result in pronounced increases in lifespan (estimated 11 d mm^-1). Remarkably, approximately 50% of reef fish individuals remain in phase two throughout their lives. Once fishes reach a size threshold of about 43 mm total length (TL) they enter phase three, where mortality rates are relatively low and the pressure to grow is presumably, significantly reduced. These phases provide a clearer understanding of the impact of body size on mortality rates in coral reef fishes and begin to reveal critical insights into the energetic and trophic dynamics of coral reefs.

Long-term exposure to elevated carbon dioxide does not alter activity levels of a coral reef fish in response to predator chemical cues

Abstract

Levels of dissolved carbon dioxide (CO₂) projected to occur in the world’s oceans in the near future have been reported to increase swimming activity and impair predator recognition in coral reef fishes. These behavioral alterations would be expected to have dramatic effects on survival and community dynamics in marine ecosystems in the future. To investigate the universality and replicability of these observations, we used juvenile spiny chromis damselfish (Acanthochromis polyacanthus) to examine the effects of long-term CO₂ exposure on routine activity and the behavioral response to the chemical cues of a predator (Cephalopholis urodeta). Commencing at ~3–20 days post-hatch, juvenile damselfish were exposed to present-day CO₂ levels (~420 μatm) or to levels forecasted for the year 2100 (~1000 μatm) for 3 months of their development. Thereafter, we assessed routine activity before and after injections of seawater (sham injection, control) or seawater-containing predator chemical cues. There was no effect of CO₂ treatment on routine activity levels before or after the injections. All fish decreased their swimming activity following the predator cue injection but not following the sham injection, regardless of CO₂ treatment. Our results corroborate findings from a growing number of studies reporting limited or no behavioral responses of fishes to elevated CO₂.

Significance statement

Alarmingly, it has been reported that levels of dissolved carbon dioxide (CO₂) forecasted for the year 2100 cause coral reef fishes to be attracted to the chemical cues of predators. However, most studies have exposed the fish to CO₂ for very short periods before behavioral testing. Using long-term acclimation to elevated CO₂ and automated tracking software, we found that fish exposed to elevated CO₂ showed the same behavioral patterns as control fish exposed to present-day CO₂ levels. Specifically, activity levels were the same between groups, and fish acclimated to elevated CO₂ decreased their swimming activity to the same degree as control fish when presented with cues from a predator. These findings indicate that behavioral impacts of elevated CO₂ levels are not universal in coral reef fishes.

Electronic supplementary material

The online version of this article (doi:10.1007/s00265-017-2337-x) contains supplementary material, which is available to authorized users.

Mechanisms underlying the control of responses to predator odours in aquatic prey

In aquatic systems, chemical cues are a major source of information through which animals are able to assess the current state of their environment to gain information about local predation risk. Prey use chemicals released by predators (including cues from a predator's diet) and other prey (such as alarm cues and disturbance cues) to mediate a range of behavioural, morphological and life-history antipredator defences. Despite the wealth of knowledge on the ecology of antipredator defences, we know surprisingly little about the physiological mechanisms that control the expression of these defensive traits. Here, we summarise the current literature on the mechanisms known to specifically mediate responses to predator odours, including dietary cues. Interestingly, these studies suggest that independent pathways may control predator-specific responses, highlighting the need for greater focus on predator-derived cues when looking at the mechanistic control of responses. Thus, we urge researchers to tease apart the effects of predator-specific cues (i.e. chemicals representing a predator's identity) from those of diet-mediated cues (i.e. chemicals released from a predator's diet), which are known to mediate different ecological endpoints. Finally, we suggest some key areas of research that would greatly benefit from a more mechanistic approach.

Conspecific and heterospecific alarm substances induce behavioral responses in juvenile catfish Rhamdia quelen

ABSTRACT The recognition of chemical information indicating the presence of a predator is very important for prey survival. In this study we tested antipredator behavioral response of juvenile silver catfish (Rhamdia quelen) against predator odor released by two different potential predators, Hoplias malabaricus and the snake Helicops infrataeniatus, and alarm cues and disturbance cues released by conspecifics and by non-predator species, Megaleporinus obtusidens and Astyanax lacustris. We used juvenile catfish that were naive to predators. The trials consisted of a 10-min prestimulus and a 10-min post-stimulus observation period. The behavioral response displayed by silver catfish exposed to alarm cues comprised a decrease in shelter use and an increase in locomotion, and also a longer latency period before feeding. Our results showed that juvenile silver catfish can perceive chemical cues released by predators, heterospecifics and conspecifics.

Behavioural consistency and group conformity in humbug damselfish

Humbug damselfish, Dascyllus aruanus, are a common coral reef fish that form stable social groups with size-based social hierarchies. Here we caught whole wild groups of damselfish and tested whether social groups tended to be comprised of animals that are more similar to one another in terms of their behavioural type, than expected by chance. First we found that individuals were repeatable in their level of activity and exploration, and that this was independent of both absolute size and within-group dominance rank, indicating that animals were behaviourally consistent. Secondly, despite the fact that individuals were tested independently, the behaviour of members of the same groups was significantly more similar than expected under a null model, suggesting that individual behaviour develops and is shaped by conformity to the behaviour of other group members. This is one of the first studies to demonstrate this group-level behavioural conformity in wild-caught groups.

Knocking on Heaven's Door: Are Novel Invaders Necessarily Facing Naïve Native Species on Islands?

The impact of alien predator species on insular native biota has often been attributed to island prey naïveté (i.e. lack of, or inefficient, anti-predator behavior). Only rarely, however, has the concept of island prey naïveté been tested, and then only a posteriori (i.e. hundreds or thousands of years after alien species introduction). The presence of native or anciently introduced predators or competitors may be crucial for the recognition and development of adaptive behavior toward unknown predators or competitors of the same archetype (i.e. a set of species that occupy a similar ecological niche and show similar morphological and behavioral traits when interacting with other species). Here, we tested whether two squamates endemic to New Caledonia, a skink, Caledoniscincus austrocaledonicus, and a gecko, Bavayia septuiclavis, recognized and responded to the odor of two major invaders introduced into the Pacific islands, but not yet into New Caledonia. We chose one predator, the small Indian mongoose Herpestes javanicus and one competitor, the cane toad Rhinella marina, which belong respectively to the same archetype as the following two species already introduced into New Caledonia in the nineteenth century: the feral cat Felis catus and the golden bell frog Litoria aurea. Our experiment reveals that geckos are naïve with respect to the odors of both an unknown predator and an unknown competitor, as well as to the odors of a predator and a competitor they have lived with for centuries. In contrast, skinks seem to have lost some naïveté regarding the odor of a predator they have lived with for centuries and seem "predisposed" to avoid the odor of an unknown potential competitor. These results indicate that insular species living in contact with invasive alien species for centuries may be, although not systematically, predisposed toward developing adaptive behavior with respect to species belonging to the same archetype and introduced into their native range.