Death feigning as an adaptive anti-predator behaviour: Further evidence for its evolution from artificial selection and natural populations

Tonic immobility behaviour does not differ between fire salamander larvae from ponds and streams

Tonic immobility is an antipredator defence in which the prey animal remains motionless after physical contact with the predator, pretending to be dead. This behaviour has been observed among a variety of taxa but has received only little attention in amphibian larvae. During our field studies with fire salamander larvae, we observed that larvae from different habitats display tonic immobility after handling. In our study site, we find larvae in pond and stream habitats, that differ in several aspects such as their stress response and their risk-taking behaviour, likely caused by the very different habitat conditions. We measured the time that the tonic immobility behaviour was displayed but found no difference between larvae from the two habitat types. Likewise, we also found no correlation between the size of the larvae and the duration of displaying the behaviour. In conclusion, we found that fire salamander larvae show tonic immobility, but found no evidence that the different habitat conditions influence the tonic immobility behaviour.

Keep the ball rolling: sexual differences in conglobation behavior of a terrestrial isopod under different degrees of perceived predation pressure

Background

Antipredator behaviors are theoretically subjected to a balance by which their display should be minimized when their benefits do not outweigh their costs. Such costs may be not only energetic, but also entail a reduction in the time available for other fitness-enhancing behaviors. However, these behaviors are only beneficial under predation risk. Therefore, antipredator behaviors are predicted to be maximized under strong predation risk. Moreover, predation pressure can differ among individuals according to traits such as sex or body size, if these traits increase vulnerability. Antipredator behaviors are expected to be maximized in individuals whose traits make them more conspicuous to predators. However, how sex, body size and antipredator behaviors interact is not always understood.

Methods

In this work, I tested the interaction between sex, body size and antipredator behavior in the common pill woodlouse (Armadillidium vulgare), which conglobate (i.e., they roll up their bodies almost conforming a sphere that conceals their appendages) in response to predator attacks. Specifically, I tested whether latency to unroll after a standardized mechanical induction was greater in animals exposed to predator chemical cues (toad feces) than in conspecifics exposed to cues of non-predatory animals (rabbits) or no chemical cues whatsoever (distilled water), incorporating sex and body mass in the analyses.

Results

In agreement with my prediction, latency to unroll was greater in individuals exposed to predator chemical cues. In other words, these animals engage in conglobation for longer under perceived predator vicinity. However, this result was only true for males. This sexual dimorphism in antipredator behavior could result from males being under greater predation risk than females, thus having evolved more refined antipredator strategies. Indeed, males of this species are known to actively search for females, which makes them more prone to superficial ground mobility, and likely to being detected by predators. Body size was unrelated to latency to unroll. As a whole, these results support the hypothesis that antipredator behavior is tuned to predator cues in a way consistent with a balance between costs and benefits, which might differ between the sexes.

Behavioural Repeatability and Behavioural Syndrome in the Dung Beetle Copris umbilicatus (Coleoptera, Scarabaeidae)

Although personality studies have primarily focused on vertebrates, the evidence showing invertebrates to be capable of displaying personalities has been steadily growing in recent years. In this study, we investigated the behavioural repeatability (repetition of a behaviour over time) and behavioural syndromes (a set of correlated behaviours) in Copris umbilicatus, which is a dung beetle species showing complex sub-social behaviour. We analysed three behaviours (activity, thanatosis and distress call emission) by measuring seven distinct behavioural traits (i.e., three activity-, one thanatosis- and three distress call-related traits). We found moderate to high levels of individual repeatability in all behavioural traits considered. The duration of thanatosis was inversely correlated with two activity traits, hinting a behavioural syndrome for thanatosis and activity, with bolder individuals exhibiting shorter thanatosis and higher locomotor activity in contrast with fearful individuals, which display longer thanatosis and poor locomotor activity. No relationships were found between the behavioural traits and body size or sex. Results of the principal component analysis (PCA) suggested personality differences among individuals. Dung beetles provide an impressive variety of ecosystem services. Since the provision of these services may depend on the personalities represented in local populations and communities, studies on the ecology of personality in dung beetles should be encouraged in future research.

Latitudinal cline of death-feigning behaviour in a beetle (Tribolium castaneum)

Death-feigning behaviour is a phenomenon in which a prey is rendered motionless due to stimulation or threat from a predator. This anti-predator defence mechanism has been observed across numerous animal taxa and is considered adaptive in nature. However, longer durations of death feigning can result in decreased opportunities for feeding and reproduction, and therefore is often associated with fitness costs as compared to environments without predators. Differences have also been observed in the frequencies and durations of death feigning within populations, and these differences are thought to be influenced by the balance between survival and other fitness costs. Furthermore, this balance is predicted to vary in response to changes in environmental conditions. In this study, we examined the death feigning in 38 populations of the red flour beetle (Tribolium castaneum). Our results demonstrate that frequencies and durations of the death feigning in T. castaneum show geographical variations and a latitude cline, indicating that this behaviour is influenced by location as well as latitude. This study is the first to demonstrate the existence of a latitudinal cline in death feigning and suggests that death-feigning behaviour might have evolved in response to environmental factors that vary with latitude.

Polygene control and trait dominance in death-feigning syndrome in the red flour beetle Tribolium castaneum

Death-feigning behavior is an anti-predatory technique used in several animal taxa and often correlates with inhibited movements (i.e., death-feigning syndrome). We performed a reciprocal crossing among strains exhibiting a genetically longer (L-strain) and shorter (S-strain) duration of death feigning. Then, we investigated related heritable factors in F₁ and F₂ populations. We also evaluated movement activities, which negatively responded to artificial selection for death feigning in T. castaneum. Our results indicated that death feigning occurred more frequently and for shorter periods in the F₁ population. However, in the F₂ population, death feigning and movement exhibited continuous segregation. Although the distribution of each trait value in the F₂ generation differed from that of the parental generation, no individuals transgressing the distribution of trait values in the parental generation emerged from the F₂ generation. Besides, chi-square analysis of the observed death feigning and movement of F₁ and F₂ progenies rejected the hypothesis of mono-major gene inheritance. These results suggested that the death-feigning syndrome was polygenically controlled, indicating the usefulness of reciprocal crossing experiments in assessing the quantitative inheritance of behavioral traits.

Increasing intensity of deimatic behaviour in response to repeated simulated attacks: a case study on the mountain katydid (Acripeza reticulata)

Abstract

How and when deimatic behaviours are performed can change during encounters between predators and prey. Some predators attack repeatedly, investigating and manipulating prey, and in response, an individual’s deimatic behaviour may intensify or may diminish in favour of escaping. The presence of a resource can further force a trade-off between displaying and escaping. Here, we examined the intensity of the katydid’s deimatic behaviour, a visual display, the propensity of their escape response under repeated simulated attacks, and how these responses change in the presence of foraging resources. We found that display intensity increased with repeated simulated attacks and that females displayed at a greater intensity than males. The presence of their preferred food plant had no significant effect on display intensity, but reduced escape probability in both sexes. Some katydids were predictable in their display intensity and at the population level we found that strong display intensity is moderately repeatable. Overall, our results suggest that 1) display intensity increases with repeated attacks and might indicate a cost in performing at maximum intensity upon first attack, 2) deploying a deimatic display while feeding can reduce the need to flee a rich foraging patch and 3) some individuals are consistent in their display intensities. Future experiments that aim to determine causal mechanisms such as limitations to perception of predators, sensitisation to stimuli and physiological constraints to display intensity will provide necessary insight into how deimatic displays function.

Significance statement

Though often regarded as success or failure, interactions between predators and prey during the attack phase of a predation event are complex, especially when predators make repeated investigative attacks in quick succession. Our study shows that in mountain katydids, intensity of deimatic behaviour increases with repeated attacks, perhaps indicating that prey sensitise or that maximal displays during initial attacks carry high costs such as conspicuousness. The intensity of the display does not change with the introduction of a valuable food resource, but the probability of fleeing decreased, suggesting that displaying may reduce the opportunity costs of leaving a patch. We also show that individuals vary in the repeatability of their display, suggesting that deimatic display may be highly adaptable, nuanced and targeted.

Responses to artificial selection for locomotor activity: A focus on death feigning in red flour beetle

Whole-organism performance, including locomotor activity, is an important fitness trait in many animals. Locomotor activity is often classified into sprint speed and locomotor endurance and differences in sprint speed and locomotor endurance affect on other traits such as life-history traits. Previous studies found that locomotor endurance, sprint speed and brain dopamine (DA) levels are correlated with artificial selection for death feigning (an anti-predator behaviour that we refer to as 'death-feigning syndrome') in some insect species. Thus, if the syndrome has a genetic basis, death feigning, sprint speed and brain DA levels may be affected by artificial selection for locomotor endurance. We artificially selected for locomotor endurance over 10 generations in the red flour beetle Tribolium castaneum, and established higher (H) and lower activity (L) strains, then compared their death-feigning behaviour, sprint speed and brain DA levels. H-strain beetles exhibited significantly shorter duration of death-feigning, and significantly higher sprint speeds, suggesting variation in death-feigning syndrome. Surprisingly, although brain DA expression affects various animal behaviours, we found no significant differences in the brain DA expressions of H- and L-strain beetles. Thus, our results imply genetic correlations between locomotor endurance, sprint speed and death feigning, but not with brain DA expression, suggesting that differences in the biogenic amine results of our and previous studies may reflect differences in behavioural expression mechanisms.

Freezing or death feigning? Beetles selected for long death feigning showed different tactics against different predators

Prey evolve antipredator strategies against multiple enemies in nature. We examined how a prey species adopts different predation avoidance tactics against pursuit or sit-and-wait predators. As prey, we used three strains of Tribolium beetles artificially selected for short (short strain) or long (long strain) duration of death feigning, and a stock culture (base population). Death feigning is known to be effective for evading a jumping spider in the case of the long strains, while the present study showed that the long-strain beetles used freezing against a sit-and-wait type predator, Amphibolus venator, in this study. The short- strain beetles were more easily oriented toward predators. The time to predation was also shorter in the short strains compared to the long strains. The results showed that, as prey, the short strains displayed the same behavior, escaping, against both types of predators. Traditionally, death feigning has been thought to be the last resort in a series of antipredator avoidance behaviors. However, our results showed that freezing and death feigning were not parts of a series of behaviors, but independent strategies against different predators, at least for long-strain beetles. We also examined the relationship between a predator's starvation level and its predatory behavior. In addition, the orientation behavior toward and predation rate on the prey were observed to determine how often the predatory insect attacked the beetles.

Genomic characterization between strains selected for death-feigning duration for avoiding attack of a beetle

Predator avoidance is an important behavior that affects the degree of adaptation of organisms. We compared the DNA variation of one of the predator-avoidance behaviors, the recently extensively studied "death-feigning behavior", between the long strain bred for feigning death for a long time and the short strain bred for feigning death for a short time. To clarify how the difference in DNA sequences between the long and short strains corresponds to the physiological characteristics of the death-feigning duration at the transcriptome level, we performed comprehensive and comparative analyses of gene variants in Tribolium castaneum strains using DNA-resequencing. The duration of death feigning involves many gene pathways, including caffeine metabolism, tyrosine metabolism, tryptophan metabolism, metabolism of xenobiotics by cytochrome P450, longevity regulating pathways, and circadian rhythm. Artificial selection based on the duration of death feigning results in the preservation of variants of genes in these pathways in the long strain. This study suggests that many metabolic pathways and related genes may be involved in the decision-making process of anti-predator animal behavior by forming a network in addition to the tyrosine metabolic system, including dopamine, revealed in previous studies.

Internal State: Dynamic, Interconnected Communication Loops Distributed Across Body, Brain, and Time

Internal state profoundly alters perception and behavior. For example, a starved fly may approach and consume foods that it would otherwise find undesirable. A socially engaged newt may remain engaged in the presence of a predator, whereas a solitary newt would otherwise attempt to escape. Yet, the definition of internal state is fluid and ill-defined. As an interdisciplinary group of scholars spanning five career stages (from undergraduate to full professor) and six academic institutions, we came together in an attempt to provide an operational definition of internal state that could be useful in understanding the behavior and the function of nervous systems, at timescales relevant to the individual. In this perspective, we propose to define internal state through an integrative framework centered on dynamic and interconnected communication loops within and between the body and the brain. This framework is informed by a synthesis of historical and contemporary paradigms used by neurobiologists, ethologists, physiologists, and endocrinologists. We view internal state as composed of both spatially distributed networks (body-brain communication loops), and temporally distributed mechanisms that weave together neural circuits, physiology, and behavior. Given the wide spatial and temporal scales at which internal state operates-and therefore the broad range of scales at which it could be defined-we choose to anchor our definition in the body. Here we focus on studies that highlight body-to-brain signaling; body represented in endocrine signaling, and brain represented in sensory signaling. This integrative framework of internal state potentially unites the disparate paradigms often used by scientists grappling with body-brain interactions. We invite others to join us as we examine approaches and question assumptions to study the underlying mechanisms and temporal dynamics of internal state.

The evolutionary origin of near-death experiences: a systematic investigation

Near-death experiences are known from all parts of the world, various times and numerous cultural backgrounds. This universality suggests that near-death experiences may have a biological origin and purpose. Adhering to a preregistered protocol, we investigate the hypothesis that thanatosis, aka death-feigning, a last-resort defense mechanism in animals, is the evolutionary origin of near-death experiences. We first show that thanatosis is a highly preserved survival strategy occurring at all major nodes in a cladogram ranging from insects to humans. We then show that humans under attack by animal, human and ‘modern’ predators can experience both thanatosis and near-death experiences, and we further show that the phenomenology and the effects of the two overlap. In summary, we build a line of evidence suggesting that thanatosis is the evolutionary foundation of near-death experiences and that their shared biological purpose is the benefit of survival. We propose that the acquisition of language enabled humans to transform these events from relatively stereotyped death-feigning under predatory attacks into the rich perceptions that form near-death experiences and extend to non-predatory situations.

Natural selection increases female fitness by reversing the exaggeration of a male sexually selected trait

Theory shows how sexual selection can exaggerate male traits beyond naturally selected optima and also how natural selection can ultimately halt trait elaboration. Empirical evidence supports this theory, but to our knowledge, there have been no experimental evolution studies directly testing this logic, and little examination of possible associated effects on female fitness. Here we use experimental evolution of replicate populations of broad-horned flour beetles to test for effects of sex-specific predation on an exaggerated sexually selected male trait (the mandibles), while also testing for effects on female lifetime reproductive success. We find that populations subjected to male-specific predation evolve smaller sexually selected mandibles and this indirectly increases female fitness, seemingly through intersexual genetic correlations we document. Predation solely on females has no effects. Our findings support fundamental theory, but also reveal unforseen outcomes-the indirect effect on females-when natural selection targets sex-limited sexually selected characters.

Divergence time estimation of genus Tribolium by extensive sampling of highly conserved orthologs

Tribolium castaneum, the red flour beetle, is among the most well-studied eukaryotic genetic model organisms. Tribolium often serves as a comparative bridge from highly derived Drosophila traits to other organisms. Simultaneously, as a member of the most diverse order of metazoans, Coleoptera, Tribolium informs us about innovations that accompany hyper diversity. However, understanding the tempo and mode of evolutionary innovation requires well-resolved, time-calibrated phylogenies, which are not available for Tribolium. The most recent effort to understand Tribolium phylogenetics used two mitochondrial and three nuclear markers. The study concluded that the genus may be paraphyletic and reported a broad range for divergence time estimates. Here we employ recent advances in Bayesian methods to estimate the relationships and divergence times among Tribolium castaneum, T. brevicornis, T. confusum, T. freemani, and Gnatocerus cornutus using 1368 orthologs conserved across all five species and an independent substitution rate estimate. We find that the most basal split within Tribolium occurred ~86 Mya [95% HPD 85.90-87.04 Mya] and that the most recent split was between T. freemani and T. castaneum at ~14 Mya [95% HPD 13.55-14.00]. Our results are consistent with broader phylogenetic analyses of insects and suggest that Cenozoic climate changes played a role in the Tribolium diversification.

Amplitude of circadian rhythms becomes weaken in the north, but there is no cline in the period of rhythm in a beetle

Many species show rhythmicity in activity, from the timing of flowering in plants to that of foraging behavior in animals. The free-running periods and amplitude (sometimes called strength or power) of circadian rhythms are often used as indicators of biological clocks. Many reports have shown that these traits are highly geographically variable, and interestingly, they often show latitudinal or longitudinal clines. In many cases, the higher the latitude is, the longer the free-running circadian period (i.e., period of rhythm) in insects and plants. However, reports of positive correlations between latitude or longitude and circadian rhythm traits, including free-running periods, the power of the rhythm and locomotor activity, are limited to certain taxonomic groups. Therefore, we collected a cosmopolitan stored-product pest species, the red flour beetle Tribolium castaneum, in various parts of Japan and examined its rhythm traits, including the power and period of the rhythm, which were calculated from locomotor activity. The analysis revealed that the power was significantly lower for beetles collected in northern areas than southern areas in Japan. However, it is worth noting that the period of circadian rhythm did not show any clines; specifically, it did not vary among the sampling sites, despite the very large sample size (n = 1585). We discuss why these cline trends were observed in T. castaneum.

The Neurology of Death and the Dying Brain: A Pictorial Essay

As neurologists earn their living with the preservation and restoration of brain function, they are also well-positioned to address the science behind the transition from life to death. This essay in pictures highlights areas of neurological expertise needed for brain death determination; shows pitfalls to avoid during the clinical examination and interpretation of confirmatory laboratory tests in brain death protocols; illustrates the great variability of brain death legislations around the world; discusses arguments for the implementation of donation after circulatory death (DCD); points to unresolved questions related to DCD and the time between cardiac standstill and organ procurement (“hands-off period”); provides an overview of the epidemiology and semiology of near-death experiences, including their importance for religion, literature, and the visual arts; suggests biological mechanisms for near-death experiences such as dysfunction of temporoparietal cortex, N-methyl-D-aspartate receptor antagonism, migraine aura, and rapid eye movement sleep; hypothesizes that thanatosis (aka. death-feigning, a common behavioral trait in the animal kingdom) represents the evolutionary origin of near-death experiences; and speculates about the future implications of recent attempts of brain resuscitation in an animal model. The aim is to provide the reader with a thorough understanding that the boundaries within the neurology of death and the dying brain are being pushed just like everywhere else in the clinical neurosciences.