Drop or fly? Negative genetic correlation between death-feigning intensity and flying ability as alternative anti-predator strategies

Observations on neurophysiological pattern and behavioural traits as death-feigning mechanism in Eucryptorrhynchus scrobiculatus (Coleoptera: Curculionidae)

In adaptation to surrounding environmental stimuli, most insects exhibit defense behaviour (death feigning) to improve survival rates in the wild. However, the underlying mechanism of death feigning remains largely unknown. Here, we tested the neurophysiological pattern and behavioural traits of the death-feigning mechanism in the forestry pest Eucryptorrhynchus scrobiculatus. Using neuroanatomy, LC-MS/MS target metabolomics detection technology and qRT-PCR, we investigated the effects of neurochemicals and metabolic pathways in experimental weevils. Excision and drug tests were conducted to verify the key regulatory body parts involved in regulating the central nervous system in death feigning. Our results reconstructed the death-feigning mechanism of E. scrobiculatus: when the effective stimuli point of arousal weevils received mechanical stimulation, the thoracoabdominal ganglion transmitted signals into the brain through the ventral nerve cord, and then the brain regulated dopamine (DA) and serotonin (5-HT) metabolic pathways, reducing the expression of dopamine (dar2) and octopamine (oar1, oab2) receptor genes, finally inducing death feigning. Our study suggests that the variation of neurotransmitters in the brain is an important indicator of the physiological response of death feigning, and the results provide ecological and theoretical information for future investigations to reveal key behaviour and target genes for pest control.

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.

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.

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.

Thermal dependence and individual variation in tonic immobility varies between sympatric amphibians

Tonic immobility (TI) is an important antipredator response employed by prey in the last stages of a predation sequence. Evolution by natural selection assumes consistent individual variation (repeatability) in this trait. In ectotherms, which experience variable body temperatures, TI should be repeatable over a thermal gradient to be targeted by natural selection; however, information on thermal repeatability of this trait is missing. We examined thermal repeatability of TI in juveniles of two sympatric amphibians, smooth (Lissotriton vulgaris) and alpine (Ichthyosaura alpestris) newts. Both species showed disparate TI responses to body temperature variation (13-28 °C). While the proportion of TI response was repeatable in both taxa, it increased with body temperature in alpine newts but was temperature independent in smooth newts. Duration of TI decreased with body temperature in both taxa but was only repeatable in smooth newts. Our results suggest that a warming climate may affect population dynamics of sympatric ectotherms through asymmetry in thermal reaction norms for antipredator responses.

Effects of host plant growth form on dropping behaviour in leaf beetles

Many leaf-eating insects drop from their host plants to escape predators. However, they must return to the leaves of the host plant after dropping, which represents a cost associated with this behaviour. In woody plants, the positioning of leaves is generally higher than that of herbaceous plants, which suggests that dropping from woody plants might be costlier for leaf-eating insects than dropping from herbaceous plants. Therefore, we predicted that dropping behaviour would be observed less frequently in insects that feed on woody plant leaves than in those that feed on herbaceous plant leaves. To test this prediction, we investigated dropping behaviour experimentally in larvae (23 species) and adults (112 species) of leaf beetles (Coleoptera: Chrysomelidae) on their host plants (86 species of 44 families) in field conditions. Larvae on woody plants exhibited dropping behaviour less frequently than those on herbaceous plants. However, this pattern was not detected in adults. Thus, host plant growth form might affect the evolution of dropping behaviour in leaf beetle larvae, but not in winged adults, perhaps owing to their higher mobility.

Anti-predator behaviour depends on male weapon size

Tonic immobility and escape are adaptive anti-predator tactics used by many animals. Escape requires movement, whereas tonic immobility does not. If anti-predator tactics relate to weapon size, males with larger weapons may adopt tonic immobility, whereas males with smaller weapons may adopt escape. However, no study has investigated the relationship between weapon size and anti-predator tactics. In this study, we investigated the relationship between male weapon size and tonic immobility in the beetle Gnathocerus cornutus. The results showed that tonic immobility was more frequent in males with larger weapons. Although most studies of tonic immobility in beetles have focused on the duration, rather than the frequency, tonic immobility duration was not affected by weapon size in G. cornutus. Therefore, this study is the first, to our knowledge, to suggest that the male weapon trait affects anti-predator tactics.

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

Death feigning is considered to be an adaptive antipredator behaviour. Previous studies on Tribolium castaneum have shown that prey which death feign have a fitness advantage over those that do not when using a jumping spider as the predator. Whether these effects are repeatable across species or whether they can be seen in nature is, however, unknown. Therefore, the present study involved two experiments: (a) divergent artificial selection for the duration of death feigning using a related species T. freemani as prey and a predatory bug as predator, demonstrating that previous results are repeatable across both prey and predator species, and (b) comparison of the death-feigning duration of T. castaneum populations collected from field sites with and without predatory bugs. In the first experiment, T. freemani adults from established selection regimes with longer durations of death feigning had higher survival rates and longer latency to being preyed on when they were placed with predatory bugs than the adults from regimes selected for shorter durations of death feigning. As a result, the adaptive significance of death-feigning behaviour was demonstrated in another prey-predator system. In the second experiment, wild T. castaneum beetles from populations with predators feigned death longer than wild beetles from predator-free populations. Combining the results from these two experiments with those from previous studies provided strong evidence that predators drive the evolution of longer death feigning.

Post-contact immobility and half-lives that save lives

A wide variety of animals become completely immobile after initial contact with a potential predator. This behaviour is considered to be a last-ditch escape strategy. Here, we test the hypothesis that such immobility should have an extremely unpredictable duration. We find that it spans more than three orders of magnitude in antlion larvae. We also analyse the second period of immobility that follows the first bout of immobility, and consider the distributions of both first and second immobility periods within the context of the intermittence that characterizes the movement of most organisms. Both immobility durations were fitted best by exponential distributions. Therefore, both were characterized by high variability and hence, unpredictability. The immobility half-life, its mean duration and standard deviation were greater for the first than the second immobility. Furthermore, individual consistency was weak or absent in repeated measures of the first immobility and between the first and second immobilities. Our quantitative approach can be replicated across taxa and would help link an understanding of immobility after an initial predator contact in both vertebrates and invertebrates. To facilitate this, we contend that the terminology should be simplified, and we advocate the use of the term post-contact immobility (PCI).

Goats adjust their feeding behaviour to avoid the ingestion of different insect species

Ungulates feed on plants that are often inhabited by insects. Goats (Capra hircus Linnaeus, 1758) can efficiently avoid the ingestion of setae-covered noxious, caterpillars while feeding, but it is unknown how they respond to non-toxic insects. We filmed and analysed the behavioural responses of goats to smooth, innocuous silkworms (Bombyx mori (Linnaeus, 1758)) while feeding. The goats successfully sorted the silkworms apart from the food despite their tendency to cling to the leaves. Although the goats exhibited behaviours similar to those displayed with noxious caterpillars, the frequency of the behaviours doubled and a new behaviour appeared. The goats detected silkworms using tactile stimulation, obtained by repeatedly touching the leaves with their muzzles. This behaviour enabled them to pick silkworm-free leaves (leaving 73% of silkworms behind). If the goats picked up leaves with a silkworm, then they shook it off. When shaking was unsuccessful, they employed a new behaviour, filtering, in which they physically blocked the silkworm with their lips while consuming the leaves. Silkworms that entered the mouths of goats (rare) were spat out. These findings demonstrate that ungulates are capable of adjusting their feeding behaviour to accurately detect and avoid the ingestion of different insect species.

Arousal from Tonic Immobility by Vibration Stimulus

Tonic immobility (TI) is an effective anti-predator strategy. However, long immobility status on the ground increases the risk of being eaten by predators, and thus insects must rouse themselves when appropriate stimulation is provided. Here, the strength of vibration causing arousal from the state of TI was examined in strains artificially selected for longer duration of TI (L-strains: long sleeper) in a beetle. We provided different strengths of vibration stimuli to the long sleepers in Tribolium castaneum. Although immobilized beetles were never awakened by the stimuli from 0.01 to 0.12 mm in amplitude, almost of the beetles were aroused from immobilized status by the stimulus at 0.21 mm. There was a difference in sensitivity of individuals when the stimuli of 0.14 mm and 0.18 mm were provided. F2 individuals were also bred by crossing experiments of the strains selected for shorter and longer duration of TI. The arousal sensitivity to vibration was well separated in the F2 individuals. A positive relationship was observed between the duration of TI and the vibration amplitude, suggesting that immobilized beetles are difficult to arouse from a deep sleep, while light sleepers are easily aroused by even small vibrations. The results indicate a genetic basis for sensitivity to arousal from TI.

Innate ability of goats to sense and avoid ingestion of noxious insects while feeding

Large mammalian herbivores regularly encounter noxious insects on their food plants. Recent evidence revealed that goats efficiently avoid insect ingestion while feeding, yet it is unknown whether this ability is innate. We experimentally examined the behavioural responses of naive goat kids to a common insect, the spring-webworm (Ocnogyna loewii). We filmed and analysed the kids' behaviour while feeding and compared it to the behaviour described in adults. Naive kids sorted the webworms apart from the food without ingesting them (all webworms survived). They exhibited behaviours similar to those displayed by adults, demonstrating an innate ability to avoid insect ingestion. The kids detected webworms using tactile stimulation, obtained by repeatedly touching the leaves with their muzzles. This enabled them to pick webworm-free leaves (leaving 93% of webworms behind). While adults frequently shook or discarded leaves with webworms or spat out webworms, these behaviours were rare in kids. The kids' mean feeding rates doubled over the trials, indicating that their feeding efficiency on plants with and without insects improved with experience. As ingesting noxious insects could be fatal, innate avoidance is critical. These findings highlight the importance of direct interactions between mammalian and insect herbivores.

Dropping to escape: a review of an under-appreciated antipredator defence

Dropping is a common antipredator defence that enables rapid escape from a perceived threat. However, despite its immediate effectiveness in predator-prey encounters (and against other dangers such as a parasitoid or an aggressive conspecific), it remains an under-appreciated defence strategy in the scientific literature. Dropping has been recorded in a wide range of taxa, from primates to lizards, but has been studied most commonly in insects. Insects have been found to utilise dropping in response to both biotic and abiotic stimuli, sometimes dependent on mechanical or chemical cues. Whatever the trigger for dropping, the decision to drop by prey will present a range of inter-related costs and benefits to the individual and so there will be subtle complexities in the trade-offs surrounding this defensive behaviour. In predatory encounters, dropping by prey will also impose varying costs and benefits on the predator - or predators - involved in the system. There may be important trade-offs involved in the decision made by predators regarding whether to pursue prey or not, but the predator perspective on dropping has been less explored at present. Beyond its function as an escape tactic, dropping has also been suggested to be an important precursor to flight in insects and further study could greatly improve understanding of its evolutionary importance. Dropping in insects could also prove of significant practical importance if an improved understanding can be applied to integrated pest-management strategies. Currently the non-consumptive effects of predators on their prey are under-appreciated in biological control and it may be that the dropping behaviour of many pest species could be exploited via management practices to improve crop protection. Overall, this review aims to provide a comprehensive synthesis of the current literature on dropping and to raise awareness of this fascinating and widespread behaviour. It also seeks to offer some novel hypotheses and highlight key avenues for future research.

Responses to relaxed and reverse selection in strains artificially selected for duration of death-feigning behavior in the red flour beetle, Tribolium castaneum

Divergent lines selected artificially for many generations make it possible to answer two questions: (1) whether genetic variation still exists within the selected population; and (2) whether the selection itself is costly for the selected strain. In previous studies, the red flour beetle Tribolium castaneum was divergently selected artificially for duration of death-feigning, and strains selected for longer (L-strain) and shorter (S-strain) durations of death-feigning have been established (Miyatake et al. 2004, 2008). Because the selection experiments have been conducted for more than 27 generations, genetic variation may be eroded. Furthermore, because another previous study reported physiological costs to L-strains, the L-strains selected artificially for longer duration of death-feigning may have suffered more costs than the S-strains. In the present study, therefore, we relaxed the selection pressure after the 27th or 30th generation of S- and L-strains. We also carried out reverse selection during the most recent eight generations of S- and L-strains. The results showed that each strain clearly responded to relaxation of selection and reverse selection, suggesting that (1) additive genetic variation still existed in both strains after long-term selection, and (2) selection for shorter and longer duration of death-feigning was costly. These results suggest that anti-predator behavior is controlled by many loci, and longer or shorter duration of death-feigning is costly in a laboratory without predators.

Aphids Playing Possum - Defensive or Mutualistic Response?

Paper presents the phenomenon of thanatosis or death-feigning in selected aphids species. This specific reaction was predominantly analysed on the example of aphid subfamily Lachninae. Individuals of this group were used in experiments, during which a thanatotic response was induced with various results. The response differed from prolonged thanatosis, lasting for several minutes (Eulachnus rileyi), through shrinking behaviour (e. g. in Stomaphis graffii) to non-responsive species such as Cinara (Schizolachnus) pineti. The large interspecies variation of observed responses can be linked to other defensive mechanisms existing in the studied species, as well as to their mutualistic relationship with ants. The behaviour of shrinking is hypothesized to be the mutualistic response, developed from thanatosis, and being adapted to transportation by ant workers.

A review of thanatosis (death feigning) as an anti-predator behaviour

Abstract

Thanatosis—also known as death-feigning and, we argue more appropriately, tonic immobility (TI)—is an under-reported but fascinating anti-predator strategy adopted by diverse prey late on in the predation sequence, and frequently following physical contact by the predator. TI is thought to inhibit further attack by predators and reduce the perceived need of the predator to subdue prey further. The behaviour is probably present in more taxa than is currently described, but even within well-studied groups the precise taxonomic distribution is unclear for a number of practical and ethical reasons. Here we synthesise the key studies investigating the form, function, evolutionary and ecological costs and benefits of TI. This review also considers the potential evolutionary influence of certain predator types in the development of the strategy in prey, and the other non-defensive contexts in which TI has been suggested to occur. We believe that there is a need for TI to be better appreciated in the scientific literature and outline potentially profitable avenues for investigation. Future use of technology in the wild should yield useful developments for this field of study.

Significance statement

Anti-predatory defences are crucial to many aspects of behavioural ecology. Thanatosis (often called death-feigning) has long been an under-appreciated defence, despite being taxonomically and ecologically widespread. We begin by providing much-needed clarification on both terminology and definition. We demonstrate how apparently disparate observations in the recent literature can be synthesised through placing the behaviour within a cost-benefit framework in comparison to alternative behavioural choices, and how aspects of the ecology differentially affect costs and benefits. Extending this, we provide novel insights into why the evolution of thanatosis can be understood in terms of coevolution between predators and prey. We offer further novel hypotheses, and discuss how these can be tested, focussing on how emerging technologies can be of great use in developing our understanding of thanatosis in free-living animals.

How goats avoid ingesting noxious insects while feeding

As mammalian herbivores feed, they often encounter noxious insects on plants. It is unknown how they handle such insects. We experimentally examined the behavioural responses of goats to the noxious spring-webworm (Ocnogyna loewii), and manipulated their sensory perception to reveal the process of insect detection. Goats did not avoid plants with webworms, demonstrating a remarkable ability to sort them apart from the plant (98% of webworms survived). Initial detection of webworms involved tactile stimulation, done by repeatedly touching the leaves with the muzzle. This enabled them to pick webworm-free leaves. If the goats picked up leaves with a webworm, they shook or discarded the leaf. They spat out webworms that entered their mouths, after detecting them by touch and taste. By using their keen senses and efficient behaviours, goats are able to feed while accurately excluding insects. These findings highlight the importance of direct interactions between mammalian herbivores and insects.

How to induce defense responses in wild plant populations? Using bilberry (Vaccinium myrtillus) as example

Inducible plant defense is a beneficial strategy for plants, which imply that plants should allocate resources from growth and reproduction to defense when herbivores attack. Plant ecologist has often studied defense responses in wild populations by biomass clipping experiments, whereas laboratory and greenhouse experiments in addition apply chemical elicitors to induce defense responses. To investigate whether field ecologists could benefit from methods used in laboratory and greenhouse studies, we established a randomized block-design in a pine-bilberry forest in Western Norway. We tested whether we could activate defense responses in bilberry (Vaccinium myrtillus) by nine different treatments using clipping (leaf tissue or branch removal) with or without chemical treatment by methyljasmonate (MeJA). We subsequently measured consequences of induced defenses through vegetative growth and insect herbivory during one growing season. Our results showed that only MeJA-treated plants showed consistent defense responses through suppressed vegetative growth and reduced herbivory by leaf-chewing insects, suggesting an allocation of resources from growth to defense. Leaf tissue removal reduced insect herbivory equal to the effect of the MeJa treatments, but had no negative impact on growth. Branch removal did not reduce insect herbivory or vegetative growth. MeJa treatment and clipping combined did not give an additional defense response. In this study, we investigated how to induce defense responses in wild plant populations under natural field conditions. Our results show that using the chemical elicitor MeJA, with or without biomass clipping, may be a better method to induce defense response in field experiments than clipping of leaves or branches that often has been used in ecological field studies.

Selection on parental performance opposes selection for larger body mass in a wild population of blue tits

There is abundant evidence in many taxa for positive directional selection on body size, and yet little evidence for microevolutionary change. In many species, variation in body size is partly determined by the actions of parents, so a proposed explanation for stasis is the presence of a negative genetic correlation between direct and parental effects. Consequently, selecting genes for increased body size would result in a correlated decline in parental effects, reducing body size in the following generation. We show that these arguments implicitly assume that parental care is cost free, and that including a cost alters the predicted genetic architectures needed to explain stasis. Using a large cross-fostered population of blue tits, we estimate direct selection on parental effects for body mass, and show it is negative. Negative selection is consistent with a cost to parental care, mainly acting through a reduction in current fecundity rather than survival. Under these conditions, evolutionary stasis is possible for moderately negative genetic correlations between direct and parental effects. This is in contrast to the implausibly extreme correlations needed when care is assumed to be cost-free. Thus, we highlight the importance of accounting correctly for complete selection acting on traits across generations.

Correlated responses in death-feigning behavior, activity, and brain biogenic amine expression in red flour beetle Tribolium castaneum strains selected for walking distance

Dispersal ability may influence antipredator and mating strategies. A previous study showed a trade-off between predation avoidance and mating success in strains of the red flour beetle Tribolium castaneum selected for walking distance . Specifically, beetles derived from strains selected for longer walking distance suffered higher predation pressure and had higher male mating success than their counterparts derived from strains selected for shorter walking distance. In the study reported here, we compared the locomotor activity, biogenic amine expression in the brain, and death-feigning behavior of the red flour beetle strains selected for walking distance. The results indicated that individuals genetically predisposed to longer walking distance had higher locomotor activity and lower intensity of death-feigning behavior than those genetically predisposed to shorter walking distance. However, no significant differences were found in the expression of biogenic amines in the brain among strains selected for walking distance, although the level of dopamine in the brain differed from that of the strains divergently selected for duration of death-feigning behavior. The relationships between walking speed, activity, death-feigning behavior, and brain biogenic amines in T. castaneum are discussed.

Burrowing energetics of the Giant Burrowing Cockroach Macropanesthia rhinoceros: an allometric study

Burrowing is an important life strategy for many insects, yet the energetic cost of constructing burrows has never been studied in insects of different sizes. Open flow respirometry was used to determine the allometric scaling of standard metabolic rate (MRS) and burrowing metabolic rate (MRB) in the heaviest extant cockroach species, the Giant Burrowing Cockroach Macropanesthia rhinoceros, at different stages of development. At 10 °C, MRS (mW) scales with body mass (M; g) according to the allometric power equation, MRS=0.158M(0.74), at 20 °C the equation is MRS=0.470M(0.53), and at 30 °C the equation is MRS=1.22M(0.49) (overall Q10=2.23). MRS is much lower in M. rhinoceros compared to other insect species, which is consistent with several aspects of their life history, including flightlessness, extreme longevity (>5 years), burrowing, parental behaviour, and an energy-poor diet (dry eucalypt leaf litter). Energy expenditure during burrowing at 25 °C scales according to MRB=16.9M(0.44), and is approximately 17 times higher than resting rates measured at the same temperature, although the metabolic cost over a lifetime is probably low, because the animal does not burrow to find food. The net cost of transport by burrowing (Jm(-1)) scales according to NCOT=120M(0.49), and reflects the energetically demanding task of burrowing compared to other forms of locomotion. The net cost of excavating the soil (J cm(-3)) is statistically independent of body size.

Weevil x Insecticide: Does 'Personality' Matter?

An insect’s behavior is the expression of its integrated physiology in response to external and internal stimuli, turning insect behavior into a potential determinant of insecticide exposure. Behavioral traits may therefore influence insecticide efficacy against insects, compromising the validity of standard bioassays of insecticide activity, which are fundamentally based on lethality alone. By extension, insect ‘personality’ (i.e., an individual’s integrated set of behavioral tendencies that is inferred from multiple empirical measures) may also be an important determinant of insecticide exposure and activity. This has yet to be considered because the behavioral studies involving insects and insecticides focus on populations rather than on individuals. Even among studies of animal ‘personality’, the relative contributions of individual and population variation are usually neglected. Here, we assessed behavioral traits (within the categories: activity, boldness/shyness, and exploration/avoidance) of individuals from 15 populations of the maize weevil (Sitophilus zeamais), an important stored-grain pest with serious problems of insecticide resistance, and correlated the behavioral responses with the activity of the insecticide deltamethrin. This analysis was performed at both the population and individual levels. There was significant variation in weevil ‘personality’ among individuals and populations, but variation among individuals within populations accounted for most of the observed variation (92.57%). This result emphasizes the importance of individual variation in behavioral and ‘personality’ studies. When the behavioral traits assessed were correlated with median lethal time (LT₅₀) at the population level and with the survival time under insecticide exposure, activity traits, particularly the distance walked, significantly increased survival time. Therefore, behavioral traits are important components of insecticide efficacy, and individual variation should be considered in such studies. This is so because population differences provided only crude approximation of the individual personality in a restrained experimental setting likely to restrict individual behavior favoring the transposition of the individual variation to the population.

When herbivores eat predators: predatory insects effectively avoid incidental ingestion by mammalian herbivores

The direct trophic links between mammalian herbivores and plant-dwelling insects have been practically ignored. Insects are ubiquitous on plants consumed by mammalian herbivores and are thus likely to face the danger of being incidentally ingested by a grazing mammal. A few studies have shown that some herbivorous hemipterans are able to avoid this peril by dropping to the ground upon detecting the heat and humidity on the mammal's breath. We hypothesized that if this risk affects the entire plant-dwelling insect community, other insects that share this habitat are expected to develop similar escape mechanisms. We assessed the ability of three species (adults and larvae) of coccinellid beetles, important aphid predators, to avoid incidental ingestion. Both larvae and adults were able to avoid incidental ingestion effectively by goats by dropping to the ground, demonstrating the importance of this behavior in grazed habitats. Remarkably, all adult beetles escaped by dropping off the plant and none used their functional wings to fly away. In controlled laboratory experiments, we found that human breath caused 60–80% of the beetles to drop. The most important component of mammalian herbivore breath in inducing adult beetles and larvae to drop was the combination of heat and humidity. The fact that the mechanism of dropping in response to mammalian breath developed in distinct insect orders and disparate life stages accentuates the importance of the direct influence of mammalian herbivores on plant-dwelling insects. This direct interaction should be given its due place when discussing trophic interactions.

Spatial sorting may explain evolutionary dynamics of wing polymorphism in pygmy grasshoppers

Wing polymorphism in insects provides a good model system for investigating evolutionary dynamics and population divergence in dispersal-enhancing traits. This study investigates the contribution of divergent selection, trade-offs, behaviour and spatial sorting to the evolutionary dynamics of wing polymorphism in the pygmy grasshopper Tetrix subulata (Tetrigidae: Orthoptera). We use data for > 2800 wild-caught individuals from 13 populations and demonstrate that the incidence of the long-winged (macropterous) morph is higher and changes faster between years in disturbed habitats characterized by succession than in stable habitats. Common garden and mother-offspring resemblance studies indicate that variation among populations and families is genetically determined and not influenced to any important degree by developmental plasticity in response to maternal condition, rearing density or individual growth rate. Performance trials show that only the macropterous morph is capable of flight and that propensity to fly differs according to environment. Mark-recapture data reveal no difference in the distance moved between free-ranging long- and short-winged individuals. There is no consistent difference across populations and years in number of hatchlings produced by long- and shorter-winged females. Our findings suggest that the variable frequency of the long-winged morph among and within pygmy grasshopper populations may reflect evolutionary modifications driven by spatial sorting due to phenotype- and habitat type-dependent emigration and immigration.

Dopaminergic system as the mechanism underlying personality in a beetle

Individuals in many animal species exhibit 'personality,' consistent differences in behaviour across time, situations and/or contexts. Previous work has revealed a negative genetic correlation between intensity of tonic immobility and walking activity levels in the confused flour beetle, Tribolium confusum, thus suggesting these beetles exhibit personality in activity-related traits. The present study investigated the mechanism underlying this correlation. We used individuals derived from two strains established via artificial divergent selection for duration of tonic immobility. "Long" (L) strains exhibited higher frequencies and longer durations of tonic immobility, and lower activity levels, while "Short" (S) strains exhibited lower frequencies and shorter durations of tonic immobility and higher activity levels. We found that the duration of tonic immobility, and activity level, could be altered by caffeine administration; L strains fed with caffeine exhibited decreased durations of tonic immobility and increased activity levels. We also found that brain dopamine levels were lower in L strains than in S strains. Consequently, this study demonstrates that the dopaminergic system plays an important part in controlling the genetic correlation between tonic immobility and activity levels in this species.

Tonic immobility in terrestrial isopods: intraspecific and interspecific variability

Many arthropods, including terrestrial isopods, are capable of entering a state of tonic immobility upon a mechanical disturbance. Here we compare the responses to mechanical stimulation in three terrestrial isopods Balloniscus glaber, Balloniscus sellowii and Porcellio dilatatus. We applied three stimuli in a random order and recorded whether each individual was responsive (i.e. showed tonic immobility) or not and the duration of the response. In another trial we related the time needed to elicit tonic immobility and the duration of response of each individual. Balloniscus sellowii was the least responsive species and Porcellio dilatatus was the most, with 23% and 89% of the tested individuals, respectively, being responsive. Smaller Balloniscus sellowii were more responsive than larger individuals. Porcellio dilatatus responded more promptly than the Balloniscus spp. but it showed the shortest response. Neither sex, size nor the type of stimulus explained the variability found in the duration of tonic immobility. These results reveal a large variability in tonic immobility behavior, even between closely related species, which seems to reflect a species-specific response to predators with different foraging modes.

Anti-predator defence drives parallel morphological evolution in flea beetles

Complex morphological or functional traits are frequently considered evolutionarily unique and hence useful for taxonomic classification. Flea beetles (Alticinae) are characterized by an extraordinary jumping apparatus in the usually greatly expanded femur of their hind legs that separates them from the related Galerucinae. Here, we examine the evolution of this trait using phylogenetic analysis and a time-calibrated tree from mitochondrial (rrnL and cox1) and nuclear (small subunits and large subunits) genes, as well as morphometrics of femora using elliptic Fourier analysis. The phylogeny strongly supports multiple independent origins of the metafemoral spring and therefore rejects the monophyly of Alticinae, as defined by this trait. Geometric outline analysis of femora shows the great plasticity of this structure and its correlation with the type and diversity of the metafemoral springs. The recognition of convergence in jumping apparatus now resolves the long-standing difficulties of Galerucinae-Alticinae classification, and cautions against the value of trait complexity as a measure of taxonomic significance. The lineage also shows accelerated species diversification rates relative to other leaf beetles, which may be promoted by the same ecological factors that also favour the repeated evolution of jumping as an anti-predation mechanism.

Initial behavior in colony fragments of an introduced population of the invasive ant Wasmannia auropunctata

We investigated in the laboratory the initial behavior of propagules of the invasive ant Wasmannia auropunctata in Cameroon where it has been introduced. Both workers and queens at first feigned death (thanatosis), and then the workers slowly moved around the experimental arena; the queens did the same about 10seconds later. Each queen antennated selected workers that then aggregated together by grasping the hind leg of another ant with their mandibles. When encountering the queen again, the lead worker climbed up the queen's hind leg and onto her back, followed by some other individuals. The remaining workers followed the queen to a location in the experimental arena. When brood was present, the workers transferred it to this location. Orphaned workers did not aggregate, but gathered the brood together and took care of it. By permitting propagules to survive, these behaviors likely contribute to the success of W. auropunctata as an invader.

Walking activity of flightless Harmonia axyridis (Coleoptera: Coccinellidae) as a biological control agent

The use of flightless strains of the multicolored Asian lady beetle, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), established via artificial selection, can be highly effective as a biological control agent for aphids. However, flightless H. axyridis must depend on walking for dispersion. Therefore, data on the walking activity levels in flightless strains are important for the development of effective methods when releasing these agents in the field. Results of measurement of walking activity levels using an infrared actograph showed that walking activity levels during the daytime (but not nighttime) in both sexes of pure flightless strains tended to be lower than those of control strains. We also found that walking activity levels during the daytime for the F1 generation of hybrid strains, produced by reciprocal crossing between two pure flightless strains, were approximately equal to those of pure strains; the reduction in walking activity levels was not recovered by hybrid vigor. Our results indicate that the reduction in walking activity levels in the pure flightless strains was not caused merely by inbreeding depression stemming from the artificial selection process. Instead, potentially flight ability and walking activity levels in this species may be controlled by the pleiotropic effect of a gene.

Biogenic amines, caffeine and tonic immobility in Tribolium castaneum

Biogenic amines are physiologically neuroactive substances that affect behavioural and physiological traits in invertebrates. In the present study, the effects of dopamine, octopamine, tyramine and serotonin on tonic immobility, or death-feigning, were investigated in Tribolium castaneum. These amines were injected into the abdomens of beetles artificially selected for long or short duration of tonic immobility. In beetles of the long strains, the durations of tonic immobility were shortened by injection of dopamine, octopamine and tyramine, and the effects of these amines were dose-dependent. On the other hand, serotonin injection did not affect the duration of tonic immobility. In the short-strain beetles that rarely feign death, no significant effects of the amines were found on the duration of tonic immobility. Brain expression levels of octopamine, tyramine and serotonin did not differ between long- and short-strain beetles, in contrast to the higher dopamine levels in short strains previously reported. Caffeine decreased the duration of death-feigning in both oral absorption and injection experiments. It is known that caffeine activates dopamine. Therefore, the present results suggest that the duration of tonic immobility is affected by dopamine via the dopamine receptor in T. castaneum.

Testing for direct and indirect effects of mate choice by manipulating female choosiness

Despite a massive research effort, our understanding of the evolution of female mate choice remains incomplete [1, 2]. A central problem is that the predominating empirical research tradition has focused on male traits, yet the key question is whether female choice traits are maintained because of direct effects on female fitness or because of indirect genetic effects in offspring that may be associated with such traits. Here, we address this question by using a novel research strategy that employs experimental phenotypic manipulation of a female choice trait in an insect model system, the seed beetle Callosobruchus chinensis (Coleoptera: Bruchidae). We show that females with increased efficiency of choice enjoy strongly elevated fitness compared to females with reduced choice efficiency. In contrast, we found no effects of female choice efficiency on offspring fitness. Our results show that female choice is maintained by direct selection in females in this system, whereas indirect selection is relatively weak at most. We suggest that phenotypic engineering of female choice traits can greatly advance our ability to elucidate the relative importance of direct and indirect selection for the maintenance of female choice.

Tonically immobilized selfish prey can survive by sacrificing others

Death-feigning, also called tonic immobility, is found in a number of animal species across vertebrate and invertebrate taxa. To date, five hypotheses have been proposed for the adaptive significance of tonic immobility. These are that tonic immobility is effective for prey because (i) avoiding dead prey is safer for predators, (ii) immobility plays a role in physical defence, (iii) immobility plays a role in concealment and/or background matching, (iv) predators lose interest in unmoving prey, and (v) the characteristic immobilization posture signals a bad taste to predators. The fourth and fifth hypotheses have been considered suitable explanations for tonic immobility of the red flour beetle against its predator, the jumping spider. In the present study, we used chemical analyses of secretions by the red flour beetles under attack by the jumping spider to reject the fifth hypothesis for this system. More importantly, we tested a selfish-prey hypothesis for the adaptive significance of death-feigning as an anti-predator strategy, in which individuals adopting tonic immobility survive by sacrificing neighbours. Findings showed that survival rates of feigners were higher when in the presence of non-feigners or prey of a different species, compared to when alone, thus confirming our selfish-prey hypothesis. In summary, our results suggest that immobility following a spider attack is selfish; death-feigning prey increase their probability of survival at the expense of more mobile neighbours.