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.     

Body-Size Dependent Difference in Death-Feigning Behavior of Adult <Emphasis Type="Italic">Callosobruchus chinensis</Emphasis>

Size-dependent differences in death-feigning behavior, an anti-predator defense, was examined in the adult adzuki bean beetle, Callosobruchus chinensis (Coleoptera: Bruchidae). A positive correlation was observed between the duration of death-feigning and adult body weight in two replicated experiments. Moreover, under starved condition, beetles reared under high (= lighter beetles) larval density showed significantly shorter longevity and a shorter duration of death-feigning than those reared under low (= heavier beetles) larval density. The body-size-dependent difference in the duration of death-feigning is discussed from the viewpoint of the difference in survival strategies between small and large beetles. Because small beetles don’t live as long, they have less time in which to reproduce and so any loss of that short time might be a greater proportion of their fitness lost. This is the first report on the body-size-dependent difference in death-feigning behavior in the adult holometabolic beetle.     

Diurnal Periodicity of Death-Feigning in Cylas formicarius (Coleoptera: Brentidae)

Diurnal periodicity of death-feigning behavior, or thanatosis, was found in adults of the sweet potato weevil, Cylas formicarius (Fabricius) (Coleoptera: Brentidae). The proportion of death-feigned weevils was significantly lower at night than in the daytime only in males. The duration of death-feint was shorter at night than in the daytime in both sexes. The influences of a weevil's behavior prior to being startled (prestimulus behavior) on the proportion of adults feigning death and on the duration of the death-feint were examined. Resting weevils feigned death more frequently and took a longer duration to recover compared to walking ones. The results suggest that, in general, death-feigning is less frequent in active weevils compared to inactive ones. A cost–benefit relationship of death-feigning behavior is discussed in relation to survival and reproduction of C. formicarius.     

A sheep in wolf's clothing: do carrion and dung odours of flowers not only attract pollinators but also deter herbivores?

Carrion and dung odours of various flowers have traditionally been considered an adaptation for attracting the flies and beetles that pollinate them. While we accept the role of such odours in pollinator attraction, we propose that they may also have another, overlooked, anti‐herbivore defensive function. We suggest that such odours may deter mammalian herbivores, especially during the critical period of flowering. Carrion odour is a good predictor for two potential dangers to mammalian herbivores: (1) pathogenic microbes, (2) proximity of carnivores. Similarly, dung odour predicts faeces‐contaminated habitats that present high risks of parasitism. These are two new types of repulsive olfactory aposematic mimicry by plants: (1) olfactory feigning of carcass (thanatosis), a well‐known behavioural defensive strategy in animals, (2) olfactory mimicry of faeces, which also has a defensive visual parallel in animals.     

Variation in Propensity to Exhibit Thanatosis in Nasonia vitripennis (Hymenoptera: Pteromalidae)

Thanatosis (death-feigning) has rarely been documented for Hymenoptera but occurs in the parasitoid wasp Nasonia vitripennis. The propensity to exhibit thanatosis did not differ with age, sex, or food deprivation. Squeezing a female’s abdomen and contacting her antennae were equally likely to trigger thanatosis. Dropping an object next to a female in order to cause substrate vibrations never triggered thanatosis, and dropping a female from a test tube rarely triggered thanatosis. Thanatosis was not seen during interactions between females. There was some tendency for females to exhibit fewer thanatosis responses on white than on colored backgrounds. Females that were least active had the greatest tendency to exhibit thanatosis.     

EFFECTS OF THE ALARM PHEROMONE (E)-β-FARNESENE ON DISPERSAL BEHAVIOUR OF THE PEA APHID ACYRTHOSIPHON PISUM

The behaviour of Acyrthosiphon pisum when dislodged from its host differed depending on the release stimulus. Aphids dislodged by tactile stimuli from a fine brush feigned death (thanatosis) longer than those dislodged by exposure to the synthetic alarm pheromone (E)--farnesene or stimuli from an approaching coccinellid predator.The aphids showed two different behavioural patterns when they resumed activity. Those that had been dislodged by a coccinellid or farnesene usually exhibited longer periods of running behaviour before they started to search and climb plant models, whereas those dislodged by touch were more likely to show immediate searching behaviour.A high percentage of aphids exhibited running behaviour when the air was enriched with farnesene, but they soon changed and searched for plants. The results indicate a rapid habituation to farnesene.Supported by the Bundesministerium für Ernährung, Landwrirtschaft und Forsten, Bonn.     

The influence of mitonuclear genetic variation on personality in seed beetles

There is a growing awareness of the influence of mitochondrial genetic variation on life-history phenotypes, particularly via epistatic interactions with nuclear genes. Owing to their direct effect on traits such as metabolic and growth rates, mitonuclear interactions may also affect variation in behavioural types or personalities (i.e. behavioural variation that is consistent within individuals, but differs among individuals). However, this possibility is largely unexplored. We used mitonuclear introgression lines, where three mitochondrial genomes were introgressed into three nuclear genetic backgrounds, to disentangle genetic effects on behavioural variation in a seed beetle. We found within-individual consistency in a suite of activity-related behaviours, providing evidence for variation in personality. Composite measures of overall activity of individuals in behavioural assays were influenced by both nuclear genetic variation and by the interaction between nuclear and mitochondrial genomes. More importantly, the degree of expression of behavioural and life-history phenotypes was correlated and mitonuclear genetic variation affected expression of these concerted phenotypes. These results show that mitonuclear genetic variation affects both behavioural and life-history traits, and they provide novel insights into the maintenance of genetic variation in behaviour and personality.