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.     

昆虫假死行为研究进展

假死是昆虫重要的防御行为,与生存策略密切相关.昆虫受到天敌或类似天敌的外部物理刺激时,足、触角等附肢收缩,强直静止,自发进入假死状态.近年来,昆虫假死行为相关的研究逐渐增多,进展明显,明确了多种昆虫假死的发生时间、发生频率、刺激方式和诱发因素等特征,涉及臭椿沟眶象Eucryptorrhynchus brandti、赤拟谷盗 Tribolium castaneum、绿豆象 Callosobruchus chinensis、马铃薯甲虫 Leptinotarsa decemlineata、甘薯象甲 Cylas formicarius、毛健夜蛾Brithys crini、长壮蝎蝽Laccotrephes robustus等40种昆虫;但昆虫假死行为的发生机制仍未明确.随着技术的发展,从昆虫生理学(生物胺)、神经生物学甚至分子生物学角度研究昆虫假死行为机制成为可能.本文总结了近年来国内外关于昆虫假死行为的研究成果,重点介绍了昆虫假死行为发生机制方面的进展,指出了存在的问题和今后需要进一步研究的方向.     

毛健夜蛾幼虫假死行为诱导因子研究

【目的】探明毛健夜蛾Brithys crini Fabricius幼虫另类假死行为的规律和特点。【方法】在实验室条件下观察毛健夜蛾幼虫体重、连续刺激、作用力的大小、背景声音和背景LED光色对假死发生率和假死持续时间的影响。【结果】(1)假死持续时间有随幼虫体重增加而延长的趋势,但相关性不显著。(2)假死持续时间有随刺激次数增加而缩短的趋势,与刺激次数呈显著性负相关;前10次刺激的平均假死持续时间(85.6 s)显著长于后10次(67.3 s)。(3)2、5和10 N力处理组间假死持续时间(分别为82.3、97.4和78.3 s)无显著差异,但三者均显著短于6 N力处理组的假死持续时间(157.0 s)。(4)风声处理组假死持续时间(65.6 s)显著短于鸟鸣声和蛙叫声处理组(分别为144.8 s和134.1 s)。(5)不同颜色背景LED光源处理下,假死持续时间有显著差异性,红色闪光、红色、黄色、绿色、白色和蓝色处理组假死持续时间分别为54.7、64.3、82.7、90.6、96.9、112.3 s。各处理假死发生率没有显著性差异。【结论】毛健夜蛾幼虫假死行为可因内外环境因素不同而异。     

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.