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

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.Publisher PDFPeer reviewe

Consequences of grouped data for testing for departure from circular uniformity

Limits to the precision of circular data often cause grouping of data points into discrete categories; but the effects of grouping on tests for circular uniformity have been little explored. The Rayleigh test is often applied to grouped circular data, despite it being designed for continuous data and the statistical literature recommending a suite of alternative tests specifically designed for grouped data. Here we investigated the performance of the Rayleigh test relative to four alternatives for testing the null hypothesis of uniformity in grouped circular data. We employed simulations grouping data into a discrete number of same-sized categories, and with samples drawn from a range of different distributions. We found that grouping had little effect on the type I error rate or the power of the Rayleigh test, and that the power of the Rayleigh test was very similar to that of the previously-recommended alternative tests designed specifically for grouped circular data. It may thus be appropriate to apply the Rayleigh test to grouped data, providing the situation is one in which the test has substantial statistical power.Publisher PDFPeer reviewe

Post-dropping behavior of potato aphids (Macrosiphum euphorbiae)

Funding: Perry Foundation, University of St Andrews, Scottish Government Rural and Environmental Sciences and Analytical Services Division.Dropping behavior is an effective antipredator defense utilized by many insects including aphids, which drop from plants to lower plant parts or underlying substrates to avoid attack from predatory invertebrates. While research commonly focusses on triggers of dropping, less attention is given to what happens to prey individuals following escape drops. In this study, the duration of tonic immobility, recovery rates, and cases of “instant recovery” (re-clinging to lower plant parts) exhibited by potato aphids (Macrosiphum euphorbiae) that dropped from potted seedlings in response to introduced ladybird (Adalia bipunctata) adults, lacewing (Chrysoperla carnea) larvae, and a standardized tactile stimulus were investigated in relation to a range of environmental factors. Air temperature had a negative correlation with the duration of post-dropping tonic immobility; as temperature increased, time spent motionless decreased. Aphids also showed a pattern of increased recovery rate at higher temperatures. Aphids may be selected to move off the substrate quicker to avoid risks of overheating/desiccation at higher temperatures; and/or higher body temperature facilitates locomotion. Stimulus type also influenced recovery rate back to the original seedling, with aphids generally recovering after the standardized stimulus quicker than after dropping triggered by a real predator. Considering cases of instant recovery onto lower-reaches of the host seedling, seedling height influenced the likelihood of re-clinging, with aphids that managed to instantly recover dropping from, on average, taller seedlings than aphids that dropped to the substrate. Plant architecture could mitigate the costs of dropping for aphids, but further studies quantifying understory foliage cover are needed.Publisher PDFPeer reviewe

Predator-prey behavioural interactions on plants, with special emphasis on aphid dropping defence and ladybird search strategies

Predator-prey interactions have significant short- and long-term implications for the individuals involved, their populations’ dynamics, and the broader ecological community. Predation is a strong selective force that has led to prey evolving a great diversity of defensive adaptations, while predators undergo strong selection to successfully locate and capture prey. For both ‘sides’ of these interactions, behavioural adaptations offer flexibility in deployment, and so are expected to be context-sensitive. In this thesis, I use the ladybird-aphid predator-prey system to examine both prey defence behaviours and predator search strategies. Considering the prey, dropping – a widespread but understudied antipredator behaviour – is my focus. My findings suggest that, although dropping seems superficially simple, the selective pressures on it are complex. Dropping is affected by internal factors, such as genotype and the presence of endosymbionts. The nature and proximity of the predatory threat also affect dropping. Further, the consequences of dropping depend on many factors, including where on a plant predator-prey encounters occur; this in turn will be influenced by predator search strategies. Considering predator search, my focus is ladybird movements when searching branched environments, particularly potential turning biases and turn alternation. My work suggests that predators do not follow simple rules when searching a plant for prey, but they may exhibit some innate turning tendencies under particular conditions. Predator search strategies will be highly influential in affecting the context (e.g. location on plant) in which predator-prey interactions take place. Future work should investigate predator search further in order to provide a deeper understanding of the selective pressures on dropping in prey. For both prey dropping and predator search strategies, researchers must also appreciate the context-sensitivity of the different behaviours, and work to understand influencing cues, triggers, and outcomes specific to the species and circumstances involved

Predator-prey behavioural interactions on plants, with special emphasis on aphid dropping defence and ladybird search strategies (thesis data)

Excel spreadsheets containing the primary underpinning data used in the analyses of the thesis