Abstract
Coordinated movement of animals is a spectacular phenomenon that has received much attention. Experimental studies of Mormon crickets and locust nymphs have demonstrated that collective motion can arise from cannibalism that compensates for nutritional deficiencies arising from group living. Grouping into migratory bands confers protection from predators. By radiotracking migrating, Mormon crickets released over 3 days, we found that specialized, parasitoid digger wasps (Sphecidae) respond numerically and prey heavily on aggregated Mormon crickets leading to loss of safety in numbers. Palmodes laeviventris paralysed and buried 42% of tagged females and 8% of the males on the final day of tracking. Risk of wasps and Mormon crickets hatching on the same site is high and may drive nymphal emigration. A preference to provision offspring with adult female Mormon crickets can be explained by their greater fat content and larger size compared with males, improving survival of wasps during diapause.
Highlights
Group living benefits animals in part by providing them with protection, whereas costs of living in groups often lead to collective movement
Increases, costs of group living increase as well owing to factors such as nutrient scarcity 2 and cannibalism [6], increased disease transmission [7], and the functional and numerical responses of predators to prey density [8,9]
We identified the predator as the sphecid wasp P. laeviventris (Cresson, figure 1), which is only known to predate on Mormon crickets [19]
Summary
Group living benefits animals in part by providing them with protection, whereas costs of living in groups often lead to collective movement. Some of the most spectacular animal mass movements, from gazelles to locusts, have been interpreted in this cost–benefit framework of protection from predation and cost reduction. This framework has been the focus of a number of theoretical and empirical studies. Empirical studies have trailed theoretical ones because of the difficulty of tracking individuals in groups, and because predation events and the causes of collective movement are difficult to observe. Advances in tracking [10] and the ability to capture, measure or manipulate phenotypes, and release individuals back into migratory bands have made Mormon crickets Anabrus simplex (Orthoptera: Tettigoniidae) and nymphal locusts (hoppers) model organisms for understanding collective motion in this cost–benefit framework [11]
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