How molting locusts avoid cannibalism

Group living has various benefits, but it also carries costs, such as risk of cannibalism. Molting is a vulnerable period of being cannibalized in juvenile arthropods, but how gregarious arthropods avoid this threat is poorly understood. Here, we examined how actively migrating gregarious nymphs of desert locust, Schistocerca gregaria, avoid cannibalism during molting, in the Sahara Desert of Mauritania. In the field, gregarious nymphs cyclically march and feed on grass during the day. Our field observations found that marching behavior helped separating pre-molting and cannibalistic non-molting nymphs. Cannibalistic non-molting nymphs marched away from roost plants, leaving sedentary pre-molting nymphs behind, creating cannibal-free spaces. Some non-molting nymphs reached a pre-molting state after daytime marching, thus both pre- and non-molting nymphs roosted on same plants at night. However, pre-molting nymphs moved away from conspecifics prior to molting. Starvation experiments confirmed that food-satiation decreased cannibalistic necrophagy. Physiological surveys of diel feeding and molting patterns revealed that nymphs molted at times when conspecifics were food-satiated rather than hungry. Hence, our results indicate that behavioral and physiological traits of gregarious locusts could function to spatiotemporally separate molting locusts from cannibalistic conspecifics, thus reducing molting-associated cannibalism. This is the first report of migration-dependent molting synchrony as a mechanism reducing costs of aggregation in gregarious arthropods.