Desiccated desert locust embryos reserve yolk as a "lunch box" for posthatching survival

Oviparous animals often prioritize the allocation of reproductive resources to egg size over clutch size, but the impact of this maternal investment on the allocation of embryonic yolks and its ecological functions is poorly understood. We investigated how desert locust embryos allocate yolk for survival energy after hatching, rather than embryonic somatic growth depending on egg size in response to desiccation stress. Crowd-reared females (gregarious phase) produced significantly larger progeny with higher tolerance to starvation than females reared in isolation (solitarious phase). Abnormally small hatchlings with residual yolk in their gut emerged from small and large eggs when exposed to desiccation. In particular, these small hatchlings of desiccated eggs survived significantly longer under starvation than those of wet ones, with larger eggs providing even greater survival benefits. Physiological analysis showed that hatchlings from desiccated eggs showed a trade-off by reserving more lipids without somatic growth than those from normal eggs. Desiccation could be a reliable signal for embryos to predict future poor vegetation, and reserved energy could increase the chance of accessing food after hatching. Our results underscore adaptive plasticity in maternal and embryonic resource allocation in desert locusts in response to unpredictably variable semi-arid habitats.

Risk of migration: not all gregarious locust nymphs reach appropriate refuges

Sedentary animals choose appropriate refuges against predators, while migratory ones may not necessarily do so. In ectotherms, refuge selection is critical during low temperatures, because they cannot actively evade predators. To understand how migratory ectotherms alter their defensive behaviors depending on refuge quality in cold temperatures, we evaluated migratory gregarious desert locust nymphs (Schistocerca gregaria) in the Sahara Desert, where daily thermal constraints occur. We recorded how roosting plant type (bush/shrub) and its height influenced two alternative defense behaviors (dropping/stationary) during cold mornings, in response to an approaching simulated ground predator. Most locusts in bushes dropped within the bush and hid irrespective of their height, whereas those roosting > 2 m height in shrubs remained stationary. These defenses are effective and match with refuge plant types because dynamic locomotion is not required. When nymphs roosted on shrubs < 1.5-m height, which was an unsafe position, nearly half showed both defensive behaviors, indicating that escaping decisions become ambiguous when the refuges are inappropriate. These results suggest that locusts display flexible defensive behaviors when finding appropriate refuges and selecting refuge before daily thermal limitations occur could be critical for migratory ectotherms, which is a risk associated with migration.