Excess offspring as a maternal strategy: constraints in the shared nursery of a giant damselfly

Maternal reproductive strategies should optimize the quality and quantity of surviving offspring. In Megaloprepus caerulatus, a damselfly that exhibits male-biased size dimorphism, larval siblicide, and a disproportionate fitness advantage from large sons, mothers lay many more eggs in water-filled tree holes than can survive to emergence. Using field experiments, I tested the siblicide advantage of excess offspring (i.e., faster development and/or larger survivors) in small and large holes and 2 alternative functions of excess offspring (predator satiation and insurance against nonpredator mortality). In small pots, the sole siblicidal survivors emerged larger than noncannibals but no sooner. However, doubling or even quadrupling a modest clutch of 25 failed to produce larger offspring. In large tubs, the size advantage that survivors gained from siblicide was constrained by a trade-off between offspring size and number. A clutch of 20 produced half as many but larger offspring than one of 100. When multiple females contributed eggs to a large nursery, size of survivors was independent of the mother's clutch size. Finally, large clutches failed to satiate dragonfly predators, and although 25 neonates were better than 2 as insurance against nonpredator mortality, a clutch of 50 provided no additional benefit. In natural and experimental holes, survivorship was female biased, suggesting that sons suffered greater mortality than daughters. Because mothers seemed unable to adaptively bias offspring sex ratio, excess offspring may compensate for the lower survivorship of sons, particularly in large nurseries where males garner a disproportionate size advantage relative to females.