Costs and benefits of maternal nest choice: Trade-offs between brood survival and thermal stress in bees.

Optimal nest site selection is crucial in animals whose offspring are completely dependent on the shelter of a nest. Parental decisions influencing nest thermal conditions are particularly important because temperature strongly influences juvenile activity, metabolism, growth, developmental rate, survival, and adult body size. In small ectotherms such as bees, maternal decisions to nest in sun-exposed or shady sites can lead to marked differences in thermal microenvironments inside nests. Small carpenter bees (Ceratina calcarata) strongly prefer to nest in sun but also prefer nesting substrates more frequently found in shade, suggesting that nest site selection is based on a trade-off between costs and benefits of warmer versus cooler nest sites. We investigated the consequences of sun and shade nesting for mothers and their offspring using a field experiment in which mothers and newly founded nests were placed in sunny or shady habitats. Maternal costs and benefits in each treatment were quantified by comparing maternal foraging effort, nest size, number of brood provisioned, and number and size of live offspring. These demographic measures allowed us to estimate fitness for mothers nesting in sun versus shade. For juvenile bees from sun and shade nests, we quantified two thermal traits, high-temperature tolerance (CT) and metabolic rate. Mothers in sun nests had significantly higher nesting success, with 59% of all nests producing brood, while mothers in shade nests experienced only 32% success. Successful sun nests actually contained fewer live brood (5.2 ± 3.0, mean ± SD) than shade nests (6.9 ± 3.3), but their higher success rates meant that maternal fitness was higher in sun than in shade. However, sun nesting entailed clear costs to brood, which were significantly smaller, less likely to survive to adulthood, and had significantly elevated CT, suggesting that thermal stress during development necessitated them to shunt resources from growth to thermoprotection. The maternal preferences for sun nesting optimize maternal fitness despite the evident costs to juveniles developing in sun-exposed nests.