Sociodemographic variation in foraging behavior and the adaptive significance of worker production in the facultatively social small carpenter bee, Ceratina calcarata

Abstract

Provisioning for young offspring is an archetypical form of parental investment. Ceratina calcarata bees provide extended maternal care to their young and demonstrate an unusual strategy of dual-phase pollen provisioning. Most bees first gather provisions as they establish nests in spring. However, C. calcarata mothers will also feed their newly eclosed young a second time, perhaps ensuring their survival during a long winter diapause. Some mothers rear a small, worker-like daughter to assist them during this second provisioning phase. We studied provisioning behavior in C. calcarata to examine patterns of maternal investment and foraging dynamics throughout the breeding season. Mothers typically made a high number of short-duration foraging trips each day, whereas late-season females tended to make fewer and longer trips. This difference in foraging duration may indicate a lower risk of brood loss in those nests where mature offspring are present. Nest demographic data revealed that an offspring laid in the first brood cell position is typically female and usually smaller than her siblings. In 29% of the nests, this small daughter was observed to adopt a forager role at maturity and provisioned for her siblings. Dwarf daughters had a higher number of active days and foraging trips per day in orphaned nests than in nests where a mother was present. The foraging behaviors of worker-like daughters were similar in length of foraging trip and handling time to mothers during this second provisioning period. We hypothesize that incipiently social foraging by this smallest daughter may act as a form of insurance against brood loss during occasions when a mother is unable to sufficiently provision for her eclosed offspring during the second phase. Parental investment in the size and sex of offspring is under strong selection for assured fitness returns. For example, many social insect mothers make an initial investment in small offspring to take on risky foraging behavior while they specialize on future reproduction. Solitary and facultatively social species provide an important baseline to understand the evolution of social complexity from natural variation in maternal care and foraging behavior. Here, we characterize the parental investment strategies of a subsocial small carpenter bee and reveal the potential adaptive significance of prolonged maternal care and worker production in this species. Mothers provide an initial investment that is extended by workers providing alloparental care to siblings. Maternal manipulation of dwarf eldest daughters may serve as an insurance mechanism in the event of maternal mortality to assure the survival of siblings.