Abstract
Division of labor within and between the worker and queen castes is thought to underlie the tremendous success of social insects. Colonies might benefit if subsets of nurse workers specialize in caring for larvae of a certain stage or caste, given that nutritional requirements depend on larval stage and caste. We used short- (<1 hr) and long-term (ten days) behavioral observations together with transcriptomic analysis to determine whether nurses of the pharaoh ant Monomorium pharaonis exhibit such behavioral and/or physiological specialization. We found that nurses were behaviorally specialized based on larval instar but not on larval caste. This specialization was widespread, with 56% of nurses in the short-term and between 22-27% in the long-term showing significant specialization. We also found transcriptomic signatures of nurse specialization on larval stage but not caste. Genes associated with nurse specialization included vitellogenin and mrjp2, which have previously been implicated in the transfer of nutrition from nurse to larvae and the regulation of larval development and caste, as well as other genes coding for secreted proteins, which may also be passed from nurses to larvae via trophallaxis. Altogether, our results provide the first evidence in any social insect for a division of labor among nurse workers based on larval stage.