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Wolbachia is a widespread maternally transmitted endosymbiotic bacteria with diverse phenotypic effects on its insect hosts, ranging from parasitic to mutualistic. Wolbachia commonly infects social insects, where it faces unique challenges associated with its host's caste-based reproductive division of labor and colony living. Here, we dissect the benefits and costs of Wolbachia infection on life-history traits of the invasive pharaoh ant, Monomorium pharaonis, which are relatively short lived and show natural variation in Wolbachia infection status between colonies. We quantified the effects of Wolbachia infection on the lifespan of queen and worker castes, the egg-laying rate of queens across queen lifespan, and the metabolic rates of whole colonies and colony members. Infected queens laid more eggs than uninfected queens but had similar metabolic rates and lifespans. Interestingly, infected workers outlived uninfected workers. At the colony level, infected colonies were more productive as a consequence of increased queen egg-laying rates and worker longevity, and infected colonies had higher metabolic rates during peak colony productivity. While some effects of infection, such as elevated colony-level metabolic rates, may be detrimental in more stressful natural conditions, we did not find any costs of infection under relatively benign laboratory conditions. Overall, our study emphasizes that Wolbachia infection can have beneficial effects on ant colony growth and worker survival in at least some environments.

•Species with simple societies are not the ancestors of species with complex societies.•The evolution of social complexity has not proceeded in a predictable stepwise manner.•Social ladder thinking is inconsistent with available data and modern tree thinking.•Modern phylogenetic comparative methods enable rigorous evolutionary inference.•Studying species in each independent eusocial lineage is invaluable in its own right.

•New technology allows probing of proximate processes underlying social behavior.•Existing social evolution theory has little to say about these new kinds of data.•We argue for integrating proximate processes with social evolutionary theory.•Existing approaches to model flexible behaviors do not go far enough.•Evolutionary modeling of gene regulatory networks presents a promising direction.

Brain evolution is hypothesized to be driven by requirements to adaptively respond to environmental cues and social signals. Diverse models describe how sociality may have influenced eusocial insect-brain evolution, but specific impacts of social organization and other selective forces on brain architecture have been difficult to distinguish. Here, we evaluate predictions derived from and/or inferences made by models of social organization concerning the effects of individual and collective behavior on brain size, structure, and function using results of neuroanatomical and genomic studies. In contrast to the predictions of some models, we find that worker brains in socially complex species have great behavioral and cognitive capacity. We also find that colony size, the evolution of worker physical castes, and task specialization affect brain size and mosaicism, supporting the idea that sensory, processing and motor requirements for behavioral performance select for adaptive allometries of functionally specialized brain centers. We review available transcriptomic and comparative genomic studies seeking to elucidate the molecular pathways functionally associated with social life and the genetic changes that occurred during the evolution of social complexity. We discuss ways forward, using comparative neuroanatomy, transcriptomics, and comparative genomics, to distinguish among multiple alternative explanations for the relationship between the evolution of neural systems and social complexity.

SummaryA new and simple technique was developed to rear honey bees (Apis melliferaL.) in vitro.One day old larvae were grafted into Petri dishes and fed a basic diet at six different time intervals. There were no differences in the larval weights, survival rates or ovariole numbers of the bees among the groups that were fed at different intervals, but they were heavier and had larger ovaries than hive reared bees. It was shown that honey bees can be reared in vitrowithout replenishing their food daily. This simple mass provisioning technique reduces the labour involved, and enables the researcher to raise large number of bees in vitro.

In some leaf‐cutting ant species, minim workers ride on the fragments of leaves as they are carried back to the nest from the cutting site. There is convincing evidence that these “hitchhikers” can protect the leaf carriers from attack by phorid (Diptera: Phoridae) parasitoids, but we consider the possibility of other functions for the hitchhiking behavior. It has been hypothesized that the hitchhikers (1) feed on leaf sap from the edges of the cut leaves; (2) ride back to the nest to save energy; (3) get caught on the fragments as they are cut, and hitchhike because they cannot (or will not) get off; and (4) begin the process of preparing the leaf to enter the fungal gardens in the nest, perhaps by removing microbial contaminants. We observed hitchhikers of Atta cephalotesin 14 nests at the La Selva Biological Station in Costa Rica. There was no difference in the proportion of leaf carriers with hitchhikers between day and night. Because the nests we observed were largely nocturnal, more than 90 percent of the hitchhiking occurred at night. The phorid parasitoids are usually considered to be diurnal, so the preponderance of nocturnal hitchhiking suggests other functions in addition to parasitoid defense. Hitchhikers spent more time in the defensive head‐up posture during the day, but spent more time in the head‐down posture at night. The head‐down posture may indicate cleaning or other leaf preparation. The hitchhikers were never observed feeding on sap. Hitchhikers frequently got onto and off of the fragments, and so they were not “marooned.” Few hitchhikers rode all the way back to the nest and were often moving on the leaf fragment; these observations make the energy conservation hypothesis less likely, although we cannot reject it. We conclude that parasitoid defense is an important function of hitchhiking but also that there are probably other functions when parasitoids are absent. Based on available data, the most likely possibility is preparation of the leaf fragment before it enters the nest.

Wolbachiais a widespread genus of maternally transmitted endosymbiotic bacteria that often manipulates the reproductive strategy and life history of its hosts to favor its own transmission. Wolbachia-mediated phenotypic effects are well characterized in solitary hosts, but effects in social hosts are unclear. The invasive pharaoh ant, Monomorium pharaonis, shows natural variation in Wolbachiainfection between colonies and can be readily bred under laboratory conditions. We previously showed that Wolbachia-infected pharaoh ant colonies had more queen-biased sex ratios than uninfected colonies, which is expected to favor the spread of maternally transmitted Wolbachia. Here, we further characterize the effects of Wolbachiaon the short- and longer-term reproductive and life history traits of pharaoh ant colonies. First, we characterized the reproductive differences between naturally infected and uninfected colonies at three discrete time points and found that infected colonies had higher reproductive investment (i.e. infected colonies produced more new queens), particularly when existing colony queens were 3 months old. Next, we compared the long-term growth and reproduction dynamics of infected and uninfected colonies across their whole life cycle. Infected colonies had increased colony-level growth and early colony reproduction, resulting in a shorter colony life cycle, when compared with uninfected colonies.