The early life of a leaf-cutter ant colony constrains symbiont vertical transmission and favors horizontal transmission.

Colonial organisms host a large diversity of symbionts (collectively, parasites, mutualists, and commensals) that use vertical transmission (from parent colony to offspring colony) and/or horizontal transmission to disperse between host colonies. The early life of some colonies, characterized by the dispersal and establishment of solitary individuals, may constrain vertical transmission and favor horizontal transmission between large established colonies. We explore this possibility with the miniature cockroach Attaphila fungicola, a symbiont of leaf-cutter ants and the mutualist fungal gardens they cultivate. The early life of a leaf-cutter colony is characterized by the dispersal of a female alate (winged "queen") carrying a fungal pellet, and the subsequent establishment of a foundress (workerless "queen") raising her incipient fungal garden and colony. Roaches hitchhike on female alates during leaf-cutter nuptial flights, which strongly suggests that roaches are vertically transmitted to foundresses and their incipient colonies; however, weak compatibility between roaches and incipient gardens may constrain roach vertical transmission. Reciprocally, opportunities for horizontal transmission between large established colonies with abundant fungal gardens may weaken selection against roach-induced harm (virulence) of incipient gardens. We use a laboratory experiment, behavioral observations, field surveys, and a transmission model to estimate the effect roaches have on the survivorship of incipient gardens and the frequency of roach vertical transmission. Contrary to traditional assumptions, our results indicate that roaches harm incipient gardens and predominantly use horizontal transmission between established leaf-cutter colonies. Ultimately, "costs of generalism" associated with infecting disparate stages of a host's lifecycle (e.g., incipient vs. established colonies) may constrain the vertical transmission of roaches and a broad range of symbionts.

Rational choice of social group size in mosquitofish.

Choice of social group can affect the likelihood of survivorship and reproduction for social species. By joining larger social groups-shoals-small freshwater fish like the mosquitofish Gambusia affinis can reduce predation risk and forage more efficiently. We tested shoal choice in mosquitofish to determine whether such choices are economically rational, i.e. consistent and optimal. Although many studies of decision-making assume rational choice, irrational decision-making is common and occurs across contexts. We tested rationality of shoaling decisions by testing the constant ratio rule, which states that the relative preference for two options should not change in the presence of a third option. Female mosquitofish upheld this rule when tested for shoal preference based on group size. Our results contrast with other studies showing violations of the constant ratio rule in foraging and mate choice decision-making contexts. These results suggest that decisions that immediately influence survivorship or decision-making along a single dimension may favour rational decision-making.

Increased hatching success as a direct benefit of polyandry in birds.

Multiple mating by females is difficult to explain in primarily socially monogamous taxa such as birds because mating outside the pair bond often provides no obvious benefit to females. Although indirect selection is often invoked to explain the evolution of polyandry, current evidence suggests that selection on indirect benefits of mating is weak. Here, I consider a direct benefit of remating in birds: increased fertilization success. I test whether increased hatching success of a female's eggs is related to rates of extra-pair paternity (EPP), a proxy of polyandry, across 113 bird species. I use two statistical approaches, control for phylogenetic uncertainty, and assess the fit of competing evolutionary models. Results show there is indeed a positive relationship between rates of EPP and hatching success in birds. I propose that by mating with many males, females may increase their fertility. I end by discussing the biological rationale for this explanation, alternative interpretations of the results, and how this study furthers our understanding of polyandry and mating system evolution.