JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Submitted July 12, 2011; Accepted February 3, 2012; Electronically published April 25, 2012 Online enhancement: appendix. Dryad data: http://dx.doi.org/10.5061/dryad.144v45c6. The University of Chicago Press andabstract: How strong is selection for cheating in mutualisms? The answer depends on the type and magnitude of the costs of the mutualism. Here we investigated the direct and ecological costs of plant defense by ants in the association between Cordia nodosa, a myrmecophytic plant, and Allomerus octoarticulatus, a phytoecious ant. Cordia nodosa trees produce food and housing to reward ants that protect them against herbivores. For nearly 1 year, we manipulated the presence of A. octoarticulatus ants and most insect herbivores on C. nodosa in a full-factorial experiment. Ants increased plant growth when herbivores were present but decreased plant growth when herbivores were absent, indicating that hosting ants can be costly to plants. However, we did not detect a cost to ant colonies of defending host plants against herbivores. Although this asymmetry in costs suggests that the plants may be under stronger selection than the ants to cheat by withholding investment in their partner, the costs to C. nodosa are probably at least partly ecological, arising because ants tend scale insects on their host plants. We argue that ecological costs should favor resistance or traits other than cheating and thus that neither partner may face much temptation to cheat.

Summary1. Ants provide variable protection against herbivores to ant-plants (i.e. myrmecophytes and myrmecophiles). The ways in which ant-plants dynamically adjust both their direct (chemical and physical) and indirect (biotic) defences in response to varying levels of herbivory are not well understood. 2. We experimentally generated a broad range of ant-attendance levels and herbivory pressures in a tropical myrmecophyte, Cordia nodosa, which allowed exploration of the inducibility of and interactions between direct and indirect resistance traits. 3. In response to increased herbivory, host plants encouraged indirect (biotic) defence by increasing domatium volume, regardless of whether ants were present on the plant. When ants were present, larger domatia housed more workers, which in turn decreased herbivory on adjacent leaves. 4. Independent of the presence of ants, plants responded to increased herbivory by inducing both chemical (phenolics) and structural (leaf toughness, trichomes) resistance traits; these traits were associated with reduced palatability to a folivorous beetle. 5. Synthesis. Our results show that both direct and indirect defences are inducible in C. nodosa, which suggests that C. nodosa may retain direct defences as insurance against varying levels of protection from its ant bodyguards. Thus, the predictions of optimal defence theory are not violated: although C. nodosa invests in multiple forms of defence, they are not redundant.

Although herbivory is widespread among insects, plant tissues rarely provide the optimal balance of nutrients for insect growth and reproduction. As a result, many herbivorous insects forage elsewhere for particular amino acids and minerals. Recent studies have shown that both herbivory and recruitment to sodium are commonplace among tropical rainforest ants, but little is known about how ants regulate their sodium intake at the individual and colony levels. In social insects, foragers may respond not only to their own nutritional deficiencies but also to those of their nestmates, who may have different nutritional requirements depending on their developmental stage, sex, or caste. Here, we investigate how salt stress among rainforest ants affects their preferences for salt and subsequent survival. We found that ants recruited more to salt than to any other bait type tested, confirming the strong preference for salt of ants in this region. Initially, we observed similarly high recruitment to salt among workers of the arboreal, herbivorous/omnivorous ant species Camponotus mirabilis. However, when provided with unrestricted access to high concentrations of salt, C. mirabilis workers suffered significantly higher mortality relative to controls, suggesting that C. mirabilis workers forage for sodium to the point of toxicity. Nonetheless, surviving workers showed reduced preference for salt at the end of the experiment, so some but not all individuals were able to regulate their salt intake beneath lethal dosages. We discuss how salt intake regulation may depend on colony members other than workers.

Field courses provide transformative learning experiences that support success and improve persistence for science, technology, engineering, and mathematics majors. But field courses have not increased proportionally with the number of students in the natural sciences. We conducted a scoping review to investigate the factors influencing undergraduate participation in and the outcomes from field courses in the United States. Our search yielded 61 articles, from which we classified the knowledge, affect, behavior, and skill-based outcomes resulting from field course participation. We found consistent reporting on course design but little reporting on demographics, which limits our understanding of who takes field courses. Cost was the most commonly reported barrier to student participation, and knowledge gains were the most commonly reported outcome. This scoping review underscores the need for more rigorous and evidence-based investigations of student outcomes in field courses. Understanding how field courses support or hinder student engagement is necessary to make them more accessible to all students.

In many organisms, phenotype and fitness are strongly influenced by both current environmental factors and maternal effects. The low genetic variation, high phenotypic plasticity, and telescoping generations seen in aphids permit us to investigate the relative importance and potential interaction of maternal and current environments on phenotype. Although past studies have identified an influence of maternal host plant on offspring phenotype and reproduction in aphids, few have demonstrated the potential for these maternal effects to also interact with the aphid's current environment. By rearing multiple generations of Aphis nerii (Fonscolombe) (Hemiptera: Aphididae) on their host common milkweed, Asclepias syriaca (L.) (Apocynaceae), we tested the relative influence and interaction of both maternal and current environmental effects of crowding and plant quality on aphid body size and reproduction. Our results indicate that aphid body size increased with current plant quality and decreased with aphid density in both generations, with an additional direct, positive relationship between body size and fecundity. We did not find evidence for adaptive maternal effects, e.g., production of fewer, larger, offspring by stressed mothers. Instead, poor maternal environments constrained aphid body size and reproduction. Importantly, these adverse maternal effects were only seen in offspring where subsequent nymph population growth was allowed to increase unchecked, likely reducing available resources. Our study thus demonstrates that the significance of maternal effects in aphid development and reproduction can depend on current resource availability, shaping the phenotype and fecundity of offspring under stressful conditions. Incorporating this framework for how aphid body size and reproduction respond to current and maternal environments may improve predictions for how aphid population growth is impacted by resource limitation across generations.