Swarming and mass migration are spectacular and sometimes devastating features of the biology of various animal species. These phenomena are typically associated with actual or anticipated depletion of food resources after an increase in population density, but the mechanisms driving such collective movements are poorly understood. Here we reveal that insects in large, coordinated migratory bands consisting of millions of Mormon crickets in western North America were deprived of two essential nutritional resources: protein and salt. The insects themselves provided a major source of these nutrients, and cannibalism was rife. We show that protein and salt satiation reduced cannibalism and that protein satiation inhibited walking. Additionally, experimentally reducing the motility or mobility of crickets substantially increased their risk of being cannibalized by other band members. As a result, the availability of protein and salt in the habitat will influence the extent to which bands march, both through the direct effect of nutrient state on locomotion and indirectly through the threat of cannibalism by resource-deprived crickets approaching from the rear. The crickets are, in effect, on a forced march. Migratory band formation and subsequent mass movement, therefore, are manifestations of specific tradeoffs between the costs and benefits of group living. Bands afford antipredator benefits to individual group members. Group movement then mitigates the resulting costs of intraspecific competition, namely local depletion of nutritional resources and the associated increased risk of cannibalism.cannibalism ͉ migration ͉ Mormon cricket ͉ swarming

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1. Extensive research has demonstrated that urbanization strongly alters ecological processes, often perniciously. However, quantifying the magnitude of urban effects and determining how generalized they can be across systems depends on the ways in which urbanization is measured and modelled. 2. We coupled a formal literature survey with a novel conceptual framework to document and synthesize the myriad of metrics used to quantify urbanization. The framework enables clear cataloguing of urban metrics by identifying (a) the urban component measured, (b) the method of measurement, (c) the metric's spatial scale and (d) the metric's temporal nature. Thus, the framework comprehensively captures the what, how, where and when of urban metrics.3. We documented striking variability in urban metrics with respect to which urban components were measured as well as how, where and when they were quantified. Overall, our survey revealed that they tended to be: (a) structurally focused, (b) methodologically simplistic, (c) spatially variable and (d) temporally static. 4. Synthesis and applications. Many metrics are used to quantify urbanization or 'urbanness'. The variation in urban metrics complicates the development of theory, comparisons of findings across studies, and the implementation of management and conservation actions. To pave a clear path forward for more efficient and policyrelevant urban research, we systematically organized urban metrics using a simple, flexible and comprehensive framework. The framework clarifies what urbanization actually means in empirical practice and identifies several crucial areas for future research, including: (a) systematic assessments of urban metrics across multiple scales, (b) an increased and judicious use of more complex urban metrics aimed at evaluating both mechanistic and broad-scale correlative ecological hypotheses, and (c) an increased emphasis on the socio-economic aspects of urban effects. K E Y W O R D S framework, human-natural systems, spatio-temporal scale, urban gradient, urban metrics, urbanization, urban-ness, variability

By means of field observations and laboratory experiments on the Malaysian stalk-eyed fly Cyrtodiopsis whitei we examined the consequences of variation in copulation duration for sperm competition. In this sexually dimorphic species over 90% of all copulations occur in nocturnal aggregations with from one to four males and up to 24 females. Copulation duration observed in both the field and the laboratory exhibited a bimodal distribution with peaks at 10 and 50 s. In the field short copulations less than 30 s long occurred frequently when more than one male was present in an aggregation but most were not the direct result of male interference. Sperm counts from female spermathecae after artificial interruptions indicated sperm are not transferred during the first 40 s of a copulation. When solitary males mated up to five times in succession to virgin females, short copulations did not occur, nor was the number of sperm transferred reduced. However, short copulations did occur when we mated isolated females within 6 rain of a previous copulation. By mating irradiated and nonirradiated males in reciprocal pairs we discovered that C. whitei exhibits both first-male sperm precedence and sperm mixing. More than half of the females mated first to sterile and then to fertile males failed to produce offspring. Such variation in copulation duration and sperm precedence is consistent with male placement and detection of a spermatophore that acts as a temporary mating plug. Our data suggest that those male C. whitei which successfully defend large aggregations of females reduce sperm waste and competition by preferentially transferring sperm to females that have not mated recently.

Mormon crickets and juvenile locusts form huge migratory bands--millions of individuals march in unison across the landscape and devastate vast agricultural areas, but little is known about why these bands form. Here we use radiotelemetry to show that band membership benefits these insects by greatly reducing the probability that they will become victims of predators. It is likely that migratory banding has evolved because it gives substantial protection to individuals within the group.

Conspecific gamete precedence, the usage of conspecific sperm by a female that mates with both a conspecific and a heterospecific male, has been found in many taxa. We construct a population genetic model to examine the evolution of conspecific gamete precedence and its coevolution with premating isolation in the process of reinforcement. Our findings suggest that conspecific gamete precedence can evolve via a process very similar to reinforcement. We explore the nature of the selection against hybridization necessary to drive this evolution. Moreover, our results confirm the prediction of Marshall et al. (Trends Ecol. Evol. 2002;17:558–563) that conspecific gamete precedence will inhibit the evolution of reinforcement between two species. We further find that reinforcement will inhibit the evolution of conspecific gamete precedence. Both reinforcement and conspecific gamete precedence increase reproductive isolation and contribute to the process of speciation. We discuss factors that may affect which of these phenomena are likely to become predominant between incipient species.

Abstract. 1. Mormon crickets form large migratory bands that march over rangeland in the western United States seeking salt and protein. Immune defence is particularly relevant to survival in migratory bands, but little is known about the role of nutrition in insect immunocompetence. We hypothesised that immune defences are compromised in these migratory bands due to nutrient limitations.2. In a migratory band in Utah, we investigated whether access to a protein relative to a carbohydrate diet would immediately reduce migratory activity, as had been shown for Mormon crickets in a previous study in Idaho, and whether the protein diet would enhance immune defence responses.3. Radio-tracking Mormon crickets in the field, we found that locomotor activity was significantly and positively associated with body mass. Body mass-adjusted locomotor activity declined marginally following access to a protein diet, whereas spontaneous phenoloxidase (PO) activity was enhanced by the same diet. The encapsulation response and lysozyme-like activity were directly proportional to body mass, but unaffected by the dietary treatments in the short term. Within 6 h of feeding on protein or carbohydrates, Mormon crickets exhibited measurable effects on the immune system. 4. We conclude that nutrition impacts immune function in migrating insects in the field. Spontaneous PO activity may be limited by dietary deficiency in a proteinseeking band of Mormon crickets.

Abstract.  1. Outbreaks of insect pest populations are common and can have devastating effects on natural communities and on agriculture. Little is known about the causes of these outbreaks or the causes of en masse migrations during outbreaks. 2. Flightless Mormon crickets (Anabrus simplex) were the focus of this study. They are a katydid species that forms large, dense, mobile groups (migratory bands) during outbreak periods, eating vegetation in their path. 3. Radiotelemetric methods were used to measure differences in movement rate and directionality in outbreak and non‐outbreak populations, testing the hypothesis that these populations differ in their travel rate and consistency of direction. 4. Daily individual movement in outbreak populations differs substantially from non‐outbreak populations that are at much lower density. In addition to large differences in distances travelled (1.6 km as compared with 1 m) and rates of travel, there is evidence for collective movement among individual Mormon crickets travelling in migratory bands. 5. These data suggest that the direction of group movement may be influenced by local environmental conditions such as wind direction and movement of nearby band members. This work forms the basis for ongoing work testing hypotheses about mass migrations in outbreak populations.