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1. Silica is crucial to terrestrial plant life and geochemical cycling on Earth. It is also implicated in the evolution of mammalian teeth, but there is debate over which type of siliceous particle has exerted the strongest selective pressure on tooth morphology. 2. Debate revolves around the amorphous silica bodies (phytoliths) present in plants and the various forms of siliceous grit—that is, crystalline quartz (sand, soil, dust)—on plant surfaces. The problem is that conventional measures of silica often quantify both particle types simultaneously. 3. Here we describe a protocol that relies on heavy-liquid flotation to separate and quantify siliceous particulate matter in the diets of herbivores. The method is reproducible and well suited to detecting species- or population-level differences in silica ingestion. In addition, we detected meaningful variation within the digestive tracts of cows, an outcome that supports the premise of ruminal fluid ‘washing’ of siliceous grit. 4. We used bootstrap resampling to estimate the sample sizes needed to compare species, populations or individuals in space and time. We found that a minimum sample of 12 individuals is necessary if the species is a browser or as many as 55 if the species is a grazer, which are more variable. But a sample size of 20 is adequate for detecting statistical differences. We conclude by suggesting that our protocol for differentiating and quantifying silica holds promise for testing competing hypotheses on the evolution of dental traits. 

Human brain size nearly quadrupled in the six million years since Homo last shared a common ancestor with chimpanzees, but human brains are thought to have decreased in volume since the end of the last Ice Age. The timing and reason for this decrease is enigmatic. Here we use change-point analysis to estimate the timing of changes in the rate of hominin brain evolution. We find that hominin brains experienced positive rate changes at 2.1 and 1.5 million years ago, coincident with the early evolution of Homo and technological innovations evident in the archeological record. But we also find that human brain size reduction was surprisingly recent, occurring in the last 3,000 years. Our dating does not support hypotheses concerning brain size reduction as a by-product of body size reduction, a result of a shift to an agricultural diet, or a consequence of self-domestication. We suggest our analysis supports the hypothesis that the recent decrease in brain size may instead result from the externalization of knowledge and advantages of group-level decision-making due in part to the advent of social systems of distributed cognition and the storage and sharing of information. Humans live in social groups in which multiple brains contribute to the emergence of collective intelligence. Although difficult to study in the deep history of Homo , the impacts of group size, social organization, collective intelligence and other potential selective forces on brain evolution can be elucidated using ants as models. The remarkable ecological diversity of ants and their species richness encompasses forms convergent in aspects of human sociality, including large group size, agrarian life histories, division of labor, and collective cognition. Ants provide a wide range of social systems to generate and test hypotheses concerning brain size enlargement or reduction and aid in interpreting patterns of brain evolution identified in humans. Although humans and ants represent very different routes in social and cognitive evolution, the insights ants offer can broadly inform us of the selective forces that influence brain size. 

Desert locustsSchistocerca gregariaare threatening the food security of millions of people and devastating economies in eastern Africa and northern India. The ongoing outbreak is the largest in seven decades.

These events give us cause to reflect on the natural history of locusts, our fraught relationship with them, and how they are represented in American popular culture and others.

Symbolic representations span millennia and most have roots in the natural life cycle of locusts—they transform, they swarm, they devastate specific food crops. There is strong tendency to exaggerate the body size of locusts and the effectiveness of control efforts. Expressions of human futility are rare except in the form of ironic humour.

We conclude by suggesting that we humans indulge in hyperbole and humour to normalize and inure ourselves to the psychologically unbearable, and that this tendency is a precondition for the techno‐optimism that drives anti‐locust technologies.

There is no substitute for effective monitoring and management programs, but the importance of new and emerging anti‐locust technologies is expected to increase with projections of increased cyclone activity in the northern Indian Ocean.

A freePlain Language Summarycan be found within the Supporting Information of this article.

 

The function of the browridge in primates is a subject of enduring debate. Early studies argued for a role in resisting masticatory stresses, but recent studies have suggested sexual signaling as a biological role. We tested associations between circumorbital form, diet, oral processing, and social behavior in two species of colobus monkey–the king colobus (Colobus polykomos) and western red or bay colobus (Piliocolobus badius).

We quantified circumorbital size and dimorphism in a sample of 98 crania. Controlling for age and facial size, we tested whether variation in circumorbital morphology can be explained by variation in diet, oral processing behavior, masticatory muscle size, and mating system. To contextualize our results, we included a broader sample of facial dimorphism for 67 anthropoid species.

Greater circumorbital thickness is unrelated to the stresses of food processing. King colobus engages in longer bouts of anterior tooth use, chews more per ingestive event, and processes a tougher diet, yet circumorbital thickness ofC. polykomosis reduced compared toP. badius. Differences in circumorbital development do not vary with wear or facial size. Greater sexual dimorphism is present inP. badius; comparisons across anthropoids indicated patterns of circumorbital dimorphism were decoupled from overall size dimorphism.

The expanded circumorbits of male red colobus monkeys evolved in response to intense male–male competition. This hypothesis is consistent with the pattern across anthropoid primates and highlights the underappreciated role of sexual selection in shaping the primate face.