Abstract: In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD) 1 . These partons subsequently emit further partons in a process that can be described as a parton shower 2 , which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass m Q and energy E , within a cone of angular size m Q / E around the emitter 3 . Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques 4,5 to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.

Abstract: Because of the uncertain long-term status of the polar bear and its charismatic nature, this species has become a focal point for discussions concerning the impact of global climate change on biodiversity. Previously, only limited genetic resources have been available to investigate questions of divergence and adaptation of bears. Here, we describe extensive genome sequence data from three different species that contribute to a better understanding of past responses during polar bear evolution and provide investigators with extensive resources that allow the formulation of testable hypotheses to establish key morphological and physiological features associated with adaptation to polar climates. Our results are consistent with an ancient split between brown and polar bears approximately 4 to 5 Mya ( Fig. P1 A ), coinciding with the Miocene–Pliocene boundary, a period of environmental change that may have launched a radiation of bear species ( 5 ). This initial split was followed by occasional admixture until recently, leaving a clear polar-bear imprint on the nuclear genomes of ABC brown bears. Genome-based analysis of historical fluctuations in effective population size (i.e., number of interbreeding bear individuals) strongly indicates that polar bear evolution has tracked key climatic events since the Middle Pleistocene; moreover, it suggests that, although the number of interbreeding individuals polar bears may have been considerably larger in the past, there have been significant population decreases during periods of long-term climate changes ( Fig. P1 B ). A prolonged and considerable bottleneck in the polar bear population in addition to recent expansions from small founder populations could explain the observed lower genetic diversity in contemporary polar bears compared with brown bears. Since their divergence from brown bears, polar bears have embarked on their own evolutionary pathway and developed their own unique genomic signatures. Thus, we identified potential regions of genes that may relate to adaptation to the Arctic environment, including those controlling fatty-acid metabolism, hibernation, and pigmentation. Our comparative analyses demonstrate that these bear species evolved largely independently over a period of millions of years, which is in sharp contrast to the more recent estimates of polar bear origin ( Fig. P1 ). Moreover, 5% to 10% of the nuclear genome of the ABC brown bears is most closely related to polar bears, indicating ancient admixture between the species. Previously used gene-by-gene sequencing of single nuclear loci lacked sufficient power to detect such ancient admixture ( 4 ). We performed deep, high-throughput sequencing of the genomes of a polar bear, two ABC brown bears, a non-ABC brown bear, and an American black bear. We also performed less-extensive sequencing of 23 other polar bear genomes, including a ∼120,000-y-old specimen. We were able to identify more than 13 million single nucleotide polymorphisms (SNPs) that vary among individuals, including 26,000 SNPs within coding regions of corresponding dog genes that result in amino acid replacements. By computational predictions of the functional effect of each amino acid substitution (i.e., single amino acid polymorphisms), 7,000 were suggested to be damaging. Based on fossil and mitochondrial DNA evidence, the polar bear has long been thought to have evolved recently from its lower-latitude sister species, the brown bear. For example, sequences of complete mitochondrial genomes have suggested a split of the maternal lineages of these species ∼150 kya and confirmed a particularly close relationship between the polar bear and a genetically isolated population of brown bears from the Admiralty, Baranof, and Chichagof islands in Alaska’s Alexander Archipelago (henceforth called ABC brown bears) ( 3 ). However, limited nuclear DNA sequence data acquired more recently suggests that polar bears may have diverged from brown bears approximately 450,000 y earlier ( 4 ). Our strategy was to examine such questions with far greater power, from a genome-wide perspective. Modern genomic tools have already provided tremendous analytical potential for increasing our understanding of complex speciation and genetic interchange among humans and domesticated species, whereas studies with whole-genome sequencing to investigate admixture in wildlife populations are only now beginning to emerge. There is overwhelming evidence that Earth’s climate is changing. These changes are most noticeable in the Arctic, which has recently experienced a dramatic loss of sea-ice cover ( 1 ). Altered environments may lead to shifts in the distributions of closely related, formerly isolated species, permitting hybridization (i.e., admixture). Such phenomena likely shaped the evolution of many organisms but may become especially pronounced in the rapidly changing Arctic, where it could have tremendous impacts on polar biodiversity ( 2 ). The polar bear, a signature Arctic species, symbolizes the threat to biodiversity from climate change. We reasoned that knowledge of patterns of species divergence, migration, and extinction across past periods of severe climatic change during the Pleistocene might inform predictions about future organismal responses. Here, we analyzed the genomes of polar, brown, and American black bears. Although our results clearly demonstrate that these bear species have had largely independent evolutionary histories over millions of years, leaving imprints in the polar bear nuclear genome likely associated with ecological adaptation to the Arctic environment, our data also indicate ancient admixture between the species. Further, our results suggest that bear evolution has tracked key climate events, including a dramatic decline in their population for the past 500,000 y.

Abstract: This study investigated the nutritional ecology of forest elephants in Kibale National Park, Uganda relative to crop-raiding behaviour, and examined nutritional differences between crops and food consumed by wild elephants. An index of dietary nutrient concentration was determined by quantifying the species and parts of plants consumed along feeding trails, collecting food items, and analysing foods for energy, fibre, protein, minerals and secondary compounds. Frequency of crop raiding was quantified over 13 mo. Energy and protein concentration was within suggested levels, but concentrations of several minerals, particularly sodium, were low relative to requirements based on captive elephants and values reported for other wild populations. The very low sodium concentrations of Kibale elephant diets and low availability of alternative sodium sources, such as soil or water, suggest that sodium drive is very likely in this population. Crops consumed by Kibale elephants had higher Na concentrations and lower concentrations of fibre and secondary compounds than wild diets. The known attraction of elephants to mineral sources throughout their range and the low mineral concentration of leaves, fruits, bark, and stems consumed by forest elephant in this study suggest that mineral nutrition is likely to be an important factor driving elephant behaviour and patterns of habitat use.

Abstract: Mistletoes are considered keystone species on woodlands and savannas worldwide, providing a food resource for a diversified fauna, as well as a nutrient-enriched litter. Infections can be large (∼1–3 m) and, in some parts of the Amazonian savannas, parasitize up to 70% of hosts locally. Despite these facts, biomass of mistletoes is rarely investigated. Here we constructed allometric models to predict the biomass stock of the shrubby mistletoe Psittacanthus plagiophyllus in an Amazonian savanna. In addition, we determined whether host size could be used as a proxy for mistletoe biomass. Finally, we compared the biomass of mistletoes with that of trees, to evaluate their relative importance. We have shown that: (1) biomass of leaves (46.1% ± 13.5%) are as important as of stems (47.8% ± 13.5%), and relative contribution of stems increases as plant grows; (2) the model including width, breadth and vertical depth was the best (SE = 0.39, R 2 = 0.9) for predicting individual mistletoe biomass; (3) mistletoe load and biomass per host had a positive, but weak (R 2 = 0.11 and 0.09, respectively), relationship with host size, and thus such host information is a poor predictor of mistletoe biomass; and (4) in comparison with trees, mistletoes constituted less than 0.15% (0.5–22 kg ha −1 ) of the total above-ground biomass, suggesting that this life-form is irrelevant to the local biomass stock despite its unequivocal biological importance.