Working memory, the ability to temporarily store and manipulate currently relevant information, is required for most cognitive faculties. In humans and other mammals, the prefrontal cortex (PFC) provides the underlying neural network for these processes. Within the PFC, working memory neurons display sustained elevated activity while holding active an internal representation of the relevant stimulus during its physical absence or retaining a motor plan for the forthcoming response. Working memory, however, is not a hallmark of higher vertebrates endowed with a neocortex. Birds also master complex cognitive problems invoking working memory, but they lack a laminated neocortex. Behavioral studies in pigeons show that the neostriatum caudolaterale (NCL) plays a central role in executive functions, such as working memory and response control. For neurons in the NCL of pigeons, we show activity changes during the delay of a working memory task, which were similar to those observed in PFC neurons and were related to the successful holding of information in memory and to the subsequent behavior. Thus, although the anatomical and morphological structure of the neuronal substrate in birds is radically different from the mammalian neocortical architecture, the neuronal mechanisms evolved to master equivalent cognitive demands seem to be very similar. (+info)
Unpredictable evolution in a 30-year study of Darwin's finches.
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Evolution can be predicted in the short term from a knowledge of selection and inheritance. However, in the long term evolution is unpredictable because environments, which determine the directions and magnitudes of selection coefficients, fluctuate unpredictably. These two features of evolution, the predictable and unpredictable, are demonstrated in a study of two populations of Darwin's finches on the Galapagos island of Daphne Major. From 1972 to 2001, Geospiza fortis (medium ground finch) and Geospiza scandens (cactus finch) changed several times in body size and two beak traits. Natural selection occurred frequently in both species and varied from unidirectional to oscillating, episodic to gradual. Hybridization occurred repeatedly though rarely, resulting in elevated phenotypic variances in G. scandens and a change in beak shape. The phenotypic states of both species at the end of the 30-year study could not have been predicted at the beginning. Continuous, long-term studies are needed to detect and interpret rare but important events and nonuniform evolutionary change. (+info)
The 'island rule' in birds: medium body size and its ecological explanation.
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Do birds show a different pattern of insular evolution from mammals? Mammals follow the 'island rule', with large-bodied species getting smaller on islands and small-bodied species getting bigger. By contrast, the traditional view on birds is that they follow no general island rule for body size, but that there is an insular trend for large bills. Insular shifts in feeding ecology are, therefore, widely assumed to be the primary cause of divergence in island birds. We use a comparative approach to test these ideas. Contrary to the traditional view, we find no evidence for increased bill size in insular populations. Instead, changes in both bill size and body size obey the 'island rule'. The differences between our results and the traditional view arise because previous analyses were based largely on passerines. We also investigate some ecological factors that are thought to influence island evolution. As predicted by the traditional view, shifts in bill size are associated with feeding ecology. By contrast, shifts in body size are associated with the potential for intraspecific competition and thermal ecology. All these results remain qualitatively unchanged when we use different methods to score the ecological factors and restrict our analyses to taxa showing pronounced morphological divergence. Because of strong covariation between ecological factors, however, we cannot estimate the relative importance of each ecological factor. Overall, our results show that the island rule is valid for both body size and bill length in birds and that, in addition to feeding ecology, insular shifts in the level of intraspecific competition and the abiotic environment also have a role. (+info)
The cellular and molecular origins of beak morphology.
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Cellular and molecular mechanisms underlying differences in beak morphology likely involve interactions among multiple embryonic populations. We exchanged neural crest cells destined to participate in beak morphogenesis between two anatomically distinct species. Quail neural crest cells produced quail beaks in duck hosts and duck neural crest produced duck bills in quail hosts. These transformations involved morphological changes to non-neural crest host beak tissues. To achieve these changes, donor neural crest cells executed autonomous molecular programs and regulated gene expression in adjacent host tissues. Thus, neural crest cells are a source of molecular information that generates interspecific variation in beak morphology. (+info)
The flowerpiercers' hook: an experimental test of an evolutionary trade-off.
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The evolution of features that enhance an organism's performance in one activity can adversely affect its performance in another. We used an experimental approach to document a trade-off associated with the evolution of the long hook at the tip of the bill of birds belonging to the genus Diglossa (flowerpiercers). In Diglossa, the more derived flower-robbing nectarivorous species have maxillae (upper jaws) that terminate in enlarged curved hooks. The ancestral frugivorous species have maxillae with relatively small hooks. We mimicked bill evolution by clipping the terminal bill hook of nectarivorous Cinnamon-bellied Flowerpiercers (Diglossa baritula) to resemble the frugivorous condition. We found that birds with experimentally shortened bills ingested fruit more efficiently, but had a reduced ability to rob flowers. Birds with intact bills, by contrast, were good flower robbers but poor frugivores. The evolution of a hooked bill endowed flowerpiercers with the ability to efficiently pierce flowers and extract nectar, but hindered their efficiency to feed on fruit. (+info)
A zone of frontonasal ectoderm regulates patterning and growth in the face.
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A fundamental set of patterning genes may define the global organization of the craniofacial region. One of our goals has been to identify these basic patterning genes and understand how they regulate outgrowth of the frontonasal process, which gives rise to the mid and upper face. We identified a molecular boundary in the frontonasal process ectoderm, defined by the juxtaposed domains of Fibroblast growth factor 8 and Sonic hedgehog, which presaged the initial site of frontonasal process outgrowth. Fate maps confirmed that this boundary region later demarcated the dorsoventral axis of the upper beak. Ectopic transplantation of the ectodermal boundary region activated a cascade of molecular events that reprogrammed the developmental fate of neural crest-derived mesenchyme, which resulted in duplications of upper and lower beak structures. We discuss these data in the context of boundary/morphogen models of patterning, and in view of the recent controversy regarding neural crest pre-patterning versus neural crest plasticity. (+info)
Carotenoid modulation of immune function and sexual attractiveness in zebra finches.
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One hypothesis for why females in many animal species frequently prefer to mate with the most elaborately ornamented males predicts that availability of carotenoid pigments is a potentially limiting factor for both ornament expression and immune function. An implicit assumption of this hypothesis is that males that can afford to produce more elaborate carotenoid-dependent displays must be healthier individuals with superior immunocompetence. However, whether variation in circulating carotenoid levels causes variation in both immune function and sexual attractiveness has not been determined in any species. In this study, we show that manipulation of dietary carotenoid supply invokes parallel changes in cell-mediated immune function and sexual attractiveness in male zebra finches (Taeniopygia guttata). (+info)
Adaptation in a plant-hummingbird association.
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Sexual dimorphism in bill morphology and body size of the Caribbean purple-throated carib hummingbird is associated with a reversal in floral dimorphism of its Heliconia food plants. This hummingbird is the sole pollinator of H. caribaea and H. bihai, with flowers of the former corresponding to the short, straight bills of males, the larger sex, and flowers of the latter corresponding to the long, curved bills of females. On St. Lucia, H. bihai compensates for the rarity of H. caribaea by evolving a second color morph with flowers that match the bills of males, whereas on Dominica, H. caribaea evolves a second color morph with flowers that match the bills of females. The nectar rewards of all Heliconia morphs are consistent with each sex's choice of the morph that corresponds to its bill morphology and energy requirements, supporting the hypothesis that feeding preferences have driven their coadaptation. (+info)