Why do female migratory birds arrive later than males?

1. In migratory birds males tend to arrive first on breeding grounds, except in sex-role reversed species. The two most common explanations are the rank advantage hypothesis, in which male-male competition for breeding sites drives stronger selection for early arrival in males than females, and the mate opportunity hypothesis, which relies on sexual selection, as early arrival improves prospects of mate acquisition more for males than for females. 2. To date, theoretical work has focused on selection for early arrival within a single sex, usually male. However, if fitness depends on territory quality, selection for early arrival should operate on both sexes. Here we use two independent modelling approaches to explore the evolution of protandry (male-first arrival) and protogyny (female-first arrival) under the rank advantage and mate opportunity hypotheses. 3. The rank advantage hypothesis, when operating alone, fails to produce consistent patterns of protandry, despite our assumption that males must occupy territories before females. This is because an individual of either sex benefits if it out-competes same-sex competitors. Rather than promoting protandry, the rank advantage mechanism can sometimes result in protogyny. Female-female competition is stronger than male-male competition early in the season, if females compete for a resource (territories occupied by males) that is initially less common than the resource of interest to males (unoccupied territories). 4. Our results support the mate opportunity hypothesis as an explanation of why protandry is the norm in migratory systems. Male-biased adult sex ratios and high levels of sperm competition (modelled as extra-pair young: EPY) both produce protandry as a result of sexual selection. Protogyny is only observed in our models with female-biased sex ratios and low EPY production. 5. We also show that the effects of sex ratio biases are much stronger than those of EPY production, explore the evidence for sex ratio biases and extra-pair paternity in migratory species and suggest future research directions.     

Why do good hunters have higher reproductive success?

Anecdotal evidence from many hunter-gatherer societies suggests that successful hunters experience higher prestige and greater reproductive success. Detailed quantitative data on these patterns are now available for five widely dispersed cases (Ache, Hadza, !Kung, Lamalera, and Meriam) and indicate that better hunters exhibit higher age-corrected reproductive success than other men in their social group. Leading explanations to account for this pattern are: (1) direct provisioning of hunters’ wives and offspring, (2) dyadic reciprocity, (3) indirect reciprocity, (4) costly signaling, and (5) phenotypic correlation. I examine the qualitative and quantitative evidence bearing on these explanations and conclude that although none can be definitively rejected, extensive and apparently unconditional sharing of large game somewhat weakens the first three explanations. The costly signaling explanation has support in some cases, although the exact nature of the benefits gained from mating or allying with or deferring to better hunters needs further study. For comments on earlier drafts, I thank Monique Borgerhoff Mulder, Mike Gurven, Ray Hames, Kristen Hawkes, Kim Hill, Robert Kelly, Frank Marlowe, John Patton, and Polly Wiessner. Rebecca Bliege Bird and Douglas W. Bird invited me to collaborate in the Meriam research and (along with Del Passi of Mer) collected the data on Meriam demography. Geoff Kushnick and Matt Wimmer ably assisted with coding and statistical analysis of these data. Eric Alden Smith (PhD 1980, Cornell University) is a professor of anthropology at the University of Washington, Seattle. His research interests include the links between production and reproduction, the ecology and evolution of collective action, and politics in small-scale societies. He has conducted fieldwork among Inuit on Hudson Bay, Meriam in Torres Strait, and Mardu Aborigines in the Australian Western Desert.     

Why do plants have phosphoinositides?

Phosphoinositides are inositol-containing phospholipids whose hydrolysis is a key step in the rapid responses of animal cells to extracellular signals. Whether they play similar roles in plant cells has not been established, and some have suggested alternative roles as direct modulators of specific proteins. Nonetheless, evidence is accumulating that phosphoinositide hydrolysis mediates transduction of some signal in plants. The evidence is strongest for a role in triggering the shedding of flagella by the unicellular alga Chlamydomonas reinhardtii under acid stress. Rapid kinetic analysis indicates that phosphoinositide hydrolysis occures within half a second and could trigger the rapid loss of flagella. Plant responses to pathogens and osmotic stress, as well as the regulation of turgor changes which underlie stomatal opening and closing and the movement of leaves and flower parts, may also be mediated by phosphoinositide hydrolysis. The evidence thus indicates that at least one reason plants have phosphoinositides is to mediate transduction of environmental signals.     

Why do some adult birds skip breeding? A hormonal investigation in a long-lived bird

Skipping reproduction is often observed in long-lived organisms, but proximate mechanisms remain poorly understood. Since young and/or very old snow petrels (Pagodroma nivea) commonly skip breeding, we tested whether they are physiologically able to breed during the pre-laying stage. To do so, we measured the ability of known-age (11-45 years old) petrels to release luteinizing hormone (LH, a crucial driver for breeding), by injecting exogenous gonadotropin-releasing hormone (GnRH). Although young petrels exhibited low baseline LH levels, they were able to elevate LH levels after a GnRH challenge. Moreover, young and very old petrels showed a stronger decrease in LH levels after the 10 min post-GnRH injection compared with middle-aged petrels. Birds that skipped breeding were as able as breeders to release LH after a GnRH challenge, indicating that they had functional pituitaries. However, the decision to skip reproduction was linked to a strong LH decrease after the 10 min post-GnRH injection. Our result suggests that the youngest and the oldest petrels fail to maintain elevated baseline LH levels, thereby do not initiate reproductive activities. Skipping reproduction in long-lived birds probably results from age-related changes in the dynamics of the hypothalamic-pituitary-gonadal (HPG) axis function.     

Why do microorganisms have aquaporins?

Aquaporins are channel proteins that enhance the permeability of cell membranes for water. They have been found in Bacteria, Archaea and Eukaryotes. However, their absence in many microorganisms suggests that aquaporins do not fulfill a broad role such as turgor regulation or osmoadaptation but, instead, fulfill a role that enables microorganisms to have specific lifestyles. The recent discovery that aquaporins enhance cellular tolerance against rapid freezing suggests that they have ecological relevance. We have identified several examples of large-scale freeze-thawing of microbes in nature and we also draw attention to alternative lifestyle-related functions for aquaporins, which will be a focus of future research.     

Why do migrating birds fly along coastlines?

If migrating birds compensate completely for wind drift over land but not over the sea, as indicated by recent radar studies, land-birds facing the sea should, under certain conditions, follow the coastline for some distance rather than depart immediately over the sea. Coastal migration is to be expected if onshore drift exceeds the angle between the birds' preferred track direction and the coastline, as will be the case when this angle is small and strong opposed sidewinds or crosswinds prevail. With offshore drift coasting should occur if the flying time to the goal along a route via the coastline is shorter than that along a direct over-sea route, as will be the case for wide sea-crossings, with strong opposed sidewinds or crosswinds and small angles between the migrants' goal direction and the coastline. Field data support this hypothesis.     

Why do fleshy-fruit plants of the mediterranean scrub intercept fall-but not spring-passage of seed-dispersing migratory birds?

Summary Production of fleshy fruits by 8 tree and shrub species, and patterns of their utilization by 6 resident and 6 transient bird species, were assessed in an Israeli Mediterranean scrub. Whereas more than half the ripe crop of plants that fruit in one relatively short burst is not removed by birds, the greater part of the crop of species that fruit throughout a long period, is removed. Of two plant species that fruit simultaneously, the one with an inconspicuous fruit is utilized by a single, year-round resident frugivorous bird, whereas the species with bright fruits is utilized by several non-resident and omnivorous bird species. A further two simultaneously fruiting species differ in fat content and the color of their conspicuous ripe fruits; the low-fat fruit being taken by a resident species and the high-fat fruit by a transient congener, just prior to the desert-crossing portion of its Fall-passage.Though more migrants pass through in Spring than in Fall, none of the fleshy-fruit plants fruit in Spring, and most fruit during the Fall. Two non-exclusive explanations for this phenomenon are (a) Fall dispersal ensures immediate germination, with least exposure to desiccation and predation; (b) Spring transients approaching their breeding territories may either be reluctant to spend much time on feeding, or mostly require proteins, amply supplied by the Spring flush of insects, typical of the Mediterranean region. Fall transients approaching the desert require mostly fats and carbohydrates, supplied by fruits. Fruits attract birds easily in the Fall, when insects are scarce.     

Why do schistosomes have separate sexes?

Paul Basch postulates that the familiar Schistosoma of humans evolved from hermaphroditic blood flukes of Mesozoic reptiles as those host became warm-blooded. The reproductive superiority that accompanied tendencies to protandry and protogyny in hermaphroditic blood flukes has led to subsequent sexual separation and dimorphism but substantial fragments of the ancestral contrasex genome persist in present-day males, as shown by common tendencies toward hermaphroditism. In present-day females the loss of the male-specific genome is far more complete and in the process of optimizing reproductive efficiency, present-day females have sacrificed many structural elements including locomotory and pharyngeal muscles. These losses have created dependency on the well-muscled male, whose primary functions seem to be compensatory; ie., physical transport of the female from the point of pairing to the point of egg deposition, stimulating growth and development by pumping blood into the female, who unpaired would starve, plus, less importantly, fertilization of the oocytes.     

Why are birds' eggs colourful? Eggshell pigments co‐vary with life‐history and nesting ecology among British breeding non‐passerine birds

The colourful appearance of bird eggshells has long fascinated biologists and considerable research effort has focused on the structure and biochemistry of the avian eggshell matrix. The presence of tetrapyrrole pigments was identified nearly a century ago. Surprisingly, how the concentrations of avian eggshell pigments vary among related species, and whether this variability is associated with either eggshell appearance and/or species life‐history traits, remains poorly understood. We quantified the concentrations of the two key eggshell pigments, protoporphyrin IX and biliverdin, from a diverse sample of eggshells stored at the Natural History Museum, Tring, UK. We explicitly tested how these two pigments are associated with physical measures of eggshell coloration and whether the pigment concentrations and colour diversity co‐vary with phylogenetic affiliations among species. We also tested a series of comparative hypotheses regarding the association between the concentrations of the two pigments and specific life‐history and breeding ecology traits. Across species, the average concentrations of protoporphyrin and biliverdin were positively correlated, and both strongly co‐varied with phylogenetic relatedness. Controlling for phylogeny, protoporphyrin concentration was associated with a higher likelihood of cavity nesting and ground nesting, whereas biliverdin concentration was associated with a higher likelihood of non‐cavity nesting habit and bi‐parental provisioning. Although unlikely to be explained by a single function, the breeding ecology and life history‐dependence of eggshell pigment concentrations in these comparative analyses implies that related species share pigment strategies, and that those strategies relate to broad adaptive roles in the evolution of variation in avian eggshell coloration and its underlying mechanisms. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 657–672.     

Why do protein architectures have boltzmann-like statistics?

A theoretical study has shown that the occurrence of various structural elements in stable folds of random copolymers is exponentially dependent on the own energy of the element. A similar occurrence-on-energy dependence is observed in globular proteins¹ from the level of amino acid conformations to the level of overall architectures. Thus, the structural features stabilized by many random sequences are typical of globular proteins while the features rarely observed in proteins are those which are stabilized by only a minor part of the random sequences. © 1995 Wiley-Liss, Inc.     

Why do mountains support so many species of birds?

Although topographic complexity is often associated with high bird diversity at broad geographic scales, little is known about the relative contributions of geomorphologic heterogeneity and altitudinal climatic gradients found in mountains. We analysed the birds in the western mountains of the New World to examine the two‐fold effect of topography on species richness patterns, using two grains at the intercontinental extent and within temperate and tropical latitudes. Birds were also classified as montane or lowland, based on their overall distributions in the hemisphere. We estimated range in temperature within each cell and the standard deviation in elevation (topographic roughness) based on all pixels within each cell. We used path analysis to test for the independent effects of topographic roughness and temperature range on species richness while controlling for the collinearity between topographic variables. At the intercontinental extent, actual evapotranspiration (AET) was the primary driver of species richness patterns of all species taken together and of lowland species considered separately. In contrast, within‐cell temperature gradients strongly influenced the richness of montane species. Regional partitioning of the data also suggested that range in temperature either by itself or acting in combination with AET had the strongest “effect” on montane bird species richness everywhere. Topographic roughness had weaker “effects” on richness variation throughout, although its positive relationship with richness increased slightly in the tropics. We conclude that bird diversity gradients in mountains primarily reflect local climatic gradients. Widespread (lowland) species and narrow‐ranged (montane) species respond similarly to changes in the environment, differing only in that the richness of lowland species correlates better with broad‐scale climatic effects (AET), whereas mesoscale climatic variation accounts for richness patterns of montane species. Thus, latitudinal and altitudinal gradients in species richness can be explained through similar climatic‐based processes, as has long been argued.     

Why do olfactory neurons have unspecific receptive fields?

Biological olfactory neurons are deployed as a population, most responding to a large variety of chemical compounds, that is, they possess unspecific receptive fields. The question of whether this unspecificity results from some physical constraint placed upon chemical transduction, or on the other hand, is beneficial to system performance is unclear. In this paper we employ the notion of Fisher information to address this question by quantifying how both the distribution and the tunings of the receptive fields within olfactory receptor populations affect the optimal estimation performance of the system. Our results show that overlapping sensory neuron tunings that respond to common chemical compounds have better estimation performance than perfectly specific tunings. Our results suggest two phenomena that might represent general principles of organization within biological sensory systems responding to multiple stimuli: maximization of the diversity of tunings and homogeneity in the distribution of these different receptive fields across the stimulus space (independent of the statistics of the input stimuli). Our model predicts that a local randomized mechanism controlling receptor specificities generates optimal multidimensional stimulus estimation, for which there is some experimental evidence from the biology.     

Why do older workers have problems with technology?

Why do older workers seem to have problems with technology? In this paper, I will review several possible reasons and illustrate them with evidence, often anecdotal, from our work as ergonomics practitioners. We find that older workers have more to "unlearn" from their accumulated experience. They may suffer from gradual or not so gradual ailing faculties of sight, hearing, dexterity, stamina, memory, and reaction time. They may exhibit a fear of making mistakes, and they may have strongly established preferences and pessimism about technological gimmicks. But the real problem is often that the designers have failed to anticipate the requirements of their users; they have failed to design for a range of abilities broader than their own; they have failed to test their designs with real people; and they have failed to learn from the experience of the market. Getting design right for older users is really only a continuation of getting design right for all.