School quality and wages

This dissertation examines the literature that attempts to measure the relationship between school quality and earnings. I begin by developing a simple economic model that predicts that, everything else being equal and with comparisons being made within a market, workers from higher quality schools will have higher earnings among those with the same level of schooling and they will have steeper schooling-earnings gradients. The remainder of this dissertation explores problems that exist in this literature for which no solutions have been presented. These problems include: 1) there doesnâÂÂt have to be a direct and positive relationship between school quality and earnings; 2) the data suggest that school quality measures are frequently mismatched to workers; 3) most school quality studies include college-trained labor while completely ignoring the quality of the college attended; 4) the omission of college quality from the estimation is especially problematic for studies that attempt to measure the school quality-earnings relationship through differences in schooling-earnings gradients for those educated in different systems; 5) state of birth wage rankings thought to capture a school quality effect are not invariant to the market (state of residence) in which they are evaluated; and 6) the evidence presented herein suggests that interstate migration is selective. These problems undermine the credibility of existing estimates of a school qualityearnings relationship

Trapped in the darkness of the night: thermal and energetic constraints of daylight flight in bats

Bats are one of the most successful mammalian groups, even though their foraging activities are restricted to the hours of twilight and night-time. Some studies suggested that bats became nocturnal because of overheating when flying in daylight. This is because—in contrast to feathered wings of birds—dark and naked wing membranes of bats efficiently absorb short-wave solar radiation. We hypothesized that bats face elevated flight costs during daylight flights, since we expected them to alter wing-beat kinematics to reduce heat load by solar radiation. To test this assumption, we measured metabolic rate and body temperature during short flights in the tropical short-tailed fruit bat Carollia perspicillata at night and during the day. Core body temperature of flying bats differed by no more than 2°C between night and daytime flights, whereas mass-specific CO2 production rates were higher by 15 per cent during daytime. We conclude that increased flight costs only render diurnal bat flights profitable when the relative energy gain during daytime is high and risk of predation is low. Ancestral bats possibly have evolved dark-skinned wing membranes to reduce nocturnal predation, but a low degree of reflectance of wing membranes made them also prone to overheating and elevated energy costs during daylight flights. In consequence, bats may have become trapped in the darkness of the night once dark-skinned wing membranes had evolved

Preface : causes of obesity, theories, conjectures and evidence

Peer reviewedPublisher PD

Empirical maximum lifespan of earthworms is twice that of mice

We considered a Gompertzian model for the population dynamics of Eisenia andrei case-cohorts in artificial OECD soil under strictly controlled conditions. The earthworm culture was kept between 18 and 22°C at a constant pH of 5.0. In all, 77 lumbricids were carefully followed for almost 9 years, until the oldest died. The Eisenia median longevity is 4.25 years and the oldest specimen was 8.73 years. Eisenia cocoons were hand-sorted every 3 weeks, washed in distilled water, placed in Petri dishes, and counted. Regular removal did not reduce breeding. Each fertile cocoon contained on average two or three embryos. The failure rates (mortality and infertility percentages) are smooth power functions where the rate at time (n + 1) captured most of the phenomenology of the previous rate at time n, as expected by the considered law, but not at both the beginning and the end of this long-term laboratory study

Does reproduction cause oxidative stress? An open question

There has been substantial recent interest in the possible role of oxidative stress as a mechanism underlying life-history trade-offs, particularly with regard to reproductive costs. Several recent papers have found no evidence that reproduction increases oxidative damage and so have questioned the basis of the hypothesis that oxidative damage mediates the reproduction–lifespan trade-off. However, we suggest here that the absence of the predicted relationships could be due to a fundamental problem in the design of all of the published empirical studies, namely a failure to manipulate reproductive effort. We conclude by suggesting experimental approaches that might provide a more conclusive test of the hypothesis

Using the MitoB method to assess levels of reactive oxygen species in ecological studies of oxidative stress

In recent years evolutionary ecologists have become increasingly interested in the effects of reactive oxygen species (ROS) on the life-histories of animals. ROS levels have mostly been inferred indirectly due to the limitations of estimating ROS from in vitro methods. However, measuring ROS (hydrogen peroxide, H2O2) content in vivo is now possible using the MitoB probe. Here, we extend and refine the MitoB method to make it suitable for ecological studies of oxidative stress using the brown trout Salmo trutta as model. The MitoB method allows an evaluation of H2O2 levels in living organisms over a timescale from hours to days. The method is flexible with regard to the duration of exposure and initial concentration of the MitoB probe, and there is no transfer of the MitoB probe between fish. H2O2 levels were consistent across subsamples of the same liver but differed between muscle subsamples and between tissues of the same animal. The MitoB method provides a convenient method for measuring ROS levels in living animals over a significant period of time. Given its wide range of possible applications, it opens the opportunity to study the role of ROS in mediating life history trade-offs in ecological settings