Delayed escape responses of male Basiliscus plumifrons (Squamata: Corytophanidae) during peak activity

Delayed escape responses of male Basiliscus plumifrons (Squamata: Corytophanidae) during peak activity. Many animals must balance their time spent active in a habitat against their perceived risk of predation. Factors that may increase that perceived risk, such as a faster predator approach, are therefore expected to cause prey to initiate escape quickly to avoid capture. At the same time, because patterns of daily activity can fuctuate throughout the day, the relative costs and benefts of initiating escape may also differ over time. Here I evaluated the escape responses of adult male emerald basilisk (Basiliscus plumifrons) lizards in two different time periods: morning (when daily activity peaks) and early afternoon (when activity is suppressed). Further, I approached each lizard at either a practiced slow or fast pace. Escape responses were recorded as fight-initiation (distance between observer and lizard prior to escape) and fight (distance travelled during escape) distance. No factor affected fight distance, and approach speed also had no effect on fight initiation distance. In contrast, time period affected fight initiation distance, with males approached in the morning delaying their escape response compared to males approached during the early afternoon. Because morning and early afternoon periods coincide with peak and suppressed periods of activity for basilisks at this study site, respectively, ambushforaging species like B. plumifrons may delay escape when active to avoid prematurely alerting the predator of their presence.Respostas de fuga atrasadas de machos de Basiliscus plumifrons (Squamata: Corytophanidae) durante o pico de atividade. Muitos animais devem equilibrar o tempo gasto em atividade em um habitat com o risco de predação percebido. Portanto, espera-se que fatores que possam aumentar aquele risco, como uma aproximação mais rápida do predador, façam com que a presa inicie a fuga rapidamente para evitar a captura. Ao mesmo tempo, como os padrões de atividade diária podem variar ao longo do dia, os custos e benefícios relativos de iniciar a fuga também podem diferir ao longo do tempo. Avaliei aqui as respostas de fuga de machos adultos do basilisco-esmeralda (Basiliscus plumifrons) em dois períodos diferentes: manhã (quando a atividade diária atinge o pico) e início da tarde (quando a atividade cessa). Além disso, eu me aproximei de cada lagarto a passos lentos ou rápidos. As respostas de fuga foram registradas como distância de início de fuga (distância entre o observador e o lagarto antes da fuga) e distância de fuga (distância percorrida durante a fuga). Nenhum fator afetou a distância de fuga e a velocidade de aproximação também não teve efeito sobre a distância de início de fuga. Por outro lado, o período de tempo afetou a distância de início de fuga, com os machos abordados de manhã atrasando sua resposta de fuga em comparação com os machos abordados no início da tarde. Como os períodos da manhã e do início da tarde coincidem, respectivamente, com os períodos de atividade máxima e de cessação da atividade para os basiliscos neste local de estudo, predadores de tocaia como B. plumifrons podem atrasar a fuga quando ativos para evitar alertar prematuramente o predador sobre sua presença

Temporal and ontogenetic variation in the escape response of Ameiva festiva (Squamata, Teiidae)

Several factors have been shown to affect lizard escape behavior (flight initiation distance or FID, the distance between predator and prey when the prey initiates escape). Patterns of daily activity, such as foraging or movement behavior, vary with respect to time of day, supporting that escape responses may vary temporally as well. However, there remains scant information regarding the effects of time of day on FID. During peak activity, FID may decrease due to increased cost of giving up resources (e.g., prey or potential mates). An alternative hypothesis is that FID may increase because lizard activity in general may serve to alert a predator in advance of its approach. A lizard in this scenario may be favored to flee sooner rather than later. Moreover, juvenile and adult lizards of multiple species may differ in behavioral, ecological, and morphological traits that could influence escape decisions. I tested the effects of time of day (in 30-min intervals) and age (juvenile or adult) on the FID of a tropical whiptail lizard, Ameiva festiva in Costa Rica. I found that A. festiva escape responses varied with time of day such that in general, their FID decreased throughout the day. In addition, I observed a peak in FID from mid to late-morning that matches published estimates of peak activity times for A. festiva. Overall, juvenile A. festiva initiated an escape response sooner than adults, which may be related to differences in perceived risk associated with differences in size and predator experience between the two age groups. I conclude that escape responses may be contingent on both the activity level of the animal at the time of approach and its age.Diversos fatores podem influenciar o comportamento de fuga dos lagartos (distância de início da fuga ou FID, a distância entre o predador e a presa quando esta inicia a fuga). Os padrões diários de atividade, como os comportamentos de forrageio ou de movimentação, variam em função da hora do dia, sustentando a ideia de que as respostas de fuga podem variar também temporalmente. Contudo, permenecem escassas as informações relacionadas aos efeitos da hora do dia sobre a FID. Durante o pico de atividade, a FID pode diminuir devido ao aumento do custo de desistir de recursos (e.g., presas ou parceiros potenciais). Uma hipótese alternative é a de que a FID pode aumentar porque a atividade do lagarto em geral pode servir para alertar um predador antes de sua aproximação. Esse cenário pode favorecer a fuga precoce de um lagarto, em vez de tardia. Além disso, lagartos juvenis e adultos de diversas espécies podem diferir em características de comportamento, ecológicas e morfológicas que poderiam influenciar as decisões de fuga. Testei os efeitos da hora do dia (em intervalos de 30 min) e da idade (juvenil ou adulto) sobre a FID de um lagarto teiídeo tropical, Ameiva festiva, na Costa Rica. Descobri que as respostas de fuga de A. festiva variaram com a hora do dia, de modo que, em geral, sua FID diminuiu ao longo do dia. Adicionalmente, observei um pico da FID do meio para o final da manhã que coincide com as estimativas das horas de pico de atividade de A. festiva publicadas na literatura. Em geral, os juvenis de A. festiva iniciaram uma resposta de fuga mais cedo do que os adultos, o que pode estar relacionado com diferenças no risco percebido associado a diferenças no tamanho e na experiência com o predador entre os dois grupos etários. Concluo que as as respostas de fuga podem depender tanto do nível de atividade do animal no momento da aproximação como de sua idade

Compulsory Deep Mixing of 3He and CNO Isotopes in the Envelopes of low-mass Red Giants

Three-dimensional stellar modeling has enabled us to identify a deep-mixing mechanism that must operate in all low mass giants. This mixing process is not optional, and is driven by a molecular weight inversion created by the 3He(3He,2p)4He reaction. In this paper we characterize the behavior of this mixing, and study its impact on the envelope abundances. It not only eliminates the problem of 3He overproduction, reconciling stellar and big bang nucleosynthesis with observations, but solves the discrepancy between observed and calculated CNO isotope ratios in low mass giants, a problem of more than 3 decades' standing. This mixing mechanism, which we call `$\delta\mu$-mixing', operates rapidly (relative to the nuclear timescale of overall evolution, ~ 10^8 yrs) once the hydrogen burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Pop I stars between 0.8 and 2.0\Msun develop 12C/13C ratios of 14.5 +/- 1.5, while Pop II stars process the carbon to ratios of 4.0 +/- 0.5. In stars less than 1.25\Msun, this mechanism also destroys 90% to 95% of the 3He produced on the main sequence.Comment: Final accepted version (submitted to Astrophys J in Jan 2007...

Deep Mixing of He-3: Reconciling Big Bang and Stellar Nucleosynthesis

Low-mass stars, ~1-2 solar masses, near the Main Sequence are efficient at producing He-3, which they mix into the convective envelope on the giant branch and should distribute into the Galaxy by way of envelope loss. This process is so efficient that it is difficult to reconcile the low observed cosmic abundance of He-3 with the predictions of both stellar and Big Bang nucleosynthesis. In this paper we find, by modeling a red giant with a fully three-dimensional hydrodynamic code and a full nucleosynthetic network, that mixing arises in the supposedly stable and radiative zone between the hydrogen-burning shell and the base of the convective envelope. This mixing is due to Rayleigh-Taylor instability within a zone just above the hydrogen-burning shell, where a nuclear reaction lowers the mean molecular weight slightly. Thus we are able to remove the threat that He-3 production in low-mass stars poses to the Big Bang nucleosynthesis of He-3.Comment: Accepted by Science, and available from Science Express onlin

What Firms Do: Gender Inequality in Linked Employer-Employee Data

This paper investigates the contribution of firms to the gender gap in earnings on average, at different quantiles of the earnings distribution, and over time to shed light on the role of firm pay policies in hindering or reinforcing the gender wage gap and to identify how their impact comes about. Using a linked employer-employee dataset for Italy, we show that the gap in firm pay policies explains on average 30% of the gender pay gap in the period 1995-2015. Sorting of women in low pay firms explains a larger fraction of the gender pay gap than differences in bargaining, on average and at the bottom of the distribution, whereas the latter dominates at the top. Moreover, differences in bargaining have increased in importance over the two decades. To explain sorting, we investigate whether women have a lower probability of moving towards firms with higher pay rates, and find that this is indeed the case. This differential mobility penalises, in particular, highly skilled women and can be related to the variability in wages in destination firms, with women not moving to those with high (unexplained) variance in pay. We also find some evidence that the firm environment as captured by exogenous changes in the gender balance in leadership positions influences the bargaining power of women, indicating that the latter is partly institution-driven

The Chemical Evolution of Helium in Globular Clusters: Implications for the Self-Pollution Scenario

We investigate the suggestion that there are stellar populations in some globular clusters with enhanced helium (Y from 0.28 to 0.40) compared to the primordial value. We assume that a previous generation of massive Asymptotic Giant Branch (AGB) stars have polluted the cluster. Two independent sets of AGB yields are used to follow the evolution of helium and CNO using a Salpeter initial mass function (IMF) and two top-heavy IMFs. In no case are we able to produce the postulated large Y ~ 0.35 without violating the observational constraint that the CNO content is nearly constant.Comment: accepted for publication in Ap

Evolution and CNO yields of Z=10^-5 stars and possible effects on CEMP production

Our main goals are to get a deeper insight into the evolution and final fates of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to investigate their C, N, and O yields. Using the Monash University Stellar Evolution code we computed and analysed the evolution of stars of metallicity Z = 10^-5 and masses between 4 and 9 M_sun, from their main sequence until the late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase. Our model stars experience a strong C, N, and O envelope enrichment either due to the second dredge-up, the dredge-out phenomenon, or the third dredge-up early during the TP-(S)AGB phase. Their late evolution is therefore similar to that of higher metallicity objects. When using a standard prescription for the mass loss rates during the TP-(S)AGB phase, the computed stars lose most of their envelopes before their cores reach the Chandrasekhar mass, so our standard models do not predict the occurrence of SNI1/2 for Z = 10^-5 stars. However, we find that the reduction of only one order of magnitude in the mass-loss rates, which are particularly uncertain at this metallicity, would prevent the complete ejection of the envelope, allowing the stars to either explode as an SNI1/2 or become an electron-capture SN. Our calculations stop due to an instability near the base of the convective envelope that hampers further convergence and leaves remnant envelope masses between 0.25 M_sun for our 4 M_sun model and 1.5 M_sun for our 9 M_sun model. We present two sets of C, N, and O yields derived from our full calculations and computed under two different assumptions, namely, that the instability causes a practically instant loss of the remnant envelope or that the stars recover and proceed with further thermal pulses. Our results have implications for the early chemical evolution of the Universe.Comment: 12 pages, 13 figures, accepted for publication in A&

Magnetohydrodynamics of Cloud Collisions in a Multi-phase Interstellar Medium

We extend previous studies of the physics of interstellar cloud collisions by beginning investigation of the role of magnetic fields through 2D magnetohydrodynamic (MHD) numerical simulations. We study head-on collisions between equal mass, mildly supersonic diffuse clouds. We include a moderate magnetic field and two limiting field geometries, with the field lines parallel (aligned) and perpendicular (transverse) to the colliding cloud motion. We explore both adiabatic and radiative cases, as well as symmetric and asymmetric ones. We also compute collisions between clouds evolved through prior motion in the intercloud medium and compare with unevolved cases. We find that: In the (i) aligned case, adiabatic collisions, like their HD counterparts, are very disruptive, independent of the cloud symmetry. However, when radiative processes are taken into account, partial coalescence takes place even in the asymmetric case, unlike the HD calculations. In the (ii) transverse case, collisions between initially adjacent unevolved clouds are almost unaffected by magnetic fields. However, the interaction with the magnetized intercloud gas during the pre-collision evolution produces a region of very high magnetic energy in front of the cloud. In collisions between evolved clouds with transverse field geometry, this region acts like a ``bumper'', preventing direct contact between the clouds, and eventually reverses their motion. The ``elasticity'', defined as the ratio of the final to the initial kinetic energy of each cloud, is about 0.5-0.6 in the cases we considered. This behavior is found both in adiabatic and radiative cases.Comment: 40 pages in AAS LaTeX v4.0, 13 figures (in degraded jpeg format). Full resolution images as well as mpeg animations are available at http://www.msi.umn.edu:80/Projects/twj/mhd-cc/ . Accepted for publication in The Astrophysical Journa

MONTAGE: AGB nucleosynthesis with full s-process calculations

We present MONTAGE, a post-processing nucleosynthesis code that combines a traditional network for isotopes lighter than calcium with a rapid algorithm for calculating the s-process nucleosynthesis of the heavier isotopes. The separation of those parts of the network where only neutron-capture and beta-decay reactions are significant provides a substantial advantage in computational efficiency. We present the yields for a complete set of s-process isotopes for a 3 Mo, Z = 0.02 stellar model, as a demonstration of the utility of the approach. Future work will include a large grid of models suitable for use in calculations of Galactic chemical evolution.Comment: 9 pages, 4 figures. Accepted by PAS