Sperm design and variation in the New World blackbirds (Icteridae)

Post-copulatory sexual selection (PCSS) is thought to be one of the evolutionary forces responsible for the rapid and divergent evolution of sperm design. However, whereas in some taxa particular sperm traits are positively associated with PCSS, in other taxa, these relationships are negative, and the causes of these different patterns across taxa are poorly understood. In a comparative study using New World blackbirds (Icteridae), we tested whether sperm design was influenced by the level of PCSS and found significant positive associations with the level of PCSS for all sperm components but head length. Additionally, whereas the absolute length of sperm components increased, their variation declined with the intensity of PCSS, indicating stabilizing selection around an optimal sperm design. Given the diversity of, and strong selection on, sperm design, it seems likely that sperm phenotype may influence sperm velocity within species. However, in contrast to other recent studies of passerine birds, but consistent with several other studies, we found no significant link between sperm design and velocity, using four different species that vary both in sperm design and PCSS. Potential reasons for this discrepancy between studies are discussed

Electrified film flow over step topography at zero Reynolds number.

The flow of a liquid film over step topography under the influence of an electric field is considered in the limit of zero Reynolds number. The particular topographies considered include a flat wall with a downward step or an upward step, or a flat wall which is indented with a rectangular trench. A uniform electric field is imposed at infinity in the direction normal to the flatwall. The air above the film is treated as a perfect dielectric. The liquid in the film is assumed to behave either as a perfect conductor or as a perfect dielectric whose dielectric constant in general differs from that in the air. Asymptotic results are derived on the assumption of small step height, and formulas are presented for the first-order correction to the free-surface deformation due to the topography. It is demonstrated that, in an appropriate long-wave limit, the solutions approach those obtained using the lubrication approximation. Finally, the small-step asymptotics are favourably compared with numerical solutions for Stokes flow over steps of arbitrary height computed using the boundary-element method. In summary, it is shown that asymptotic models based on small-amplitude step topography provide simple formulas which are effective in describing the flow even for moderate step amplitudes, making them an efficient analytical tool for solving practical film-flow problems