Size and shape of sperm cells vary tremendously throughout the animal kingdom. The adaptive significance of this variation is not fully understood. In addition to sperm-female interactions and the environmental conditions, the risk of sperm competition might affect number, morphology and other "quality" traits of sperm. In the male-diphenic ant Cardiocondyla obscurior, winged sneaker males have limited sperm number, because their testes degenerate shortly after adult emergence, as is typical for males of social Hymenoptera. In contrast, wingless fighter males continuously replenish their sperm supply due to their exceptional lifelong spermatogenesis. While winged males usually have to compete with several other winged males for virgin queens, wingless males are able to monopolize queens by killing all other rivals. Hence, this presents a unique system to investigate how alternative reproductive tactics and associated physiology affect sperm morphology and viability. We found that sperm-limited males invest into sperm number instead of sperm size. Variance in sperm length is smaller in winged males, probably reflecting that they have to compete with several other males. Finally, sperm viability is equally high in both male phenotypes.
In a self-consistent semi-empirical numerical approach based on abinitio-calculations for small samples, we evaluate the GMR effect for disordered (001)-(3-Fe/3-V) ∞ multilayers by means of a Kubo formalism. We consider four different types of disorder arrangements: In case (i) and (ii), the disorder consists in the random interchange of some Fe and V atoms, respectively, at interface layers; in case (iii) in the formation of small groups of three substitutional Fe atoms in a V interface layer and a similar V group in a Fe layer at a different interface; and for case (iv) in the substitution of some V atoms in the innermost V layers by Fe. For cases (i) and (ii), depending on the distribution of the impurities, the GMR effect is enhanced or reduced by increasing disorder, in case (iii) the GMR effect is highest, whereas finally, in case (iv), a negative GMR is obtained ( "inverse GMR").PACS: 75.50R-Magnetism in interface structures (incl. layers and superlattice structures); 72.15G-Galvanomagnetic and other magnetotransport effects (metals/alloys); KEYWORDS: GMR-effect; magnetic multilayers; ab initio calculations; * based on the diploma thesis of A. Moser, Regensburg 1997 (present address: Siemens Co., Munich) ;
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