Variation of pecking rate with sward height in wild wigeon Anas penelope

We examined how pecking rate changed with sward height in wintering European wigeon Anas penelope in a nature reserve of the French Atlantic coast. Pecking rates were recorded as the time taken to perform 10 or 50 pecks. We found a negative correlation between pecking rate and sward height over a range of heights from 1.0 to 3.5 cm, but no sex effect. Equations based on the time taken to perform 10 or 50 pecks were slightly different, suggesting that scanning (scans of the surroundings) and moving (search for plant items of good quality) had an effect on pecking rate. However, these costs were lower (i.e. a decrease of 6–10% of pecking rate) than the effect of handling time, which remained the dominant foraging process constraining pecking rate (i.e. a decrease of 24% with each 1-cm increase in sward height).     

Potentiation by steroids of the β-adrenergic agent-induced stimulation of cyclic AMP in isolated mouse thymocytes

There is an increasing amount of evidence suggesting that glucocorticoids may modulate the responsiveness of various cell types to β-adrenergic agents. In some systems, it has been shown, in addition, that steroids potentiate the elevation of cAMP induced by catecholamines. Little is known however of the mechanism underlying steroid action. We have studied this ‘permissive action’ in isolated thymocytes which have specific receptor sites for both glucocorticoids and β-adrenergic agents. The glucocorticoid compound dexamethasone did not alter intracellular cAMP level but markedly enhanced the stimulation produced by isoproterenol. This effect was instantaneous and was still measurable at 10^?7 M dexamethasone. A similar potentiating action was observed in the presence of corticosterone but also in the presence of sex steroids. Determination of β-receptors after cell preincubation in the presence of dexamethasone showed that rapid alterations in β-receptors are not involved in this permissive action. Experiments done in the presence of the calcium chelator, ethyleneglycol bis(β-aminoethyl ether)-N,N′-tetraacetic acid, suggest that dexamethasone action could be related to a modification of calcium mobilization.     

Sexual specialization and inbreeding avoidance in the evolution of dioecy

Dioecy has evolved independently, many times, among unrelated taxa. It also appears to have evolved along two contrasting pathways: (1) from hermaphroditism via monoecy to dioecy and (2) from hermaphroditism via gynodioecy to dioecy. Most dioecious plants have close cosexual relatives with some means of promoting outcrossing (e.g., herkogamy, dichogamy, self-incompatibility, or monoecy). To the extent that these devices prevent inbreeding, the evolution of dioecy in these species cannot logically be attributed to selection for outcrossing. In these cases, the evolution of dioecy is, we believe, due to selection for sexual specialization. However, in other species, that lack outbreeding close relatives, dioecy may have evolved from gynodioecy (males and hermaphrodites) as an outbreeding device. Subsequent disruptive selection and selection for sexual specialization may have also shaped the evolution of dioecy from gynodioecy in these species, resulting in two genetically determined, constant sex morphs. Both pathways for the evolution of dioecy require the operation of disruptive selection, though the gynodioecy route involves more restrictive disruptive selection and a genetic designation of gender. In contrast, the monoecy route is not dependent on the genetic designation of two sex morphs, but, rather, allows the possibility of sexual intermediates and sexual lability. Both pathways produce one morph in which maleness is suppressed and another in which the female function is negligible or nonexistent—the reproductive mode recognized as dioecy. Evidence is presented here to support the thesis that instances of sexual lability, the presence of an array of sexual intermediates, sex-switching, and sexual niche segregation can be explained in terms of the pathway that was taken in the evolution of a particular dioecious species. In addition, the degree of sexual dimorphism seen in dioecious species is correlated with mode of pollination (insector wind-pollinated) and other ecological factors.     

Rheological properties of cross-linked hyaluronan-gelatin hydrogels for tissue engineering

Hydrogels that mimic the natural extracellular matrix (ECM) are used in three-dimensional cell culture, cell therapy, and tissue engineering. A semi-synthetic ECM based on cross-linked hyaluronana offers experimental control of both composition and gel stiffness. The mechanical properties of the ECM in part determine the ultimate cell phenotype. We now describe a rheological study of synthetic ECM hydrogels with storage shear moduli that span three orders of magnitude, from 11 to 3 500 Pa, a range important for engineering of soft tissues. The concentration of the chemically modified HA and the cross-linking density were the main determinants of gel stiffness. Increase in the ratio of thiol-modified gelatin reduced gel stiffness by diluting the effective concentration of the HA component.