In a marsh-breeding population of red-winged blackbirds, the sex ratio of offspring that survived to leave the nest varied with maternal age. Old mothers produced an excess of male fledglings, middle-aged mothers produced almost equal proportions of males and females, and young mothers produced nearly twice as many females as males. More males than females hatched from the eggs of old.mothers, whereas among newly hatched progeny of middle-aged and young mothers the sex ratio did not differ from unity. The.hatching rate of eggs of old mothers was unusually low,. suggesting that the biased sex ratio of their hatchlings may have been caused by more frequent death of female embryos, although other possibilities can be imagined. Star--ation of nestlings after hatching also affected the sex ratio among young that left the nest; When starvation occurred, it fell principally on young produced by the last and next-to-last eggs laid in the clutch. Because old mothers allocated relatively more energy .to those eggs than to earlier-laid eggs, whereas young mothers apportioned energy equally to their eggs, few nestlings of old mothers but many nestlings of young mothers starved. Most nestlings that died were male. It followed that the. male bias in sex ratio of progeny of old mothers did not change between hatching and nestleaving, but the ratio among progeny of young mothers shifted after hatching to a strong bias favoring females at nest-leaving.Even though the age of breeding birds within a population may vary considerably (1-3), little advance has been made toward a comprehensive empirical analysis of-the effects of individual age on reproductive performance. Adequate knowledge of how individuals of different ages vary in reproductive output requires long-term field research on single populations, and the difficulty of this requirement has impeded progress. Nevertheless, arriving-at-a better understanding of age-dependent reproductive-processes is of great importance. Age structure is a major element in several key theories pertaining to population biology (4-6), and what few data exist support the view that reproductive performance and age are often related (1, 2, 7).We searched for and found age'dependent reproductive patterns in a marked population of red-winged blackbirds (Agelaius phoeniceus) that had been under continuous study during 10 previous breeding seasons. Redwings, which are widespread throughout North America and have been extensively studied because-of.their polygynous mating system (8), have several characteristics that rendered them a-promising species for this investigation. First, nearly all-parental care bestowed directly on the nest and its contents, from building until young leave the nest, is provided by the female. If parental age significantly affects reproductive performance, we postulated that such effects would be most easily detected when a single-parent bore most or all of the direct burdens of supplying energy to eggs and nestlings. Second, the annual survival rate and the fideli...

We investigated the influence of ambient temperature (Ta) and food availability on seasonal timing and extent of physiological responses to short photoperiod (SP), in particular daily torpor, in Djungarian hamsters (Phodopus sungorus). Exposure of hamsters to cold temperature (Ta = 5 degrees C), relative to warm Ta (23 degrees C), resulted in: 1) a significant advance (P < 0.05) of the first occurrence of torpor among cold-exposed hamsters (days 52-97 vs. days 83-99 in SP); 2) a higher (P < 0.01) incidence of torpor (48% vs. 20% torpid animals/day); 3) a higher (P < 0.05) degree of molt into the winter pelt; and 4) an accelerated reduction of body weights (P < 0.001). However, within SP/cold-Ta exposed groups, individual hamsters clearly showed different tendencies for torpor (torpor on 0-95% of days observed). Therefore, we evaluated the effects of small changes in Ta on torpor frequency and extension by subjecting the same SP-adapted individuals to varying temperatures. Lowering of Ta from 15 degrees C to 10 degrees C and 5 degrees C caused significant (P < 0.05) increases in the incidence of torpor (20%, 33%, and 40%, respectively) and lower minimal body temperatures during hypothermia (P < 0.05). When the same animals were subjected to 24-48 h lasting periods of food restriction (60% of the ad libitum intake), torpor frequency further increased 1.8- to 2.6-fold at all Tas. These results show that Ta and food availability are effective in modifying both seasonal timing and extent of photoperiodically controlled adaptations. This integration of multiple environmental cues, combined with a pronounced within-species variability of winter adjustments, indicates that Djungarian hamsters are capable of flexible physiological responses towards unpredictable climatic changes in the environment.

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SummaryWe propose a method to image the surface topography of transparent objects. The space between the object and the opposite closely positioned surface (such as a cover glass or a slide) is filled with a strongly absorbing dye. The contrast is generated by recording a transmission image at a wavelength where the dye absorbs. Since the transmitted intensity depends on the depth of the dye layer, it carries information about the relief of the tested surface. With sufficiently concentrated dyes, nanometre unevenness of a surface can be detected. By using less-concentrated solutions, it is possible to image and measure larger objects, such as biological cells. At the present stage, biological applications of the method are only semiquantitative, but the method still provides detailed information about cell shapes that is not readily obtainable with other imaging techniques.Conversion of the image grey scale into the units of vertical distance requires knowledge of the absorption coefficient of the dye. The same method that is used for imaging can be adapted to measure the absorption coefficient of concentrated dyes. The solution to be analyzed is placed between a glass slide and a spherical lens of known radius. The absorption coefficient is determined from attenuation of transmitted intensity as a function of the distance to the centre. At the same time, the interference pattern in the reflected image allows measurement of the refractive index of the dye.

Seasonal breeding is common in mammals, particularly in habitats outside the tropics. Climate and availability of food are the ultimate factors that usually dictate the optimal time of year for a mammal to breed; however, day length (photoperiod) often serves as the proximal cue to signal the onset or cessation of seasonal reproduction. Some individuals in some populations of deer mice are reproductively responsive to photoperiod, while other individuals in the same population are not. As shown here, selection can dramatically alter the frequency of photoresponsiveness in a laboratory population in only two generations. To our knowledge this is the first demonstration of selection for reproductive photoresponsiveness in any mammal. By implication, some wild populations of deer mice must use multiple, genetic-based reproductive strategies, and the degree to which each such strategy is exhibited must be subject to rapid change in response to both seasonally and momentarily changing climatic and dietary conditions.