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991.
992.
The present study was conducted in the Laboratory of Animal Physiology and Biotechnology, Department of Animal Production, Faculty of Agriculture, Mansoura University, Egypt. The present investigation aimed at studying effects of ages, pubertal stages and seasons of the year on testosterone concentrations in blood plasma and tissue homogenate of the testes. The testes used in the current study were collected from a total of 104 one-humped male camels (Camelus dromedarius). Samples were taken from pre (1–3.5 years) and post (3.5–13 years) pubertal camels. Testes were studied for a two consecutive seasons. The freshly prepared homogenate of the testicular tissue and blood plasma were used for determining the concentrations of testosterone in plasma and testicular extract. The concentrations of testosterone in blood plasma and testicular tissue were significantly increased during the breeding season compared with that of non-breeding season; the concentration of testosterone was higher in testicular tissue than in blood plasma.Testosterone concentrations in plasma and testicular tissue were increased in breeding than in non-breeding season. In addition, the testosterone concentrations were closely related with seasonal changes, stage of puberty and advancing age. 相似文献
993.
McCall GS 《Journal of human evolution》2006,51(4):429-439
This paper examines variability and the emergence of the Howiesons Poort within the Middle Stone Age lithic technologies of Klasies River Mouth. Here I present a pattern recognition study using multivariate statistical techniques examining the data presented by [Singer, R., Wymer, J.J., 1982. The Middle Stone Age at Klasies River Mouth in South Africa. Chicago University Press, Chicago]. Using this model, the Middle Stone Age (MSA) lithics from Klasies River Mouth load on three main factors: (1) a Levallois flake manufacture factor, (2) a blade manufacture factor, and (3) a worked point factor. The MSA I assemblage from Cave 1 and the MSA II assemblage from Shelter 1a correlate strongly with the worked point factor, the MSA II assemblage from Cave 1 correlates strongly with the Levallois factor, and the Howiesons Poort assemblage from Shelter 1a correlates strongly with the blade factor. The Howiesons Poort from Shelter 1a is differs more from all other industries than the remaining industries do from each other. In addition, the Howiesons Poort from Cave 2 groups closely with the MSA III assemblage from Shelter 1a due to similarities in raw material frequencies. 相似文献
994.
This paper assesses selective pressures that shaped primate life histories, with particular attention to the evolution of longer juvenile periods and increased brain sizes. We evaluate the effects of social complexity (as indexed by group size) and foraging complexity (as indexed by percent fruit and seeds in the diet) on the length of the juvenile period, brain size, and brain ratios (neocortex and executive brain ratios) while controlling for positive covariance among body size, life span, and home range. Results support strong components of diet, life span, and population density acting on juvenile periods and of home range acting on relative brain sizes. Social-complexity arguments for the evolution of primate intelligence are compelling given strong positive correlations between brain ratios and group size while controlling for potential confounding variables. We conclude that both social and ecological components acting at variable intensities in different primate clades are important for understanding variation in primate life histories. 相似文献
995.
Webb AR 《Progress in biophysics and molecular biology》2006,92(1):17-25
The synthesis of vitamin D in skin is a two-stage process that begins with the production of previtamin D after irradiation of 7-dehydrocholesterol by ultraviolet (UV) radiation. A number of personal and environmental factors control the probability of a suitable UV photon reaching a molecule of 7-dehydrocholesterol in the skin. These are astronomical factors that govern the solar zenith angle (SZA), and the local state of the atmosphere, determining the available solar UV radiation; skin pigmentation and age, determining competing absorbers of UV radiation and available 7-dehydrocholesterol; individual behaviour in the local surroundings, determining exposure of unprotected skin to available UV radiation. The only one of these influences that can be determined unequivocally for any situation is the SZA. The other influences must be considered either as individual case studies, or be represented by "typical" and "idealised" situations for the weather, skin and behaviour. At large SZAs there is insufficient solar UV radiation to initiate significant vitamin D synthesis. At smaller SZAs assessment of solar exposure necessary for vitamin D synthesis can only be indicative and application of any such assessment necessarily requires awareness of both self- and the local environment. 相似文献
996.
Václav Pavel 《Journal of Ethology》2006,24(2):175-179
It has been suggested that the brood defence by parents of altricial birds should increase during the breeding attempt until the young depart from the nest. The two proximate hypotheses provide alternative predictions about the peak of brood defence intensity: (1) the vulnerability hypothesis predicts a rapid rise in brood defence after hatching of the chicks, with maximum defence intensity just before fledging and strong decline afterwards; (2) the feedback hypothesis predicts that brood defence intensity will, after a rapid rise, reach a plateau at the end of the nestling period and early after fledging and then slowly decline. I compared brood defence behaviour of altricial meadow pipit (Anthus pratensis) breeding in the Czech Republic during the late nestling stage and during the fledging time. A stuffed stoat (Mustela erminea) was placed 5 m from a meadow pipit nest and the defence behaviour of parents was recorded for 10 min from a hide. Brood defence intensity was higher during the fledgling time than during the late nestling stage, and this trend was more evident in males than in females. Regardless of the proportion of already fledged chicks and those still present in the nest, brood defence did not significantly decrease during the fledgling time in males or females. The results do not agree with the predictions of the vulnerability hypothesis and support the predictions of the feedback hypothesis. 相似文献
997.
998.
T B Gage 《American journal of physical anthropology》1988,76(4):429-441
A five-parameter competing hazard model of the age pattern of mortality is described, and methods of fitting it to survivorship, death rate, and age structure data are developed and presented. The methods are then applied to published life table and census data to construct life tables for a Late Woodland population, a Christian period Nubian population, and the Yanomama. The advantage of this approach over the use of model life tables is that the hazard model facilitates life-table construction without imposing a particular age pattern of mortality on the data. This development makes it possible to use anthropological data to extend the study of human variation in mortality patterns to small populations. 相似文献
999.
1000.
从不同年龄(20天,30天,1年)的小白鼠全脑制得细胞质混合氨酰tRNA合成酶。用异源体系(即用酵母tRNA和小白鼠全脑氨酰tRNA合成酶)测定了氨酰tRNA合成酶分别载运~3H标记的Asp、Gly、Glu、Lys和Ala的活力。结果表明除未检出tRNA~(Glu)的合成酶活力外,对其余四种氨基酸都有明显的活力,特别是年龄20天小白鼠的氨酰tRNA合成酶对~3H-Gly具有高达35%的载运活力。对~3H-Gly、~3H-Lys和~3H-Ala的载运活力有随增龄而下降的趋势,但对~3H-Asp的载运活力则随年龄增长而增高。 相似文献