26 research outputs found
Food availability, energetic constraints and reproductive development in a wild seasonally breeding songbird
1. In many organisms, food availability is a proximate cue that synchronizes seasonal development of the reproductive system with optimal environmental conditions. Growth of the gonads and secondary sexual characteristics is orchestrated by the hypothalamicâpituitaryâgonadal (HPG) axis. However, our understanding of the physiological mechanisms by which food availability modulates activity of the HPG axis is limited. 2. It is thought that many factors, including energetic status, modulate seasonal reproductive activation. We tested the hypothesis that food availability modulates the activity of the HPG axis in a songbird. Specifically, we foodârestricted captive adult male Abert's Towhees Melozone aberti for 2 or 4 weeks during photoinduced reproductive development. A third group (control) received ad libitum food throughout. We measured multiple aspects of the reproductive system including endocrine activity of all three levels of the HPG axis [i.e. hypothalamic gonadotropinâreleasing hormoneâI (GnRHâI), plasma luteinizing hormone (LH) and testosterone (T)], and gonad morphology. Furthermore, because gonadotropinâinhibitory hormone (GnIH) and neuropeptide Y (NPY; a potent orexigenic peptide) potentially integrate information on food availability into seasonal reproductive development, we also measured the brain levels of these peptides. 3. At the hypothalamic level, we detected no effect of food restriction on immunoreactive (ir) GnRHâI, but the duration of food restriction was inversely related to the size of irâGnIH perikarya. Furthermore, the number of irâNPY cells was higher in foodârestricted than control birds. Food restriction did not influence photoinduced testicular growth, but decreased plasma LH and T, and width of the cloacal protuberance, an androgenâsensitive secondary sexual characteristic. Returning birds to ad libitum food availability had no effect on plasma LH or T, but caused the cloacal protuberance to rapidly increase in size to that of ad libitumâfed birds. 4. Our results support the tenet that food availability modulates photoinduced reproductive activation. However, they also suggest that this modulation is complex and depends upon the level of the HPG axis considered. At the hypothalamic level, our results are consistent with a role for the GnIH and NPY systems in integrating information on energetic status. There also appears to be a role for endocrine function at the anterior pituitary gland and testicular levels in modulating reproductive development in the light of energetic status and independently of testicular growth
Current Trends In Educational Publishing: A Personal View
Perhaps the most distinctive characteristic of our times is the
rapid increase in the rate of change. In whatever field one cares to
look, the pace of change is accelerating. This is certainly true of
education.
Because educational publishing serves education, the really
important trends in educational publishing are the result of changes
in education or in society's view of education. My thesis, stated
briefly and straightforwardly, is that there are two major trends at
work in American education. The first is a shift in society's view of
education: once considered an expense, education is now seen as an
-Investment. The second is a shift in the emphasis from teaching to
learning.
Education as investment. Education as learning. These two
phrases embody the ideas I hope to clarify and embroider. It will
quickly be seen that the most important trends in educational publishing
are a consequence of these two trends in education.published or submitted for publicatio