ABSTRACT
Rhythms are ubiquitous in nature, from single-celled organisms to complex species, and they are thought to help individuals adapt to a particular ecological environment and physiological situation. The rabbit is an excellent example of how the neuroendocrine system changes to fulfill the specific needs of reproduction, particularly lactation. Hormone levels in blood have a rhythm that reaches peaks and troughs at specific hours during the day. For example, in the rabbit maintained in normal light/ dark conditions, the plasma glucocorticoid level is highest in the evening, before the onset of the period of maximal locomotor activity (Szeto et al., 2004), while lowest values occur at 6:00 in the morning. The adaptive significance of the high glucocorticoid concentration is to promote alertness to daily activities of this nocturnal animal during its active period. Similar changes are found across mammalian species, both diurnal and nocturnal. As a result, the levels of corticosteroids and other hormones fluctuate at particular hours to support daily, and even seasonal, activity patterns of individuals and populations. These rhythmic changes are controlled in an orderly way by a clock mechanism located in the brain, specifically in the suprachiasmatic nucleus (SCN).
