ABSTRACT
Endocrine systems regulate and integrate physiological systems to maintain homeostasis during dynamic changes in environmental conditions or organismal demands. These changes can include photoperiod, temperature, nutrient and water availability, exercise, and reproduction. Hormones are chemical messengers secreted into circulation by endocrine glands and tissues. These chemical messengers interact with receptors on target cells that alter gene expression, or activate second messenger systems that modify cellular function. Endocrine systems are typically regulated through stimulatory and negative feedback mechanisms, often involving separate endocrine glands in a cascading sequence of hormone release originating from central neurological structures. Other internal biochemical stimuli, such as changes in blood glucose or electrolyte concentrations in plasma, are also capable of directly eliciting endocrine responses from the structures that are responsible for maintaining those constituents within appropriate physiological limits. In general, endocrine systems in marine mammals follow the basic organization and biochemical characteristics typical of other mammals. However, the unique physiological challenges faced by marine mammals as part of their life histories have led to some important differences from terrestrial mammals that are of interest to both researchers and clinicians. The extensive and growing work on the stress responses (see Chapter 9 ) and reproductive endocrinology (see Chapter 10 ) of marine mammals is reviewed in separate chapters.
