Photo credit: Daniel J. Cox @ NaturalExposures.com
A BRIEF INTRODUCTION TO PHYSIOLOGY
Physiology is an intersection of several biological fields: evolution, behavior, ecology, and anatomy (13). The following pages discuss the evolution, behavior, and ecology of polar bears and the connection of these three disciplines to polar bear physiology, specifically thermoregulation. To reach a complete understanding of this relationship, it is important to first understand the field of physiology.
Physiology is the study of the way animals, vertebrates and invertebrates, function; this includes their growth and development, absorption of nutrients, synthesis of proteins, function of different tissues, and role of organs (14). Where the anatomy of an organism focuses on how the organism is structured, physiology seeks to understand how these structures interact (15). These studies can range from a focus at the cellular level to whole organism studies. Often times the field of physiology is broken down further into four subsections: cellular physiology, neurophysiology, comparative physiology, and environmental physiology.
Similar to other fields, physiology relies on three basic themes (13). The understanding of these three themes is crucial to understanding the four elements of physiology in the polar bear.
1. Function is based on structure.
2. Homeostasis is the maintenance of the internal environment through internal regulation mechanisms, specifically negative feedback and positive feedback systems.
Negative feedback: the absence of something prompts the production of what is missing. For example, temperature regulation in endotherms operates like a thermostat, when temperatures fall below a set point thermoregulation mechanisms kick in to raise the temperature.
Positive feedback: an increase in a stimulus triggers the production of another chemical, protein, or substance. For example, upon injury blood pressure decreases causing an increase in heart rate, an increase in blood flow, and an increase in blood lost.
3. Animal structure and function are the result of evolution through natural selection.
Polar bears have four particularly interesting elements of their physiology: liver toxicity, thermoregulation, locomotion, and circulatory system.
As previously discussed, seals are the primary food source for polar bears. Seal’s livers store large amounts of vitamin A. As a result, the livers of polar bears are also cable of maintaining very high concentrations of vitamin A.
Figure provided by Steven C. Amstrup.
Photo of a ringed seal (Phoca hispida). Ringed seals make up the most of a polar bear’s diet. Typically these seals will weigh more than 100 kg (8).
The consumption of the liver is toxic to humans. A review compiled by Dr. Amstrup lists the side effects experienced by humans after (early Arctic explorers) eating the liver: drowsiness, headache, irritability, and severe skin peeling (8). Because vitamin A concentrations vary between polar bears and the amount consumed varied among explorers, symptoms also show differing levels of severity. Some individuals only suffered localized skin peeling in the hands, feet, or torso while other individuals experienced full body peeling (8).