Question

Walter Cannon, who coined the term homeostasis, argued that lizards and amphibians are less highly evolved than mammals and birds because they are not as fully homeostatic as are mammals and birds. Considering water-salt relations and any other aspects of physiology you find to be pertinent, explain in detail why you agree or disagree.

Short answer

Whether one agrees or disagrees with Cannon's viewpoint boils down to how they perceive evolution. If high degree of homeostatic control is viewed as the pinnacle of evolution, then yes, mammals and birds could be considered 'more evolved'. However, if the ability to endure and adapt to variable external conditions is seen as evidence of evolution, then one could argue that reptiles and amphibians are similarly evolved. The key is understanding the diversity of life and recognizing that different kinds of organisms have evolved different strategies for survival, all of which are successful in their own right.

Step-by-step solution

Understanding Homeostasis

Homeostasis is a concept in biology that refers to an organism's ability to maintain a constant internal environment despite changes in external conditions. Walter Cannon credited with the formulation of this concept, argued that the ability to maintain homeostasis is a sign of an evolved organism.

Comparing Vertebrate Classes

Next, we have to consider the classes of vertebrates that are being compared: reptiles and amphibians versus birds and mammals. Reptiles and amphibians are ectothermic organisms, meaning they rely on external heat sources to control their body temperature. One aspect of homeostasis in these creatures is water-salt balance. They can usually tolerate larger deviations in internal conditions as compared to birds and mammals. On the other hand, birds and mammals are endothermic organisms, meaning they produce heat internally to maintain body temperatures. They tend to have more complex homeostatic mechanisms that tightly control internal conditions. An example is their ability to control water-salt balance using hormones like vasopressin and aldosterone.

Forming an Opinion

The argument put forth by Cannon is that the ability to achieve tight homeostatic control evidences evolution. While reptiles and amphibians exhibit some level of homeostasis, it's not as highly developed as in birds and mammals, thus suggesting to some that these organisms are 'less evolved'. However, one's agreement or disagreement with this view depends on their understanding of what constitutes 'being evolved'. An alternative perspective could argue that the ability to survive in a wide range of external conditions, as seen in reptiles and amphibians, is another form of adaptation (and therefore evolution).

Key concepts

Water-Salt Relations

Homeostasis involves the regulation of internal conditions to maintain stability. When it comes to water-salt relations, organisms need to balance the intake and excretion of water and salts to maintain physiological functions. This balance is crucial for the maintenance of osmotic pressure and cellular function.

• Ectothermic organisms like reptiles and amphibians have fewer homeostatic mechanisms for water and salt balance. They rely more on behavioral adjustments to regulate these aspects.
• The water-salt homeostatic process in endothermic organisms is more complex, involving hormones to regulate kidney functions and maintain a narrower range of internal conditions.

This difference means that reptiles and amphibians can tolerate wider variations in their internal environment, while birds and mammals require a more controlled balance to support their higher metabolic rates.

Ectothermic Organisms

Ectothermic organisms, often referred to as 'cold-blooded', have body temperatures that fluctuate with their environment. These organisms rely on external heat sources for thermoregulation.

• Reptiles and amphibians are ectotherms. They exhibit a 'behavioral thermoregulation', by basking in the sun to warm up or finding shade to cool down.
• This strategy allows them to live in a variety of environments, but it also means they are less efficient in extreme conditions.

From a physiological perspective, their internal processes are slower than those of endothermic organisms, which is why their overall metabolic rates are lower. This adaptation allows ectotherms to survive longer periods without food.

Endothermic Organisms

Endothermic organisms, also known as 'warm-blooded', maintain a constant body temperature through internal heat production. This group includes mammals and birds, which possess more advanced homeostatic mechanisms.

• They generate heat metabolically, which allows them to remain active in a wide range of external temperatures.
• Their higher and more consistent body temperatures support more efficient cellular activity and higher rates of metabolism.

Endotherms also have complex mechanisms involving the nervous and endocrine systems to regulate their internal environments. This complexity provides them the flexibility to thrive in environments where ectotherms might struggle.

Hormonal Control in Homeostasis

Hormonal control is key to maintaining homeostasis, especially in endothermic organisms. Hormones like vasopressin and aldosterone are critical for regulating water-salt balance.

• Vasopressin, also known as Antidiuretic Hormone (ADH), helps concentrate urine and retain water in the body by acting on kidneys.
• Aldosterone is involved in sodium retention and potassium excretion, which is vital for maintaining fluid balance and blood pressure.

These hormones allow for precise adjustments to be made in response to varying internal and external conditions, demonstrating the complexity of the homeostatic system in birds and mammals.