A-1 - Encouraged by What You Read?

I feel cold all the time.

When I was younger, I was always hot. I needed to be in air conditioning everywhere I went. I could never get the temperature cold enough. But now that I am older, I feel like a frail person who is always chilled, needing to drape myself with a blanket to keep warm.

Nevertheless, like all human beings, I am still warm-blooded. I am an endotherm, as are all mammals and birds.

For many biologists, endothermy represents a bit of an enigma. Maintaining a constant body temperature requires an elevated basal metabolic rate. But the energy needed to preserve a constant body temperature doesn’t come cheap. In fact, warm-blooded animals demand 30 times the energy per unit time compared to cold-blooded (ectothermic) creatures.

Though biologists have tried to account for endothermy, no model has adequately explained why birds and mammals are warm-blooded. The advantages of being warm-blooded over being cold-blooded have not seemed to adequately outweigh costs—until now.

Recently, a biologist from the University of Nevada, Reno, Michael L. Logan, published a model that helps make sense of this enigma.1 His work evokes the optimal design and elegant rationale for endothermy in birds and mammals—and ectothermy in amphibians and reptiles.

An Explanation for Endothermy

For endothermy to exist, it must confer some significant advantage for animals constant, elevated body temperatures.

Logan argues that endothermy maintains mammalian and bird body temperatures close to the thermal optimum for immune system functionality. The operations of the immune system are temperature-dependent. If the temperature is too low or too high, the immune system responds poorly to infectious agents. But an elevated and stable body temperature primes mammalian and bird immune systems to rapidly and effectively respond to pathogens. When birds and mammals acquire a pathogen, their bodies mount a fever response. This slight elevation in temperature places their body temperature at the thermal optimum.

In other words, the fever response plays a critical role when animals battle infectious agents. And warm-blooded animals have the advantage of possessing body temperatures close to ideal.

Temperature and Immune System Function

A body of evidence indicates that the immune systems components display temperature-dependent changes in activity. As it turns out, fever optimizes immune system function by:

  1. Increasing the flow of blood through the bloodstream because of the vasodilation (blood vessel expansion) associated with fever. This increased blood flow accelerates the movement of immune cells throughout the body, giving them more timely access to pathogens.
  2. Increasing binding of immune system proteins to immune cells, assisting their trafficking to lymph tissue.
  3. Increasing cellular activity, such as proliferation and differentiation of immune cells and phagocytosis.

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Figure: The Human Immune System. Image credit: Shutterstock

Other studies indicate that some pathogens, such as fungi, lose virulence at higher temperatures, further accounting for elevated body temperatures and the importance of the fever response. Of course, if body temperature becomes too high, it will compromise immune system function, moving it away from the temperature optimum and leading to other complications. So, the fever response must be carefully regulated.

Heres the key point: the metabolic costs of endothermy are justified because warm-bloodedness allows the immune systems of birds and mammals to be near enough to the temperature optimum that infectious agents can be quickly cleared from their bodies.

Fever Response in Ectotherms

Cold-blooded animals (ectotherms) also mount a fever response to infectious agents for the same reason as endotherms. However, the body temperature of ectotherms is set by their surroundings. This limitation means that ectotherms need to regulate their body temperature and mount the fever response through their behavior by moving into spaces with elevated temperatures. Doing so places them at the mercy of environmental changes. This condition means that cold-blooded creatures experience a significant time lag between the onset of infection and the fever response. It also means that, in some cases, ectotherms can’t elevate their body temperature to the immune system optimum if, for example, it is night or overcast.

Finally, in an attempt to elevate their body temperatures, ectotherms need to be out from under cover, making themselves vulnerable to predators. So, according to Logan’s model, endothermy offers some tangible advantages compared to ectothermy.

But endothermy comes at a cost. As mentioned, the metabolic cost of endothermy is extensive compared to ectothermy. Pathogen virulence marks another disadvantage. Logan points out that pathogens that infect cold-blooded animals are much less virulent than pathogens that infect warm-blooded creatures.

Endothermy and Ectothermy Trade-Offs

So, when it comes to regulation of animal body temperature, a set of trade-offs exists that include:

  • Metabolic costs
  • Immune system responsiveness and effectiveness
  • Pathogen virulence
  • Vulnerability to predators

These trade-offs can be managed by two viable strategies: endothermy and ectothermy. Each has advantages and disadvantages. And each is optimized in its own right.

Regulation of Body Temperature and the Case for a Creator

Logan seeks to account for the evolutionary origins of endothermy by appealing to the advantages it offers organisms battling pathogens. But, examining Logans scenario leaves one feeling as if the explanation is little more than an evolutionary just-so story.

When endothermy presented an enigma for biologists, it would have been hard to argue that it reflected the handiwork of a Creator, particularly in light of its large metabolic cost. But now that scientists understand the trade-offs in play and the optimization associated with the endothermic lifestyle, we can also interpret the optimization of endothermy and ectothermy as evidence for design.

From my vantage point, optimization signifies the handiwork of a Creator. As I discuss in The Cell’s Design, saying something is optimized is equivalent to saying it is well-designed. The optimization of an engineered system doesn’t just happen. Rather, such systems require forethought, planning, and careful attention to detail. In the same way, the optimized designs of biological systems like endothermy and ectothermy reasonably point to the work of a Creator.

And I am chill with that.

Resources

Check out more from Reasons to Believe @Reasons.org

Endnotes
  1. Michael L. Logan, “Did Pathogens Facilitate the Rise of Endothermy?” Ideas in Ecology and Evolution 12 (June 4, 2019): 1–8, https://ojs.library.queensu.ca/index.php/IEE/article/view/13342.

 

About The Author

Dr. Fazale Rana

I watched helplessly as my father died a Muslim. Though he and I would argue about my conversion, I was unable to convince him of the truth of the Christian faith. I became a Christian as a graduate student studying biochemistry. The cell's complexity, elegance, and sophistication coupled with the inadequacy of evolutionary scenarios to account for life's origin compelled me to conclude that life must stem from a Creator. Reading through the Sermon on the Mount convinced me that Jesus was who Christians claimed Him to be: Lord and Savior. Still, evangelism wasn't important to me - until my father died. His death helped me appreciate how vital evangelism is. It was at that point I dedicated myself to Christian apologetics and the use of science as a tool to build bridges with nonbelievers. In 1999, I left my position in R&D at a Fortune 500 company to join Reasons to Believe because I felt the most important thing I could do as a scientist is to communicate to skeptics and believers alike the powerful scientific evidence - evidence that is being uncovered day after day - for God's existence and the reliability of Scripture. [...] I dedicated myself to Christian apologetics and the use of science as a tool to build bridges with nonbelievers. Fazale "Fuz" Rana discovered the fascinating world of cells while taking chemistry and biology courses for the premed program at West Virginia State College (now University). As a presidential scholar there, he earned an undergraduate degree in chemistry with highest honors. He completed a PhD in chemistry with an emphasis in biochemistry at Ohio University, where he twice won the Donald Clippinger Research Award. Postdoctoral studies took him to the Universities of Virginia and Georgia. Fuz then worked seven years as a senior scientist in product development for Procter & Gamble.

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