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 system’s components display temperature-dependent changes in activity. As it turns out, fever optimizes immune system function by:
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.
Increasing binding of immune system proteins to immune cells, assisting their trafficking to lymph tissue.
Increasing cellular activity, such as proliferation and differentiation of immune cells and phagocytosis.
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.
Here’s 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 Logan’s 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 inThe 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.
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