Reasons to Believe

The Fungus is Not Among Us: Optimization of Mammalian Body Temperature

I did a lot of work with bacterial cultures in graduate school. On occasion the cultures would get contaminated with fungus, ruining the experiment. The only consolation was the opportunity to say, “The fungus is among us, and it’s humongous.”

Fungal contamination presents a real problem in microbiology labs. Fortunately, though, as new work by researchers from Albert Einstein College of Medicine in New York City attests, fungal infections are not nearly as bad as they could be for humans and other mammals.1

Fungi commonly cause diseases in plants, insects, amphibians, and reptiles, but rarely in mammals. No one really knows why, but some investigators suspect that it has to do with the fact that fungi are susceptible to high temperatures.

Mammals display the related properties of endothermy and homeothermy, which basically means that these creatures have an elevated body temperature that is tightly regulated around a set point of 37 °C. The body temperatures of other organisms vary and are dictated by the environment. Scientists believe that the higher body temperatures of mammals prevent fungal infection, because the growth of these organisms is retarded as the temperature increases. In other words, the mammalian body temperature creates a temperature exclusion zone, preventing fungal growth.

But maintaining body temperatures higher than the ambient conditions costs metabolic energy. So even though the mammalian body temperature promotes survivability (because it restricts fungal growth), it is a disadvantage for the organism (because it requires that mammals consume more food than other organisms).

The research team explored the relationship between energy cost and reduced mycoses by developing a simple mathematical model that calculated the fitness of mammals as temperature varies. They discovered that fitness reaches an optimal value at 36.7 °C and that the optimal range resides between 35.9 °C and 37.7 °C.

In other words, mammalian body temperature appears to be optimized to accommodate the trade-off between metabolic costs and protection against fungal infections.

The researchers speculated that endothermy and homeothermy were critical at the beginning of the Tertiary when the mammalian radiation took place—a time of explosive diversification of mammals right after the dinosaurs went extinct. The fungal protection afforded by high body temperatures helped mammals to flourish in the face of another radiation event documented from the fossil record at the end of the Cretaceous called the fungal bloom.

As I discuss in The Cell’s Design, optimization signals intelligent design. In fact, 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 mammalian body temperature, reasonably point to the work of a Creator.

1. Aviv Bergman and Arturo Casadevall, “Mammalian Endothermy Optimally Restricts Fungi and Metabolic Costs, mBio 1, no. 5 (2010): e00212–10; doi:10.1128/mBio.00212-10.


Dr. Fazale Rana

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. Read more about Dr. Fazale Rana