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Juan G. Rubalcaba: Read the article
Why do endotherms spend so much energy? A theoretical model shows that species followed one of two possible evolutionary patches: live fast while spending a lot (homeotherms), live slowly and keep your costs low (heterotherms)
A s I write this article, I am huddled at my desk with my portable space heater blasting into my face, warming the soft blanket that I have wrapped around my entire body like a cocoon. It is summer. I just really dislike air conditioning.
Despite my aversion to AC, however, if I were to actually measure my body temperature, it would be relatively consistent with what it always is: about 98 degrees Fahrenheit. This is because I – like all humans – am an endotherm, meaning I maintain a high, stable body temperature using metabolic heat production. This strategy is in contrast to ectotherms, which rely on external sources of heat.
Endothermy has developed in many different types of organisms, including fish, birds, and plants. However, it has a major downside compared to ectothermy—to produce heat, endotherms must burn resources in an inefficient way, costing the organism a lot of energy. Researchers aren’t sure how this ineffective strategy evolved in so many organisms, so Juan Rubalcaba set out to explore which energy balance regulation strategies could feasibly support this energetically costly trait while also still benefitting the organism.
The study of how organisms allocate energy to processes like growth and reproduction in order to maximize their fitness (their ability to survive and reproduce) is known as life history theory. Rubalcaba wanted to determine if an evolutionary stable endothermic strategy could have emerged from organisms optimizing their life history, and so he created a modeling framework to explore the idea. He developed a life history model for the balance between how an organism assimilates energy and how it allocates that energy to survival, growth, thermoregulation, and reproduction. He hypothesized that the temperature-dependent generation of energy performed by endotherms can produce a surplus of energy, which could then be used to fuel both thermoregulation and reproduction.
The results, however, were quite interesting. Although the higher body temperature in endotherms allows them to assimilate more energy, a really large amount of this energy must be allocated to producing heat. Thus, the energy left for growth and reproduction is actually similar to the ectothermic strategy. Contrary to the common argument that endothermy is somehow the “ultimate goal” or more advanced evolutionary strategy, this model suggests that endothermy does not necessarily outperform ectothermy; rather, it simply provides an alternative strategy that would be beneficial in certain environments, like colder regions. Rubalcaba’s model shows that a drop in temperature can force populations to reach a breaking point, allowing endothermy to rapidly expand; while ectothermic organisms would be eradicated once the temperature drops to a certain point, endothermic organisms could continue to thrive in those climates without competition from ectothermic organisms. In this way, birds and mammals have expanded into many new environments that would be far too cold for an ectotherm. There are more details on the formation of the model and its implications in Rubalcaba’s paper.
Me? I might prefer to bask in the sun like a lizard. But if I ever decide to move to Alaska, it’s good to know that my body’s homeothermic strategy will be up to the task of keeping me alive.
Kaleigh Remick is currently a PhD student in the Department of Molecular Biology at Princeton University, where she studies replication of influenza A virus in the Velthuis Lab. She graduated from Cornell University with a B.A. in biological sciences and a minor in English. When she’s not in lab, you can find her bartending, salsa dancing, tutoring at prisons, conferencing at the Writing Center, eating ice cream, or reading a book!
